A Thesis Entitled Engineering Geology of the London Clay in The

A Thesis Entitled

Engineering Geology of the London Clay in the London and Hampshire Basins

Submitted for the degree of Doctor of Philosophy in the Faculty of Science in the University of London

by Alister Douglas Burnett

Geology Department, February, 1972 Royal School of Mines, Imperial College. Abstract The aim of this study has been to investigate relationships between geological and geotechnical properties of the London Clay in the London and Hampshire Basins. This has been carried out on three scales, i.e. a laboratory or "one dimensional" scale; a single engineering site or cliff exposure, a "two dimensional scale"; and a regional scale from top to bottom of the deposit, i.e. "three dimensional scale."

It was possible to relate the more factual geological information, thickness and structural contours, lithology and a sedimentqklgical and palaeontological biased zonation system of the London Clay to the detailed mineralogy of the clay. The mineralogy and lithological (mainly grading) characteristics were found to have close relationship to the mechanical properties of the clay and by this means geology and geotechnical properties were linked.

The geological and geotechnical data was largely gathered from the literature either published or private, whereas the mineralogical data was accumulated by the writer on analysis of over 300 samples.

The study has been corroborated and linked with strong sedimentological evidence which suggested a rhythmic pattern of clay deposition giving rise to regional lithological, palaeontological and mineralogical variations both horizontally and vertically. These variations have been analysed and plotted using several techniques, both manual and computerised. Geo- technical property variations have been similarly plotted and comparison of the two sets of data reveals significant correlations at varying confidence levels.

Regional plots of clay zonation, palaeogeography, structural contours and isopachytes and mineral distribution maps and plans are presented. Geological and: geotechnical depth profile sections are used to establish qualitative relations between lithology, zonation, mineralogy and geotechnics.

The magnitude of influence of changes in lithology and mineralogy on soil properties has also been investigated by means of a series of laboratory experiments designed to simulate ii

in-situ variations within the deposit. These experiments found the grading variations within the London Clay to account for some 14 percent of its plasticity variation whereas the minera logical distribution accounts for the complete plasticity variation range found throughout the London Clay of the London and Hampshire Basins.

A testing programme to investigate the effects of weathering on London Clay produced results which indicated that this phenomena has played only a small role in soil property variations. iii

Contents Page Abstract List of Figures vii List of Plates xi List of Tables xii Chapter 1 Introduction 1

Chapter 2 Geology of the London Clay Sea Basin 6 A. Regional Stratigraphy 6 B. Palaeogeography and Structure 0 C. Sedimentology 9 1)Principals and Early Views 9 2)Current Views and Regional Palaeogeography 11 D. Conclusions 18

Chapter 3 Review of the Chemistry and Mineralogy of Clay 20 Deposit Genesis A. Sedimentary Clay Mineral Environments 20 1)Chemical Aspects 20 2)Mineralogical Aspects 22 B. Sediment Deposition 25 1)Chemical Aspects 25 2)Mineralogy Aspects 25 3)Discussion 29 C. Diagenetic Processes 29 1)Chemical Aspects 29 2)Mineralogical Aspects 32 3)Discussion 33 D. Weathering 34 1)Chemical Aspects 34 2)Mineralogical Aspects 36 3)Discussion 38

Chapter 4

Processes Culminating in the Formation of the London 42 • Clair

I. Geological History, Structure and Deposition 42 iv Page A. Palaeogeography and Strui:ture 42 B. London Clay Zonation 48 C. Depositional Characteristics 50 II. Mineralogy 54 A. Regional Mineralogy 54 B. London Clay Mineral Suite 58 C. The Vertical Relations between Lithology and 59 Mineralogy D. The Regional, 3 Dimensional, London Clay 61 Mineral Distribution 1) Puartz Distribution 62 7) Total Clay Mineral Distribution 63 3)1Caolinite Distribution 64 4)Illite Distribution 65 5)Montmorillonite Distribution 65 6)Cation Exchange Capacity Distribution 66 E. Regional London Clay Isopleth Distribution 67 1)Quartz Isopleths 67 2)Total Clay Mineral Isopleths 67 3)Ka1inite Isopleths 68 4)Illite Isopleths 69 5)Montmorillonite Isopleths 70 III. Environment of Deposition 70

Chapter 5 Geologica: :factors Influencing Engineering Properties 74 A. Fabric and Mineralogy 75 B. Void Itio 77 C. Permeability 78 D. Plasticity 79 E. Sensitivity 81 F. Compressibility and Consolidation 87 G. Shear Strength 84

Chapter 6 Experiments to Relate Geological and Soil Index Parameters 86 A. Sedimentation Simulation 86 B. Removal of Overburden 91 C. Investigation into Weathering 92 1)Mineralogy 93 2)Chemistry 94 3)Cation Exchange Capacity 95 Chapte 7 Page Geotechnical Investigation of Principal Sites and 100 Lateral Trends I. Detail of Principal Sites 100 A. Ashford-Sunbury 101 B. Bradwell-Orford Ness 104 C. Streatham-Central London 108 D. Hoak-Hill 109 E. Herne Bay 110 II. Lateral Trends 113

Chapter 8 Regional Trends in the Engineering Index Properties 117 of the Deposit A. Liquid Limit 119 B. Plasticity Index 121 C. Dry Density 122 D. Undrained Shear Strength 124 E. Discussion 125

Chapter 9 Engineering Implications of London Clay Geology 127 A. Implications Drawn from Clay Deposit Literature 127 B. Implications of the Regional Geology lns C. 2tructural Implications 130 D. Depositional Implications 131 E. M.:--alogical Implications 132 F. Implications Drawn from the Experiments Conducted 134 G. atiwriag Tests Implications 136 H. Conclusions Drawn from Single Site Implications 136 I.Regional Trend Implications 138

Chaptcyr 10 Summary and Conclusions 141 A. Results of the Geological Literature Review 141 B. Structural and Sedimentological Development of 144 the London Clay Basin C. London Clay Mineralogy 145 D. Geotechnical Literature Review and Experimental 147 Results E. Studies at the Principal Sites and Regional 150 Geotechnical Trends F. Analytical Techniques 154 vi

Cnapt, 11 ApperZliz 1 Page A. Choice of Analytical Techniques 157 B. Sampling and Sample Preparation and Treatment 162 1)Sampling 162 2)Sample Preparation 162 3)Sample Treatment 163 C. Interpretation and Quantification of X-Ray 165 Data 1)Whole Rock 165 2)Clay Fraction 167 3)Notes on the Calculation of Whole Rock 171 Mineral Percentages 4)Notes on the Calculation of Clay Fraction 174 Mineral Percentages D. Reproducibility of X-Ray Results 178 1)Diffractometer Characteristics 181 2)Metal Holder Characteristics 184 3)Porous DiSc Reproducibility 185 E. Accuracy 18C' 1)Quartz Determination 18C 2)Chlorite Determination 193 3)Cation Exchange Capacity 194 4)Carbonate Determination 196 5)Full Chemical Analysis 197 F. Summary and Conclusions 201 Ac~~nccvlo:~ tints 203 neferences 205

Appendix 2 Vol.II Mineralogic..71 124-,st Data

Appendix 3 Vol.II Geotechnical Test Data vii

List of FiLas Chapter 1 Figure No.

1.1 Location and General Geology Map

Chapter 2 2.1 Diagramatic Representation of 3 Cycles of Deposition n n ea. /.3 Palaeogeography of the Lower Landenian Sea 2.3 Palaeogeography of the Upper Landenian Sea 2.4 Section through the N.W.Cent Area 2.5 Palaeogeography of the Basal Ypresian Sea 2.6 Palaeogeography of the Uppermost Ypresian Sea 2.7 Palaeogeography of the Upper Ypresian Sea 2.8 Palaeogeography of the Lower Lutetian Sea

Chapter 4 4.1 Chalk Top Contours and Zonal Boundaries 4.2 Trend Surface Chalk Top Contours 4.3 That Bed Isopachytes 4.4 Wool'Ach and Reading Isopachytes 4.5 London Clay Computer Isopachytes 4.6 Computer London Clay Base Elevations 4 7 London Clay Lithology and Zonal Boundaries 4.8 Lower London Tertiaries Depth Grading Profiles 4.9 Vineral Distribution Within a Single Sample alum Bay London Clay Depth Profiles /,11 Lonclon Clay Quartz Distribution 4,12 London Clay Total Clay Minc..-al Distribution 4.:3 London Clrty Kaolinite Distribution 4.14 London Clay Illite Distribution 4.15 London Clay Montmorilionite Distribution 4.16 London Clay Cation Exchange Capacity Distribution 4.17A Upper London Clay cultrtz Distribution 4.17E Middle London Clay Quartz Distribution 4.17C Lower London Clay Quartz Distribution 4.18A Upper London Clay-Clay Mineral Distribution 4.18B Middle London Clay-Clay Mineral Distribution 4.18C Lower London Clay-Clay Mineral Distribution 4.19A Middle Londdn Clay Kaolinite Distribution 4.19B Middle London Clay Xaolinite and Chlorite proportion distribution viii

4.ncA Middle London Clay n.Ae Distribution 4.20B Middle London Clay Illite Proportion Distribution 4.21A Middle London Clay Montmorillonite Distribution 4.21B Middle London Clay Montmorillonite Ptoportion Distribution

Chapter 5 5.1 Particle Orientation in Clays 5.2 Comparison of Activities of Clay Mixtures 5.3 Principal Types of e Log P Relationship for Cohesive Sediments 5.4 Correlation of Compression index with Percent Clay Fraction 5.5 Pressure-void Ratio Curves for Montmorillonite Xaolinite 5.6 Rate of Consolidation of Montmorillonite and Xaolinite 2 at 4 to 8 Tons/Ft. 5.7 Coefficient of Consolidation Versus Pressure 5.8 Preoure-Void Ratio curves for Hydrogen Clay Minorals 5.9 Li(1,'_dity Index Versus Shear Strength for Remouldt-c: Clay 5.10 Shear strength Versus Depth for normally Consolidated Clays showing surface drying Zone 5.11 Residual Shear Strength of Mineral Mixturos

Chapter 6 Graded Cmartz added to London Clay 6.2 Liz.,'1.id Limit Variations on Addition of Quartz to Orford Ness London Clay 6,3 Clay Conti nt Versos Liquid Limit on Addition of Quart to Orford Ness London Clay 6.4 Linear Relationship :Jotween Liquid Limit and Percentage of Added Xaolinite 6.5 Plot showing the Separate Ao-Uvities of the London Clay-Xaolinite Mixtures 6.6 Relationship between Activity and Percentage Xaolinite added to London Clay from Walton on thei;-aze 6.7 Relationship between Liquid Limit and Percentage Montmorillonite added to Herne Bay London Clay 6.8 Relationship between Activity and Percentage Montmorillonite added to a Herne Bay London Clay ix

6.9 Plasticity Chart for London Clay-Montmorillonite Mixtures 6.10 X-Ray Versus Optical Orientation Measurelhents of Flocculated Consolidated Xaolinite Samples 6.11 X-Ray Versus Optical Orientation Measurements on Natural Clays 6.12 Undrained Shear Strength of Unweathered Whitecliff Bay London Clay 6.13 Undrained Shear Strength of Weathered Whitecliff Bay London Clay

Chapter 7 7.1 Ashford Common-Sunbury Section 7.2 Orford Ness Section 7.3 Streatham-Central London Section 7.4 Noah Hill Section 7.5 Herne Bay Section 7.6 Clay Mineral Percentage Versus Plasticity Index 7.7 Montorillonite Percent Versus Activity 7.8 Clay Mineral Percent Versus Effective Residual Strength 7.9 Percentage Montmorillonite Versus Effective Residual Strength 7.10 Casagrande Chart of Data from Published Literature 7.11 Cacagrande Chart of Writers Data

Chapter 8 8.1 Plot of Regional Liqrid Limit Data 8.2 Piot of Regional Plasticity Index Data &,3 Plot of Regional Dry Density Data 8.4 3-Dinsional Computer Trend Surface Map Showing Regional Liquid Limit Distribution 8.5 3-Dimensional Computer Trend Surface Map Showing Regional Plasticity Index Distribution

8,6 3-Dimensional Computer Trend Surface Map Showing Regional Dry Density Distribution

8.7 3-Dimensional Computer Trend Surface Map Showing Regional Undrained Strength

8.8 3-Dimensional Computer Trend Surface Map Showing Regional Cuartz Distribution

8.9 3-Dimensional Computer Trend Surface Map Showing Regional Clay Mineral Distribution 8.10 3-Dimensional Computer Trend Surface Map Showing Regional Montmorillonite Distribution 8.11 3-Dimensional Computer Trend Surface Map Showing Regional Xaolinite Distribution 8.12 3-Dimensional Computer Trend Surface Map showing Regional Liqtadity Index Distribution 8.13 Plot of Regional Undrained Strength Data

Chapter 9 9.1 Idealized Mineral Distribution Within a Basin 9.2 Theoretical Liquid Limit Increase Seaward

Chapter 11 11.1 Typical London Clay Whole Rock X-Ray Diffraction Trace 11. X-Ray Diffraction Truces of an Oriented London Clay Sample After Various Pretreatments 11.3 Diagram showing Complete Resolution for a London Clay Minus 2 Micron Fraction 11.4 Subdivision of Simple Sample for Statistics Tests 11.5 Summary Plot of X-Ray Statistics Tests 11.6 Effect of Grinding on the Estimated Mineral Percent.ge 11.7 Quartz Percentage Working Curve 11.8 Grinding Curve for A.D.Burnett 11.9 Standard Curve for Quartz Determination 11,10 Typical Infra Red Spectrum showing the 12.5u Quartz Doublet 11,11 Infra Red Versus X-Ray Quartz Percentage 11.12A) CT2'7tz Percent Versu; X-Ray Peak Height 11.123) Cuartz Percent Versus X-Ray Peak Height 11,13A) Quartz 1- ent Versus X-Ray Peak Height 11.13B) Quartz Percent Versus X-Ray Peak Height 11.14A) Quartz Percent Versus X-Ray Peak Height 11.14B) Quartz Percent Versus X-Ray Peak Height 11.15A) Quartz Percent Versus X-Ray Peak Height 11,15,3) Quartz Percent Versus X-Ray Peak Height 11.16 Chemical Versus X- Ray Carbonate Percentages 11.17 Chemical Versus X-Ray Carbonate Percentages 11.18 Al203 Versus X-Ray Clay Percentage 11.19 Chemical Versus X-Ray A1703 Percentages xi

List of Plates

Chapter 2

Plate No. 2.1 Woolwich Beds Overlying Thanet Sands 2.2 London Clay Overlying Woolwich Beds 2.3 Lower London Tertiaries 2.4 Fales Bedded Coarse Woolwich Beds Base of the Reading Beds 2.6 Current Bedded Sandy Blackheath Beds Pebbly Blackheath Dods n.a London Clay Overlying Blackheath Beds 2.9 Basement Beds of the London Clay Overlying Reading Beds 2.10 Claygate Beds Overlying London Clay 2.11 Current Bedded Bagshot Sands

Chapter 4 4.1 Pebble Zone in the London Clay 4.2 Ripple Marks in the London Clay 4.3 Plane of Energance and Erosion Within London Clay 4.4 Whitecliff Bay-Looking East 4.5 Sandy and Clayey London Clay Zones 4.0 London Clay Landscape

Chapter 6 6.1 Tranoition from Clay to Sand in a London Clay Zone 6.n smAll Scale Sand-Clay interlyers in London Clay C.3 Weathored London Clay

Chapter 7 7.1 London Clay Overlying Oldhaven Beds n Oldhaven Sands Over Woolwich Beds Over Thanet Sands at Bishopstone Gap 7.3 Heavily Fissured Brown London Clay 7.4 Stiff Fissured Grey -Blue London Clay

xii

List of Tables Chapter 9J Table No.

9.1 Summary of Published Stratigraphy

P.2 Summary of Published Strucural and Palaeogeographical Papers

Chapter 3 3.1 Summary of Papers Dealing with Clay Mineral Deposits

Chapter 5 5.1 Void Ratio of Crushed Quartz 5. Permeability of Clays and Sand-Clay Mixtures 5.3 Influnce of Grain Size of Crushed Minerals on Atterberg Limits 5.4 Atterberg Limit Values 5.5 Compressive Strength of Sand and Clal, Mixtures

Chapter 6 6.1 Results of Grinding Test la 6.2 Results of Milling Test lh

Chapter 1C, 10.1 List of Work Undertaken and Results Obtained

C4twtoT 11 11.1 Factors for X-Ray ,Quar 4 tati7e Interpretation of Whole Rock Mtnerals (Ghult...., 1064) 11.2 Working C x..r for the Calculation of Whole Rock Mineral rercentages 11.3A Whole Rock Reproducibility Teet Results 11.33 Oriented Disc Reproducibility Test Results (Five Discs from 1 Bulk Sample) 11.30 Oriented Disc Reproducibility Test Results (Five Disc from 1 Split) 11.3D Oriented Disc Reproducibility Test Results (1 Disc each Treament Run Five Times) 11.3E Oriented Disc Reproducibility Test Results (1 Disc,l Set of Treatments, Interpreted Five Times) Summary of Methods )5 2), and 4) for Quartz Determinations Summary of all Methods used for Quartz Determinations Comparison of Cation Exchange Capacities Determinations Cation Exchange Capacities Calculations Full Chemical Anaysis of 6 Samples Mineral Percentages in Samples Analysed Comparison of X-Ray and Chemical Illite Percentages Factors for X-Ray Quantitative Interpretation of Whole Rock Minerals (Burnett1197S) 1

CHAPTER 1 Introduction This thesis relates a regional scale engineering geology study on the London Clay of the London and Hampshire Basins. The London Clay is a sedimentary deposit of Eocene age and has for many years fascinated geologists and engineers alike. This is I:4rdly surprising as the clay underlies L=don and many other important centres in S.E.England. The Than-:s Estuary, Solent and Southampton Water all lie within its ou-crop area and take advantage of lines of structural weakness within the deposit and its bbdrock thus placing several large ports within its boundaries. When the large outcrop area of the London Clay is coupLad to its geographical position, it is easily seen why such interest has been shown in both the geological and engineering aspects of the deposit.

From a geological point of view, the London Clay is an interesting subject to study as a great deal of structural and palaeontological zonation control can be obtained for it. This control largely stems from the fact that the deposit outcrops ever a very large area in both the London and Mampehire Basins, is not too extensively overlain by younger beds and is heavily penstrstod by boreholes. Exposures and thus samples are plentiful foa' examination. As the clay is a sediment use may be made of scaf_mentological principles by which the mode of depositi-ma and property variations within the clay may be readily understoI.

Prow a geotechnical point of view, the deposit has been uaed as a British "testbed" of the subject for many years and in fact English soil mechanics can be said to have largely grown up cn the London Clay. This has resulted from the high density of structures to be founded on the clay, its extensive outcrop area and troublesome nature, all of which have played their part in inducing the numerous investigations of the clay. Also the fact that centres of research such as Imperial College, Road Research Laboratory and the Building Research Station consider London Clay as their "local" material.

Geological observation and discussion has taken place for more than a century and little now remains unknown of the general lithology and stratigraphy, structure and palaeontology of the Eocene deposits cf S.E.England. Betwz,en 1846 and 1855, 7,!-I-f)stwich published a series of papers dea:ing with a variety of topics ranging from the stratigraphy and correlation of the Lower London Tertiaries, the London Clay age and structure to a discussion of the Bagslot Sands. These classic studies were followed between 1874 anU 1889 by Whitakerts publication of two memoirs and other works on the London Valley and Bas-n. Boswell (1916) undertook a detailed study of the stratigraphy and petrology of the Lower Eoc'me deposits of the north easts!rn part of the London Basin and thfs was followed by important papers by Stamp (1921) on cycles of sedimentation and correlations in the Eocene strata of Ang)o-Franco-Belgian Basin. Wooldridge (1924) Hawkins (1955), Curry (1965), Davis and Elliot (1957) and Hester (1965) have all wince made valuable contributions to Eocene stratigraphy.

The structural geology of the London Sea Basin was brought to its ?resent state of knowledge firstly by Prestwich in 1854, then 137' Boswell in 1916 and finally by Wooldridge between 192 and 1938. Similarly, palaeontological studies have a,:lvanced from the p.oneering 7,irk of Wrigley (1924 and 1940), an_w even prior o tint Wetherell (1836) to studies by Chandlcr (1262), Davis T..= Elliot (1957), Curry (1965) and (1970). These wor,ir, with the help of many other contributors, have succeac::ed it ,JLitablishing a fivefold palaeontological zonation of tho ILdoll Clay which is closely related to and supported by fiedimentnlogioal features. The palaeontologists, through a study of the flora and fauna have also been Ithle to advance a tentative environmental picture of the land and sea conditions during Eocene depos.ltion.

The history of London Clay soil mechanics began about 1940 with the laboratory testing of specimens of clay mainly from engineering sites in London by Cooling (1940), Cooling and Skempton (1942) and Bishop (1948) and the publication of the results. Since then, numerous site investigations have been undertaken but these are largely unpublished. In 1957, however, Skempton and Henkel completed a comprehensive work involving tests on London Clay from deep borings at Paddington, Victoria and the South Bank. Ward, Samuels, Butler and Marsland followed shortly in 1959 and then 1965 with papers on testing prograkmes from 3

Ashford Common and varic,us London cites. Bishop, Webb and Lewin (1965) also discussed test results from the Ashfor,_! Common . The latest well described site was that of: BradwelL, Essex, which proved extremely interesting and was described in 1961 and 1965 by Skempton and Skempton and La Rochelle respectively. All of these papers were somewhat divorced from the more classical geological approach to the subject.

Cne of the main aims of this engineering geology study is to investigate the gap that exists between the lithological and palaeontological studies and even more important to relate the unified geological picture to the rather more localized geotechnical field. Hence it is hoped to be able to draw together and rzllate qualitative geological information and detailed quantitative geotechnical data into a simple broad picture which will result in a better understanding of the deposit.

In order to carry this out, four important steps have been undertaken. The first step was to assemble, access and reduce to a simple comprehensive concept the vast amount of literature dealing with the lithological, stratigraphical, tructural and palaeontological spects of the London Clay. These individual fields were then brought together to reconstruct the history of the sediment from its depositional pattern lnd environment to the stratigraphical and structural changes it under:ant th.f.',..,shout its lifespan.

With the overall geological picture established, the second step was --,..rred out. This involved a collation of relevant gf.,cechnf_cal data, interpretation of this data ar.d finally its 1,iy into suitable fc,r1-1 such that an orderly pattern or trend emerged. Each parameter was then related to the already established geological form of the deposit.

However, this relationship was not easy to accomplish as certain essential intermediate facts were found to be missing. The third step was thus to recognise these missing links and to set up the necessary techniques for their retrieval. In this case the missing information was judged to be of a sedimentological nature and which when found would bind the separate fields of deposition, lithology, palaeontology and soil mechanics together. This evidence could only be established from a detailed sedimentological examination of the London Clay. Therefore, the following studies havr.:, bf.n recently conducted-- the general mapping of litholotry,couf3ed with a regional palaeontological examination of -t,ne deposit, b) a regional scale zonation system based on a) with units being reasonaly easily identified by visual means, c) a regional scale mineralogical examination of the clay concentrated on the whole rock constituents and not only the heavy minerals. This study was of great importance in relating lithology to stratigraphy and palaeontology but even more important in correlating geotechnical properties with geological parameters, and d) experimention cliLic;ned to establish with greater certainty the relationship between mineral grain size and content and soil index properties. The effects if mineral weathering were also studied by means of actual experiment. An appraisal could then be made as to which geological process or mineral characteristic was of greatest importance with respect to soil property origin and variation.

The fourth and final step was the linking and correlation of all the concepts and data produced in the first three step: into a comprehensive sensible model which would a:C1‘.w the history of the s4,climent to be traced from its deposition to its :anal uplifted and weathered form. In doing so, it wsta possible to account for the principal soil properties and their regional va7;_tion.

These four steps form the bulk of this thesis but before arriving at these, a systematl.c, reasoned programme of study had first bet=a carried out. Without such a programme, it is doubt- 7°:;1.L whether the results and conclusions would have been x-Lached, k:1: indeed whether the interested reader could have followed the background information and current reasoning.

The programme of study commenced with a comprehensive literature review of four subjects, namely London Clay geology, London Clay soil 1,echanics, general clay deposit mineralogy and chemistry and analytical mineralogical techniques. The condensed and refined results of these reviews are to be found principally in Chapters 2, 3, 6 and 7 and 11 respectively. Hundreds of samples were collected from main locations, as illustrated in figure 1.1, and over 30C of these were examined fairly thoroughly mineralogically. Selected samples and sections were then subjeced to further testing and examination before all the data was compiled and plotted in svt:eral forms, using both manual ant] covutnr techniques. These results are t be found in Chapters 4 and 5. Applied sedimentology principles guided the programrp of study through these initial phases to its completion shown in Chapers 8 and 9 where the results, their implication2, and the final conclusions are discussed in detail. 6

CHAPTER 17. Geology of the London Clay Sea &-s.in It must be stressed that th,- London clay is only one facies• of a large suite of freshwater and shallow marine deposits which were laid down over much of North West Europe in Ralaeogene times. For a full appreciation of the London Clay geology, it is necessary to fully review the literature concerning these alaeogene deposits where significant to the present work.

A.Regional Stratigraphy One of the earliest works of importance in English Tertiary geology is a monograph of fossils on the Mardwell (Barton) cliffs by Solander in 1766 (in Brander). In 1814, after some initial work had been published by William Smith, Webster subdivided the Lower Tertiaries by recognising the alternations of marine and freshwater formations in the cliff sections of the Isle of Wight. Similar alternations had also been noted above the Chalk near Paris about this time. Between 1812 and 1846 the Sowerby family published their excellent descriptions of Lower Tertiary fossils. Other authors, e.g. Lyell, Sedgewick, Bowerbank and Martell, also studied these ralaeogene deposits but no real progress on sn' division and correlation was made.

Between 1846 and 1857 Josh Prestwich contributed nine papers on the succession of.:!...Dcene beds in Britain and two on their relation to similar beds in the Belgian and Paris Basins.

Prestwich first divided the then "London '71ay Formation" into an upper London clay and lower Bognor beds, but shortly afterwards, realizing a mistake, renamed the latter London Clay and introduced the names Bracklesham Sands and Barton Beds for the upper division. Also in 1847, he subdivided the Bagshot elands and correctly correlated these with the Bracklesham Beds. Next, in a series of three papers, Prestwich dealt with the basement beds of the London clay (1850), then described and named the Thant Sands (1852) and lastly (1854) recognished the beds we now know as the Woolwich and Reading Beds and demonstrated that they are of the same age. In 1854(h) he contrasted the faunas of the London clay and Bracklesham Beds and in the 7 same year (1854g) attempted a zonal subdivision of the London clay.

Further, papers between 1350 and 139C resulted in thE?, final subdivision of the Englich Lower Tertiaries, Lower London Tertiary deposits. The stratigraphical names of neadon, Osborne, Bembridge and Hempstead Beds were created by Forbes in 1853, Whitaker in 1866 separated the Cldhaven Beds from the London Clay basement Beds, Fisher (1862) described the distribution and subdivisions of the Bracklestam Beds as did Gardner, seeping and Monkton (188C) en teee Barton Beds.

Meyer (1870) while examining deposits at Portsmouth recognised a basic rhythmical aspect to the sediments which was eecontuated by faunal associations.

Two memoirs on the London Basin (Whitaker 1372, 1829 a and b) end three on the Isle of Wight and Strahan 1889) we_'e also published by the Geological Survey.

After a lull of some twenty years, the sub,foct was again scrutinized in a flurry of papers beginning f_a, 'YA.6 with Boswell, who studied the lecene Beds of Essex and Suffeik in great detail, relying heavily on structural evidence and sedinentary petrology.

Stamp in 1921 published two papers (a and b) enlarging on the cork of Meyer (1870) in connection with cycles of sedimentation and showed existence 32 successive ma::oe renrine inundations in the nandenian (Thanet and Woolwich and Reading times), YpresE-Imn (London Clay tines) and Lutetian (Lower Brackleffham) tLAos. e:, ramp also attempted detailed correietion of specific eorizons over the Anglo-17ranco-Belgian Basin, which he regarded as a single geographical basin of deposition as well as developing Prestwich's idea of a "Wealden Island."

Wooldridge (1924) described the Bagshot Beds of Essex as well as studying the history of the London Basin (1923 and 1926). A tabulated summary of these important stratigraphical papers has been drawn up (See Table 2.1)

Wrigley (1924) erected a five-fold bioctratigranhical division of the London clay in the London area which he was subsequently (1940) able to extend and correlate over much of the London Basin. In 142 this same author wrote a brief description of the Thanet gnnds. Table na Summary of Published Works (Important Strat- igraphy Papers

Author Date Subject Solander 1766 Fossil monograph Webster 1814 Subdivision of Lower Tertiaries Sowerby 1812-46 Lower Tertiary fossil description Prestwich 1847 Age of London Clay 184 7 Structure E. Age of Bagshot Beds

IMO 1850 London Clay Basement Beds 1854 The Thanet Sands

Owl 1854 The Woolwich and Reading series 1854 The Thickness of London Clay YIN -854 The London clay EL Bracklesham Sands Forbes 1853 The Headon, Csborne, Bembridge Hempstead Beds Prestwich 1855 N.W.European Eocene Correlation Whitaker 1866 The Cldhaven Sands Fisher 186f,' The Bracklesham Bed subdivision Gardener et al 1888 The Barton Beds Whitaker 1872, The London Basin Memoir 1889 Reid & Strahan 1889 Isle of Wight Memoir Meyer 1870 London Clay pebble bands Boswell 1916 London clay Stamp 197A Cycles of Sedimentation 19n1 The Lower London Tertiaries of N.W.Europe Wooldridge 1924 The Bagshot Beds Wrigley 19:14 Faunal divisions of the London clay

AMP 19-10 The faunal succession in the London clay White 1931 Bishopstone Beds Hester 1965 Woolwich & Reading Beds Williams 197C Subdivision of the London clay 8

White in 1931 proposed the r%„-!enBishopstone typenfor the 5ft. bed of grey and green sand exposed near Herne Bay below the Cldhaven Sands and correlated this with the estuarine Woolwich Beds further west

D. Palaeogeagraphy and Structure The structure of the London and Hampshire Basins has been studied for many years '(See Table !;.2). Pr-estwich in 1847 examined the structure of the Bagshot Beds as well as that of the London Clay. Between 1850 and 1854 this author published works on the structure of the London Clay Basement Bed, the Thanet Beds, the Woolwich and Reading Beds and the London and Bracklesham Sands. In 1854, Prestwich published the first isopachyte map of the London Clay and made comment on the contour gradients approaching outcrop margins and probable shore line positions.

Boswell in 1916, during a study of lower Mocene deposits in Essex and Suffolk, proposed lines of structure and isopachytes which have subsequently been shown to be rezarkably accurate.

A few years later in 1923 and 19!.. 6 Wooldridge in a series of farrows papers, described the minor structures of the London Basin. He was able to demonstrate the existence of a series of folds.°and monoclines in the region which were active before after London clay deposition.

Davis and Ellie 41958) in their paper on the palaeogeography of the London Clay Sea examined 14,1aspects of the deposit and cc minted cm the C,,,;rman, Dutch, Belgian and French successions. They also gave their interpretations on sea transgression and regression timing.

The stratigraphy and palaeogeography of the Woolwich and Reading Beds was extensively discussed by Hester (1965) who produced isopachyte maps of these deposits as well as the Thanet Beds.

Hawkins (1954) described ani_important series of post- depositional structures affecting Tertiary deposits in the Enborne region, and it appears that these sort of structures are developed in many areas.

valaeogene beds of South East England are discussed

Table Summary of Published Works (Important Structural and Palaeogeograohical Papers)

Author Date Subject Prestwich 1847 Structure of Basement Beds 1854 The thickness of London clay Boswell 1916 London Clay Structure & Thickness Wooldridge 193 Minor Structures of the London Basin Structural Evolution of London Basin - & Linton 1938 Structural Evolution of S.E.England Chandler 195-63 Lower Tertiary Floras Hester 1965 Structure and thickness of Thanet Beds & Woolwich & Reading Beds Hawkins 1954 Tertiary Structures Davis & Elliot 195C Palaeogeography of London Clay Sea Stamp 19'1 Palaeogeography of Lower London Tertiaries "in; 1954 Geology of English Channel Curry 1965 Palaeogene beds of S.E.England 1,47- 1970 Palaeogeography of London Clay '1:accheiter1 1961 Isopachytes of N.W.Europe Tertiaries 9

by Curry (1965) who summarized previous works and presents some informative sections in A paper blending stratigraphy, palaeontology and palaeoecology.

Chandler, working over a period of many years (1925 to 1963) has produced an important study on the plants, especially the seeds of the Lower Tertiaries. These studies have important implications with regard to depositional environment.

Recently Williams (1970) in a Ph.D. thesis of London University has suggested a stratigraphical zonation of the London Clay based upon environmental characiwiristics re-sealed by sedimentological and palaeontological criteria.

C. Sedimentology (1) Principles and Early Views With this brief review complete and by way of intrcduction to a more detailed bed by bed description of the London Clay Sea BarIA, the outstanding work of Stamp (1971), in connection with the princifles of sedimentation in a basin, will now be reviewed in greater detail. This applied sedimentology approach is of paramount teportance in understanding the vertical and lateral sequence of rocks at any one, or indeed varying times, within a basin undergoing transgressive and repressive marine inundations. It helps explain for example why basal conglomerates or sands are followed vertically by clays, or why nearshore sands can be of the save age as offshore clays.

Three important premises must be borne in mind to make understanding of the cycle of sedimentation concept easier. Firstly, to create marine inundation or transgression, either the land must sink or the sea must rise (the inverse is true for regression) or both. In the following discussions, the land will be assumed to move rather than the sea. Secondly, in the areas covered by the Palaeogene Basin, the regions inundated were generally flat and of low elevation (such as the Low Countries are now). Hence it was not necessary for deposition to cover such dramatic topographical features as the Chalk Cliffs for example. Thirdly, the fact that present .day sedimentation areas are noticeably increasing shows that over a period of a few million years regions as large as the North Sea can easily become silted up. 10

Much of the following description comes direct from Stamp (1.91), "If one considers a sea transgressing over a steadily sinking (flat) landscape, the first, stage in its progress will be marked by coarse, conglomeratic deposits of coastal Origin." (This is because with shallower near-beach conditions the water energy is high and the churning action disturbs and removes to seaward areas of lesser energy all but coarse sand and pbbbles.) "The Sediments pass laterally, in a sentward direction, into sands and finer deposits." In a later stage as the beach advances landward V:.: conglomerate it created at a certain location ands itself covered by deeper (less high enJ1-,:;7) water where sand and then eventually silts and clays are being laid down. "This process continues during the transgression and so in deposits of a .simple overlap of this nature one has, at any point, -a vertical transition from pebble beds tha'cugh coarse sand to finer sands and clays." Thece materials are of course not of the same age but young upwards. "That the (whole, of the) basal conglomerates are not of the same age throughout (its aerial extent) is .)roved by a study of the faunas. It is found that paln.iontological (and hence time) zones pass laterally from clays into sands and coarse sands and probably into the ccastal pebble beds (from seaward to landward . "Such zones are represented in the diagram (Fig.2.1) by horizontal layers marked Zi, Z2, Z3 etc.

'Then the transgressive movement of the sea ceases, there is generally a tendency for a basin to become "silted up."" Either from continental or oceanic organic sources. As the rivers deposit debris into the sea, the coarse fraction is dropped near the shore with the finer sediment being carried further out to sea (See Fig.2.1). This silting up results in the sea shallowing or regressing seaward as that, "the marine deposits pass gradually upwards into estuarine beds with a brackish-water fauna. These are succeeded in turn by lagoonal (brackish or fresh water), lacustrine, fluviatile or even aeolian deposits." This regression of the sea is thus marked by a recurrence of cosmser sediments lying over marine clays. Shallow water sedimentary features such as ripple marks are seen in plate 4.2

"The succeeding marine invasion may remove a portion of the continental or coastal deposits, the coarse or hard materials being rearranged to form a new basal conglomerate. Where the C3

74 Cl A Z3 _ _ _ ".:• • Tf •

A B

T T T T T R R R R S Dil L,------L___>-- ---k,..._ „..------A L ?.vel I I 1st Cycle 2nd CyclE 3rd Cycle F 71 Z2 Z3 Z4 Z 5 Z 6 R . ,...._. B . , 1st Cycle 2nd Cycle 3rd Cycle

(After Stamp, 1921)

Fig. 2.1 DIAGRAMATIC REPRESENTATION OF 3 CYCLES OF DEPOSITION.

M. MARINE DEPOSIT, C = CONTINENTAL DEPOSIT, Z = PALAEONTOLOGY ZONE, R =POSITION OF RAVINEMENT, T. JUNG TION PLANE BETWEEN ZONES THE 2 GRAPHS SHOW THE OSCILLATIONS OF THE LAND SURFACE RELATIVE TO SEA LEVEL AT THE POINTS A AND B. 11 latter are fine-grained or unconsolidated the now basal conglomerate may be feebly developed and inconspicuous, but the plane of erosion between the new marine deposits and the underlying beds whether of continental or marine origin, is almost invariably well-marked. In other words there is a "ravinement" at the base of each marine formation. Such a plane of erosion or emergence may be seen in plate 4.3.

The period between two successive ravinements constitutes a. cycle of sedimentation, with each oscillation including a positive Illase of marine invasion and a negative phase of regressic,n. 0:1 Current Views and Regional Palaeogeography

The Palaeo gene deposits occur in four main localities .10 the so called basins of Paris, Northern France and Belgium, London and Hampshire. Uplands of Cretaceous or older rocks separate these four areas of Dacene deposition which 'wire once almost certainly part of one great Basin - the Anglo-Franco- 1::;elgian Basin. A small amount of pre-depositioned folding of the chalk has been noted but the major folding which separated the basins was mainly post-Eocene.

A general description of the palaeogeography is given by Stamp (191) - "The Paris Basin is situated mainly to the North of Paris. Southwards the Eocene series becomes incomplete, since some of the marine invasions did not penetrate beyond the latitude of Paris. The Paris Basin is separated from the eocene region of Northern Franco and Belgium by the chalk uplands of Artois Picardy and Champagne. A few intermediate patches of lower Eocene rocks are also found.

In Jelgium, Eocene rocks occupy the whole of the northern and western parts of the country and this basin extends into the adjoining regions of Northern France. It stretches northward into Holland, but the Eocene rocks are there covered by a great thickness of later deposits. To the south east the Ardennes apparently formed an area of dry land during most of the Socene period, and the deposits tend to assume a littoral character as they are traced in this direction.

Cn the extreme west of the North France-Belgium region near Calais and St.Cmer, the succession is so similar to that on the opposite side of the Straits of Dover on the Isle of Thanet and

I - , -11,•41 I. •

• .'' like

- 4

Plate 2-1. .7ine sandy Woolwich Beds overlying Thanet Sands at Upnor, Kent. Pebbly Woolwich Basement Bed is present.

I) • '•• 41.41. • • • -7-'`.,. -:;. ..' 2*/ • - . : -, 7 0, ... . 4-•-• • - ..t‘. 1:, /1', , ., ., . ,. f- ;--,.-. -,..4e, 214:-- ,.c.-- .-f-i.v" .',"-- -4' -1•;. -. 1 .. .,...... ,,,. -' .-'•'•?:;r-r,... , ‘ ,.;•.;,....,... i: 43 r.„.:,;4C,.i.;,_,.:.. , .._tr,.:..,,.. 4.„.....„..„,,t.,.i-,... „,i,,,...... ,.„ t ., ...., ,.. . -, • ,., ., - "--. ---V-. .' 4 ., 44.:,;.,,,, , I''l: ,•./r-/-. 4rS , . . • 1",-• ":04 ), ,-:, --‘--- ...... — • • ....., . -- ,..x ,,, 1 'I's . .• a• 7-f;:1,, e -r , . 4 ., ' ' s' . ' .. •.7 ...^' 4•1; ,..".it --gr • ,..i10140 l'...r. ... -0'''.0.- 04.'")+DiPolir ...e . . . , ,r..,...- ' ,I: ..: ,th 44, ..c..,,. -..,,- --(4attn - „.••••• r-r• Z. ...V;,'', 4, - ' ''''' 4":'''''. ...-,- ,— ,.. L ,-. -;,....-e '- ' - • -1-4- 4 . . 'f7"-z. Mr, '0 - LI

1•'' ii - , -,--.- - --r- -c,opp,-.., -;••••.1.--,--P2'4,-. 4.--", -.""--

f "

• . . . • •

Pl:te 2.7. Top few feet of Plate 2.1. London Clay is overlying the light coloured Woolwich Beds. 12

near Herne Bay that there seems little doubt concerning the former continuity of the deposits.

There is every reason to believe that the Hampshire Basin was originally joined to the extreme western end of the London Basin. The existence of a few small outliers as well as relict blocks (sarsens) on the intervening Chalk lands and the close lithological resemblance between the two areas are sufficient proof.

The outliers at the eastern end of the Hampshire Basin, especially near Newhaven, form a connecting link win the small outliers on the other side of the Channel near Dieppe (as does the recently proven Mid-Channel Eocene). In many respects, the -3cene beds of Dieppe resemble the HampshLxo deposits more clearly than they do the Paris deposits. Not only are the Dieppe deposits connected with the main part of the Paris Basin but also with the deposits of Norihera France. r,) The Lower Landenian or Thanet Beds The oldest Tertiary beds in South East England are the Thanet Beds whichoccur in a rough triangle bounded by Rams- gate, 17-d'om and Ipswich. Those beds are essentially marine lauc3nitic fine grey sands with seams of clay, whi,:;h become less clayey westwards and thin out in this direction :°::om a maximum of 12C feet, to a feather edge just to the m.ist of London. The basal conglomerate is known as the Bullhead Bed.

These English beds are part of the remains of the first major post-Cretaceous marine inundation. This Thanetian or Lower Landesian Sea also covered much, if not all, of the Belgian-North France Basin as well as depositing the marine Sands of Brachequx in the Paris Basin. See fig.n.2. As in the English beds, the French sands are glauconitic, often argillaceous towards the base, tending to be harder on the lower part and to form beds of "tuffeau." Glauconite becomes less abundant in the higher zones.

All these deposits rest unc nformably directly on various zones of the Upper Chalk. Chalk extending at least into the Naestrichtian stage already been laid down over this enormous area. Uplift followed, which was especially concentrated in the area of the London Palaeozoic Platform and the Artois Axis. Erosion then set in removing more than 500 feet of Chalk over the greater part of the London Basin during a 15 million year period. Plate 2.1. Lower London Tertiaries, probably sandy Reading and pebbly Thanet ]3eds overlying Upper chalk at Harefield, Middlesex. 13

The Lower Lmndenian Sea, which was supposedly shallow (250m) and cool to warm at first probably occupied a minor area but under the influence of regional depression, gradually transgressed. During this regional subsidence and toward Middle Landenian times the Wealden buckle probably experienced a slight folding movement or a minor and rapid uplift, giving rise to a ravinement within the marine Landemian. This "kick" is marked by the basal pebble bed of the Woolwich Bottom Bed. This Woolwich Bottom Bed (Cyprina scutelloria zone of the Continent) marks the greatest extent of the Lower Landemian Sea and is found virtually without break in the East, Centrli and West London Basin, East and West Hants.Basin, 7orthern Ye'rance and Belgium as well as in the central Paris Basin. b) Upper Landenian or Woolwich and Reading Beds As a result of the cessation of subsidence (and hence of the marine Woolwich and Reading Bottom Bed), the Lower Landenian lea became silted up (from the margins inward) with sediments grcliped together, as Upper or continental Landenian (Woolwich and Reading Beds). Figure 1.3 shows this phase of deposition of fluviatile sediments (Reading and Argile Plast1Aue type) from the rarginal landmsses which possibly extended over the Weald -Artcis axis, lagoontl. azA esturine (Woolwich and SparnacfA,n type) brackish shallow water sediments town.2'els the centre of the two basins and finally a marine and (Bishop- stone and Marine Sparnacinn type) at the centre.

The lowest member of the series is almost everywhere * glauconitic pebbly marine sand which is known as the Woolwich or Reading Bottom bed, where appropriate. Plate n.3. In East Kent the whole series is of marine sands and is called the Bishopstone Beds. In north-west Kent the Bishopstone facies is confined to a few feet of sand at the base of the series and a new facies appears above; one of 'ark estuarine clays and sands - the Woolwich Beds. Falsebedded Woolwich Beds are shown in plate

In the London 3asin to the west of London and throughout the Hampshire Basin a third facies was recognised, that of the Reading Beds. This comprises predominantly reddish, grey and green pure clays with lenticular beds of sand. Plate 2.5 shows the base of the Reading Beds.

The picture presented by the Woolwich and Reading series

,--)••4111111o1r.....

• 5.fS. • , • 44 ••f.-)r ... .4 ' •• , ...... , $ c -.1. ,. ' 4 - f ' ... - .,:-. ,:-••••toor--- 7r"..

. .... , / .., ' , 2 ,.... . 4, f • ' ...• - . , .: • .4'; ...4. . -,-. - ,...... ". '-. -.7-4- I . ;

i ' • - " •

F - , •:.: . „ . : , ,.• , , ., t7s.,,. ;I:',4:;,":f) ..,.....:3',:;...,1: 1

. ,.. 1 , . z.,__ -, .. , . -••••- , • ,,.,- .„„•••••,,,..:,,,•,,,,..4--- •.-r A• 1 I •-.. __, 0.: • - --,•,:,,,• r., +r,, ,,”. •"' ":" - '. ' '" "r ":7;-';'-- ' *":" ' ' - ' • ;' ' • ''' . ' ',...."`. ' '' -j.r ; ?"' •-.,•- ' -',r .... .f. ' , , ' ;;,„,' s r -7;ilf ..,. :-••--'fr - • , , • •••- . '. - •-;, •,--- ,.. . '0‘.14' P"'. OW ,f",• . + ,,r if , '•4'. e .."--' 1 •e c,(-1. A -, 0 ' • - :- '') .1:* i :''''' ': ' , ' •S: ;.; I 7...,...r.7*- , ,„,;,,,..., ,• ,'41,,,.41,14•1 ", "' ' otiv-' t, :" ••-‘-'•-•..,-/ ' ' ' ' ' i .i• .r ' • , ., , I , .... y . ''' r , ,

• ,-.. -. ' • • ' • • ) '

•, ' • '•;-- 7' • t•

Plate 2.4. False bedded coarse sands of the Woolwich Beds from Fernhill, Kent. Pr ,1 44#‘14,17219, r •••,. .5 e '.C4 -fA=444 4

S1*

s eft Pigs.

-71

• -es44

Plate 2.5. Base of the Reading Beds showing lowermost current bedded glauconitic sand with contemporaneous erosion plane and overlying sands at slough, Bucks. 14 as a whole is thus of a shallow 3 to- the east and south, fringed by brackish mud-flats surrounding the sea and which stretcb -7rom London to Northern Belgium and Bognor to Paris (i.e.S4nith of the shallower Weald-Artois Axis). These, in turn '-acked by a lowland country stretching in a great arc to t_?.) north west of London fro!i• Suffolk to Hampshire thence into the channel and the South Paris Basin. North France ar.c: ,)uth Belgium lying on the somewhat uplifted Artois Axis are above sea level and fluviable. Upper Landenian Sands are deposited here. Table summarizes all rocks of Lutetian Age. c) Basal Ypresian or Oldhaven and Blackheath Beds Woolwich and Reading Bed timos were terminated in the eastern region by a new marine transgression probably caused by a deepening of the Thames Valley syncline. Figure shows a section through N.W.Kent during these times. The -trey invaded approximately corresponds with that occupied by the Thanet Beds though the southerly limits are unknown. The destruction of much of the underlying Woolwich and Reading Beds in this region bears witness to the vigour with which the sea returnd. The Blackheath Pebble Beds appear to form a great 11-: c,-escent made up of individual crescendic banks of pebbles, see plate somewhat resembling a series of sand dunes. The crescents are convex to the east. Cn this outer side no Cidhaven Beds were being deposited, while on the inner western side the topmost fluviable Reading clays were being laid down.

Lithologicilly these thin beds (less than 30 feet), which are rather rich faunally, are more marine to the east. Here they comprise about 2,5 feet of current Bedded fine samd.(1500 Plate 2.7) with seams of molluscs and are really more estuarine than truly marine. This is typically the Cidhaven Beds. At the western limit of their range (See fig.1.5) the sands contain quantities of pebbles which are exclusively flint. This is the facies known as the Blackheath Beds.

In the Paris Basin, the waters of the invading Ypresian Sea flooded very rapidly over the Upper Landenian Lagoons. There are extensive accumulation of well rolled flints and everywhere very fine or well washed sands, showing the disturbed conditions which prevailed at the time. For a short time the incoming marine fauna lived side by side with the existing fauna

Table 2.3

Summary of Landenian Age Roezz

Lower Middle Upper

West Reading Reading 1 (Fluvaitile) London Bottom Central Marine Woolwich (Estuarine) Basin Thanet Woolwich Bottom Bishopstone East Sands I Bed (Marine) N.France Sands d CstricourtI Upper Fluviatile Landenian becoming Belgium Lower Marine Landenian Estuarine near Cctend Channel Estuarine Beds i (Woolwich Type)

Rants 1 West Marine Reading k (Fluviatile) I. Bottom Basin East Bed Woolwich (Estuarine) Paris (Centre Lower Marine Landenian,Tuffeau 1 Upper Landenian and Sands of Bracheaux (Sparnacian) Basin Margins Calcaires Upper Landenian Lacustres (Argile Plastique)

S N

Thanet Sands Chalk

• — _ Pfc. .• , _--- Woolwich Shelly Clay . ..O. .. •E"--Woolwich Bottom Bed . , • • . • b

—Blackheath Beds

Sea Level

London Clay with Basement Bed

(After Stamp, 1921)

Fig. 2. 4 SECTION THROUGH THE N.W KENT AREA Plate 2.6. Current bedded sands with pebble and shell layers of the Blackheath Beds from Elmstead, 7ent.

Plate 2.7. Sandy and pebbly bedded Blackheath Beds from )3rickley, Kent. 15 of the lagoons and the curious mixed fauna of the sables de Sinceny resulted.

This Lower Ypresian Sea did not appear to extend over the Hampshire and North France-Belgium Basin and only a few scattered pebble beds and sands are noted in the latter region. d) Upper Ypresian or London Clay (Fig.2.6) The local movement responsible for the curious Blackheath deposits soon gave place to a slow, gentle subsodemce diromg which the London Clay Sea transgressed widely. As far as is Lnown, it originally extended over the whole of South 777st England and the English Channel. Equivalent beds, similar in fauna and lithology, extend through northernmost France into the Low Countries and north-west Germany. Davis and Elliot (1958) are of the opinion that the London Clay 7-4a was almost land-locked with an opening to the north, between Scotland and Scandinavia. This, of course, is consistent with the cold watur aspects of the marine fauna. They also E:-.,ggest a connection was opened south-westwards to the Tethys about halfway through London Clay times but Curry (1965). believes it to hs.Y!e occurred in Lower Bracklesham tires.

The lowest few feet of the London Clay arc referred to as the London Clay Basement Bed and are almost invariably sandy and glauconitic with accumulations of marine molluscs. The microfossils also suggest deposition in shallow sea water with no estuarine or fresh water species known from these beds. The London Clay Basement Bed overlies from east to west Oldhaven, Blackheath and Reading Beds. This is shown in plate 7.1, 2.8 and

The diatoms, radiolaria, unbored wood and general lack of other fossils indicates that the lowest fifty feet of London Clay above the Basement Bed experienced foul bottom conditions in a deeper sea.

The main mass of London clay in its unweathered state is a dark brownish or bluish-grey marine clay which seldom shows obvious macrostructure. It is not a pure clay but sometimes contains large auantities of silt and/or fine sand. The sandy beds becoming more prominent westwards, while the middle beds are almost invariably less sandy than those at the top or bottom of the series. Plate 2.8. Light coloured :lackheath sands and pebbles overlain by London Clay from Brickley, Kent.

Plate 2.0. Basement Beds of the London Clay overlying the lighter coloured current bedded Reading Bed sands near Watford, 16

Although the main mass of London Clay was apparently laid down in cool water which was quite deep at various stages (greater than 100 fathoms) Chandler (1961) has concluded that the lar3e flora were derived from a nearby continental region of tropical rainforest with heavy rainfall with high temperatures (over 70019.

The very uniform and fine grained nature of London Clay lithology contrasts with the Bracklesham Beds -but resembles the Thanet Beds. This leads to the conclusion that even near presout day margin of the outcrop the actual shoreline is qu to distant as well as indicating an ancient sea which was free of streng currents.

The London Clay is thickest to the east of London ti re it attains a thickness of about 550 feet in Essex, progressively to the west ia‘,Ing, about 100 feet thick N;-,ding, 30C feet at Portsmouth and about 100 feet thick in north--west Hampshire and Dorset.

The 7presian of Belgium and Northern France consists of a mass of clay of identical lithology to the London Clay. At the base there is usually a bed of sand (often at least 1 metre in thic&nea) and below that some pebbles occasionally forming a thick pebble band but frequently only a sparse layer. The whole mass in the plains of Flanders generally consists of clay, but towards the east and more especially the south, beds of fine sand are to be found at the top of this mass. Tracing the continuation of these beds by means of a number of outliers which connect the Belgian and Paris Basins, it is thought that the whole of the Ypregian is represented in the latter basin by the Sables de Cuise which are a series of shallow marine fossiliferous sands, generally very fine grained.

As the top of the Ypresian (and the lowermost Lutetian) is reached this vast London Clay or Ypresian Sea became silted up. In the Hampshire Basin, the marine shoreline retreated, leaving lagoonal tonditiOns and a brackish-water sandy Upper London Clay zone. Landward of this zone, i.e. to the west, the continental lower Bagshots was laid down. See fig. 7.

In the London Basin, the silting up resulted again in sandy conditions but this time the Claygate Beds (see plbte 2.10) Table 2.4 Summary of Ypresian Age Rocks

Lower Middle Upper (:West (Topmost Reading Basement Bed 1 Lowest of the London Bagshots London --- Clay 'Central Blackheath 3eds London Basin (Pebbly types) Clay East. Cldhaven Beds (Basement (Sandy tyke) Bed) Pebble Bed & Argile N. France Sand d'Ypres Belgium Pebble Bed & Argile Ypres- Sand Jenne Sable Ypresian

Channel Pebbles (Galets Clay de Oldhaven) + sand I Hants West 1 Basement Bed London Basin East of the London Clay Clay Centre Sables de Basement Bed Marine S incery of the Cuise Cuise Paris Sands Sands Margins Topmost Argile Continental Basin Plastique Sands, Urias, Herouvol, t Belleu

..., --. • :,- - . _ • ,`Pf.„-_, "... .. • ,-.r..--„,_,,,,.-,„•,;,?,.,,,,-, _ ..--1-7,..-4.4. .. ,.. -r-",,--- r- 1.r.. :-.- -,--- - ,;,--,. ...--- --''.. • - 4'' ....•,47*--,-:,--"--"- ,.. • -- - ' -- : ---.. .,.,,,. ; • -•,. •- - _ -z----biriOrQ:7"-P' .----- .._,;:--.-,,,,. r -.., .-.-- _ -ter-7-iX.-1,-. , .... • ,-,...... -_... , • 4F - ' r,,-:-.,,,- - ,.4.4- - ‘:. ' "' • - , -..'"";-' -'-' -

- .., - ' M. e • • -14 •".-v •

- .-`-` • , •

Plate 2.10. Sandy seams of the Claygate Beds overlying London Clay at Claygate, Surrey.

Plate 2.11. Current bedded Bagshot Sands, 1 r_ile HW of Wareham., Dorset. 17 overly the topmost sandy London Clay and grade into the Lower Bagshot Sands of partial freshwater environment (Curry 1965).

The argillaceous Argile de Flandres in Belgium, gives way On sea shallowing to the Sands of Mons en Pevele or Ypresian Sands with a local deeper basin of Roubaix clays. These marine beds are equivalent to the Lower Bagshot of the London Basin and are of Lower Eocene cage. Table 2.4 summarizes all rocks of Ypresian a.ge.

In the Paris Basin, the shallow Cuise Sands Sea retreated ,1L,3rthwest leaving ingoonal and continental sands and ,-Am!,-Es in the fon:LI of the Sands of Unia, Sands of Brasles, Sands of Herouval and clays of Laon and St.Gobain. Tite connection to the warm tropical Tethyan Ocean in the south west appears now to have been accomplished (Stamp, 1981), e) Lutetian or Bracklesham During this cycle large portions of the four basins again LAecame inundated by marine transgression. This transgressive sea appears, from palaentological evidence, to have been open to the cold north and warm south west oceans. This means that the sa would have been converted from one similar to the Baltic, with small tides, into something more like the present North Sea. With thec strong tides and heavy marine erosion a conroer and lore variable litholo_a, would be expected. These features are best displayed by the Lower Bracklesham Beds of the Hampshire Basin.

The Lutetian invasion is marked in the above region by a basal pebble-bed or pebbly glauconitic and at Bracklesham and Whitecliff Bay which is followed by a great thickness of marine glauconitic sandy clays, the Bracklesham Beds. At the same time, a large amount of sediment was being dep,:_sited to the west under mainly fluviatile conditions as the Bagshot Sands and Bournemouth Beds. Typical Bracklesham Beds can be traced to the New Forest but are not seen again until their lateral equivalent, the London Basin Marine Bagshot Beds are encountered. Plate 2.11, illustrates these coarse grained beds. These beds' are considerably thinner (100 feet as compared to 50C feet) than the Bracklesham and are essentially a series of marine sand with subordinate clayey bands. All these Bagshot divisions are equated by Curry (1965) with the Hampshire Bracklesham divisions. See Fig.2.8 and Table

Table 2.5

Summary of Lutetian Age Rocks

Lower Middle Upper 4 1 London West Marine Marine Bagshorts Basin East Claygate Beds I Bagshots N.France Facies Calcaires Belgium Sands of Mons Paniselion Bruxellian en Pevele I --- — Sands Channel Uppermost London clay Hants West Bournemouth Boscombe Upper Beds Sands Basin East 3agshot 1Bracklesham Bracklesham Beds !Beds Beds Paris Centre Sands & clays of i Calcaire Upper Laon St.Gobain Calcaire Basin Margins Continental Sands Grossier Grossier Uniae, Herouval, Belleu 18

The Belgian Lutetian lies above the Sands of Mons en. Pevele and there are good reasons to believe that the PaniseliaM is a non-calcareous 16611 shallow water representative of the lower part of the Lutetian. Its fossils seem to be intermediate between the Ypresian and the normal Lutetian. It passes up gradually by loss of glauconite into the paler and more calcareous Bruxellian sands which overlap the Paniselian in an easterly direction. Thus it appears the Paniselian is a facies of the lower Lutetian, deposited in a shallow sea stretching eastwards as far as Brussels. Possibly the Lutetian Sea trariLgressed from the north towards the Paris Basin so that these Bruxellian deposits are slightly earlier than ti.; lowermost Calcaire Grossier division of the latter region. (Stamp 1921 and Prestwich 1855).

The Paris Lutetian is the famous Calcaire Grossier. It generally commenced with a coarse glauconitic sand, containing pebbles or rolled fossils at the base. The marine Lutetian consist of varied calca ous beds, of which the best known is the shelly, foraminiferal limestone the "facies calcaires." Sandy limestones and calcareous sands are of common oocurence. The Lutetian is divisible into four zones, the first of which only just reaches the Paris Basin, while the higher zones successively overlap to a much wider distribution.

It is not intended to discuss beds higher on the seouence as they are not widely distributed except in the llampshire Basin and the principles involved in their sedimentation are the same as discussed above. Re eremce to all these beds described above have been made from the following public- ations Boswell 1916; Stamp 1921; a; Stamp 1921, b- Prest- wich 1855, i; Hester 1965; Davis and Elliot 1957; Curry 1965; Itaascheiten 1961;Pomerol and Feuguer 1968; Lericke 1915.

D. Conclusions The principles of sediment deposition are particularly important in this study as they not only give rise to the major variation between deposits but may also be used to explain and predict more subtle lithological variations within the LOndon Clay itself. These variations include a slight vertical and horizontal silt and clay fraction change. The occurrence of occasional but critically important pebble bands 19 within the clay and individual clay mineral distributions. Al6 these latter factors form part of the study at present being undertaken as does the environment of deposition. With all thos9, factors known and supplemented by the extremely nicropalaeontological study by rI.WilliaLs (197C) which rr:',ducod a regional zoning of the London Clay as well as comming on its depositional environment, it is possible to produce the most complete geological picture of this clay to date. This information can now be statistically and visually Compared with soil mechanics porameters in the hope of obtaining a correlation, thus showing the influence of geology on civil engineering. 20

Chapter 3

Review of the Chemistry and Mineralogy of. Clay Deposit Genesis The topics dealt with in this cahpter are extensive. Therefore, in order to do justice to a study such as the one at present being described it is imperative to have a good and thorough understanding of the work undertaken by previous authors in all related fields. Hence the object of this ri,!arch through the literature is to obtain a thorough but :concise impression of most of the important work carried out within the reasonably short lifetime of the subject. An important proviso, however, is an appreciation of the fact that all the papers and works read, should fall within a skeleton or frau.ewark which is pertinent to the s.Aimentary environment under study. The review is set out in such a way as to make use of the typical processes and stages which a sediment must undergo during the formation of a deposit. These stages include. provinance (or source rock and area), transportation (almost exclusively fluviatile in this instance), environment of deposition, deposition, dingenesis and weathering. As the first two stages were not dealt with in the present work, the literature review deals with them only briefly when and if they occur under one of the other processes. As this chapter heading indicates both the chemical and mineralogical aspects are to be dealt with and hence each of the four remaining stages are divided into these two sub- sections. Table 3.1 may help explain this review scheme as well as show how the various topics are related. It may be noted that a similar layout has been used in Chapter 8 where all these stages and processes have been enlarged upon but from the viewpoint of their engineering implications and importance.

A. Sedimentary Clay Mineral Environments Firstly in reviewing the effects of environment on clay minerals, it is important to show that valuable clues to the environment may be obtained from the minerals themselves. 1) Chemical Aspects In dealing with the chemical aspects of environment, it was established as early as 1946 by Zobell, that each deposit Table 3.i

Environment Deposition Diagenesis Neathering Degens,Williams&Kieth Griffin (1960 Lynn&Whittig(1966) Griffin (1962) (1957) Grim&Johns(1954) Powers (1959) Lynn&Whittig(1S66) Zobell(1946) Porrenga(1966) Nelson(1958) Hensel&White(1S58) Xrumbein&Garrels(1952) Groot&Glass(1958) Carrol&Starkey(1956) Ferrel&Grim(12E7) Nicholls(1963) Olnuma&Nobayashi,Sudo Scott&Smith(1966) Budo(1962) Nicholls&Loring (1961) (1959) Burst (1959) Droste (1959) Mineralogy Frederickson&Reynolds. Milbe&Earley(i953) GrimIDeitz&Bradley Battacharya(1962) (1959) Whitehouse,Jeifrqy& (1949) Earrison&Murray(1957) Simons&Taggert (1953) Debbrecht (19601 Grim (1970) Gjems (1967) Taggert&Xaiser (1960) Biscaye(1965) El Wakeel&Riley(1961) Eroshchev-Shale(1962) Burst(1959) ...... 0V••••.INIS Degens,Williams&Mieth Grim, Deitz&Bradley Harriss (1967) Jacison(1948) (1957) (1949) Powers(1959) FreCarickson(1952) Turekian (1955) Grim&Johns(1953) Powers (1953) Jorgensen (1965) Kulp,Turekian&Boyd Milne&Shott(1958) Keller(1904) Rosenqvist&Jorgensen (1952) Millot(1942) Chemistry Grim&Johns (1953) Walker&Price (1963) Tourtelot,Schultz&C • - Huffman(1961) Landergren (1945) 21 or sediment type had its own Eh and pH characteristics on sedimentation. The pH varying from 6.4 to 9.5 and the Eh from +0.35 to -C.50 with a general pH increase in depth and Eh decrease. That is, burial inducing more alkaline and reducing conditions.

In 195n, Krumbein and Garrels, in a paper on the origin and classification of chemical sediments in terms of pH and Eh defined three environments representing deposition in normal marine open-circulation, restricted humid (euxinic) and restricted arid (evaporite) environments. They contend that pH and Eh afford two basic controls which largely determine the kinds of chemical and members produced by both inorganic and biochemical reactions, The deposition of calcium carbonate, iron minerals, manganese minerals, phos- phates, evaporites and organic matter are shown within the framework of these controls in their relation to variations in pH and Eh of the environment. pH, Eh and salinity graphs and diagrams are used to accomplish this.

In 1963, Nicholls puts forward two figures showing Eh-pH stability diagrams for all forms of iron minerals at 20°C and 1 atmosphere pressure as well as discussing in great chemical detail relative bond strengths related to detrital and non-detrital minerals under varying environments. This author also discussed the use of boron and organic material as salinity and general environmental indications.

That pH, salinity, (and probably Eh) conditions do, in fact, vary from area to area within a depositional region was vividly displayed by Grim and Johns (1953) who plotted pH and chlorinity against environment (inland bay and open gulf) for the central Texas coastal region. The bottom sample and surface water pH as well as chlorinity were seen to increase seaward.

Since Landergrens (1945) suggestion that marine and non- marine sediments might be distinguished by their boron contents, this possibility has been examined by numerous geochemists. Degens, Williams and Keith (1957) found that boron and rubidium were more abundant in marine sediments than in fresh water ones and that gallium was more abundant in the fresh water carboniferous sediments they were studying. They also 22 related the abundance of these elements to clay mineral composition.

Within the range of salinities tested, Frederickson and Reynolds (1960) found that the boron content varied proportion- ntely and that clay size i]lite minerals in many sedimentary rocks of known salinity of depisition bore proportional boron contents.

A relation between boron content and total abundance of clay minerals, sources raterials, boron content of sea water, and rate of sedimentation was established for the Pierre shale by Tourtelot, Schultz and Huffman (1961).

Walker and Price (1963) discussed the uptake of boron according to potassium content, grain size and depositional salinity for illites.

Strontium and even magnesium has been suggested as a chemical salinity indicator and Xulp, Turekian and Boyd (1952) and later Turekian (1955) have investigated this possibility without :arriving at any clearcut correlations. 2) Mineralogical Aspects It has long been established that various mineral suites are more "at home" or in equilibrium under certain environmehts, hence it is eaually feasible to reverse the roles and obtain environmental information from the mineral suites. Much general information or trend data can be gathered regarding pH, Eh salinity and temperature by this means. Millot (1942) has carried out mach work in this respect and found that in marine sediments illite was invariably present, making up from 5C-10C percent of the total clay mineral content. Higher illite contents being noted in calcareous marine sediments (70-10C percent). Xaolinite was less frequent and made up from C-50 percent of the clay minerals. The presence of calcite tended to inhibit, and pyrite promote, the recurrence of kaolinite. Chlorite and vermiculite minerals being minor constituents. In lagoonsl sediments, illites again mainly predominated but smectite and attapulgite-sepiolite occasionally assumed major proportions. Very noticeable was the total lack or very minor proportion of kaolinite. Chlorite and vermiculite again frequently occurred as minor constituents. 23

%u lacustrine sediments of "aggressive water" origin, i.e.active 1eaohinz or lownH, kaolinite is the dominant mineral totalling from 70 to 100 percent.

In sediments of lacustrine origin of the nonaggressive type, i.e. little movement and slight alkalinity, the dominant minerals are illite, smectite and sepiolite-attapulgite.

This phenomena of mineral suite (or mineral) stability, orpreference for certain environments has been studied by numerous other authors who generally come to the conclusion that illite contents, relative to kaolinite, are greater in marine than contInental sediments, that shales as opposed to sandstones contain more illite. Large, well-formed particles Of kaolinite are characteristic of continental deposits and small, poorly formed plates of marine origin. The converse is true of illite particles.

Not only are these broad observations well established but greater sophist:_cation is being attempted in the realms of environmental conditions versus mineral stability. Pryor and Glad (1961) studied Zretaceous and Tertiary clay mineral compose too 2rom the Upper Mississippi embayment. They found that clays deposited in a fluviatile environment were dominantly ktolinitic. Those in the outer neritic environment dominantly smectitic and those in the inner neritic region comprised equal kaolinite, illite and smectite. Hayes (1962) in a study of Mississippian strata from Iowa found that kaolinite was near shore, chlorite intermediate and illite off-shore.

Grim and Johns (1953) substantiated and developed this micro-environment versus clay mineral stability relationship with mineral, pH and chlorinity versus environment (from the source to open gulf) plots showing the general diminution of montmorillonite and kaolinite from inland to sea and increase of illite and chlorite. It must be noted that X-ray diffraction c spacings are particularly useful in observing mineral lattice depredation and crystallization. Degraded, weathered minerals, e.g. continental illite are seen to pick up potassium, within the lattice, and improve their lattice configuration as salinity pancreases. Degens, Williams Xeith (1957) remark on, and in fact use the shape of the illite (001) peak which becomes broader and more assymetrical towards the sea. This 24 peak also tends to shift from 10.1A to 10.4A on moving from fresh to saline waters.

In 1962, Nicholls and Loring, working on an individual Welsh coalseam in a cyclothem of about 40 ft. thickness plotted thickness versus log weight percent siderite and FeS (norm) as well as the chlorite and kaolinite proportions. Both the iron curves and the clay mineral curves "kicked" inversly at the sails depths obviously showing a change in environment. An Eh, pH, iron minerals stability diagram was then used to .7:1x. the Lh Er pH of tae older beds. TLa chaucse in (:,1, ironment to the upper beds was then ascribed to a stilal variation, as to have CMILIG the variation on pH grounds would have necessitlted a large change which would have been reflected in the phosphate content. The postulated Eh chanT'i,1 was z%:,ufirmed by the predicted decrease of carbonates being observed.

In conclusion to.this review of environment indicators, it is obvious that many have been tried; carbonate and iron mineral form; or?:anic content; evaporate form; phosphorite content; boron, strontium and magnesium cotent; individual and ILLile:cal suite content; EIL. well as present ^.:y 2h, pH and chloriraity measur:Jments; but none are really proven or entirely satifactory. The indicators, uEod singly, neithur give very good absolute or even relative values but when used collectively, a reasonable environmental picture may be obtained and explanations for various mineral changes found.

Two other common and reasonably reliable indicators not mentioned so far are fossils and the mineral glauconite. The presence or absence of various fossil species indicates in a general manner the environment of deposition, as certain of these animals can only survive under limiting, well known conditions. In addition to acting as rough Eh and p11 guides, different fossil species afford some clues as to the depth, temperature and salinity of the sea water at the time.

The mineral glauconite is setected for special consideration as it is a diagenetic clay mineral which, if encountered, indicates an unchanging reducing environment (-Ye Ph) maintained by bacterial action. It also appears to very sensitive to magnesium content and is formed mainly at depths of between 1C and 400 fathoms with preference to warm waters and slow sediment.-, ation. 25

B.Sediment Deposition 1) Chemical Aspects This section deals with sediment deposition, and the small number of papers covering the chemical aspects will be discussed first, followed by the numerous works relating mineralogy to sedimentology. It must be emphasozed that there is a very slender line of division between sediment deposition and sediment diagenesis both from a mineralogical and chemical point of view. Aiiboth processes may occur in the same environment and over the same span the literature tends to grade from one topic to the other.

Grim, Deitz and Bradley, as early as 1949postulated that potassium was being taken up by accumulating Pacific Ccean rlediments, mostly by degraded or partially weathc.rL2 illite to form reconstituted illite and similarly for magnesium in the formation of illite or chlorite.

Grim and Johns. (1053) corroborated thes-,) ideas when work on clay sediments frets the central Texas coast cowed an increar Inc) and MgO sedimen'L content seaward tarough various Emb-eilir.:_-:mentE;, This correzonded with an iito increase toward 1=y) open sea and a chlorne increase to the "br„ys" region followed by a decrear,a., to the open sea. To verify tLis, Gr141- and Johns (1953) carried out elemental pore water chemistry on the samples to ascertain if these elements had, in fact, been absorbed. Graphs of element concentration versus environment showed a potassium and magnesium pore water decrease in the delta region corresponding to the illite and chlorite increase.

Johns and Grim (1958) while working on Mississippi Delta samples carried out analyses similar to those of the progmimse above and arrived at almost identical conclusions.

Milne and Shott (1958) found that chemical analyses of sediments from the Mississippi Sound and Mobile Bay area showed a progressive increase in magnesium, potassium and sodium and a decrease in calcium from fresh water to the Gulf. n) Mineralogy Aspects Mineralogical studies on clay mineral distribution patterns were begun as long ago as 1891. After the Challenger samples were collected and this was followed in 1926 by the Meteor sample studies. In 1949, studies conducted on samples collected from the E.W.Scripps by Grim, Deitz and 3radloy, off the Californian coast and in the Gulf of California showed illite, smectite, kaolinite and occasionally chlorite to be present. Illite was the most common mineral, kaolinite the least. Mixed layer minerals were also noted. Some of the chemical results have been described in the section preceding this.

Recent sediments in the Western Pacific were examined by Cinuma, rtobayashi and Sudo in 1959 who found small amounts of /_-_aolinit.:) in all t!1...;. sanples, chlorite in all the samples, illite increasing seaward and being the dominant mineral with smectite increactng northward and in ocean trenches. Little verticn, variation in minerals was noted. In the shallow Eastern Sea (7C-130m) illite, chlorite, kaolinite and span amounts c,f smectite all d71:pplaying good crystallinity.

Zn a series of samples from the Atlantic, Pacific and Indian Oceans,e1 Wakeel and Riley (1961) found that illite, chlorite and smalle:,' amounts of smectite were the principal deep-sea sediments and advocated a continental c;T-igin for them, They :111.2 found that the kaoli'lite content appeared to increase landwarl.

Biscaye, in 1965, after an analysis of five hun0;,..ed sa lea from the Atlantic, Antarctic and Western Indian Cceans determined the relative abundances of montmorillonite, illite, kaolinite, chlorite and various mixed layer minerals. He ruled out diagenesis in the Atlantic but not the South Western Indian Oceans and maintained that the deep sea clay is mainly continental detritus. Sediment provinance for the Atlantic Ccean is aided by the mineralogical analysis of the fine-fraction of deep-sea sediments and kaolinite, gibbsite, chlorite and mixed layer minerals contribute greatly toward provinance information due to their restricted sources of continental origin.

Eroshchev-Shale (1962), also working on Atlantic Ocean sediments, concluded that illite predominates at latitudes corresponding to subarctic, temperate and subtropical climates; kaolinite prevails in tropical, subequatorial and equatorial zones with smectite occurring in less regular form and originating mainly from the alteration of volcanic material. 27

Porrenga (1966) during a study of Niger Delta samples found kaolinite, montmorillonite and illite in that order, to be the main constituents, with some halloysite. The montmorillonite was used as a sedimentological Larker and its content in the sediment increased seaward in contours which ran parallel to the coastline. This seaward increase showed the interesting phenomena of smectite not settling in proportion to its source concentration. Hence differential settling and distribution is noted.

en the question of differential settling velocities of clay minerals in saline waters, Whitehouse,Jef2rey and Debbrecht (1960) determined these velocities for kaolinite, snectitu, illite (and chlorite) and noted that illite settled faster than kaolinite and that both velocities increased markedly with salinity to then reLained constant. Smectite was constantly but slowly affected by salinity from 0-18% and settled much more slowly than illite or kaolinite, it also had the effect, in mixtures, of reducin:s slightly the last settling rate oz illite.

:ivi,are and Vernhet (1951) found that traczo of humic matter greatly retarded the floculation of kaolinite clays. This phenomena was not noted with illite and smectite clays and may partly account for the diminution of kaolinite content with increasing distance from fresh water.

Studies in the Gulf of Paria off South America by Van Andel and Postma (1954) noted illite to be .the most dominant mineral and called upon differential settlement to explain the uniform kaolinite content and abundant open sea smectite.

Groot and Glass (1950 in a study of the mineralogy of the North Atlantic coastal plain found kaolinite and mica with some chlorite present in deltaic, lacustrine and fluviatile environments. Montmorillonite and glauconite then increased dramatically as the neritic environment was reached. Prefer- ential settling and floculation was called upon to partly explain this mineral distribution.

Grim (196C) provides an excellent review on the Gulf of Mexico and Mississippi Delta studies and quotes from his book are given hence. 28

"All investigators (Grim to Johns (1954), Milne and Earley (1958), Johns and Grim (1958), McAllister (1958), Griffin (196n), and Taggart and Kaiser 1960) ) agree that the sediments are composed of variable amounts of illite, chlorite, smectite and mixed layer minerals, and that close to shore the clay- mineral composition reflects, in general, variations in source material. Murray and Harrison (1955) showed that abyssal deeps contain relatively large amounts of smectite."

There is aconsiderable difference of opinion regarding changes of composition as fresh water muds accumulat in a marine environment. Thus Grim and Johns (195) reported the gradual loss of smectite material with the ultimate formation of poorly crystalline illitic and chloritic phases stn sediments from the vicinity of aockport, Texas. These same authors stated that th..1,7oughly degraded micaceous material is being regraded into illite and chlorite in the Mississippi Delta seAiments.

Milne and Earley (1958) have reported the apparent Formation of illite from smectite in areas of very slow sedimentation ln the Mississippi River Delta. They also stated that Pleistoc6me clays in cores from depths of more than 500 2t. off the Louisiana coast contain more illite than present botto_ sediments in the same area, and the increase in illite is attributed to diagenesis.

McAllister (1958) studied sediments in the Gulf of Mexico, west of the Mississippi River Delta and concluded that there was only minor change in composition on contact with saline waters. Griffin (196) reported on clay minerals in sediments of the northeastern Gulf of Mexico and concluded that the alteration in the clays produced by marine waters was minor. He also showed that in the western drainage basins, erosion and transportation of essentially unaltered uontmorillonitic sediment prevails. Eastward weathering becomes more effective and kaolinite gradually becomes more abundant in the soils and river sediment. Consequently, the Mississippi River is contributing a montmorillonitic clay suite, the Apalachicola River is contributing a kaolinitic suite. The Mobile River, between these rivers, is contributing an intermediate clay mineral suite. FJ

Taggart and 'wiser (1S3C) :found no appreciable alteration as the result of diagenesic in the Quaternary Mississippi River Delta sediments off the coast of Louisiana. They further concluded that clay mineral composition of source sediments determines the clay mineral composition of these marine sedimentary rocks. 3) Discussion Summarizing this section on sediment deposition, it appears that the minerals kaolinite, illite and smectite are universally present in varying proportions with occasional smaller amounts of chlorite and mixed layer minerals. The pro7ortions of the source minerals is governed mainly by pi"-,,vInance but the •Iroportion of the sediment minerals may vary from the err ated intim , mixture for one or more of the following reaso.:4o a) Tidal action having a sorting (and disaggregation) effect on the :Jinerals, b) action ;...eansporting minerals selectively, c)Sizes effects, whereby the finest particles are carried furthest, d)Settling velocities experiments (see above), e)r1c4_ ,Altion of minerals upon reaching saline: waters, f)Diagoautic effects.

Kaolinite appears to have a definite near shore or cont inental affinity, illite is neritic (with higher concentration for various reasons), chlorite may be near-shore or deep sea as may smectite and it is these minerals which give the major clues to the sedimentological history of the deposit. Each new deposit conforming only generally to basic rules but detailed mineralogical distribution being largely unpredictable.

C. Diagenetic Processes Diagenetic processes include mineral changes occuring on transportation, deposition and burial of a sediment. The intensity of such changes are, however, not well defined. 1) Chemical Aspects Much chemical evidence and reasoning has been put forward to substantiate claimed diagenetic processes. The marine environment being alkaline and non-leaching, favours the formation or at least stability of smectite, illite and chlorite rather than kaolinite. The presence of much dissolved calcium also tends to inhibit the formation of kaolinite. 30

This is.also, aoted in weathering process studies whereby kaolinite does not form from a calcareous parent until all the carbonate is removed. (Note the close but reverse relationship between diagenesis and weathering).

Harriss (1967) discusses the evolution of the oceans in relation to clay minerals and concludes that the concentrations of Si to react with Al in order to maintain an equilibrium oceanic condition are too low for all clays except kaolinite. He also poses the problel; of the high oceanic Ka/N ratio whic22 should, in theory be reducing (but isn't) if no diagenesis is occurring.

Keller (1964) noted that the NaPS ratio for igneous rocks averaged 1.0 but on weathering, transportation and deposition an average shale ratio is about 0.5. The sodium thus appears to go into solution in the sea water while the potassium is prefereL.tially held in the clay sediments. In this respect of potassium and magnesium uptake, it must be noted that this io made easier by the fact that some of the minerals are degraded in such a fashion as to be alkaline and alkaline earth deficient. This deficiency results from the continental weathering process.

Certain authors (Milm and Earley (1958) and Johns and Grim (1958) ) have proposed that the uptake of potassium and sodium in smectites, together with a change in structure induces the formation of illitic and chloritic phases.

Whitehouse and McCarter (1958) supported these ideas as their research work on artificial sea water containing various clay minerals showed a change from sinectite to illite and chlorite beginning at the settling and floculation stages and dependent more upon the Mg-II ratio than total salt concentration. Other inorganic ions promote, whereas organic material, retards these changes.

With regard to the Si/Al relation in sea water as discussed by Harriss (1967) it had already been pointed out by Correns, Barth Lk Eskola (1939) that at pH's of less than 51 silica is insoluble and R20 is soluble. With pH from 5 to 9, Rno3 is very slightly soluble and silica becomes more and more soluble. Thus at low pig values kaolinite is stable but at 31 higher values minerals with higher Si02, R203 ratios become more stable in preference.

It has also been shown by Millot (1942) that silica sole are relatively stable even at high cation concentrations. Alumina sols are not very stable, being floculated at low cation concentrations and hence in low cation environments there is a lot of available alumina with consequent low silica to alumina ratio minerals, e.g. kaolinite. The inverse is true for high cation concentration environments with the development of smectite.

In 1967, Ferrel and Grim in a series of tests to establish the effect of MOH solution on clay minerals at different acidities, set out a table of "frayed edge" development which decreased from the mineral kaolinite through illite, illite mixed layer and then to montmorillonite.

In 1966, Scott and Smith Lnd established the suaptibility c interlayer potassium in micas to exchange with sodium. The order of susceptibility was muscovite > ›, biotite ) phlogopite . vermiculite.

Carroll and Starkey in 1958 had already completed these tests but using sea water held in contact with clay minerals for varying lengths of time. They found that the amount of reaction followed the order of total exchange capacity, i.e. kaolinite > halloysite illite $ mixed layer x montmorillonite. This result was in fact expected,as ion exchange is the first stage of adjustment of clays to sea water. The removal of SiC2 and Al2C3 was established by titrations as being the reverse of this series except that kaolinite replaces halloysite.

In 1959, Powers put forward his concept of the "equivalence level" in which he tried to explain why minerals such as chlorite, vermiculite and even montmorillonite, which formed by diagenesis were largely absent in ancient rocks and had not in fact "built up" or accumulated. Ile suggested, with much theoretical and experimental chemical backing, that after being laid down in a normal sea with a Mg:N ratio of 5 a clay would, upon burial, increasingly absorb potassium and desorb magnesium and that within a few hundreds of feet an equivalence level would be reached below which : a) Mg and Fe replace Al in the octahedral layer and It replaces these gaps so created resulting in an Si/A1 decrease, X increase, no Mg loss, and less smectite expansion, also b) X directly replaces Mg with a compositional but no mineralogical change. 7) Mineralogical Aspects Proceeding to mineralogical evidence presented in favour of diagenesis, Powers in 1953 published a paper on clay diagenesis in the Chesapeake Bay area. Me studied samples entering the bay from two long river estuaries and found that chlorite was forming from degrathi.1 illite by addition of brucite to illite basal surfaces. The brucite structure continuing to grow across the illite surface with increasing tile and salinity. Most of the chlorite seems to start forming soon after contact with water and may progress throu711 a mixed layer vermiculite stage. The thermal stability of the chlorite increases with increasing salinity and also somewhat with depth. This reflects a build up of the brucite structure in 4CCC years. Powers (1953) also found that the flock size decreased and the crystal sfJzo increased toward the open sea and also that the crystal shapes became more abundant seaward, possibly due to crystal growth. Chemical evidence presented by Powers (1953) supported evidence of chlorite and vermiculite formation.

Powers reiterated these feelings in 1957 when he found smectite, and to a lesser extent illite, being altered to chlorite in the Gulf of Mexico and along the Pacific Coast. He again presented favourable chemical evidence and discussed the theory of equivalence level at some length. In 1959 Powers examined samples from several deepT ertiary boreholes from the Gulf of Mexico in which he found that at depths of less than 5000 feet smectite predominated over illite, chlorite and mixed layer minerals. With increasing depth smectite contents diminished and illite-smectite and even illite values increased to a point when below 9c00-1,0CC feet no smectite was detected.

Burst (1959) studied postdiagenetic clay mineral environmental relationships in the Gulf Coast Eocene and found smectite decreasing in depth to 3000 feet then formation of a 33

smectite-illite interlayer to 14,CCC feet, then virtual elimination of this mineral in deeper sediments. Chlorite also appears to increase in depth as does its thermal stability.

Nelson in 1958 virtually repeated Powers' Chesapeake Bay study and with minor reservations agreed that diagenesis occurred in the form of increased illite and chlorite content and enhanced crystallinity, montmorillonite and kaolinite decreasing seaward.

An increase in the crystallinity of kaolinite as well as its diminishing content in roc:mi of pre-Devonl_an age (Fuechtbauer and Goldschmidt 196) as well as chlorite inrease with age (Eckhardt 1959) are also noted as proof of diagenetic Processes.

In papers published as early as 1949, Grim, Deitz and bendl?y advocated tLJ formation of an illite or chlorite mineral due to the significant structural changes of ":".aolinite in the Gulf of California and off nle coast of California. same basic conclusions were reached by Griffin and Ingram (1955) while studying bottom samples in the Neur-) River estuary along the Atlantic coast.

A great deal of interesting wor has been carried 'our recently Caul-ley, Mart, Pinson ts Fairbairn (1958), Hurley, Brookins, Pinson, Hart and Fairbairn (1961). Hower (1961) in connection with diagenetic evidence using potassium-argon age determinations. The essence of these experiments is the age determination of different clay mineral size fractions. With the age of deposition known (by other means) it is thus possible to establish detrital (older age) sediments from diagenitic (younger age) ones.

1) Discussion In conclusion to this section on diagenesis, there seems little doubt that this process occurs. For the purposes of this study, it is not essential to know at what stage in the sediments life such processes may have occurred. What is important is a guide as to the possible percentage alteration of the clay minerals, and this is little discussed in the literature. It would appear to be a difficult problem, given only samples of some ancient sediment to establish whether 34 diagenesis has occurred at all, to what extent, and when, conceding that normal sedimentological clay mineral distribution may have also taken place. Diagenetic mineral changes appear to be rather subtle and definitely of less importance than primary distribution variations. Nevertheless such information is important from a geological and geotechnical point of view. For example, any process altering the swelling capacity of smectite, either past or present, is important.

D. Weathc!-Ing Tuathering processes are lei.t . ly subaerial. t.nd may IL:t.c.2.-ace ninoralogical or fabric altafations of th,J roe:c they are flcting 12,23n. These processes thus begin once the sediment has emerged from the subacueous environment of deposition and lingenesiu. 1) Chellal Aspects The formation of clay minerals can take place 7; _eans of precipitation from ionic solutions, crystalIArmtion o,iloidal mixtures or by a direct structural charge. It is this last process which is of mu-ticular inte2dst in clay mineral weathering. A direct structural change can involve merely a relatively slight modification of the structure, eg. smectite formation from mica, or a substantial chamse, es. kaolinite and mica formation from felspar. The former change commonly occurs through an intermediate mixed-kayer transition stage.

As early as 1948, Jackson et al. published their fundaLental generalizations on the weathering sequence of clay-size minerals in soils and sediments. Their stability sories or weathering sequence is considered both from the standpoint of the minerals found, and from the standpoint of the basis in crystal chemistry for the SOCIUQUZ30. The seouence of thirteen stages is represented by the type minerals, The weatherill; stage is considered to be a resultant of intensity factors and capacity factors, together with time. The following generalizations are made on the basis of oJserved data 1) From three to five minerals of the sequence are usually present, one or two often being dominant. !1) With increased weathering, the percentage of minerals of the early stages decrease and successive members increase. 35

3)Intermedinte stages may be absent. 4)The weathering is considered reversible moving to the right (in the Jackson (1948) weathering sequence) in soils and left in sediments. 5)The mineralogical composition varies according to climatic conditions, particle surface and proximity to the free surface. 6)The stability factor is a resultant both of crystal structure and of the specific isomorphous elements.

Within the framework of these generalizations much theoretical and experimental ch—lical work hasp been carried out to elucidate theca direct structural changes. Keller (1.) discussed the chemical consequences of weathering under different environments and climates (mainly pH, rainfall and drainage). He explains the variable processes of formation of kaolinite, illite and montmorillonite by means of A1203 and SiC solubility and cation movement. Kaolinite requiring low pH and removal of cations and illite and montm=111onite requiring higher pH's and concentration of caAons due to waterlogging or evaporation.

Frederickson (1952) reaffirms the view that clay minerals are very sensitive to changes in composition, pH, temperature and environment and that they are not stable end members of weathering processes but rather part of a dynamic equilibrium. He outlines their formation by means of flow diagrams and considers illite mainly marine, kaolinite to form under acidic strongly leaching conditions and montmorillonite in the presence of alkaliesand alkaline earths.

The formation of vermiculite by weathering of chlorite was explained by Stephen (1952) in England, and Jorgensen (1965) in Scandinavia, as an exchange between Mg from the brucite layer and protons in exchangeable positions. The proton fastening onto an CH group to form a water molecule which, being about the smil-e size as an CH group creates vacant sites in this layer.

Jorgensen (1965) found that the dioctahedral illite was much less influenced by weathering than chlorite and explained the 10A low angle "tail" of weathered illite by potassium leaving, and water entering, interlayer positions with a 36 contemporary entrance of protons into vacant octahedral positions. (Rosenquist and Jorgensen,(1963) )

This process is thus different from that described by Brown and Norrish (1952) and Brindley and Sandalaki (1963) who believed that the process was an exchange of 113&. for e in interlayer positions, this exchange leading to some possible interlayer expansions.

Hence without going much deeper into the chemical changes and reasoning brought about by direct structural alteration of, one clay mineral to another (refer further to Bradley (1945), Brindloy (1951) and Grim (1970) ) it is necessary to discuss briefly the findings of various authors with regard to the actual mineral changes which occur on weathering of clays. 2) Mineralogical Aspects Sudo et al.(1962) studied the formation of many types of mixed layer minerals from Japan. These minerals, with a regular alternation of illitic and montmorillonitic verm- i3ulitic layers, were probably formed by differences in isomorphous substitution in the "bacMone" layers.

Soveri (1956) found that mineral size is important as the 14A mineral in some Finnish clays gradually changed from. a vermiculite to a montmorillonite type mineral with decreasing particle size (i.e. expanded on glycolation).

Bhattacharya (1962) found that illite changed to a montmorillonite-like mineral on weathering of the Indiana Glacial Tills.

Droste (1956) while examining the Wisconsin Tills found chlorite to weather through an intermediate stage to vermiculite, and illite to "degrade" to an illite-montmorillonite mixed layer.

Jonas (1960) determined the factors which governed the expansion of clay minerals with mica-like backbone-layers. He found that size of mineral, charge density, character of ions in interlayer positions and type of absorbed liquid were crucial factors. Such data could explain why Droste (1956) and Bhattacharya (196) found an illite to montmorillonite alteration and Jorgensen (1965) an illite to vermiculite change on weathering. 37

Harrison and Murray (1957) while working on Pennsylvanian age shale and Mississippian Borger group rocks found that each weathered from a parent illite, kaolinite, chlorite parent to a kaolinite and mixed layer illite + expandable mineral and chlorite + expandable mineral mixture. These types of mixed layers being called segregated mixed layers, i.e. parent + expandable.

In 1966 Lynn and Whittig carried out work on Cat Clay development in California and found that on oxidation, various different aged profiles containedless and less chlorite and vermiculite while the montmorillonite content increased. On reduction the profiles, to time, showed an increase in chlorite.

Hensel and White (1950) established a rate of weathering of TV?© from the 0.2-"7,.0 micron Wisconsian Till fraction at 0.1 percent per thousand years and they could therefore gauge the total age of the respective formations.

The weathering of mixed layer clays in soils was investigated by Tamura (1955) who established that a vermiculite- illite mineral weathered to a randomly interstratified chlorite- illite.

Murray and Leininger (1955) concluded from their work on Wisconsin and Illinein tills that weathering is proportional to intensity and capacity factors with montmorillonite as an end result from the weathering of illite and chlorite. According to the weathering sequence of Jackson et al (1948) kaolinite should be next to appear if leaching and acid conditions prevail. Its absence indicates the establishment of mineral equilibrium under conditions not severe enough to advance weathering processes further.

Bayliss and Laughnan (1954) in a study of mineralogical transformations accompanying the chemical weathering of clay- slates from New South Wales found that interstratified minerals with chlorite as a component are the least stable and are destroyed in the leached zone, thus kaolinite increases relatively. Montnorillonite, having a lower surface charge than vermiculite, is more stable and persists into the leached zone but not the ferruginous zone. Illite, of the highly crystalline 2M1 variety is relatively stable in the leached zone 38

but the no contentdrops in the ferruginous zone showing partial destruction.

While conducting clay mineral studies, on Scandinavian soils, Cdd Gjems (1967) found no postglacial formation of kaolinite, but noted that the amount of vermiculite and mixed layer illite-vermiculite increased upward in the semipodsol profiles inversely wkth the amount of trioctahedral illite and interstratified chlorite. The podsol profiles showed montmorillonite, vermiculite and mixed layer illite-vermiculite increasing upwards in the profile inversely with trioctahedral illite, chlorite, interstratified chlorite and mixed layer minerals. 3) Discussion It thus appears from this review of clay mineral weathering that almost all these minerals are prone to rather rapid chemical alteration, the direction of which is rather difficult to predict, and may even be reversed mirth changing environment. Theoretical chemical explanations for these alterations seem rather tenuous and almost as many of these exist on the alteration processes themselves.

In conclusion to this review dealing with the chemistry and mineralogy of clay deposits, it is hoped that the framework within which each topic was discussed has helped draw these subjects together under a sedimentological theme with a fluviatile-marine environment undergoing clay sediment deposition with poceible conter'poroneous diagenesis followed by uplift and consequent weathering. It is only really when viewed from this single standpoint that all the relevant literature may be advantageously understood and assimilated.

Two excellent recent examples of exactly such studies involving teams of scientists investigating single deposits on a regional scale are those of the Pierre Shale in America and the Keuper Marl in Britain.

The aims of the former study are quoted from Tourtelot (1962), "Shale is the predominant sedimentary rock in the crust of the earth. Knowledge of the expectable chenical composition of shale and its variations is an important part of the data needed to interpret the chemical and physical processes involved in the erosion of source rocks, the mode of 39 transportation of the ro,2ulting detritus to depositional sites, and the subsec.Jent modification of the material during diagenesis and lithification, Type of source rock, type and degree of weathering, mode and distance of transportation, and environments of deposition and diagenesis all influence the composition of the shale, and characteristics and interactions of these factors must be deduced in large part from the composition of their products. In addition, data on the composition of the shale aid in evaluation of economic potential of the shale itself and, when related to the physical nropertfLos of the shale, in the planning and construction of engineering works located on shale terraces.

The Pierre Shale of late Cretaceous age is thick and eztensive...therefore, interpretations based on its study would be widely representative. The stratigraphy of the units is fairly well krown, and abundant fossils permit biostratigraphic zoning throughout the extent of tiu.- In addition, the shale is entirely :imrine in the eastern Isart and passes westward into entirely non-marine rocks," hence, "the Piers? Shale and its (Nraivalent stratigraphic units seem to offer excellent opportunities of obtaining data. It

It need hardly be said that the observations described above are also valid for the London Clay.

The aims of the second "single deposit" case history, that of the Keuper Marl, was set cut in the 1964 interim report as follows, 'That is Xeuper Marl? Where is it found? How variable is it and how does weathering affect it? How does one recognise it and distinguish it from similar deposits? What are its major structural features and stratigraphic associations? What minor teztural features does it present and what other physical and chemical mineralogical characteristics govern its behaviour as an engineering material?

These are some of the more important questions in the engineering geology of the Keuper Marl. With answers to all if them, the foundation engineer would be well equipped tc deal with the material at every encounter.

The reports which follow attempts to find answers to at least some of the above questions and try at the same time to 40 relate engineering prop3rtiz and treatment 14/ 2:1:, the fundamental properti()G wherever possible."

Team studies, such as the two above, facilitate the investigation of a very wide spectrum of topics in great detail within one particular theme or problem. The ciriter hopes to use such precedents selectively, in order to follow a somewhat narrower spectrum in equal detail. 42

Chapter 4 Processes rulminatinc in the formation of the London Clay In this chapter, an attempt is made to examine the detailed nature of the London Clay.

The aim of this work was firstly to study the physiography of the basin in which these clays were laid down. ' This necessitated analysis of the structural geology of the basin and its control of the shoreline positions, bottom gradients and timing of successive inundations. Depositional cIlaracteristics of the clay were secondly consired by investigating the detailed stratigraphy, as defined recently by its palaeontological zonation, and mineralogy. This was undertaken mainly from a sedimentological viewpoint. Thirdly, a picture of the environment of deposition is presented which is based on these new palaeontological and mineralogical concepts.

I. Geological History, Structure and Deposition A. Palaeogeography and Structure Attention is first given to the Palaeozoic floor under- structure and outer framework on which the basin is situated. This floor extends continuously beneath the area, emerging at the surface on its margins in Devonshire, Somerset, Gloucester and the Midland coalfield. It is thought that the younger formations; the Upper Cretaceous, extended continuously across the region but of importance is the fact that the Mesozoic formations are found in the major hollows in the floor. It has also been shown that the form and structure of the floor have influenced the deform- ation of the cover rocks and that many ancient structures are transmitted through the younger cover (Wooldridge, 1938). It has been shown that the floor to the north and south of the Thames is significantly different and that the line of the Lower Thames is probably a continuation of one of the major tectonic boundaries of Europe which separates Palaeo and Meso-Europe. Hence north of the Thames the floor is gently sloping towards London from about 45C feet in Bucks to 1000 feet in London. Considerable undulations occur on this floor which generally trend NW-SE as they do in the Midlands. Wooldridge's "London Ridge" is an example of these undulations. In this region, pre-Devonian rocks analogous to the Brabant Dftssif of Belgium are encountered. 43

South of the ThAmes, the floc: 'r plunges relatively steeply and below the centrsl and southern Weald is roughly a mile deep, (Wooldridge 1938, fig.47) with the rocks encountered being continuations of the Deveno-Carbo tracts of Devonshire and the Ardennes. These are traversed by powerful folds of general E-W direction thrusting on the more compacted palaeo rocks.

During a great portion of Mesozoic times, the rocks beneath London and East Anglia projected as a "massif" above the tracts lying to the north and south of it. The floss- the latter regions was depressed in deep troughs allewin the ac:umulation of great thicknesses of soei_ment, the dersits of the Triassic desserts and Jurassfn Seas. The intervening massif was part of a considerable mountain range but was subject to long subaerial destruction and was base-levelled before the end of Jurassic times. It is probable that part of the Upper Jurassic rocks were deposited over Ail or most of the former upland area and the peneplain surface was thus cut by marine erosion. There- after, the London massif was subject to at least local uplift and most of the Jurassic cover stripped from it. It was at this time that the earliest recognisable movements along the line of the London ridge occurred. The sedimentary investment of the massif was resumed in Lower Cretaceous times, and at 'rio beginning of Upper Cretaceous times the sea extended completely :across it before it was finally buried beneath the Gault. (Wooldridge and Linton, 1955).

During the Albian Stage (Gault and Upper Greensand) isopachyte maps (Wooldridge, 1938, fig.49) show a notable east and west marginal thinning with a marked central Wealden basin of deposition. Localized E-W thickness variations in Kent and the Thames Valley cculd relate to movements along the Armorican Axis in the buried floor.

Cenomanian isopachytes show more uniform thicknesses but with some thinning over the London Platform. Sedimentation is heaviest in the Upper Thames any Wessex regions with a middle upland over the Weald. A narrow belt of thinning from Marl- borough to Thames Estuary breaks in two the depressed zone which is recognised as the ancestor of the London Basin. (Wooldridge, 1938, fig.50). 44

Turonian times (Midc 'hall) saw a ,-ever-.11,nn to Albian contions with temporary subsidence over the ',7t-Ilden atfJa, the greatest thickness being on its southern nargin. Sea depths were great at about 500 fathoms. Again, however, on the line of the Thames Valley axis local thinning occurred indicating continued uplift.

The Senonian Stage must be viewed with care as variations in thickness reveal the combined effects of contemporone&us and later (pre-Tertiary) movements. The sea slowly shallowed during these times as a result of regiona2 uplift and resulted in widespread emergence of the bottom. MM gentle and uniforr uplift allowed the crests of, folds to planed off with th3 removal of hundreds of feet of chalk to produce a sub-Eocene peneplain surface whftct was further trimmed by the advaLeing Eocene Sea. By considering iie thickness of the Upper Chalk as a whole and also by mapping to distribution of its zones, an idea is obtained of the sub-Ecene structure.

Dec; basins of Upper Chalk are noted in tae South Hampshire Basin and from East Anglia to N.7Cent. Between these two U.-as a shallow area of which was subject to pre-Eocene uplift and erosion, embracing the Weald, Chiltern Y nd South Midland areas. Within this uplifted area is noted a shallow elongate depression (Wooldridge, 1922, fig.52) which is the forerunner of the London Basin. An axis of upwarping of Charnian trend crosses the London district from Watford to Streatham, following the line of the London Ridge. South of the Thames Valley, E-W trends are more dominant probably indicating the underlying Mesozoic structures. Studies of the zonal features of the Upper Chalk (fig. 4.1) reinforce the structural picture in that the main central uplift and NE and SW depressions are seen stratigraphically lower and higher zones respectively, i.e. Coranguinum in the central ridge flanked by Mucronata and Lunata to the NE and SW.

In the SW, the Eocene base oversteps from Mucronata to Puadratus Chalk in an easterly direction in the Hampshire Basin but thence northwards to the edge of the London Basin, there is a more or less regular northward overstep of the E-W trending zones. Hence after emergence of the chalk sea bottom, its erosion and subsequent sinking, the Eocene Sea transgression was 45 subject to almost exactly Lame uniform southerly t Icing motlf7,ns, which when projected northwar tho Woalden, suggests pr Eocene uplift. Also the Eocene Sea thus filled the same structural depression which created the great Upper Chalk thickness.

In the London Basin, the higher divisions are more extensive at the eastern and western extremities, the central NW-SE trending section reflecting an uplifted region or "cross-warp" flanked by two deeper basins, the Reading basin and the E.Anglia basin. The Cenomanian initiated Wealden uplift appears to have been strongly enhanced in pre-Tertiary ox very early Tertiary times or else the Reading and Hampshire basins would have ben permanently joined, Similarly, the prediction of a uniformly deepening NE basin folinwing the upper chalk zone thickness lines is proved wrong 27 an szamination of the sub-Eocene chalk top contours, fig.4.1 and fig.4.P, which show an E-W trending, Upper Thames centred, creep basin plunging east. Inflection points being noted at various localities, e.g., the Medway and Harwich. Thus it becomes obvious that the U7;, .s Chalh basins still acted very generally as basins in Eocene times, especially in Hampshire and Suffolk, but that overstepping because of super imposed structure controlled the major porticn3 of the London Saa Basin although the Chalk shorelines are only infrequently breached.

In order to follow the growth of the basin, stratigraphic horizons of uniform lithology, which should therefore hav.., accumulated on a fairly level surface and be contemporoneous throughout, must represent time planes. Lithologic characters and facies maps also contribute to the study.

The Thanet Beds offer the first opportunity to examine the shape and form of the ;Chalk basin left after pre-tertiary buckling and erosion. The Thanet transgression entered from the east with a relatively deep water facies, the "blue clay." The sea then became shallower on silting up and in extent eventually reached somewhere near Guildford. Points of interPst are the steep south gradient as compared to the north, the flexure in axis and isopachytes (fig.4.3) west of the Medway; a possible E-W line of contemporaneous warping in the Thames Valley, the curious "island" of Harwich and its complementary "deep" lying somewhat inland. The general gradient from E-W appears 46

to have been rather uniform and the London Ridge has apparently not hindered the influx of the sea.

Without detailing all the available evidence for W3olwich and Reading times, the salient features appear to be ;- a marine transgression overstepped the silted Thanet Basin depositing a sandy bottom bed over an area shown in figure 4.4 to under- lie the complete Woolwich and Reading Series. This bottom deposit was continuous from Norfolk to Hampshire and overstepped both Thanet and Chalk at times, especielly in Hampshire breaking through the Chalk periphery. The Wealden extension although almost certainly a "high" at one time did not stop the advancing sea inundating it. Evidence of the Charnian NW trending London and Harwich ridges are also present as seen in reduced thicknesses of Woolwich and Reading beds. Areas of maximum deposition or ,eeps" ware seen in the E.Hampshire basin and near Chichester, in the London Basin near Che:rtsey and Newbury and east of Lowestoft. It appears auite 1:Aely that in the region of the Thames Estuary a further deep was to be found, this area having been eroded upon uplift by ',Le advancing Blac&heath-Oldhaven Sea.

The Blackheath-Oldhaven facies are a thin ( 3C feet) marine incursion almost exactly overlying the Woolviich lagoons and hence represent yet further but very slight subsidence along the eastern end of this now well established axis of subsidence through the Thames Valley.

The foregoing discussion has given a picture of the structural episode leading to the formation of the basin within which the London Clay was to be deposited. Factors and features which operated before these times are thus expected to continue into London Clay times. From figures 4.5 of the writer and 19 of Wooldridge (1926) it becomes plain that this deposit thins both westwards and towards its margins. If full stratigraphic thicknesses or trend surfaces are drawn it becomes obvious that four "deep" areas exist, a) E.of Lowestoft, b) the Thames Estuary, c) the Reading basin and d) the Hampshire Basin. Thus the relatively quiescent period of the Blackheath Beds was terminated by renewed downwarping alongthe line of two older synclines (London and Hampshire) thus initiating the transgression of the London Clay Sea over an area not unlike that of the Chalk and 47

Woolwich and Reading Beds before :A. Howov~:,.•, the four basins sunk to gather their individual sediment piles. The "basins" are separated by submerged, but nevertheless high areas of previous 7tructural activity, the NW-SE trends through Watford to Streatham, through Harwich and an E-W "high" through the Wealden tract. The uniformity of the deposit and of the overlying Bagshot Beds in the eastern end of the Basin during late Eocene times preclude the possibility of any marked deepening of the syncline. Hence this latter period appears to be one of quiescence. Within the London Clay, however, temporary halts in subsidence gave rise to slight shallowing conditions which resulted in the rhythmic stratigraphy to be discussed later in the chapter. Comparison of the computer trend surface maps of London Clay isopaohytes and present day basal contours shows a remarkable likeness whi-s,h indicates that the post London Clay deposition movements were either rather small or in the same direction as previous movements. See Figs. 4.5 and /.1.6.

St1',3tural knowledge of the Oligocene beds is very scanty, es?ecially in the London Basin and even in the Hampshir,-; Basin. These deposits appear over a limited area and form an almost insignificant addendum to the Eocene record. They reflect a phase of gentle widespread uplift and warping which may well have been premonitory of the more violent movements which followed in Cligo-Miocene times. Flat land and sea gradients meant large shoreline changes on uplift and eventually all but the central Hampshire Basin emerged from the sea.

The Alpine movements began to be felt about this time and are judged to be post-Middle Oligocene and pre-Middle Pliocene in age. The most obvious result of this folding is that it completed the major structures. Only very slight movements have since been felt and thus the disposition of the strata of England was settled on their completion. The main synclines v'ere deepened, rather in the form of synclinoria with several parallel axes, to almost their present depths, while anticlines were raised only to be subsequently planed to their present low gradients. The movements represented the culmination of very long term tendencies. Even these relatively young and violent movements, however, have been harnessed by the position and variance of depth of the Palaezoic floor and in the relative ease with which the structures could perpetuate themselves through the sedimentary cover. Hence Wooldridge and Linton (1955) were able to distinguish between • a)The main belt of Alpine folding comprising the Weald and Wessex, and b) the London Basin, the region of gentle "foreland" flexuring under the control of the structures of the ancient undermass.

These foreland flexures may be analysed into four separate overlapping and interfering elements (Wooldridge, 1923, plate 14 and Wooldridge and Linton, 1955, fig.9.) :- 1)The Thames Valley anticline comprising a series of domes en echelon in character which are remnants of earlier structure, e.g. Thanet dome, Cliff dome, Purfleet-Grays dome, Charlton dome, Windsor dome. 2)NE-SW folding lines in the central London Basin region. These minor structures are often responsible for inlying tracts of and Reading Beds near Northow and Pinner. Nn_Nerous other similar trends are noted in the Chalk, London C1.y and Bagshot Beds. 3)A NE-SW belt of faulting, for example, near Greenwich, from Surbiton to New Cross, from Dagenham through Hornchurch to Brentwood. 4)District NW-SE Charnian elements are noted in the London region which are probably accommodation structures linked with the main Charnian cross-warp.

B. London Clay Zonation This section will be dealt with under two headings, namely stratigraphy and mineralogy. The two are closely linked and it is hoped to show that both have t!'eir origins completely rooted in the sedimentological aspects of the deposit. These sedimentological aspects can be used almost exclusively to explain the origin and disposition of both the stratigraphy and mineralogy of the London Clay. This premise is not necessarily obvious as not all stratigraphic zonations are so intimately and directly related to sedimentology.

The preceding structural section illustrated how, when and with what rough geographical limits the London Clay Sea basin was 49

formed. The Eocene sediment pile which resulted from this single transgression will now be examined with particular reference to the effects of this tectonic activity. Hence it is intended to study the results of these relatively minor tectonic disturbances within the framework of SE.England having undergone great previous (post Cretaceous) and subsequent (Oligocene) uplift. The sedimentary concept that a short halt in downwarping causes a silting up and hence coarsening of the material still holds good on this local scale. This fact is mentioned as many zonation systems are based on the rhythms produced by sudden submergence (basal conglomerate or coarse sand) followed by steady(or slowing) downward movement (clays) with a final halt (silt).

Wetherell (1836) -,-as the first to zone the London Clay. Hs proposed three divisions characterised exclusively by index fossils with sedimentological features not being considered. The Upper division was to be seen at Highgate Archway, the Middle at Reg.4.7,1ts Park and the lower at Islington and the cl!!I sectons between Whitstable and Herne Bay. This clas2ification was a good one and although Prestwich (1854) added a fourth zone which included London Clay from both the London and Hampshire Basins, this classification did not really improve Wetherell's but merely extended it.

In 1871, Meyer, while investigating London Clay in the region of Portsmouth-Bognor and the Isle of Wight, noted intraformational pebble layers which could be correlated over the area. Ile set up a threefold zonation based mainly on these pebble bands with some fossil backing. Another interesting observation was that each band was followed upwards by a stiff clay which gradually became silty and finally sandy before the rhythm was repeated. This has been correctly interpreted by White (1915) as shallowing or "shoaling" conditions following deep water ones. Thus it becomes obvious that where sedimentological features cannot easily be seen. Relatively far away from the shorelines, palaeontology techniques must be resorted to in order to recognise the same stratgraphy.

Wrigley in 1924 and 194C proposed five divisions based on the succession of species and connected and reinforced them with 50 their stratigraphioal position above the Basement Bed. The first (lowest) division is a zone of stiff clays some 50 to 70 feet above the Basement Bed, diatoms and radiolaria have been noted but no characteristic fauna established. The second division occurs at about 100 feet above the base and contains stiff clays with occasional flattish septaria. The third division usually is noted at about 200 above the base and more normal spheroidal concretions are noted as well as a more varied fauna which occupies most of the central thickness of the London Clay. The fourth division which contains stiff clay with nica-ual septaria occurs some 300 to 35C feet above the base. The uppermost fifth division is roughly 50 feet thick and contains abundant fossils in its sandy beds which grade into Claygate Beds once sand layers are encountered.

In a recent Ph.D. thesis, Williams (1970) has studied the subzonation of the London Clay with reference to both microfossils and sedimentology.

C. Depositional Characteristics Cn examination of over 400 samples, using both palaeontology and grading characteristics, derived from 21 long stratigraphic exposures in the London, Hampshire and French Basins, a firm stratigraphic zonation has euerged, see fig.4.7. The zonation recognises and is based on the palaeontological and sedimentological principles involved and reconciles both in its final form. The zonation is based primarily on the sedimentological principles governing deposition in a large sea "basin" which has very low bottom gradients and is undergoing and reacting to constant sea level movement, mainly as a :fesult of regional tectonism. tectonics hence result in sea depth and shoreline movements and the consequently deposited sediments reflect this d.z,positional and environmental history. As explained in detail in Chapter 2 these teconic movements give rise to rhythms and subrhythms. An idealized rhythm is given below -- relative Movement Environment Sediment Sudden sinking Transgression-Lithoral Basal pebbles & sand Continued sinking Deepening sea.-muddy Silt then clay Slow sinking Deep sea-cold & still Clays Halt in sinking Shallowing sea-slow F.Silts & clays or slow uplift currents Plate 4.1. Plane of scattered pebbles in clayey London Clay beds. These pebbles mark the base of rhythms of deposition, Whitecliff Bay, Isle of Wight.

Plate 4.2. Small scale ripple marks in sandy London Clay beds indicative of shallow water deposition, Whit9c1iff Bay: Isle of Wight. 51

Uplift Full silting up - M. IN :.Silt aeration

The zonation system as established by Williams (1970) consists of three main divisions, i.e. Lower, Middle and Upper. Each is further subdivided into a lower (1), Middle (2), and upper (3) subzone. These subzones are particularly marked in the Upper clay mass and only poorly developed in the Lower and Middle London Clay. Hence the most easily recognisable zones are the Lower, Middle and Upper 1, 2 and 3 zones. These 5 zones correspond roughly to Wrigleys (1924) original system. Williams Wrigley

-^"."— 4 5 0

a00' 3cc, 1 3 0...efe*r• 5'.00 •••••••••••••••mb 11/ 100' 100' L 1 L041103/$14:/_. Basement Bed 0Q0000000 The London Clay Basement Bed is usually recognisable because of its pebbly and sandy nature and contrast to underlying beds. Observation of the 2 pebble layers which separate the Middle and Upper zones is not always easy, especially in borehole derived samples. Careful examination is needed to locate the plane containing these sparsely scattered small black flint pebbles. Plate 4.1 shows this zone of pebbles. The task becomes increasingly more difficult asone enters the London Basin from the Hampshire Basin. In such areas, a better zonal indicator is the grad* characteristics of the rock, the use of shallow water indicators, e.g.ripplemarks (see Plate 40) and the presence of planes of emergance and erosion (see Plate 4.3). Silty horizons occur near the top and bottom of rhythms with fatty clays inbetween. If the total deposit thickness ismocnkidered, it is often possible to establish which zone is present from the rough depths shown in the diagram above. The problem again becomes more difficult in the east away from the coarser shoreline materials and palaeontological Plate 4.?. Plane of emergence and erosion within the London Clay at Whitecliff Day, Isle of Wight.

Plate 4.4. Whitecliff Bay looking East. Red Reading Beds and London Clay Basement Bed provide the foreground while deep water clays and then sandy upper and basal Middle London Clay follow. 52

examination of a series of samples covering at least 3C feet might be required.

The basal pebble zones rarely attain 1,foct thickness and because of the very flat gradients and apparently sudden down- warps in the region, it seems reasonable to assume that these layers were substantially completed over the whole region in less than 140C years, i.e. this is a sufficiently small time for them to be used as time planes. As such, the thickness of deposit between them indicates the relative rate of deposition and hence subsidence of the region.

The Lower London Clay displays a relatively constant thickness and thus rate of deposition and sinking. Thinning is noted near Reading and in the French section. The former being fairly near a shore line and the latter on an uplifted or shallower section. In the far eastern region e.g. near aldon, faster subsidence is noted by the thicker sediment pile. The Lower London Clay of the Hampshire Basin shows Studland to be near a shoreline because of its coarse sediments and small amount of sinking. Further east slightly faster subsidence than the main London Basin took p lace and no real thinning can be seen towards the Wealden axis. Hence there is no reason to suppose that this region was emergant during this period.

The Middle London Clay in the London Basin is again of rather constant thickness from west to east but it may thicken in France. Although of similar thickness to the London Clay, it probably took longer to accumulate, being of deeper water fine grained material. In the southerly basin, a great thickening took place during these times indicating a greater rate of deposition and sinking. A W-E axis of subsidence through the Isle of Wight seems possible. Thinning to the north does occur, e.g. Swanwick section but this only brings the thickness in line with the London Basin figures so that again a continuous sea and deposition seems likely.

Upper London Clay data is scanty ss few full exposures exist or have been examined. It is evident that subzones 1, 2 and 3 were deposited over the whole region from the Isle of Wight to Suffolk to a total thickness of some 950-30C feet before Bagshot Bed times. From the Upper 1 zone, it appears 5.3 that roughly the same trends as for the Middle cla# pertained except that thicknesses were greater and sediments coarser in the shallowing sea.

Although the same stratigraphy is noted in the Hampshire and London Basins, any equivalent beds in the former will be coarser grained because of a combination of factors. They will be nearer a shoreline (sedimentological evidence suggests this), nearer to more coarse grained elastics (the granites) and have been deposited in shallower water (faunal evidence).

These stratigraphic and petrographic variations must be borne in mind to be extremely subtle and detailed when viewed in comparison with the almost contemporaneous Thanets and Brackleshar Beds which can be relatively easily distinguished. This subtlety is caused by the fact that even at the present outcrop margin the shoreline was many miles distant, the homogenising influence of this deeper water and the fact that currents were very slow because of the closed nature of the basin.

Interpretations based on isopachyte maps of the London Clay show that the contour map of Wooldridge (l9'.5) selected boreholes displaying very full stratigraphic sequences. Figure 4.5 shows computer trend and hand drawn isopachytes based on about 400 data points of randomly selected nature. Therefore, present day thicknesses are presented which are usually far from representing the total stratigraphic thickness. However the maps agree well and illustrate four areas of great present day thickness, i.e. Hampshire basin, Reding basin, Thames Estuary basin and a possible basin off Yarmouth and Lowestoft. Also both maps show the London Ridge platform trending NW-SE from Watford to Streatham, the NW-SE Harwich ridge and the Wealden anticline extension striking roughly E-W.

The basin off Lowestoft is only tentative, but may be up to 300 feet deep lying north of the Harwich axis.

The Thames Estuary regi.n has been an area of consistent subsidence on the E-W line of the::alaeo4lesozoic boundary. The sea has always advanced from the east to cover this inevitably marine basin which is to be found between the present Weald, Harwich and London ridges. Some 600 feet of 54 sediments were deposited here through London Clay times. It is not really proven, however, that any, except possibly the Wealden ridge, really existed or had any effect on the stratigraphy at the time of deposition. The zonation appears to indicate no separate Lowestoft, Thaes, Reading or Hampshire Basins but rather that these were created later with the Alpine movements. The total thickness does, however, thin rapidly to the south and much more slowly to the north where the Chalk shoreline might even have been reached.

The present-day Reading trough may also not have existed as such during deposition but it is clear that about 500 feet of sediments were laid down in an ESE trend which lines up remarkably with the northern Hampshire basin outcrop. This trough has a very steep bottom gradient to the south and a more gentle one to the north. It can easily be argued that the contours do not in fact form concentric circles on the southern outcrop but rather cusp-like figures which join on to the Hampshire Basin. The computer trend contours certainly conform to this view (figure 4.5).

The Hampshire Basin is difficult to study because of its extensive cover of sediments. The axis of subsidence appears to be E-.W through the Isle of Wight. Great thicknesses of coarse sediment indicate proximity to a shoreline and a great supply of sediment from the W to SW.

II. Mineralogy A. Regional Mineralogy In considering the regional mineralogy aspects of deposition certain background facts should be borne in mind. The reasons for developing mineralogy in this study are mainly twofold. Firstly, the submicroscopic effects of provenance, transportation and deposition on this thick, apparently extremely homogeneous deposit require to be investigated from the point of view of their constituent minerals. Secondly, long established geotechnical trends within the London Clay had been put down to mineral variations, this hypothesis has to be investigated.

The aim of this section is to present a 3-dimensional qualitative and quantitative picture of each major mineral in 55 order to establish their relationship to the detailed sedimentology and stratigraphy and hence be able to correlate, explain and even predict property variations with minerals.

The literature to date deals mainly with evidence of provenance derived from the sand fraction heavy minerals, such studies include Davies (1915, 1916), Boswell (1915, 1923), Baker (1920), Groves (1928) and Walder (1964). The last author, after an exhaustive study, concluded, "mineralogical and mechanical analysis suggest contributary drainage systems from the south-west, west and north, each contributing its own suite of minerals."

Blondeav and Pomorol (1968) in a contribution to the sedimentological study of the Palaeogene of England detailed the heavy mineral characteristics of these deposits as well as naming their clay minerals. They suggested three source areas of sediment : a) the Hinterland, b) the N.Seas and c) the Armorican Massif.

In a recent paper Weir and Catt (1968) studied the sand, silt and clay fraction minerals of N.E.Kent Palaeogene sediments mainly from a provenance concept while revealing at the same time much information of use in the present study. These authors postulate 3 main sources of detritus: a) the Chalk, b) metamorphic rocks and granites, possibly of the Armorican Massifs, and c) other metamorphic rocks. Elany of the tabulated results in the paper have been summarized and replotted in the form of figure 4.3. This figure shows the sample locations, stratigraphy, sand, silt and clay fraction depth profiles and clay fraction mineralogical proportions. Samples traverse and include the Bullhead Beds, Thanet Sands, Woolwich Beds, Oldhaven Sands, London Clay Baseiient Bed, London Clay, Claygate Beds and Bagshot Beds. The sand and clay fraction depth profiles are in agreement with findings of the writer and magnificently illustrate the large scale or mega Italaecgene rhythm within which the present study forms a part. The rhythm starts with sudden sinking of the calk with resulting high energy sand and pebble strand line deposits. These include the Thanet, Woolwich and Oldhaven Beds. These shallow water deposits are followed on continued subsidence and rapidly deepening water by the intermediate depth, thin London Clay 56

Basement Bed and then by the deep water London Clay. This deep water situation prevails with the minor movements (as discussed previously) for roughly 1 million years or about 500 feet of deposition. The top third of which shows marked coarsening as the areas of subsidence silt up upon cessation of sinking. These trends of coarsening and shallowing under marine conditions continue through the Claygate and lower Bagshot Bed times.

The clay fraction minerals for each bed are presented on the right hand side of figure 4.8. The Thanet Beds contain between 65 and 95% montmorillonite, 5-25% mica, but almost no kaolinite. Small amounts of quartz and felspar also occur, but these are common in about all the clay fractions anyway. The fine silt fraction contains all these named minerals but the q quartz and felspar contents now dominate. The medium silt fraction contains quartz with minor felspar, muscovite, glauconite and flints. Montmorillonite has also been consistently identified.

The Woolwich Beds contain a very similar mineralogy to the Thanets with the exception that as London is approached kaolinite makes its appearance. This mineral marks the beginning of a continental influence on the suite.

The Cldhaven Beds sand is similar again to that of the Thanet Beds but is slightly finer. The clay fraction minerals are montmorillonite with small amounts of mica and kaolinite.

The London Clay clay fractions are reported to average 60% montmorillonite, 25% mica, 5-10% kaolinite and '",-.5% chlorite. The wliter agrees with the qualitative interpretation but not the quantitative. The differences will become obvious in the following pages. ruartz also occurs in this fraction. The detrital sand and coarse silt minerals are similar to the Oldhaven and Thanet Beds. The main non-detrital sand and silt minerals are glauconite, calcite, pyrite and occasionally siderite.

Claygate and Bagshot samples contain, in the sand and coarse silt fractions, 7(-85% quartz, 9-13% felspar with minor glauconite, muscovite and flint fragments. The clay fraction contains 80-90% montmorillonite with snail amounts of mica and kaolinite.

With this brief literature review complete, reference is made to the writer's own techniques of analysis, interpretation of results, accuracies and reproducibility errors and limitations. These topics are dealt with extensively in Appendix 1 and it suffices to say here that the techniques, interpretations and results obtained by the modification of accepted methods produce data concerning the London Clay which is considered reliable, acctrate and reproducable. Nearly 300 samples from about:"' sites have been analysed by various techniques to produce a bank of data which has been used in the present study. All this mineralogical data may be found in Appendix 2 in tabulated form.

Due to the large number of samples analysed and the engineering geological bias of the project certain of the more subtle mineralogical tests were only carried out on selected samples. In general, therefore, every sample has its mineral constituents expressed as a weight percentage of the whole rock but the proportions of the clay minerals in the clay fraction can also be easily obtained by simple calculation, if so desired. Of more limited interest and thus only selectively tested for, was the exchangeable cation present in the in-site London Clay. This testing was carried out by comparing untreated clay fraction X-ray traces with previously saturated samples. The saturating solutions being rich in Ca, Mg, Na and X respectively. Samples of Upper London Clay from Noak 71111, Essex and Lower London Clay from Herne Bay and Whitecliff Bay all showed the exchangeable cation to be divalent in character and almost certainly calcium. This is hardly surprising when the ubiquitous presence of probably authigenic calcite is noted. It is important however, from the engineering geology point of view since the montmorillonite is thus of the less plastic variety, i.e. it is not the sodium or Bentonite type.

The second form of data which is only occasionally tested for and collected is the size-percentage-mineral distribution type diagram such as presented in figure 4.9. This diagram illustrates the relationship between mineral distribution and size. It shows also the weight percentage of four fractions. The graph shows that all the minerals grade from one size

CUMULATIVE 60 PERCENT

620/ >2 <-1170 5< 110/o X20E-15°47' SIZE (Microns) Log Scale

Fig. 4.9 MINERAL DISTRIBUTION WITHIN A SINGLE SAMPLE fraction into adjacent ones. Each mineral usually reaches a maximum proportion (but not necessarily content) in a size fraction which may vary from locality to locality. Hence the quartz content in the -nu fraction (62% of sample) averages about 6% of the minerals. The total content is thus 3.72% of the whole rock. Similarly, clay minerals in the medium silt range form about 4% of the total rock. The size standard fraction cutoffs are thus seen to be arbitrary when considered from a mineralogical point of view as certainly not all the clay minerals fall within the clay fraction range. Other interesting features are the decrease in kaolinite content with size and the slight increase of quartz and pyrite (and even felspar and carbonates) in the very fine fractions possibly indicatina. authigenesis.

B.London Clay Mineral Suite Before discussing the aerial distribution of the minerals it is necessary to take note of the local vertical sequence at any one place in order to gain a qualitative picture of the mineral suites encountered and see how these tie in with the detailed stratigraphy and palaeontological zonation system. Of the twenty or so localities studied only one will be dealt with here. This is to avoid a repetition of what turned out to be the general case over all the basin, Further depth profiles are dealt with in chapter 7 where the same theme is noted even when the profiles are discussed from an engineering standpoint. The site to be dealt with here is the excellent London Clay exposure at Alum Bay, Isle of Wight. The site was chosen for discussion because the whole London Clay thickness is exposed in this near strand-line region so that grain size variation pebble beds, and general display of sedimentological features are at a maximum. Also some 4C samples have been collected and analysed, both palaeontologically and mineralogically, from the 420 feet thickness of clay at this locality hence giving excellent cover for the site.

An indication of the London Clay mineral suite has been obtained from a single analysis by R.E.Grim (1951) and several analyses by Weir and Catt (1868). The qualitative interpretations given correspond well with the writer's extensive work to confirm the presence of quartz, clay minerals, carbonates, 59 felspars and pyrite in the clay. Figure 4.9 represents a fairly typical mineral distribution with size for London Clay. The quartz dominates the coarse fractions with a minor clay mineral content while the reverse is true for the fine fractions. The carbonate minerals encountered are calcite, dolomite and siderite calcite and dolomite sometimes reverse roles as the dominant carbonate but are usually equal in amount. Siderite is always subsidiary and sometimes absent. The felspars are of the potash type with plagioclase occasionally assuming importance. Pyrite in small quantities is often present. Very occasionally, gypsum (or selenite) and clinochlor is found. The carbonates, felspars and pyrite are always found as minor constituents to the ubiquitous quartz and clay minerals. The clay mineral suite is typically marine and qualitatively constant over this site at Alum Bay and indeed the whole London Clay. It consists of a poorly crystalline kaolinite, illite, chlorite and montnprillonite. All four minerals acting as though they are reasonably pure but with poor (or degraded) lattice structures. This suite can be recognised from the underlying and overlying beds only with some difficulty by virtue of the fact that their clay minerals seldom assume major proportions and their montmorillonite proportions are significantly greater in the clay mineral suites. The Reading Beds, however, contain high clay mineral proportions but their colouration and high kaolinite proportions are usually decisive in their recognition.

C. The Vertical Relationship between Lithology & Mineralogy The close relatioinship between sedimentology and palaentological zonation has already been discussed.. This relationship is again noted in figure 4.10 by comparing the geological log with the zonation. To relate these to mineralogy at the Alum Bay site, it is intended to discuss each mineral profile in turn. Taking the quartz percentage profile first, the contents are seen to vary from 28% to over 80% of the whole rock. It is also obvious that the variations occur as trends and are not random. Thus each time a high context csp is reached, e.g. at 0', 85', 200', 260', 34C' and 420 feet, a zonation boundary is crossed. Hence the Basement Bed, Lower to Middle, Middle to Upper 1, Upper 1 to Upper 2, Upper 2 to Upper 3 and top of Upper 3 boundary is indicated. These mineralogical boundaries correlate extremely well with the visual features expressed by the log, the palaentological boundaries and the rough grading analyses. Plates 4.4 and 4.5 show the grading variations and hence zonation. quartz contents are at a minimum during the middle portion of each rhythm when deeper still water conditions prevail. Although these sediments are a rather coarse-grained representative of London Clay, the same results have been found in the fine grained equivalents at, for example, Herne Bay and Crford Ness where the correlation by visible rock logging is extremely subtle and difficult to detect. The subzonation of the Lower and Middle Clay is not of major importance as explained previously and their boundaries cannot be recognised from this Particular quartz profile although occasionally it is possible to do so in other profiles.

The profile of clay mineral percentages is roughly the inverse of the quartz one. This is because the whole rock content is dominated by these two groups with the other minerals only forming a minor fraction. Hence the same zone boundaries are indicated, but this time by a decrease in clay mineral content. Ho uniform trend in depth over the whole thickness is seen and the mid-rhythm clayey materials all reach roughly the same clay mineral maximums. See figure 4.10. Examination of the individual clay minerals may be carried out either by means of their content expressed as a percentage of the whole rock or as a proportion of the clay minerals or both. The dominant feature in the kaolinite profile appears to be the minerals close continental affinities. Thus when a particular rhythm is undergoing uplift or siltation near the end of its duration and continental sediments and influences are great, the kaolinite contents are high. These effects are noted on the clay mineral proportions profile but concealed on the whole rock profile by the fact that a whole rock clay mineral percentage decrease drags down the kaolinite whole rock percentage automatic- ally. Thus high kaolinite clay mineral proportions are noted at about 85, 200, 270 and 470 feet. The 270 foot "high" correlates with a period of great silting up and even nossible emergence of the deposition surface. See plate 4.3.

The illite profiles are greatly influenced by the whole rock .., • ,

Plate 4.5. Sandy and clayey portions of a single London Clay stratigraphic zone, Whitecliff Bay.

Plate 4.C. London Clay landscape in Eocene times. (An artists impression) clay mineral percentages as this mineral is the dominant clay mineral. The profiles are thus similar and show the same features. The illite proportion (of clay minerals) curve is rather constant and uninteresting except that the erosion paane is clearly demarcated by a fall off in mica proportion.

The montmorillonite whole rock percentage and clay mineral proportion curves are rathek similar and appear to be controlled by two major factors. The first is a narked general decrease in montmorillonite from the base to the top of the London Clay. This large scale phenomena must be connected to the overall rhythm of which the London Clay forms only the deep water marine portion. (Figure 4.8) That is, after the initial fairly rapid dramatic subsidence which created the Basement Bed, the montmorillonite content slowly decreased. The smaller superimposed "highs" and "lows" are due to small intraformational movements. High contents being related to the more marine aspects, the 10, 90, 200 and 350 foot positions. The period after resubmergance of the erosion plane also shows high montmorillonite contents. It should be noted that the Upper 1 to Upper 2 boundary is not absolutely fixed at either the 255 foot pebble bed or the 28C foot erosion plane. The boundary remains conjectural.

The remaining minerals, i.e. the carbonates, felspars and pyrite, only make up about 1C percent of the whole rock and hence do not play a significant part in the material. These minerals also appear to be of little use in demarking zonal boundaries or of varying in any systematic manner with zonation. Cn the whole, however, the major minerals bear direct relation to zonation and hence the mineralogy and palaeontology are tied up directly with sedimentology to the palaeogeography and climate of deposition.

D. The Regional, 3-Dimensional, London Clay Mineral Distribution Now that the intimate, yet complex, relationship between sedimentology, palaeontology and mineralogy has been analysed using single location profiles of which Alum Bay is the chosen example, it becomes possible to combine these locations and concepts to investigate the London Clay on a regional scale 62 but with the knowledge that the critical third dimensional effect of depth variation is recognised. Hence it becomes possible to obviate the previously bewildering effects of cross- correlation and depth variations. This prime factor is more fully discussed in Chapter 7 where it is shown that for years engineering properties of one zone have been cross-correlated with those of another zone from some other locality and where the variation with depth of mineralogy and stratigraphy and hence properties has not been fully understood. Only by full use of strati graphic zonation can material characteristics from one location be correctly cumpared with those of the same strata from another location. It is meaningless, even in a supposedly homogeneous deposit, such as the London Clay to compare remote materials unless they are from the same stratigraphic horiman.

Two methods have been used to present the mineralogical data necessary to study this regional 3 dimentional model. The first is a colour column technique using the map of figure 4.1 as a basis. The zonation and hence stratigraphy is shown in depth. The columns are used not to show lithology but whole rock mineral percentage by colour. Each of the 6 colours used represents a different mineral percentage range. Thus when proceeding up a column from say the Basement Bed the colour is governed by the average profile line percentage over that section and when this changes from one category to another, the colour changes.

1) quartz Distribution Figure 4.11 thus represents the 3-dimensional quartz variation for 18 localities over the London, Hampshire and part of the Belgian Basins. Features illustrated in this figure are as follows - a) Using this averaging method of presentation, the zonation boundaries are not so clearly distinguishable but a clearer mean value emerges. b)The Upper London Clay values are consistently higher than the Lower and Middle Zone values, irrespective of location. c)Usually 3 colours dominate each location so that not much variation occurs per borehole. d) Neighbouring locations (within the correct stratigraphy) 63 usually show a gradual and not rapid trend. e)The lateral trends, although gradual, are of equal if not greater importance than the vertical ones. f)The E-W extremities show great variation between similar zones, e.g. from over 40 percent in the Western Upper London Clay to between '3:5 and 30 percent at Noak Hill. g)Ouartz values in the west are generally higher than the east. h)Hampshire Basin values are greater than London basin values, e.g. comparison of the uppermost Alum Bay, Bracknell and Noak Hill values, all three of which are roughly equivalent stratigraphically. i)Three possible material transportation pictures arise (i) A 'large cometant depth basin with a major entry point from the scnth west. constant depth, contrluous ?basin with laulle pelipheral entry points. The south-west material being granite derived and the north-west material Jurassic and Cretaceous derived. (iii) A variable depth, continuous basin with a major entry point to the south west. j)A region of shallowing, or at least higher quartz concentration, seems probable over the London Platform, e.g. shown by the Lower London Clay variation from Horton through Streatham and Aveley to Herne Bay.

2,) Total Clay Mineral Distribution Figure 4.12 will now be examined. It represents the clay mineral percentage of the whole rock. Many of the points to emerge are similar to the quartz distribution. The main features of this map are : a)The zonal boundaries are not very well distinguished but each location is dominated by 2 colours. The vertical clay mineral percentage range at any particular locality is reasonably narrow. b) Neighbouring locations show a slow gradation of increasing clay mineral content from W to E from values of about 50 percent to nearly 80 percent. c)The total lateral change iS thus of about equal magnitude to the vertical change at any particular locality. d)A fairly clearcut line of separation between silty or sandy sediments and clayey sediments is noted along the direction of the London Ridge. Hence Ripley, Bracknell and Reading 'Ire distinctly coarser grained than the rest of the London Basin. e)East of this London Ridge line, the sediments, be they Lower, Middle or Upper, are very uniform in clay mineral content except for a slight coarsening over the London Platform. f)The Hampshire Basin values show a slight coarsening in the Upper London Clay but very little directional trend and thus there is no reason to assume any break in sedimentation characteristics between this and the Western London Basin. g)Again the problem of sediment source arises as it is necessary to explain the fine grained nature of the central and Eastern London Basin which are not much, if any, more remote from the previously proposed shorelines (Williams, 1970).

3) Kaolinite Distribution A discussion of the Kaolinite distribution follows and this is aided by reference to figure 4.13. This mineral has a peculiar distribution within the basin the main points of which may be summarized below •- a)Although zonal boundaries are often difficult to distinguish by this presentation method, the basic pattern of kaolinites affinity for the coarser, upper rhythm, continental influenced or regression type sediments is well illustrated, e.g. Upper London Clay from the Alum Bay, Herne Bay and Orford Ness sites. b)A large variation can occur in each location from the kaolinite depleted base upwards to about SO percent. c)Lateral variations are not so drastic thus the vertical variations are often more important than the aerial ones. d)The London Basin has a reasonably constant kaolinite distribution ranging between 10 and no percent. The eastern coastline sites at Walton on the N'aze, Orford Ness, Herne Bay and even N. France display low values for this typically continental mineral simply because they are located at previously extremely marine areas. e)The Hampshire Basin is an apparent anomaly, being very near shore and coarse grained and yet displaying low kaolinite percentages. The explanation is, however, that the kaolinite clay mineral proportion is high but the total clay mineral whole rock percentage is low, hence the kaolinite whole rock percentages are also low. If the figures are adjusted to an equivalent London Basin whole rock clay mineral percentage, 65 all the colours move up one place so that browns and reds predominate and even blues are seen. These type of adjustments for percentage clay minerals thus even out to a large extent the lateral variation of each clay mineral but the vertical variations still remain.

4) Illite Distribution

Figure 4.14 represents the regional 3-dimensional illite distribution and it is this mineral that will be discussed next. a)This mineral, being the dominant clay mineral, is closely related in distribution to the whole rock clay mineral percentages. b)The red, blue and brown colours dominate the map and thus the illite range is rather narrow falling generally between 20 and 40 percent of the whole rock. c)7ertical distributions, although difficult to relate to zones, are as variable if not more so than lateral ones. The complete range of values from less than 20 to about 40 percent being found in the Alum Bay section. d)The Hampshire Basin values are lower than the London Basin, but on increase eastwards and northeast suggests a fining of sediments in these directions and a probable gradation into the western London Basin. e)The London Basin distribution shows a major illite concentration in the central to western portion, i.e. between central London and Reading. Sediments further to the east tend to show a slight decrease in values probably due to the fact that the very fine grading is on the small side for illite.

5) Montmorillonite Distribution The montmorillonite distribution is presented in figure 4.15. a)The relation of this mineral to the basal more marine or transgressive phases of the whole London Clay rhythm is apparent in quite a few of the localities, e.g. Herne Bay, Orford Ness and Hammersmith. b) Each locality comprises two major colours and the montmorillonite range per locality is thus fairly narrow, usually varying from 10 to f_P5 percent of the whole rock. c)This vertical variation, as with all the mineral variations both vertical and lateral, is a gradual one being directly related to sedimentology. No individual bed or area has a sudden or unexpected jump in montmorillonite content. 66 d) Neighbouring locations show a slow and gradual mineral change. e)The total west to east range is from about le to 35 percent montmorillonite. This total lateral variation, although much more gradual than the vertical distribution, is about twice the magnitude of the latter. f)All 22 sample locations can be treated on the basis of one large basin for the montmorillonite distribution and a constant increase of the mineral is thus noted from the west and south- west to the east, e.g. Studland and Whitecliff Bay and Reading to Orford Ness and France. g)The eastern montmorillonite rich sediments would thus appear to be finer grained, further from a source material entry point and not particularly affected by floculation on contact with saline water.

6) Cation Exchange Capacity Distribution In discussion of the cation exchange capacity, it is important to remember that this phenomena embraces the whole mineral suite but is particularly biased towards samples with either high clay mineral content, high montmorillonite content or both. The parameter indicates the capacity of a suite to exchange cations, this process thus also gives a strong indication of the samples ability to gain and lose interlayer water and hence to swell and shrink or display high plasticity characteristics. The cation exchange capacity determinations may thus be used as an index test related directly to mineralogy but having engineering implications. Features noted on figure 4.16, which presents the C.E.C. distribution at 9 localities, are as follows a) The zonal boundaries are not well demarcated by the averaging technique but fairly large ranges of values are encountered at certain sites, e.g. Alum Bay indicating lithology and mineralogy changes. Values ranging from 5 to nearly 10 milliequivalents per 1e0 gram are found. b) A gradual lateral increase is seen from Studland Bay in the west to Orford Ness and France in the east. This increase is from 10 to 40 meg./100g. and thus is of the same magnitude as the greatest vertical variation, c) No easily recognisable impediments to a continuous deposition sequence are noted and the distribution is similar to, and in fact dominated by, the clay mineral percentage distribution. Fig. 4.17a UPPER LONDON CLAY QUARTZ DISTRIBUTION

Scale 1/t860,000

0M0 K ilometers

• • N

Fig. 4-17b MIDDLE LONDON CLAY QUARTZ DISTRIBUTION

Scale : 1/1,860,000

0 2770 Kilometers

(40)

( ) = Average of Lower, Middle and Upper Values Fig. 4.17c LOWER LONDON CLAY QUARTZ DISTRIBUTION

Scale : 1/1,860,000 19

20 40 Kilometers

29 2,p •26 •

•

70 • • • 67

E. Regional London Clay Isopleth Distribution The second method of presentation used to study the regional mineral distribution is a numeric averaging technique by which means the average Upper, Middle and Lower zone mineral percentage is plotted for each locality. Hence average quartz contents, for example, of the Lower London Clay can be compared over the whole basin.

1) Cuartz Isopleths Figures 4.17 a, b and c illustrate this technique as it is used in a discussion of the quartz distribution. The main points of interest are :- a)This mineral accounts for between a fifth and a third of the whole rock percentage in all the London Clay divisions. b)The Middle London Clay, particularly in the London Basin, contains the least amount of quartz but generally no large scale change in content is noted. The vertical averaging is very severe by this method and hence the rather small differences in zonal content. c)With respect to aerial distribution, the Hampshire Basin is rich in quartz throughout all the zones while the London Basin is relatively depleted. An easterly and north-easterly directional decrease in the Hampshire Basin appears to be in with western London Basin values reasonably well, However, on progressing eastwards a region of rather low values is encountered before an increase occurs in the probably Upland London Platform region. Thereafter in Essex, 'Cent and Suffolk very low quartz contents are encountered as a result of the remoteness from source material and deep, still deposition conditions. The Staines, Hammersmith and Streatham "low" probably also indicates a deeper portion of the basin reasonably far from a shoreline. d)In an attempt to draw lines of equal quartz content, an overall average (circled figures) was taken and contours drawn. These are shown on the Middle London Clay isopleth map.

2) Total Clay Mineral Isopleths Figure 4.18 shows the clay mineral whole rock percentages. The main points with regard to distribution are :- a) These minerals account for between one half and three quarters of the whole rock percentages. They vary in a I I N I Fig, 4.18a UPPER LONDON CLAY- CLAY MINERAL I DISTRIBUTION /

Scale : 1 /1,860,000

0 20 40 Kilometers 68 70

75 • 71

•

50 Fig. 4.18b MIDDLE LONDON CLAY- CLAY MINERAL 1 DISTRIBUTION / (710). Scale 1/1,860,000 5) 74)

0 20 40 Kilometers • 66k66)

-.6)" /•72 (71) (7:2) 71(6 71) 74 X74 .C70

-(70)/ • (65) N

(60)

56(57 ( Average of . Lower, 1 39 Middle and Upper Values V1-1) (55) (50) N I Fig. 4.18c LOWER LONDON CLAY-CLAY MINERAL I DISTRIBUTION 1 I

Scale : 1/1,860,000 74

0 20 40 Kilometers

• 62 7.1 • • 0. 74 •64

(

66 57 38 63 strictly inverse fashion from the quartz contents. b)The Lower and Middle zones display the highest averages and are roughly equal in clay mineral content. c)The aerial distribution is the exact inverse of the quartz one and such needs little discussion. The total overall average values contour in a similar manner to those of quartz, again showing a shallow Hampshire Basin, a "deep" centred on Staines, the London "upland", the Thames Estuary deep and the Harwich anticlinal axis. d)Maximum average variation is from about 50 to 70 (i.e. PC percent) in a lateral sense while average vertical variations between zones is only about half this amount.

3) Kaolinite Isopleths In discussing the kaolinite, illite and montmorillonite distributions, it is important to remember that the data may be presented in the form of clay mineral proportions or whole rock mineral percentages. The former testifies to the real provinance and sedimentological distribution trends while the latter is proportional to the whole rock total clay mineral percentage and may be related more to engineering interpretation. That is, the patterns produced are similar but the figures differ. Reference to figures 4.19 and b, which represent the whole rock kaolinite percentage and the clay mineral kaolinite proportion, illustrates the above discussion. Points of interest noted from interpretation of these maps are as follows a)The kaolinite proportion figures are almost twice the kaolinite percentages and this increase is particularly marked in the sandy lampshire Basin. b)The Upper London Clay zone is slightly richer in the mineral than the roughly equal Middle and Lower zones. That this is discernable on a regional scale and using this averaging technique reinforces the idea that the kaolinite affinity lies with the sandy nearshore or shallow water facies. c)This affinity is clearly shown on the kaolinite proportion map as a decreasing W to E trend of values. That is, from high shoreline values, e.g. Reading CT, Studland, to low deep- water regions, e.g. Orford Ness, Walton on the „aze. d)This general decrease is shown by the contour lines which were drawn using values for each site created by averaging the Fig. 4-19a MIDDLE LONDON CLAY KAOLINITE DISTRIBUTION

Scale : 1/1,860,000 [(8)]

0 ZO 40 Kilometers 51(5)

212 :13:

iir7047.4413). ' 15 2]

7 (43) 6[101 ( ) = Average Va lues [ = Lower L.C. Values (10) : : = Upper L.C. Values N

Fig. 4.19b MIDDLE LONDON CLAY KAOLINITE AND CHLORITE PROPORTION DISTRIBUTION

(11)[11] Scale : 1/1,8 60,000 —(10) =-Acimoi 20) // 0 20 40 Kilometers 9(9)

• (5) 24 [2l( 91 • 24:25): • • 19(20) 21 19 \ 171 • 1 •

( ), Average Value [ l= Lower L C. Value . • =Upper L Value (17) 3k 15 [17] [201(2b1 (20) :2 15 S9

Upper, Middle and Lower mean values. The decreasing trend contours are modified by both the ancient shoreline direction, i.e. NW-SW, and by what was probably submarine (bottom) topography which resulted from structural geological consider- ations. The NW-SE London Ridge axis of Wooldridge (1923) can be seen as well as the Thames Estuary deep and the Hampshire basin structure. e)The western end of the London Basin and the Hampshire Basin can be connected relatively easily but contours between the eastern end and Hampshire or even France are somewhat tenuous but nevertheless probable. Lack of information from the English Channel makes these corrections hypothetical. f)High kaolinite contents on the western extremities, howev3r, (i.e. Studland Bay and Reading) testify to a nearby shoreline and source material entry points. The contour lines, though only guide lines, show a decrease eastwards which may be taken as indicating increasing distance from shoreline and increasing depth and salinity. The low average French site values appear to indicate that no material is yet entering from an easterly source, i.e. the locality is probably still nearer a western shoreline than an eastern one!

4) Illite Isopleths Presentation of the illite data follows that used for kaolinite. Interpretation of maps 4.`'.0n, and b is difficult and the following points arise a)The illite proportion figures are about 1.5 those of the whole rock illite percentages. b)Very little vertical variation between zones is seen. c)Very little lateral variation is noted over the whole region and the illite content and proportion seem remarkably constant. d)No meaningful contour lines can be drawn as trends do not appear to occur. e)The supply of this mineral to the basin thus seems to have been very constant and the distribution ubiquitous. The grading seems also to have been very broad so that ubiquitous distribution may have occurred. This is to a large extent proved by figure 4.9. N Fig. 4.20a MIDDLE LONDON CLAY ILLITE t DISTRIBUTION I (33) Scale : 1/1,860,000 3)]

0 20 4.0 Kilometers 29 (29)

33) 34

3) [37 3 (33) 39

30 i(32)] 32 ( ) = Average Values [ = Lower L.C. Values 28 (28) : = Upper L.C. Values :26: [301 (28) 2E [3a '(26)' [2 8]

Fig. 4.20b MIDDLE LONDON CLAY ILLITE PROPORTION DISTRIBUTION I

Scale : 1/1,860,000 (44)] =mom 0 20 40 Kilometers 43 (43)

47 (47) (43].(44) 45 43)] :(44): 49 (512)* .53 • • liiM.,11.4 5): 53 -6 :51: 91

:44: Average Values •{4 [ ]= Lower L.C. Values : : = Upper L.C. Values 45).- [47] 7C

5) Montmcrillonite Isopleths The montmorillonite distribution maps, i.e. whole rock content and clay proportion are shown as figures 4.21 a and b. Points of interest are a)Lower London Clay values generally exceed both the Middle and Upper zones. This is particularly true in the Hampshire Basin. b)The montmorillonite content increases relatively steadily, from west to east, i.e. from shoreline to deep water. Increasing salinity does not appear to affect the distribution by floculation processes. c)Again the contours appear to be controlled by the NE-SW ancient shoreline and also possibly by submarine topography. d)The western contours apparently join the Hampshire and Western London Basins without break. The picture becomes more confused over the Weald and Channel areas but a steady mineral increase seems likely until France is reached. Whole rock percentages thus having increased from about 17 to 33 percent. e). Interesting palaeogeographical factors following from these maps are that the closer the contours the steeper the ancient bottom gradient in reaching deeper water. Inversely, little mineral variation with corresponding widely spaced meandering contours indicated low gradients and uniform sea depth. Study of the Studland Bay and Alum Bay-Whitecliff Bay section shows (as does the kaolinite distribution) a steadily shallowing sea and eastward shoreline migration (because of decreasing mont. content) from Lower through Middle to Upper London Clay times.

III. Environment of Deposition To conclude the chapter on the structure, deposition and final form of the basin, a short review and discussion of the environment of deposition is pertinent.

The picture that has been presented of the land-masses which surrounded the basin is based heavily on evidence derived from the ancient flora found in the deposits. The most characteristic plant is the Nipa Pain, the modern day equivalent of which only grows in muddy estuaries and coastal swamps in salt or brackish water, embedded in thick mud from what was probably an old Palaeocene and Cretaceous soil. Also found Fig. 4/1a MIDDLE LONDON CLAY MONTMORILLONITE DISTRIBUTION (30 Scale 1/ 1,860,000 33)1 / 0 20 40 Kilometers (f )( 33 (33)

:(2P: • 23):24: ) (26)[27] (P 2(24 L 26): 19 [241

( ),Average Value •32[301 [ ], Lower L.C. Value (31) : := Upper L.C. Value 23 :22: 18 1-(18)1 • 1 :10: Fig 4-21b MIDDLE LONDON CLAY MONTMORILLONITE (PROPORTION) DISTRIBUTION -(45) Scale 1/1,860,000 45)] (35) 0770 Kilometers )30) ;(33): (25) / 1333 34 [42](38) / \:26i2)7 • 5' 435); ? 5): 27 • 125)/ • - [34]-

( ) Average Value a [ Lower L C. Value : : = Upper L.C. Value 37: 17 18, (38) f3p) {35] 0, 114 13 6'4 38 71 are the fruits and pollen of mangroves including Aricennia and the coastal fern Acrastichum (Bagshot and Benbridge Beds). This environment is illustrated in Plate 4.6.

The mangroves often extend some distance inland in brackish swamps and lagoons, forming a fringe or occupying inlets between which run sluggish tidal streams. During high tide, the palms and foliage extend into the water while at low tide, muddy tangled roots are exposed.

Behind the strandline lies the main body cf the lowland tropical rain forest, characterised by the size of vegetation, variety of species and number cf forms. Lofty trees form thick overhead canopies of foliage below which, in the shade is found thick undergrowth of creepers and lianes. The dominant climate would be tropical with high and even all year temperatures o averaging 21oC-2,6 C, high rainfall distributed evenly throughout the year and a high humidity of about 5C percent (Curry 1965, Davis and Elliot, 1958). Fossils of fruits of higher land trees have also been found, i.e. the pre-Mesozoic uplands, exampl:x include Magnolia, Walnut and Conifers. This general form of -7C1 :etation and physiography with forested Mesozoic lowlands and upland pre-Mesozoic regions is thought to have prevailed (Reid (4 Chandler, 1933) for some q,0 million years around the London-Hampshire Basin.

The first Palaeogene sea to transgress the heavily eroded and pereplainedChalk surface in S.E.England deposited the thin glauconitic fine sands with clay seams known as the Thanet Beds. Haynes (1958) from a study of the Foraminifera has concluded that these beds were laid down in a cool sea which was for the most part only about 3C metres deep. Calcareous algae, found in the highest Thanet Beds, are well as certain mollusc genera, however, indicate shallow warm water in late Thanet times.

The Thanet Beds are succeeded by the complex series of sands and clays known as the Woolwich and Reading Beds. The picture presented by Hawkins (1946) is thus a shallow sea to the east, .e. Herne Bay to Erith in which the marine glauconitic pebbly sands of the Bishopstone Beds were deposited, fringed by brackish mud-flats which stretched from London well into the Paris Basin, i.e. the estuaries clays and sands of the Woolwich es

beds. These worms bmohm.d ]by a lowland country stretching in a great arc to the north west of London from Suffolk to Hampshire and the English Channel. This low-lying country contained freshwater marches interspersed with dunes and temporary pools. The climate was possibly semi-arid and much of the sediment wind-borne, the finer dust becoming trapped in pools and the coarser material migrating slowly in dunes from a westerly direction.

The Cldhaven and Blackheath beds are marine sands with subordinate beds of flint pebbles. They form the western or landward end of a large estuary, being shallow marine to the east at Herne Bay and brackish nearer London where the pebbly Blackheath facies are evident, roughly marking a strand line by means of N-S trending arcuate shingle beaches.

The sandy and glauconitic London Clay Basement Bed contains marine mollusc accumulations such as Pitar and Dosiniopsis together with restricted foraminiferal fauna in which polymorphinids and protelphidium are dominant, both of which suggest deposition in shallow sea water. This westward sea transgression being an extremely gentle but rapid one as very little disturbance and/or erosion appears to have taken place. No estuarine or freshwater species are known from these beds. A study of the basement material pebbles shows these to diminish in size and frequency from the west until they disappear alnost completely in E.Xent and France. The same may be said for the intraforwational zonation pebbles of the London Clay itself.

The lowest fifty feet above the Basement Bed (and sporadic- ally at higher levels) has yeilded no fossils other than abundant diatoms and some Radiolaria. In such beds unbored wood may occur. These facts suggest the existence from time to time of foul bottom conditions, with organisms confined to thin sheets of water at the sea surface. (Curry, 1965 and Williams, 1970.) Indications of environment culled from mineralogical and chemical data for this 50 foot zone suggest shallow water conditions with fairly rapid deposition (i.e. muddy seas). This is because the detrital mineral content (quartz and felspar) is high, clay minerals low and sand size fractions high. High (relatively) 73

pyrite percentages and low carbonate contents probably reflect an Eh of -0.25, a pH of 17.6 and medium salinity of 35. Thus an open advancing sea with lowish temperatures and slow moving waters of slightly alkaline nature and foul reducing conditions is envisaged. The low kaolinite and relatively high montmorillonite contents relate well with a marine transgression in a steadily sinking basin.

Prestwich (1847) noted that the Middle London Clay in the London area contained a macrofauna indicative of deep water. The Foraminifera confirm this (Curry, 1968). Protelphidium and polymorphinids are rare and nodosariids occur. Globigerina occurs commonly up to 30% of some samples and this association suggests a depth of the order of 100-200 fathoms. With these Foraminifera occur crinoids and Terebratulina as well as Thyasira and Cocculina all of which suggest deep water. Arctica and Astarte, both with a predominantly boreal distribution indicate cold (because deep) water. The mineralogy shows low detrital mineral contents and small grading sized which indicates deep water, still, clearer seas and a constant rate of sediment inflow and deposition. Low pyrite and siderite contents and high calcite and dolomite percentages indicate Lore oxidizing conditions with Eh 0.0 to -C.2 and an alkaline pH of about 8.0 with salinities approaching 40. A slow moving, open sea, well oxygenated and fairly clear is envisaged.

The sandy Upper London Clay contains molluscs which recall species found in the shallow water Basement Bed. The Forariniferal fauna tend to confirm this type of environment. High quartz, felspar and kaolinite values also suggest shallowing tendencies with fairly rapid and constant deposition. 74

Chapter 5

GuoIo'olcal factors Influencing Engineering nroper/ies In this chapter, a review is made of the geological factors which influence soil properties and civil engineering practice. The arguments and hypotheses expressed in the following pages are an extension of the review carried out in Chapter S, but deal with applied engineering geology rather than theoretical mineralogy and sediMentology.

In order to keep the complete picture within view, it is proposed to discuss briefly the 3 major geological processes which a sediment undergoes during its lifetime. This may help show at what stage and with what results such processes take place.

The first process, involving sediment source, deposition and diagenesis virtually creates the new sediment as it governs the initial, and dominant, fabric and mineralogy. The subsequent uplift and erosion, often referred to by en,!:ineers as "removal of overburden" may effect the material fabric but in all probabilty does not alter the mineralogy. The final weathering stage could theoretically have an effect on both the fabric and mineralogy. As every sediment undergoes these irreversible Processes in reaching its final state, they are subjects of investigation in that they help control the engineering characteristics of the rock mass. To summarize

Geological Process Affects Environment Weathering Weathering Fabric r5 Minerals (Sub Erosion (Removal of Aerial Weathering ((Cverourden , Fabric Tectonics Diagenesis (Deposition Fabric & Minerals Sedimentation Sub Provenance eiqueous It is now intended to discuss more detailed aspects of geotechnical relationships with particular reference to clay deposits. It is borne in mind that the geological origin of a deposit determines not only the physical properties of its constituents, but also its pattern of stratification. It is not intended to dwell on the latter point here but merely to stress that an awareness of the pattern of stratification is of importance when assessing the degree of accuracy with which the performance of a subsoil can be predicted on the basis of boring records and test data. The subsoil performance depends primarily on the physical properties of the deposit in its present state. Some of these properties are liable to modification due to environmental changes but a knowledge of the processes responsible for physical properties during the depositional period reduces the margin of error involved in the interpretation of boring records and test results.

The physical factors which may have a significant effect on the mechanical properties of sediments includes particle fabric, mineralogical composition, grain size distribution and shape, the pore water and gas chemistry, the organic Content and the chemical composition of the absorbed layers (Terzaghi,1955 and Grim,1950). Not all of these factors apply in any one material and their relative importance is not always the same. It has also been found that certain components, even though present in small amounts, may exert a very large influence on properties.

A. Fabric and Mineralogy Before proceeding with a point by point discussion of geological influence on soil mechanics parameters a few general words on material fabric and mineralogical composition is required.

"The fabric of an earth material may be defined as the appearance or pattern produced by the shapes and arrangement of the mineral grains, independently of the external boundaries of the material." (Mitchell 1956). Clay particles are usually colloidal and generally have the shape of thin flat plates and this forms the basis for many theoretical and observed soil structure models. Some underlying concepts are 7'- 1) A clay suspension will floculate when attractive forces exceed repulsive forces, n) Interparticle repulsive forces generally decrease with increasing electrolyte concentration and increasing cation content 3) Clay particles usually floculate when suspended in solutions of high salt content.

Lambe (1953) postulated that when a sediment enters a salt water body the increased electrolyte content and the reduction in velocity of the transporting water floculates the clay 76

particles causing them to settle simultaneously with the silt and fine sand, forming a loose, porous fabric. Clay particles entering fresh water, remain dispersed and settle with a slower velocity and a much higher degree of parallel orientation, at least within small areas (See Fig.5-1) Mitchell (1956) noted some important differences between a dispersed and floculated clay:- 1) At any consolidation pressure , a given weight of clay occupies a smaller volume in the dispersed (oriented) state than in the floculated (random) state, 2) The particles within a dispersed clay are distributed more evenly throughout the voYvLaethan are the particles in a floculated clay, 3) A given increment of pressure applied to both clay types, previously consolidated to the same pressure, causes a greater shifting of particles relative to each other in the floculated clay, this shifting is toward more parallel orientation. Mitchell (1956) also reached certain conclusions which were evidently independent of grain size, com►_3ition or mode of formation of the clay. These conclusions were -- 1) silt particles within the clay did not touch each other in either the undisturbed or remoulded states. This was even true for clays with a clay fraction of only !;'5 percent. Since these silt particles "float" in a clay matrix they would appear to have little influence on the materials strength properties, f) Remoulding of clays reduces the variability between the size and intensity of oriented areas, 3) Cne dimensional compression resulted in a greater improvement in parallel clay orientation for remolded clays than for undisturbed clays, 4) In remoulded clays at natural water content, orientation of clay particles parallel to one plane is not good over large areas, however the orientation within small areas (<350u) is good. It is this orientation of particles within snail areas which Mitchell (1956) believes to be of prime importance.

A considerable unbalance exists when the composition of a soil and the techniques for measuring its engineering properties are compared. The former falls far behind the latter for four important reasons .1) soil mineralogy is a relatively new and to date imprecise science, !) soil analysis requires special techniques and eauipment, 3) natural deposits usually consist of a complex mixture of minerals and, 4) it is not usually Possible to predict the engineering behaviour of a clay by using the principle that the contribution to a given soil property

UNDISTURBED SALT WATER DEPOSIT

UNDISTURBED FRESH WATER DEPOSIT

REMOULDED

FIG. 5-1 PARTICLE ORIENTATION IN CLAYS (LAM LI E, 1953)

KAOLINITE/BENTONITE 4

ACTIVITY ILLITE BENTONITE

O 20 40 60 80 ICO BENTONITE

FIG.5-2 COMPARISON OF ACTIVITIES OF CLAY MIXTURES CSEED, ET AL, 1964) 77

is in proportion to the percentage of that component present in the soil. For example, Winterborn (1950) quotes the dry tensile strength of Putnam clay at 90 p.s.i. and the strength of Cecil clay at 5C p.s.i., but a 9th-lam', mixture of Putnam to Cecil clay did not have a strength of 86 p.u.i., but one of 167 p.s.i.: Lambe and Martin (1954) have also stressed the importance of recognising and allowing for differences in mineral crystallinity as well as the dangers related to even'small percentages of free iron oxides and organic matter, both of which act in a completely disproportionate relationship to their percent*ge. These authors, however, conclude that compositional data should be used considerably more than at present especially on large contracts and whenever unusual soil behaviour is encountered. They propose two forms of study, both of which are being used by the writer :- 1) determination of the engineering properties of known clay minerals alone or as added components and making mineralogical analyses of soils of known properties and drawing conclusions between composition and properties. Terzaghi (1955) has also pointed out that the mineralogical composition of the different size fractions is commonly found to be very different. Fresh or only moderately altered rock forming minerals such as quartz, felspar, calcite and mica make up the sand and silt sized fractions whereas the weathering products of the rock forming minerals make up a large proportion of the clay fraction. Minerals such as kaolinite, illite and montmorillonite being very common. Hence the ratio of platy to equidimensional minerals increases with decrease of grain size. Grim (1949) has presented many frequency distribution curves which demonstrate this mineralogy - grain size relationship.

The engineering properties of sediments, and the way in which various significant geological factors influence these properties and which are relevant to the London Clay are now discussed.

3. Void Ratio This is defined as the ratio of the volume of voids to the volume of solids. It is easily related to porosity, a term which is better understood by the geologist (i.e. ratio of pore volume to total volume). Both these parameters obviously vary both aerially and vertically within a single sedimentological cycle. This is because sediments generally become finer from 78 a shoreline seawards and also upwards from a basement bed. Not only is the overall grading important but also the proportions of silt to clay.

The void ratio of a well sorted cohesionless aggregate of equidemensional grains can vary from C.35 to 1.CC. If a sediment is formed from a suspension of quartz powder its void ratio. can vary from 1.0 to 2.66 with decreasing size. Such structures with void ratios greater than unity are metastabic› . The phenomena of increasing void ratio with decreasing size is accounted for by assuming that intergranular bonds are established, the number of which increase with decreasing size, thus "binding" the grains in positions which are increasingly unstable. (See Table 5-1)

If a quartz powder is mixed with powdered mica the void ratio at a given size increases rapidly with increasing mica content and the effect of vibrations on the density of the sediment decreases, i.e. the bond strength increases. (Terzaghi 1955). From these observations, it would appear that the void ratio could increase seaward with clay content. The void ratio is such a fundamental property that if it is altered all other properties of the material change, hence the importance of fabric alterations and studies cannot be underestimated.

C. Permeability This important property of a sediment affects both the rate of consolidation and the shearing resistance - rate of shear relationship. The factors that affect permeability are mineral corposition, particle size distribution, texture, void ration, exchangeable cation composition, fluid characteristics and degree of saturation.

Examination of table 5.2 illustrates the variation in permeability on addition of certain clay minerals to a quartz sand. Addition of only 1C percent mica virtually reduces the sands permeability to that of the mica. If kaolinite is sub- stituted for mica lower permeabilities are reached and the trend continues with montmorillonite. Samuels (1950) has shown the effect of exchangeable cations and load on montmorillonite and kaolinite. Permeability generally decreases in the order Al, Ca, WI for both minerals, with montmorillonite reaching imperm- eability at 6 tons per square foot whereas kaolinite still Tablc 5.1 Grain Size (u) 750-250 kCC-2C n0-6 6-2 2 eo after sedimentation 1.0 1.21 2.23 2.57 2.66 r. 1.5C 2.16 e1 ” a vibration 0.67 C.80 1.1C Ratio e1/eo 0.67 0.66 0.49 C.58 C.81 Void Ratio of Crashed Quartz (Terzaghi 1925)

Table 5.2 Clay and Mixture Permeability cm/min -3 ruartz sand 1 x 10 Puartz sand . mica 4 9 1 4.6x 10 ,4 7 3 4•t x 1C 4 1 .• 1 5.8 x 1C -4 C • 1 4.9 x 10 Cuartz sand . g.holin 75 9 1 9.5 x 1C -6 7 : 3 8.9 x 10 -6 1 : 1 2.5 x 10 -6 C : 1 3.0 x 10 Quartz sand . Ca-Montmorillonite 75 9 .• 1 4.3 x 10 7 3 2.1 x 10-6 -7 1 • 1 5.0 x 10 -7 C .• 1 23C x 10 ruartz-sand : Na-Montmorillonite 9 •. 1 1.6 x 10-7 -8 7 •. 3 3.0 x 1C 1 .• 1 Impermeable C •. 1 Impermeable

Permeability of Clays and Sand-Clay Mixtures (Endell et a1.1938) 79

retains some permeability at this load.

Permeability has been shown to decrease with increasing particle surface area and increasing polarity of the permeating fluid.

The permeability within a sedimentary deposit should thus act in a regular, understandable and even predictable manner being strongly related to mineralogy, size distribution and texture. Permeability in general decreasing toward the basin centre and from the basement beds upward, i.e. toward the fine grained, closer knit, predominantly clayey materials.

D. Plasticity Plasticity is that property of a material which, when present, gives rise to the compressibility and cohesion characteristics of the material within certain ranges of water content. Plasticity has also been found to correlate with most of the other physical properties as well as providing a convenient method of classification of sediments, by use of such quantities as the liquid limit, plastic limit and plasticity index. More- over the natural water content in a clay stratum is significant when considered in relation to the Atterberg Limits. This relation is expressed by the Liquidity Index. The liquid and plastic limits are influenced by the mineral composition and crystallinity, grain size distribution and shape, organic cv-tent and the absorbed ions of the material. It appears that the solid particles (even at their plastic limit) are separated from each other by absorbed layers and that the seat of strength and cohesion lies in these layers. As the grain size or mineralogy change the ratio between absorbed and free water also changes invoking an Atterberg limit alteration (Terzaghi 1955). The use of a plasticity chart showing liquid limit versus plasticity index, together with the Casagrande "A" line helps in classifying the material as well as characterizing its properties. Geologically speaking, the chart is most useful because points representing two samples in the chart which fall on very different lines are almost certain to have been derived from different sources.

Plots of activity, defined by Skempton in 1953 as plasticity index versus clay fraction, are also extremely useful from both a geological and engineering point of view in that they are Co

fearly independent of grain size and hence may be used to give an indication of the mineralogy and even geological history of the material (Skempton 1953). They also help relate mineralogy to true cohesion, and explain and relate sampling difficulties, sensitivity and liquidity index. Clay activities are also used as an aid to material classifications.

Davis (1967) examined the concept of activity from a mineralogical point of view and used as his subject the Keuper Marl. This nathor found the marl to exist in an aggregated state and created the "Aggregation Ratio" i.e. ratio of clay mineral particle percentage to percentage particles less than n micron e.s.d.. By plotting activity versus aggregation ratio Davis concluded that in an aggregated state, the clay particles absorb nearly as much water as in a disaggregated state, therefore exhibiting roughly the same plasticity, whilst yielding a much reduced clay content.

Inspection of tables 5.3 and 5.4 shows how the limits of the equidimensional minerals are lower than the flaky ones and also how decreasing size has little effect on the former but significantly increases the latter. The effect of various exchangeable cations is reflected in table 5.3 which shows a limit decrease in the order• attapulgite, montmorillonite, halloysite 4mc, illite, halloysite nmo, and kaolinite. Also, exchangeable sodium and lithium cause high limit effects, calcium, magnesium, potassium and ammonium roughly eaual effects and aluminium, hydrogen, ferric iron and thorium ions show the smallest effects.

Dumbleton and West (1966) carried out some important research into the relation between clay minerals and plasticity. They concluded.-1) that reasonable agreement existed between artificial mixtures and natural montmorillonitic soils but this was not so for kaolinitic soils and mixtures due to the effects of mineral crystallinity and the presence of iron oxide, in artificial mixtures involving quartz, higher limits than expected were obtained because of the water necessary to wet the quartz. A certain amount of interaction was also suspected, 3) the behaviour of various types of soil which plot below the "A" line explained in terms of the concept of "ineffective water," i.e. water contained in voids produced by the interlocking of Table 5.3

Grain Size (u) Mineral 1CC-2C 2C-6 6-2 L2 Lw Pw Ip Lw Pw Ip Lw Pw Ip Lw Pw Ip cuartz 34 34 C 34 34 C 34 34 C 35 35 C Felspar 37 37 C 38 38 C 38 38 C 39 3) C ]3iotite 46 46 C 53 45 Serpentine 41 41 C 76 51 Muscovite 49 49 0 91 77 14 Hematite 36 20 IG Chlorite 33 33 0 44 44 C 7 47 n5 Talcum 33 33 C 46 48 C 76 48 28

Influence of Grain Size of Crushed Minerals on Atterberg Limits (Atterberg 1913) Table 5.4

Ca++ mg++ K+ NH"' Na"' , Le Clay Pw Lw Ac 'w Lw Ac nw Lw Ac Pw Lw Ac Pw Lw Ac nw Lw A c Montmorill- 1 65 166 1.:'!6 59 158 1.24.57 161 1.30 75 214 1.74 93 334 3.14 CC 638 6.98 onite 2 05 155 1.20 51 199 1.97 57 1'5 0.91 75 114 C.52 89 443 4.71 5S 535 6.75 3 63 177 1.34 53 162 1.24 6C 297 2.79 60 323 2.09 97 700 7.09 6C 600 0.35 4 79 1'3 0.44 73 138 6.65 76 1C8 0.32 74 140 0.66 86 280 1a2 8-'5 '9" 2.10 Attapulgite 124 nac. 2,..ce 100179 0.7C104 161 C.57 97 150 0.61 100 "12 1.1f, 1C3 T'S 1.23 Illite 1 40 90 C.5C 39 83 0.44 42 81 0.38 4n 82 C.40- 34 61 0.27 43. 68 C.:"7 33 69 C.32 35 71 0.36'46 72 0.32 37 6C 0.23 24 59 C.25 33 63 0.'5 423 106 0.58 43 98 C.55'41 7' 0.31 39 76 6.371 41_ 75 0.34 40 89 0.49 gaclinite 1 36 73 0.37 36 60 0.36 38 69 0.31 34 75 0.41 :;,6 52 0.26 33 67 0.34 - 26 34 C.08 28 39 0.11C8 35 C.07,28 35 C .07, 28 '9 0.01 fC 37 C.CS Halloysite =0 38 54 C.16 47 54 0.67 25 39 0.04 32 43 0.11 f9 36 0.07 37 42 .0.1 4120 53 65 0.07 60 65 0.05155 57 0. C'2 56 61 C.05 54 56 0.CS 47 49 C.01.

Atterberg Limit Values (White,1955) 81

plate-like particles of muscovite. Both artificial and natural silty soils containing muscovite plot below the "A" line whereas illitic mixtures and soils do not, 4) Clay flake aggregation and large individual clay flakes sometimes cause difficulties in interpreting activity figures for soils.

In a later publication, Dumbleton and West (1966,a) while investigating the influence of the coarse fraction on the plasticity of clays found that angular glass, angular quartz and platy mica caused increasingly greater limits to be found. The effects being more noticable on the montmorillonite mixtures.

Seed, et al.(1966) in two impressive papers analysed the fundanental and clay mineralogical aspects of Atterberg limits. They examined the physical significance of these tests from a theoretical and practical viewpoint and came to the following conclusions 1)The relationship between liquid and plastic limits and clay content is in good agreement with actual test results. 2)The linear relatiobship between plasticity index and clay content does not hold rigorously below 4C percent clay fraction. 3)Activity accurately classifies a soil with regard to its liquid limit versus clay content relationship regardless of the clay mineral composition. 4) Illite, although of higher activity than kaolinite, when mixed with bentonite has substantially lower activities than kaolinite- bentonite mixtures. A marked assymetry in the activity vs. percent bentonite added plots also being noted. See fig.5..

The relevance of this discussion on plasticity and its implications within a sedimentary basin will be discussed at length later when sone of the writer's experiments on this topic will be related.

E. Sensitivity The sensitivity of clays is a complex topic with many differences of opinion among the published works. A well established fact, however, is that sensitivity is roughly proportional to liquidity index, which means that water contents well in excess of the liquid limit are required for these sensitive clays to exist. The loss of strength can be jointly attributed to a metastable grain arrangement and the disappearance 82 of hardening which had been caused by thixotropy. This latter strength being largely dependant on mineralogy. (Terzaghi 1955). Skempton and Horthey (195) however, could find no obvious relation between mineralogy and sensitivity nor any connection between particle size distribution and sensitivity although these materials usually have a medium to low activity.

Mitchell and Houston (1969 and 1969,a) have presented two excellent papers summarizing the state of this topic and concluded that at least eight mechanisms exist for the development of sensitivity. These are-- metastable particle arrangement, silt skeleton-bond clay, cementation, ion exchange, salt leaching, weathering, thixotropic hardening and dispersing agent addition, Sensitivity may thus be developed at almost any stage of a sediments history be it depositional, during reEoval of overburden or while undergoing weathering,but it is noticeable that this property decreases during consolidation and is virtually unknown in overconsolidated clays.

F. Compressibility and Consolidation Hoth these terms refer to the relationship between the increase of load on a laterally confined specimen of a sediment and the corresponding decrease in its void ratio. This relationship is conveniently represented by plotting the void ratio e against the logarithm of the unit load p. For every sediment the e-log p curve starts with the horizontal tangent and at certain unit loads joins an inclined straight line with the equation: e -Cc logiop + C where Cc is the compression index and C is a constant (See Fig. 5.3). The value of Cc is equal to the decrease of void ratio produced by an increase of unit load from p to 1Cp. This value ranges between about C.15 for lean sandy clays to more than 1.0 for highly colloidal clays. According to Skempton (1944) the compression index increases with increasing liquid lfe_t :Clir in accordance with the emperical equation- Cc = C.0C7 (Lw-1C%) Skempton (1944) has also shown the relationship between compression index and clay fraction in natural sediments, see fig. 5.4. This relationship however, may be expected to vary with the mineral composition of the clay fraction. • UNDISTURBED, EXTRA SENSITIVE

VOID RATIO CORRESPONDING TO LIQUID LIMIT VOID .\\\

RATIO •N, MEXICO CITY CLAY E UNDISTURBED REMOULDED

UNDISTURBED LOW SENSITIVITY

0 01 0.1 1.0 10 100 1000

EFFECTIVE UNIT LOAD P (KG PER SQ. CM)

FIG. 5-3 PRINCIPAL TYPES OF E- LOG P RELATIONSHIPS FOR COHESIVE SEDIMENTS (TERGAGHI, 1955)

1-0

CC 0.5

0 40 80 % CLAY FRACTION

FIG. 5-4 CORRELATION OF COMPRESSION INDEX WITH PERCENT CLAY FRACTION (SKEMPTON, 1944) MONTMORILLONITE

VOID RATIO E

2 4 6 8

EFFECTIVE PRESSURE P CONS/FT)

3.0

VOID RATIO 2.0 E

I 0 2 4 6 8 EFFECTIVE PRESSURE P (TONS /FT9

FIG.5-5 PRESSURE-VOID RATIO CURVES FOR MONTMORILLONITE & KAOLN1TE (_SAMUELS, 1950) 83

In a comprehensive study in 195C, Samuels presented compression and consolidation results some of which are given below. In a series of °odometer tests, samples of liquid limit consistency were subjected to pressures from 0 to 9 tons per sq.ft. Fig.5.5. shows the results for various cationic slurries of montmorillonite and kaolinite. Sodium montmorillonite shows a very large reduction in volume at low pressures and that after about 5 tons per so.ft. very little compression occurs. Calcium montmorillonite shows reduced compression by comparison with large void ratio reductions up to about 1.5 tons per sq.ft. The Al and Th varieties show less compression still, even at low pressures. The compressibility of the kaolinite sample is Considerably less than the montmorillonites With small cationic differences. Compression is seen to decreage in the order Al, Ca, H and Na.

The rate of consolidation curves (Fig.5.5) are interesting in that they show a slow initial rate of consolidation for montmorillonite, particularly of the Na and Ca varieties. All the other curves being of roughly similar shape and form.

With increasing loads on montmorillonite the rate of consolidation decreases initially. After a short time pressure increases cease to effect the rate. Hence the coefficient of consolidation decreases with increasing load. The converse is noted with kaolinite samples. This effect is shown in figure 5.7. Data presented by Cornell University (1951) illustrates (see fig.5.8) the decreasing compressibility of montmorillonite, attapulgite, kaolinite and illite, as well as showing the effect of particle size and crystal variation upon this property.

Explanation of the above phenomena may be sought from an undentanding of the physical nature of the clay materials. In montmorillonite clays much of the total water is oriented absorbed water (c.f. nonoriented pore water) and this is particularly true in the Na form. Hence the swall loss of water at low pressures with increasing loss at higher pressures. Also the oriented enverlope of water acts more as a bumper than a lubrication in attaining orientation of flakes. The effect of sodium is mainly twofold:- 1) It tends to destroy any orientation of particles, especially with respect of kaolinite and, it acts as a dispersing agent, aiding in the development of smaller

NA MONT 0

AL MONT % OF CA MONT 40 TOTAL TH lirlikkiiiiL MONT CONSOLIDATION CA KAO 80 NA KAO

0I 1 10 100 1000 TIME CMINS)

FIG. 5-6 RATE OF CONSOLIDATION OF MONTMORILLONITE AND KAOLINITE AT 4 TO 8 TONS/FT.2 (. AMUELS, 1950)

,n 100 ' 0 TH 60 MON TMORILLONIT E AL COEFE 20 2 CA U OF .`NAT

CONSOLIDATION n, 0 40 AL

CA z KAOLINITE 'f 20 NA

2 H —

2 3 4 5 6

PRESSURE (TONS/F T 9

FIG.5-7 COEFFICENT OF CONSOLIDATION VERSUS PRESSURE CSAMUELS, 1950) MONT

ATTAPULGITE

VOID RATIO KAO. RAT ONAT D

E 2

ILL. FRACT ION TED

KAO. U NFR AC-10NA D

0:1 02 0.4 0.8 I 2 4 6R

PRESSURE (TO NS/FT2)

FIG.5-8 PRESSURE -VOID RATIO CURVES

FOR HYDROGEN CLAY MINERALS (CORNELL UNIVERSITY DATA, 1951) 84

particles. Slow starting consolidation characteristics may be explained as a result of the slow break up of oriented water and resistance of interparticle adjustment due to rigid water envelopes, fine particle size and low permeability.

G. (Shear) Strength In simple terms, shear strength is measured by the shearing stress at maximum displacement before failure. The shear strength of a soil varies with the clay mineral composition, particle size distribution, shape and arrangement of the particles and geological stress history. Meilenz and Xing (1955) have shown that small additions of clay minerals to sand or silt greatly increase the compression strength and that the maximum strength of such mixtures may exceed the strength of either the sand, silt or clay alone. Table 5.5 illustrates these conclusions and also shows that montmorillonite (which is weaker than kaolinite) when mixed with sand yields greater compressive strengths than sand kaolinite mixtures. It has also been shown that this strength increases with increasing amounts of clay in the following order•- montmorillonite (weakest), illite, (chlorite halloysite) and kaolinite (strocat)

Winterborn and Moorman (1941) found that the shear resistance of cohesive soils is approximately a logarithmic function of the moisture content, while figure 5.9 shows the relationship of liquidity index to shear strength for several remoulded clays. Skempton and Northey (1952) have also shown how strength varies with depth and hence effective overburden pressure (see Fig. 5.10). Also strength increases relatively more rapidly for clays with higher plasticity index values.

Samuels (1950) has shown that in a sample of montmorillonite shearing resistance in relation to exchangeable cation composition increases in the following order.- calcium, sodium and aluminium. lie has also shown that the shear resistance of sodium, calcium and aluminium kaoltuite is essentially identical to that of Al montmorillonite.

The shear strength of a clay is made up of two parts, cohesion Cr and the coefficient of internal friction tan Or, according to the expression - tf s Cr +n tsn Or where drii is the effective pressure normal to the shear plane. The cohesion is independent of the loading pressure and is caused H RION

LOND( 1.4

LWj LIQUIDITY I 0

INDEX SHELL HAVE 0.6 r- GASP1ORT

0.2 H PW

0 02 01 I 5 20 SHEAR STRENGTH (LB/INO

FIG. 5-9 LIQUIDITY INDEX VERSUS SHEAR STRENGTH FOR REMOULDED CLAY (SKEMPTON & NORTHEY, 1952)

SHEAR STRENGTH

ZONE I

ZONE 2

DEPTH ZONE 3

FIG. 5-10 SHEAR STRENGTH VERSUS DEPTH FOR NORMALLY CONSOLIDATED CLAYS, SHOWING SURFACE DRYING ZONE (SKEMPTON & NORTHEY, 1952)

VOA° 5.5

Mixture, Wt5J Moist Specimens Air Dried Specimens Ha. nholinite Sand Water Comp. Water Content Comp. Mont. Content Strength Initial Final Str.ngth p.s.i. p.c.i.

'CC 17.1 55.5 f 20.0 7.0 195.2 25 75 14.4 4`.0 f 42.0 1.3 541.0 25 5C, 13.9 85.8 t 85.8 2.6 533.5 n5 75 13.6 9.1 t 9.1 0.1 76.5 1CC 14.2 100.3 11CC.3 0.4 65.6

Compressive Strength of Sand and Clay Mixtures (Meilenz and King,1955) 85 by a cementing or binding force between particles. In a bentonite sample this cohesion contributes about 8C percent of the shear strength, in illitic samples from 40 to 60 percent an d in kaolinitic samples less than3 percent of the shear strength.

In 1967, Kenney carried out an extensive set of experiments to relate mineralogy to residual strengths of mixtures as well as natural clays. Some of his conclusions were as follows 1) The samples of layer lattice minerals exhibited smaller values of tan Or than the samples of massive minerals, °,7') Tests on crushed quartz and S102 showed that tan Or is dependent on particle size and shape, and that the massive minerals including felspar and calcite have similar tan 0 rts of about 0.7C, 3) Of the layer lattice minerals tan Or increased from montmorillonite (0.10, kaolinite (0.73), muscovite (1:0.35), hydrous mica (0.40) to attapulgite (0.56). These values are slightly affected by cation type and concentration, 4) No unique relationship exists between tan Or and plasticity or grain size for either minerals, mixtures or natural clays, 5) Binary clay mixtures often show non-proportional strength effects (see fig. 5.11) which probably indicate that shear resistance depends on the relative volumes of wet minerals rather than dry weights, 6) The residual strength of a natural soil is dependent on mineral composition and to a lesser extent on the system chemistry and normal effective stress.

The above brief discussions serve to introduce some of the experimental and theoretical aspects of the wads in which geological factors influence and are related to the more mechanical properties of soils. Further reference may well be necessary if any points of detail are to be perused by the reader. QU

0 6 —30

TA N 0- 4 R DEGREES C\) R —20

KAO

0 2 — 10

NA MONT

20 40 60 BO

CONTENT OF MINERAL B

Cio DRY WET)

FIG. 5-11 RESIDUAL SHEAR STRENGTH

OF MINERAL MIXTURES (,,KENN EY, 1966) 86

CHAPTat 6 Experiments to :_elate _ alegical and Soil "ndexrarameters As the study of the engineering geology of the London clay progressed, it became increasingly obvious that more than just a general relationship between geological and soil index parameters was essential. It was decided to design three sets of experiments to study the relationships, but keeping these broadly within the framework already developed in this and previous chapters, under the headings of sedimentation, removal of overburden and weathering. The design of these experiments was to actually carry out studies in the clay material itself to obtain direct results instead of attempting to correlate the results of tests performed as separate exercises, which forus the bulk of this thesis. A. Sedimentation Simulation This set of tests was designed to study clay index property reactions to the material variations which might be encountered in a newly forming sedimentary basin. It is reasoned that these experiments have a good chance of exposing these relations for two principal reasons :- 1)The basic constitutent, to which small amounts of other minerals was added, was London clay. Hence, it was not a case of testing artificial nixtures (as has often been done in the past) but of testing modified London Flay. The various additional minerals simulating known sedimentary or mineralogical variations. 2)The quality and ouantity of additive minerals was chosen to represent natural London clay variations as closely as possible. The aims of this particular set of tests are set out below:- a) To simulate sedimentary processes, i.e. to test London Clay mixtures which cover the complete range of in-situ material, mineralogy and grading. The index tests are independent of fabric and hence this aspect of the material is not accounted for in these experiments. Two fortis of testing have been carried out. The testing of size effects on London Clay has been conducted in two separate experiments. The first, test la, involved crushing silt sized London (lay for varying time intervals and then testing each of these samples. The second size effects test, ib, involved adding a constant percentage of different gradings of quartz to a standard London Flay sample. The first test simulated a constant source of material entering a basin with the effect of fining toward deeper Ater 6 87

The Pozond test assumojl .rins7tant provenance, i.e. constant proportions of minerals but this time emphasized the effect of near shore quartz being coarser grained. Plate 6.1 shows the grading variations which occur over short distances at Whitecliff Bay.

The testing of mineralogical variations was effected by adding from 0 to 50 percent of a particular mineral to a natural London clay sample which had a known deficiency that mineral. By this means, stmlation of known mineralogical variations had been accounted for using only small additives to natural London Clay, hence avoiding the use of artificial mixtures which often bear no likeness to natural clays. Plate 8.fl illustrates these in-situ mineral and grading variations from Whitecliff Bay. b)To establish qualitatively the direction in which the characteristics (mainly plasticity) are trendin-, following the various treatments. c)To determine quantitatively the alteration of material characteristics following treatment. d)To assess, by means of the results of c) whether the natural in-situ material variations (known from the writer's many X-tay analyses) could account for all, or only part of the established London Clay property variations. e)To act as yet another rough check on the writer's mineralogical techniques, i.e. use liquid limit and activity data to estimate mineralogy and detect any obvious errors, e.g. samples containing illite, kaolinite and a very little montmorillonite should not have activities in excess of about 0.9 to 1.0

Test la was carried out by crushing the whole mineral suite contained in two air dried samples in a porcelain end-grinder for periods varying from 0 to SC minutes. Silty, basal 1,...adon Clay samples from Herne Bay and Oxford Ness were chosen in an effort to make crushing easier. As is shown in Table 6.1 mechanical size reduction proved extremely difficult. The Uerne Bay sample showing almost no grading change after 30 min. grinding and the Oxford Ness sample displaying very small size reductions. This reduction in grain size tends to result in an increased liquid limit with conseauent plasticity index increase (for the Orford Ness sample at least). These results tend to plot on the plasticity almost as a straight line parallel to and above the "A" line but further from the origin. This is exactly what Plate GA. Transition from clayey to sandy beds in a London Clay stratigraphic zone at Alum Day, Isle of Wight.

Plate 3.2. Small scale sand-clay interlayers indicating shallow deposition of the London Clay at Whitecliff Day, Isle of Wight. PURE SAMPLE (I • 02)

CC L AY (I.08) PLASTICITY /

IDEX

55 MED. SAND La 99) re

FINE SAND (I • 00)

50 80 85 90 LIQUID LIMIT FIGURES IN BRACKETS = ACTIVITY

FIG 6- I GRADED QUARTZ ADDED TO LONDON CLAY. 88 wotzid be expected from a rlingle mineralogical suite which is decreasinT in size. Neaa-ze the further` the progress toward the deeper central parts of the basin, the more pl.iztle becomes the clay. This result assumes a constant mineralogy as was borne out in the test results by the'constant activity figures.

In an attempt to firm up on these rather tentative conclusions another silty Orford Ness sample was ground up for 30 minutes and then wet milled for periods of 10, !7,0 and 30 minutes. The results of these tests are identical with the previous ones but far more definitive in their nature (See Table 6.n). They confirm the conclusion that London Clay of constant mineralogy becomes more plastic with decreasing grain size.

The second "size effectp" test ib, was carried out by adding 15 percent (by weight) quartz of four different gradings to a standard "fatty" London Clay from Walton on the Naze. This experiment simulated a constant source material entering a basin but assumes the coarser quartz fraction is deposited in a clay matrix of a near-shore position and the clay size quartz is swept further into the basin. This sedimentary process is a common one and by adding only 15 percent cuartz to the samples a realistic simulation of sedimentary processes is hoped for. The results of this test are summarized in figure 6.1 when the following points are noted :-a) The Atterberg limits increase as the quartz size decreases. The activity remains relatively constant as would be expected from a mineralogy which remains unaltered. b) The plasticity increases parallel to the A line, and above it away from the origin again indicating constant mineralogy but showing up size effects. c) A roughly 10% increase in liquid limit is noted from the medium sand addition to the clay size quartz addition. Hence the maximum variation of limits which size effects could be called upon to explain is roughly 10 percent. d) An interesting feature is noted when the plots are compared with the original "non contaminated" sample. The addition of medium (and even fine) sand size quartz decreases the limits roughly parallel to the A line, but the clay size quartz addition line decreases at a high angle to the A line and would soon cross below it with more quartz additions. This feature has been previously encountered bu Dumbleton and West (1966,a) using mica silt. The direction of this move suggests an increase of "ineffective water" within the structure of the Table 6.1 Results of Grinding Tust-la Herne Bay Crford Ness Mins.Grinding 1( 15 25 3C 1C 15 'C E5 3C C.+ M. Silt% 'C".3 "1.9 `C.8 '1.5 `' .C' .0 49.7 47.5 47.9 48.0 46.1 F.Silt% 14.8 15.1 15.1 15.9 15.6 7.5 8.5 8.4 8.5 C.0 Clay% 64.9 63.0 64.( 6",.6 64.4 4'.8 44.1 43.7 43.5 45.9 Liquid Lim. 88 89 91 88 89 92 91 93 95 96 Plastic Lim. 28 n9 30 3C 31 "71 5 sS '6 '77 Plasticity Index6C 60 61 58 58 65 66 67 69 69 Activity 0.03 0.95 0.05 0.03 0.00 1.52 1.50 1,53 1.58 1,5G

Table C.viford Ncss Mins.Milling 1C f'C 30 C.+ M.Silt% 35.319.5 19.5 F. Silt% 10.6 13.9 '1.1 Clay% 54.1 56.6 59.4 Liquid Lim. 89 94 95 Plastic Lim. "C r7 30 Plasticity Index 61 67 65 Activity 1.13 1.18 1.10

Results of Milling Test-la 89

Lo.adon Clay. This directional trend is not noted in nature and hence an artificial structure most have been cruatod.

The first section of test involved adding varying proportions of quartz silt to a fatty, active Orford Ness London Clay containing 8=, percent clay fraction. The results may be summarized in a series of charts-- a) The liquid limit decreases roughly proportionally with increasing quartz content (fig.6.) but it should be noted that a 1C percent quartz addition results in a 15 percent limit reduction. This illustrates the importance of relatively small quartz variations. Insitu quartz variations may reach as much as 40-50 percent! b) a plot of plasticity index versus clay fraction shows the points to plot on a straight line almost through the origin. This is in accordance with the findings of Seed, et al (1966) and indicates a constant activity of about 1.20 which is reasonable as the proportions of clay minerals have not altered with respect to each other. c) The liquid limit versus clay fraction graph (fig.6.3) after Dumbleton and West (1966) shows that as the clay fraction decreases with addition of quartz the liquid limit remains unexpectedly high due to an uptake of water in wetting the additional ouartz surface area as well as possible quartz-clay interaction. d) The liquid limit - plasticity index (or liquid limit) plot runs above and parallel to the A line indicating that the quartz is inactive and the mineralogical regime is not disturbed.

The second portion of this test was a repeat of the first but using kaolinite instead of quartz. This test bears resemblance to nature in that the in situ kaolinite distribution was found to vary between C and about 40 percent. It was carried out on a rather silty Walton on the Daze sample whose natural kaolinite content was less than 5 percent. The results may be summarized as follows a) The Atterberg limits decrease with increasing kaolinite content. See fig.6.4. This decrease takes place smoothly or proportionally as a straight line in the plot. Cuantitatively the effects are lower than for quartz snd it is noted that a 1C oercent addition of kaolinite lowers the liquid limit by only 5 percent. b) The activity chart plot of plasticity index versus clay fraction shows that the points do not fall on a straight line through the origin but that a separate line is needed for each point, See fig 6.5. c) Hence

QUARTZ ADDED 30 0 (PERCENT)

20 0 NN N N 10 0 N N N N N

60 70 80 90 100 110 120

LIQUID LIMIT FIG. 6-2 LIQUID LIMIT VARIATIONS ON ADDITION OF QUARTZ TO ORFORD NESS LONDON CLAY. 140

120 0 ?

100

LIQUID 80 LIMIT

NON PROPORTIONAL PLASTICITY

20 40 60 80 100

CLAY CONTENT CPERCENT)

FIG, 6-3 CLAY CONTENT VERSUS LIQUID LIMIT ON ADDITION OF QUARTZ TO ORFORD NESS LONDON CLAY

50 0

KAOLI NI TE 30 ADDED (PERCENT)

10 0

n 60 70 80 90 100

LIQUID LIMIT

FIG. 6-4 LINEAR RELATIONSHIP BETWEEN LIQUID LIMIT & PERCENTAGE OF ADDED KAOLINITE

1.02 (ACTIVITY) 60

.910-83 0.77 50 // Q 0.71 / 0.59 0.54 PLASTICITY /// 40 INDEX //

20 40 60 80 100 CLAY FRACTION (PERCENT)

FIG. 6-5 PLOT SHOWING THE SEPERATE ACTIVITES

OF THE LONDON CLAY- KAOLIN1TE MIXTURES 90

tho activity decreacos in a constant manner as each le percent increment of kaolinite is added. See figure 6.6. This is in accordance with the findings of Seed et al (1966). The activity decrease is rather large, changing from about 1.0 to about 0.5 for 5C percent kaolinite addition. d) On the Casagrande chart of plasticity index versus liquid limit the points form a straight line above and parallel to the "A" line tending toward the poorer plasticity of pure kaolinite.

The final part of this test involving mineralogy consisted of adding 10 percent increments of montmorillonite to a fatty middle London Clay sample from Herne Say. The results are as follows :- a) the liquid limit increases dramatically in a assymtotic fasion with increasing montmorillonite. Fig.6.7. illustrates that the first SO percent added has only a 17 percent effect on the London Clay, whereas the following 20 percent added increases the liquid limit by some '1,2 percent. The form of this line is exactly in keeping with the findings of Seed et al. (1963) who worked on artificial mixtures. This highlights a strange phenomenon that although montmorillonite is an extremely active mineral its distributional va*iation within the basin, i.e. roughly 20 percent, only alters the clays properties slightly. The hypothesis of Seed et al (1966) that illite (or London Clay)- montmorillonite mixtures form "cemented" interstratified complexes with low initial plasticities may well be correct. b) The activity plot resembles that of kaolinite except that higher activities are generated, reaching 1.8 for 50 percent montmorillonite added. c) The activity versus montmorillonite added graph is again remarkable in that the activity remains initially constant at about 0.85 for the first 20 percent clay added then doubles in the following 3G percent. See fig.6.8. d) The plasticity chart shows all the points plot on a straight line which lies above the"A"line but converges on it slightly at the lower limits. See Ag.6.9.

This latter test was repeated using a commercial nat*ral clay comprising some 70 percent montmorillonite and 3C percent kaolinite. The results were identical to those for the montmorillonite test showing again that the activity remains constant (or even decreased slightly) for the first 2C percent montmorillonoid mineral added. This phenomena being dUe either to montmorillonite absorbing the exCeS8 Ater hefitii6 ,t,4/ -.0: itself becoming "saturated" or perhap6 th‘ kbAtibrillonite 1.2

0.8

ACTIVITY

0, 6

0.4

0.2

I0 20 30 40 50

KAOLINITE ADDED (PERCENT)

FIG.6-6 RELATIONSHIP BETWEEN ACTIVITY & PERCENTAGE KAOLINITE ADDED TO LONDON CLAY FROM WALTON ON THE NAZE 50

MONTMORILLONITE ADDED 30 (PERCENT)

90 110 130 150 170 190 LIQUID LIMIT

FIG.6. 7 RELATIONSHIP BETWEEN LIQUID LIMIT & PERCENTAGE MONTMORILLONITE ADDED TO HERNE BAY LONDON CLAY 1.8

1 .4

ACTIVITY 1.2

1 - 0

0.8

0 • 6

10 20 30 40 50

MONTMORILLITE ADDED (PERCENT)

FIG.6.8 RELATIONSHIP BETWEEN ACTIVITY & PERCENTAGE MONTMORILLITE ADDED TO A HERNE BAY LONDON CLAY / 140 / / / / 120 / PLASTICITY /

IDEX 100 (s / / ,\-.4c -i- BO

I 1 BO 100 120 140 160 180 200

LIQUID LIMIT FIG. 6-9 PLASTICITY CHART FOR LONDON CLAY - MONTMOR1LLORITE MIXTURES. 91

becomes imirolvof. in clay-water interactions or mixed layering reactions.

3. Removal of Overburden The Eocene sedimentary deposits of London Clay would appear because of their relatively young age, to be excellent subjects for an analysis of their stress history. They are overlain only by Claygate/3agshot beds and locally by Pliocene deposits which testifies to the low magnitude of their merburden load. However, a quantitative reconstruction of the geological stress history is complicated by three factors :- a) alpine tectonic forces have uplifted and folded the whole sequence, b) Pleistocene glaciers have ridden over the northern half of the deposit causing low loading and unloading cycles within the stress history and, c) analytical methods, even for these low loadings, are not sufficiently accurate to allow a clear picture to emerge. These methods are mainly twofold. The first involves a geological interpretation of the former full thickness of the deposit and overlying deposits at that locality but cannot really incorporate an ice loading estimate. The second requires the construction of an e-log p curve for an °odometer tested sample and the method fails for London Clay because the loadings required to simulate natural in situ conditions often exceed the capacity of the normal oedometers (i.e. about 25 tons/ft)

In recent years, methods of studying clay particle orientation have advanced to a stage where it may be possible to use these techniaues to gain a auantitative estimate of the overburden renoved by erosion from a particular locality. The principal being to measure a clay particle orientation ratio for the undistrrbed soil sample and hence relate it to other localities where the full stress history is known or perhaps even compute the overbuden removed directly. With this intention partly in mind, the writer, in collaboration with Dr.J. Tchalenko and J.Hung, made a study of particle orientation in clays. (Tchalenko, et al. 1971). A three dimensional mathematical model defining a basic orientation ration was devised by Tchalenko. Tbis was extended to invoke a birefringence ratio to account for optical microscope light readings and a diffraction ratio for X-ray intensity readings. Using floculated kaolinite slurries consolidated to varying loads the optical and X-*ay ratios were measured and plotted as shown in ti3.6,1C# 92

The relationship between plotted values and the theoretical line while considering the relevant consolidation loads, shows the general validity and application of the technique. Figure 6.11 shows the plot for natural clays, both London and Lias, of different overconsolidation loads. Again the plot agrees with theory and an indication of the maximum load endured by each sample can be roughly gauged on comparison with figure 6.10. It must, however, be remembered that much further research concerning this new technique is required to eliminate or account for such difficulties as the effects of floculation on orientation, differential mineral orientation, etc. Nevertheless, the method has already proved, a) that a series of overconsolidated clays lie on a predicted linear plot, b) that the order of preconsolidation in a set of samples as well as a rough quantitative estimate can be established and, c) that different minerals orient to varying degrees under the same loading conditions (See Tchalenko et al.1961).

C. Investigation into Weathering Weathering is found to occur in the London Clay whenever a free surface is presented to the atmosphere. This may either be a fossil or present day phenomena, and is most easily observed by the marked colour change which the weathered material undergoes. The scale on which these processes proceed is largely twofold. Firstly, the large scale cliff or quarry face size,. where the upper two or three meters from the surface nay display the colouration effects which indicate weathering. Secondly, the hand specimen size scale, where an identical colour change is to be observed penetrating the clay lumps from the bounding fissure surfaces.

An examination of the large scale weathering will be continued in the following chapter using depth profiles of numerous parameters proceeding from the land surface downwards. The investigation to be discussed here concerns the second, smaller scale weathering process.

The aims of this investigation are thus to examine in detail the small scale weathering effects in order to clarify a large scale phenomena. An isolation of the weathering process is attempted to establish a qualitative and quantitative estimate of its effect on London Clay. 1.0

THEORETICAL CURVE

0.B 0.05) on 6

BIREFRIGENCE

O.6 7 RATI 0 (o . I o

(OPTICAL) 8 on 0.4 (0.40)

(1.•60) OD I0 0 WHOLE ROCK OD n THIN SECTION O.2 25-00)

(25 oc) n. 4

0 2 0.4 0.6 0.8 1.0

DIFFRACTION RATIO (X RAY)

(0.10) MAXIMUM CONOSLIDATION LOAD (AFTER TCHALENKO ET AL,1971)

FIG. 6-10 X RAY VERSUS OPTICAL ORIENTATION MEASUREMENTS OF FLOCCULATED CONSOLIDATED KOALINITE SAMPLES

1. 0

58 92

370 O. 8

BIREFRINGENCE 3831 382

0 .6 RATIO 65

(OPTICAL) 671 3791

0.4 1 66 109 376

1 381

THEORETICAL RELATIONSHIP O.2

I I I I I I I I I I 0.2 0 4 0.6 0 8 1.0 PEAK RATIO (X RAY)

AFTER TCHALENK 0 ET AL,197) = LONDON CLAY = LIAS CLAY

FIG. 6-I I X RAY VERSUS OPTICAL ORIENTATION MEASUREMENTS ON NATURAL CLAYS

Uuch of the investiTlttlba invCived tests on weathered and fresh material for comparative purposes. The two types of material being obtained by carefully separating the weathered brown rim and fresh grey core in hr..1d- specimens of London Clay from the cliffs of Whitecliff Day, Isle of Wight. Plate 6.3 illustrates weathering in hand size specimens.

1) Mineralogy Whole rock and clay fraction (-2u) mineralogical examination of each material was carried out.. The results of the former analysis are tabulated below :-

Whitecliff Day, London Clay Mineral, Fresh Core% Weathered Rim% Cuartz 40.0 37.0 T1 Felspar 1.0 1.6 Plagiochase 1.1 1.7, Mixed Calcite 1.0 0.7 Dolomite 1.5 1.7 Siderite 0.2 0.4 ryrite 2.2 1.4 Clay minerals 53.0 56.0

As both materials originated as a single hard specimen sized lump of clay their former composition was probably identical. Hence mineral changes could be considered to have developed on weathering. It must also be borne in mind (See Appendix) that there is a high error coefficient of variation in minerals with percentages less than approximately 5 percent. Thus the general conclusions with respect to these samples appear to be a) a marked colour change from chocolate brown to orange brown, b) an approximately 3 percent decrease in quartz content and an increase in clay mineral content on weathering. This clay mineral increase cannot be followed through any mineralogical process such as decomposition of the felspars etc., in fact the felspars appear to increase slightly in percentage and this is cualitatively borne out in the clay fraction traces as well, c) the total carbonate content remains roughly constant, indicating that little severe leaching has occurred, d) The pyrite percentage decreases slightly and may be caused by an oxidation of the sulphide to a chemical sulphate.

The mineralogical analysis concerning the clay fraction minerals is summarized in the following table Plate 6.3. Nand specimen sized weathered London Clay showing fresh grey core and weathered brown rim material, Whitecliff Bay, Isle of Wight. 94

Mineral Fresh Core % Weathered Rim To Kaolinite 17 17 Chlorite 1 3 Illite 46 47 Montmorillonite 36 33 Felspar Present Present ruartz Present Present

Analysis of these results shows a) no major change in the proportion of clay minerals is noted, b) small amounts of chlorite might form at the expense of montmorillonite in the weathered material, c) small amounts of felspar and quartz are present in the clay fraction of both materials, and, d) the crystallinity of the minerals as judged from their peak shapes appears to be very similar.

1) Chemistry A full wet chemical analysis has been carried out on samples of the two materials in order to compare purely chemical variations between the two. The results are tabulated below •

Weathered Fresh i•ror Rim Core -w/o Moisture 105°-11G°C ';'.43 '7.1' C.C? Ignition Loss 11Co -10CCo C 6.03 6.5" 0.05 SiC", 62.83 6".4C• C.05 A19C3 15.19 14.53 0.05 Fes,C3 6.C5 5.74 0.0% TiC' 0.1' C.10 0.C1 CaC r.-6 9.89 C.C6 142:( '.49 r'.53 0.C' ir;C. '.65 3."'3 0.09 Na'C 0.05 C.C7 C.C1

Interpretation of these figures leads to the following conclusions - a) greater ignition losses of the fresh material indicate that more organic carbon has its source in this material and/or more water of dehydration is available for release from the fresh rock (probably being stored in the larger percentage montmorillonoids),b) The increased Al2C3 content of the weathered rim is a reflection of the larger clay mineral content as proposed in the discussion mineralogy, c) the iron increase on weathering is not accounted for easily when considering the 95

mineralogy but this, coupled with the fact that it is most probably in the ferrous state, could account for the colouration effects. No ferrous oxide minerals such as goethite are note, however, d) The CaC contents (along with the MgO contents) reflect a possible slight reduction in the Ca and MgCa carbonates on weathering. Hence a mild leaching may have occurred. This effect may be somewhat masked by the slight increase in Mg rich chlorite in the weathered material, e) the very low weathered rim it content cannot be accounted for in the mineralogical analysis as neither the felspar nor illite content is particularly low. Calculations using the fresh core results indicate an illite content of about 47 percent which is correct. The weathered rim 7C"C percentage only indicates about 36 percent illite.

The wet chemical elemental analysis thus generally confirms the mineralogical conclusions of mild chemical and mineralogical leaching, probable oxidation as well as a slight breakdown in the carbonate and sulphide minerals.

Duplicate samples oI both weathered and fregh material were tested for carbonate content using a volumetric method involving the reaction of gaseous carbon dioxide with a solution of barium hydroxide. This resulted in a CC1 percentage for the weathered material of 1.36 and 1.77 for the fresh core material. Converted to (Mg,Ca)CO3, i.e. a mixed calcite cum dolomite composition for comparison gives ''.84%7 and 3.7C. for the two materials respectively. This again indicates a slight breakdown and leaching of the carbonates in the weathered rim.

3) Cation Exchange Capacity Cation exchange capacity determinations were carried out on the fresh and weathered materials to act as both a check on the mineralogical analyses and also to indicate the general "activity" and surface area characteristics of the materials. The determinations were carried out following the methylene blue d- method explained in Appendix 1. The results may be tabulated as follows :-

"ore , $.hered. Rim Mine.zal Caic Chem. Whor- them. C..111 C. 1 "ock% C.E.C. C.E.C. RockT C.E.C. C.E.C. Kaolinite 5 9.0 0.45 J.5 C.48 Chlorite le 0.5 0.05 1.7 C.17 Illite 15 n4.4 3.66 76.3 3.95 Montmorillonite 80 19.1 15.70 18.5 14.10 53.0 19.36 19 56.0 19.4C '70

It is seen that the calculated C.E.C's agree well with the chemically found C.E.0 's hence verifying the mineralogical analyses to a large extent. Also a slight increase of C.E.C. in the weathered material is noted in both cases even although the percentage of montmorillonite in the fresh core exceeds the rim material. This phenomena illustrates the importance of an increased clay mineral percentage on the total properties of the clay material. From these results, it would thus be anticipated that the soil testing programme could illustrate a more "active" nature for the weathered material.

Soil Testing Two small programmes of soil testing were carried out on weathered and fresh London Clay from Whitecliff Bay to establish whether any change in index properties could be found from one sample to the other.

The first set of tests was conducted on hs:;1 specimen size samples divided into fresh and weathered portions in the same manner as the foregoing studies. The assumption again being made that due to the small size of the original sample no primary property variation could be expected to exist before weathering. The results of these tests are given below

Test Fresh Core Weathered Rim Specific Gravity 7.69 7,.69 Liquid Limit 64 66 Plastic Limit 71 ni Plasticity Index 43 45 Clay Fraction % 41 4C Activity 1.05 1.17

Again the results show only marginal changes in properties. The specific gravities remain constant and probably reflect the single material origin. Atterborg limits increase slightly cn 97

weathering and th-7, is bec.?Ise of the increased mineral content in the rim material, as already noted in the whole rock mineralogy- section. The fraction of the rock less than 2 microns in size is slightly but unexplicably reduced by weathering Which results in activities of 1.05 and 1.12 for the fresh and weathered materials respectively. This increase in activity, especially with the quoted clay fractions, can only be explained by more active minerals' making their 'appearance or a higher-clay mineral content. As is obvious from the mineralogical section the latter process is favoured here.

The second testing programme was conducted on samples obtained'to represent the second or larger scale of weathering phenomena, i.e. a cliff or' face size scale. Great difficulty is usually encountered in obtaining nearloy samples of fresh and weathered rock of exactly the same stratigraphical level in an attempt to eliminate natural lithological variations. This' problem was overcome at Whitecliff Bay where, due to the vertical nature of the strata, fresh and weathered material from identical stratigraphic positions could be obtained by sampling vertically from near beach and cliff top levels respectively.

As a first stage of the testing and to ensure represent- ability six quick undtained triaxial tests for both fresh and Weathered rock were carried out on undisturbed' specimens hand trimmed from clay lumps. The mohr circle plots for these tests are shown in figs. 6.17-and 6.13. The unweathered cliff bottom samples have a range of undrained strengths from 40 to 54 p.s.i. while the weathered cliff top strength range is from 35 to 52 p.s.i. As the range of values is considerably in excess of any strength variation between materials very little can be concluded with regard to these visibly differing rocks. The cliff top rocks are what would be simply described as weathered brown London Clay whereas the lower samples are fresh grey London Clay.

ror the remaining index tests on these samples of large scale weathering, the grouping of the strengths into weathered samples 1, ':. and 3 and 4, 5 and 6 and fresh samples 1, 7, 4 and 5 and 6 provided a convenient sub division. The following table summarizes the results of these tests ,:- UNWEATHERED CLIFF BOTTOM SAMPLE

SHEAR

40 STRESS

Q? S.1) 30

20 30 40 50 60 70 BO 90 100 110 120 130 140 NORMAL STRESS 6 (PS.1) FIG. 6-12 UNDRAINED SHEAR STRENGTH OF UNWEATHERED WHITECLIFFE BAY LONDON CLAY WEATHERED CLIFF TOP SAMPLE

•

SHEAR

STRESS 40

CP S.1) 30

20 30 40 50 60 70 80 90 100 HO 120 130 NORMAL STRESS 6 CPS. I)

FIG. 6-13 UNDRAINED SHEAR STRENGTH OF WEATHERED WHITECLIFFE BAY LONDON CLAY J

Fresh Bottom Weathered Top Sample Mixtures Samples Mixtures Test 6 1 ,, , Specific Gravity '7.CC .79 ".8C r.74 Liquid Limit 6" 60 63 64 Plastic Limit '11 93 19 `1 Plasticity Index 41 37 44 43 Clay Fraction To 39.3 38.6 37.5 38.0 Water Content(insitu) '0.1 18.6 1.6 '1.0 Activity 1.04 C.96 1.17 1.13 Liquidity Index -0.03 -C.1 +0.06 0.CC

Before drawing conclusions from these results, it is necessary to consider the respective mineralogical analyses carried out on the mixtures of sample material making up the four groups tested above. Fresh Sample Mixtures Weathered Sample Mixture Mineral 1,;3,4,5 r 6 , 1,2,3, 4,5,6 0uartz 33 34 47 38 .0 Felspar 1.6 1.8 1.9 1.0 Plagioclase 4.1 1.5 1.."' C.7 Calcite C.8 ' C.5 C.9 3.4 Dolomite 1.8 1 2.6 2.1 3.1 Siderite C.4 - - C.7 Pyrite ,.3 '.0 - 2.6 Clay Minerals 55 57 48 5C Xaolinite 11(0) 11('C) 8(17) 8(16) Chlorite 7(4) 1(``) r,(4) ,='(4) Illite 31(56) 32(56) '',6(54) ?7(54) Montmorillonite 11(7C) l(',1) 1":(''5) 13(96)

The specific gravity tests do not reveal any significant variations although weathered sample mixture 4, 5 and 6 appears to be enormously low for some unexplained reason. The Atterberg Limit results are interesting in that they show a. consistent trend toward higher values for the weathered samples. This trend is in the c -der of between 5 to 1C percent, and ray be accounted for on examination of the clay fraction percentage figures in brackets. These indicate a slightly higher montmorillonite content for the weathered material. The minus c micron fraction is roughly constant but as with the rin versus core test the weathered material tends to show slightly reduced figures. This surprising result rernins largely unexplained 99 but has the effect of the activity of nose materials from an average of 1.00 for fresh rock to 1.15 for weathered rock. This ±11% increase is fairly significant Adoring the variation of London may activity is only of the order of ±30 percent over the whole basin. Hence for this parameter weathering plays an important role in the total variation whereas it is not so significant (10 percent variation compared with over 80 percent) for the Atterberg Limits. Further study of grading figures shows that although the fresh material has a higher clay fraction than the weathered it also has a higher coarse silt plus fine sand content and this may considerably help in the plastic property reductions already noted. Especially when considered in the light of results noted earlier in the chapter.

Fraction Fresh Sample Mixtures. Weathered Sample MixtuA 1,.2,3,4,5 6 1,2,3 4,5,6 1 C.Silt + F.Sand% 26.4 n9.1 23.7 24.5 Med.Silt% 273.0 20.9 25.8 25.7 Fine Silt% 17..3 11.4 13.0 11.8 raay% 39.3 3C.6 37.5 ns.c 1C0

Chaoer 7 Geotechnical Investigation of Principal `51.tes and itegional Trends I. Detail of Principal Sites The aim of this chapter is to link the small scale investigations described in the preceding chapter and the regional scale studies described in a following chapter. The small scale investigation used detailed experimentation, and the regional scale used collation and correlation techniques.

This study lies between these extremes and involves an examination of individual engineering sites or geological exposures. It is felt that on this scale, which is obviously of most interest to the geologist and engineer alike, a more complete understanding of soil property origins and variations must be attempted. Hence from an inspection of soil, chemical, mineralogical and mechanical parameters on this scale, it is hoped that correlations will emerge which relate geological history to present-day properties.

Published Data Gnly three London Clay sites have been discussed in detail in published geotechnical work. These are located at Ashford Common, Bradwell and Central London. A great deal of soil mechanics detail has been discussed in these published works but their value, from an engineering geological viewpoint, is somewhat limited for three important reasons :- 1)Applied sedimentclogy principles, as described in detail in Chapter 2, have not been fully implemented resulting in a somewhat poor recognition and comprehension of stratigraphical implications, 2)Stratigraphical zonation, and hence control, has to date been incowlete leading to cross-correlations between zones, 3)Besides a single early analysis by Grim (1951), detailed mineralogical data was lacking, this made the establishment of mineral property relationships virtually impossible. Studies of the palaeontological and mineralogical aspects have partly removed barriers and produced data which, it is hoped, will be able to further the knowledge of these and other sites.

Each site will be discussed in turn and an attempt made to relate the published data and the writer's results in such 101

a way as to link the litholcgy, mineralogy and mechanics of the soils in a vertical profile. This will be followed by two sections on investigations conducted at Noak Hill and Herne Bay. For soil data on each, the writer is indebted to Nuttalls Geotechnical Services, Dr.P.Vaughan, Dr.D.Chandler, Dr.J. Hutchinson and Dr.D.Petley.

A. Ashford-Sunbury The Ashford Common site has been described by Ward, et al. (1959) and comprised a shaft sinking and tunnelling operation for the Metropolitan Water Board. During the shaft sinking the authors mentioned above, in conjunction with Prof.Bishop 01 Imperial College, undertook soil mechanical studies on in situ and block samples derived from six levels within the shaft. It is these results which will be summarized and used in conjunction with mineralogical data obtained by the writer on samples from a nearby borehole at Sunbury Cross. These borehole samples, also from a M.W.B. site were also used by palaeontologists as part of their study in zoning the London Clay (Williams, 1970).

Figure 7.1 shows the more detailed geological section in which it is seen that the Middle and Upper London Clay Are exposed, that certain silt and septaria layers tie in with Wards et al (1959) section, and that in fact almost the total Middle London Clap (i.e. 1, 2 and 3) as well as Upper London Clay 1 is recognised. The suggestion that the Claygate Beds may have been intersected just below the gravels (Ward et al, 1959) is hence proved wrong as Upper London Clay 2 and 3, which represent some good few hundred feet are missing. The palaeontological boundaries show close relationship tc sedimentological features and principles, in that they are generally to be found at or near the base of a silty horizon which itself represents the finer grained top of the underlying cycle and the basement bed of the new cycle. This feature is admirably brought cut on comparison of the boundary positions with the X-ray quartz percentages and the sedimented clay fractions. Thus the site has been accurately stratigraphically positioned within the London Clay geological column and subdivided on sound palaeontological and sedimentological grounds and the -geological log (silty horizons, etc.) adequately related to mineralogical 102 variations. With this know:r,dge, it becomes possible to re- examine the geotechnical aspects as well to consider the recurring question of a regional trend of properties. For example, Bishop et al (1965) reasoned that a comparison of the Ashford and Bradwell site properties suggested mineralogical and/or pore water electrolyte differences on moving across the London Basin. It should now be clear but further emphasized that such a comparison can only be accurately made on sections of stratigraphically continuous strata and not those separated vertically by some 300 feet as mentioned above.

The fact that non stratigraphic comparisons could be made at all is fortuitous in that the sea has always transgressed from the east resulting in relatively deep water in this direction.

The first property to be discussed (see figure 7.1) is the clay fraction (or percentage -2u). This property has previously been regarded as having "no significant trend" on' this site (as have the Atterberg limits), but some interesting observations emerge from these variations on comparison with the detailed geological column and the mineralogical results. It may first be noted that the sedimentation clay fraction figures relate well with the X-ray clay mineral profile and both generally show reductions at the silty top and bottom of each cyclic zone. Secondly, a fairly large absolute discrepancy exists between the two sets of figures resulting in an (aggregation) ratio of 1.4. The mineralogical figures appear to be confirmed by the cation exchange capacities whereas the Atterberg limits and activity back up the sedimentation data. It is not felt that cementing is the only cause of this paradox but also that the locality is fairly near shore and that a significant percentage of clay minerals be in the silt size range. A similar ratio was found for Whitecliff Bay (see Chapter 5) whereas for Herne Bay, the ratio drops to nearly 1.0.

The plastic limit profile follows that of the liquid limit, both of which average a rather low 40 and 67 percent respectively. They do, however, as does the plasticity index profile, vary in good agreement with usual and X-ray observations (See fig.7.1). This whenever silty horizons are intersected, as at 20-30 feet and 70-90 feet, the limit profiles decrease noticeably. This effect appears to be dominated by the granular materials and is

1C3

predicted from the writer's experiments as discussed in Chapter 5. Thus the montmorillonite percentage which averages 2C cannot be related to Atterberg limits in this near shore profile where quartz content is the overriding factor.

The activity profile is noted to be roughly the inverse of the clay fraction. This is because it depends almost entirely on the latter and not on any particular mineralogy. In fact, this parameter bears no relation to montmorillonite content for example, even although this mineral increases steadily in content from about 15 percent at the profile top to 25 per cent at the bottom. This is because its effect is offset by the quartz content variations. The absolute activity valves are rather low especially when compared to the usually quoted value of 0.95 (Skempton,1961). This is, however, found to fit in with regional trends of this particular parameter which appear to be governed by mineralogy and sedimentology. These regional aspects will be discussed at greater length toward the end of the chapter.

Although the montmorillonite content cannot be directly related to activity on this section, it strongly resembles the cation exchange capacity profile and a relationship is suspected. Cation exchange capacity data are only recently becoming common and their use as a amide and check on the mineralogy as well as a reliable index parameter cannot be too strongly recommended in certain situations. An example of this properties usefulness is given for depth level C :-

Mineral Mineral% C.E.C. Product% Chem C.E.C. Montrnorillonite 20 120 24.00 Illite 4C 30 12:.00 Chlorite C ,_ (17 10 1.7C Kaolinite (

71 37.7C 38 These figures confirm the mineral analysis to some extent as well as proving the existence of considerably more clay minerals than indicated by the clay fraction alone (i.e.59%) which would have resulted in considerably lower values of C.E.C. It is suggested that activity data might be used in exactly the same manner, for example level B 104

Mineral Mineral% Act. Product% Lab Act. Montmorillonite 20 2.00 0.40 Illite 40 1.00 0.40 Chlorite ( ' . ) Xaolinite ( 17 0.45 .) 0.07 ) 0.87 0.79 The C.E.C. hence reflects grading curve as well as mineralogical changes to a far greater extent than the foregoing properties for the reason that the property is so extremely sensitive to montmorillonite. Regional C.E.C. trends (in three dimensions) discussed in previous chapters have illustrated its enormous value from the sedimentological viewpoint.

The natural water contents measured by Bishop, et al. (1965) also tend to pick out the drier silty layers but on the whole are, rather regular and lie on the dry side of the plastic limit. This results in a negative liquidity index very close to zero which gives further proof of the overconsolidated nature of this clay. The preconsolidation pressure deduced from the geotechnical data by Bishop is equivalent to some 1,200 feet at level E. However, it becomes obvious from the ribbon map (fig.4.7) that in the order of 400 feet of London Clay is thought to have overlain the level (judging from the Ripley Borehole) before the Bagshot Beds are even intersected! These latter deposits can also reach thicknesses of the same order of magnitude and once the Bracklesham and Barton Beds thicknesses are added, the order of preconsolidation load is being approached.

The result of this preconsolidation stress on the clay particle orientation (as well as the effect of X° on the site which varies in depth from 3.4 to 2.0) has not been studied on this site by means of either an optical or X-ray orientation ratio. This is somewhat unfortunate in that Bishop et al.(1965) found low O'values of around 160 which recorded well with Skemptons (1964) figures which he accounted to good particle orientations created on slip planes. It was thus speculated that the low Of values on this site might have arisen from overall good particle orientation induced in the clay because of the high preconsolidation pressures.

B. Bradwell-Orford Ness The investigation of the two short term London Clay failures in the excavations for the Bradwell nuclear power station by Skempton (1961 and 1965) proved extremely fortuitous for those engineers and geologists interested in London Clay. These investigations found variations in this deposit which were until then hardly suspected. These variations were found to fit in to a regional pattern which up to then had not been studied in any detail. The-site location and details are given by Skempton (1961) who in this paper used samples from a deep boring of 12C feet to calculate Xo values and reconstruct the complete stress history of the site. In a second paper in 1965, Skempton and La Rochelle examined the topmost 50 feet of clay in great detail while analysing excavation failures at site.

The present writer, through Mr.T.Mellor and the Engineering and Power Development Consultant obtained samples and technical data from the deep Borehole 19 on the Orford Ness site some 40 miles to the northeast. The section of London Clay hero penetrated was only 65 feet below the Pliocene Crags. Mineral- ogical analyses were carried out on some 14 samples.

The stratigraphical position and zoning of these two sites has been made possible by use of a third nearby site at Malden. The sedimentology of this latter site, with a total London Clay thickness of some P,50 feet, has been investigated by Williams (1970). The correlation of sites was made relatively easy by the intersection of the Basement Bed and Oldhaven Sands in the case of the Bradford site and the strong indication of approach to the Basement Bed as well as known local London Clay thicknesses at Orford Ness.

Figure 7.2 illustrates the full geological column with the stratigraphy and subzones superimposed as well as the vertical position and correlation of sites as best as can be estimated. That is, with a vertical error of ±10 feet. It thus becomes clear that the Orford Ness site with its mineralogical and soil mechanical data overlaps the lower portion of the Bradwell exposure by some 40 feet. The full column hence displays the Basement Bed, the roughly 90 feet of Lower London Clay with subdivisions 1, 2and 3 and almost all of the Middle London Clay thickness, which forms part of the major 1965 investigation and includes some 30 feet of brown weathered London Clay. 106

The individual clay fraction figures have not been quoted in either of the references but an average value of 52 percent with narrow range is stated for Bradwell. This figure is surprisingly low for a supposedly deep water locality but is borne out by even lower figures for Orford Ness. This is understandable in a relative sense being nearer the silty basement. On examination of the palaeogeography maps of the London Clay Sea in Chapter 2, it is noticed that this locality is not really very much further offshore than Ashford. However, following a line perpendicular to the assumed shoreline, the clay fraction figures for Central London vary:between 45 and 65 percent (Skempton and Cooling, 1942) while Herne Bay averages 65 percent (Dr.J.Hutchinson, Imperial College personal communic- ation). The Orford Ness X-ray clay mineral percentage averages 75 and so an aggregation ratio of roughly 1.4 is again noted. The reasons for this discrepancy are difficult to understand as this time, unlike Ashford, a strong floculation effect is proposed to account for the very small clay fraction percentage, a large proportion of which is montmorillonite.

The high proportions of this mineral (at these low clay fractions) is confirmed by an examination of the Atterberg Limit results. A high but reasonably constant plastic limit of between 30 and 35 is noted for the complete profile. The liquid limits vary from a low of 85 to 90 for the silty Basement Bed and the Lower to Upper basement horizon to a high, averaging about 100 and reaching 115 percent in places. Plastic- ity index values hence range from about 55 to 80. The limits, although exceptionally high, still relate well to the stratigraphical boundaries based on visual and palaeontological evidence (Williams, 1970). The general liquid limit low around -70 0.D. or the Lower to Upper Clay boundary being an example (See fig.7.2). The relationship between the limits and the mineralogy is dominated by the quartz percentage as was the Ashford site. The clay mineral percentages increase steadily up the profile. Illite and kaolinite proportions also increase upwards. Montmorillonite on the other hand decreases in proportion as more fatty sediments are encountered. Hence where the montmorillonite is at its highest concentration, the quartz content is also high and having a lowering effect on the plasticity characteristics.

These factors lead on to a discussion of the activity. If an average clay fraction value of 52 percent is assumed, the plasticity indicies may be calculated using the quoted activity value of 1.5. Now using the calculation as before:-

Mineral Act. Product Lab Act. Montnorillonite 4C 2.0 0.8C Illite 35 1.00 0.35 Chl. Kao. 4 0.45 0.0 1.17 ±1.25 These calculated and experimental values correspond reasonably well. What is more important is the absolute value of 1.25 which increases to even higher values at Orford Ness. This adequately illustrates the considerable montmorillonite increase of some 2C percent over Ashford. The results of the tests discussed in Chapter 6 should, however, not be forgotten as they suggested that initial quartz and kaolinite variations were equal to, if not more important, than those of montmorillonite. This point is brought out in the following calculation of predicted cation exchange capacities : Mineral % C.E.C. Product% Chem C.E.C. Mont. 40 120 48.00 Illite 35 30 1C.50 Xao. 86 Chl. 4 10 0.40 58.90 38 The water content, determined by Skempton (1961) plots as a curved average, not varying much with lithology, but decreasing smoothly from 36 percent to 29 percent at about 100 feet, hence the liquidity index is positive in the top half of the profile and negative in the lower, the values remaining close to zero. The density correspendingly increases from 118 to 1°2.3 lb/cu.ft. as does the undrained shear strength from 1000 lb/cu.ft. to about 4000 cu.ft. at 100 ft. depth.

For geological and geotechnical reasons, Skempton (1961) has quoted an approximate preconsolidation pressure of 30,000 lb/sq.ft. This represents an overburden, which has since been removed by erosion, of some 500 feet. The calculated No values range in depth from 2.17 to 1.46.

Review of the 1965 publication concerning Bradwell proved 108

interastinc, without being o_sically a shallow investigation the weathered brown London Clay and its transition to blue clay was exposed and studied. Also interesting is the fact that about half the site is covered by a soft Postglacial deposit of Marsh Clay lying on a late Pleistocene weathered erosion surface of London Clay. "The top 20 to 30 feet of the latter is oxidized, showing a typical brown colour, The junction between the brown and the blue clay is not sharp, and it does not correspond to any abrupt change in mechanical properties." (Skenpton and La Rochelle, 1965). This statement tends to confirm the results discussed in Chapter 7 which could find almost no variation between stratigraphically identical fresh and weathered clay. However, in-situ properties may well vary and water contents increases, vertical stress relief expansion, clay fabric disturbance, increased fissure intensity, decreased strength etc. have in fact been proved.

C. Streatham - Central London This section, like that of the previous two, is a collation of data from various soil mechanics investigations in Central London (Skempton and Cooling, 1942) and (Skempton and Henkel, 1957) and the mineralogical analysis of a 140 ft. borehole at Streatham. Once again the stratigraphical zoning is superimposed on the original logs (See figure 7.3) with the result of clarifying and subdividing the sections. The Lower to Middle London Clay division is suggested by palaeontological evidence and the mineralogical data, especially quartz content, verifies this transition. It is not intended to discuss each and every profile in detail for the sake of repetition but rather to bring the following general points to notice : 1)No direct clay fraction figures are quoted by the various authors but an activity of 0.95 for London Clay is referred to. Hence by the relation CF = 0.95 x WPI the individual clay fractions may be obtained. These are seen in figure 7.3 to average 50-55 percent for these basal layers of London Clay. Hence the ratio of X-eay clay minerals to sedimentation fraction = 1.3 - 1.4. 2)The plastic limit averages about 27 and the liquid limit about SO. Both these values as well as the clay fraction and activity are somewhat higher than Ashford but lower than 109

Bradwall, 3) No cation exchange capacity values are available for this section but the theoretical predicted activ ity calculation Is e,o follows : Mineral % Act. Product% Lab Act. Mont. 25 2,00 0.50 Ill, 34 1.00 0.34 Xao. & Ill. 16 C.45 0.t7 0.91 -0.954- 4)The water contents lie almost exactly on the plastic limit profile hence the Liquidity Index is 0.00 5)An estimated preconsolidation pressure based on both geological reasoning and pedometer testing (Skempton and Cooling, 1942) is quoted as between 500 and 700 feet of previous overburden. 6)Much of the removal of overburden occurred during the Pleistocene by the Proto Thames. Following each downcutting, terrace gravels were deposited ranging from the Taplow (-70 ft OD and 150,000 yrs. B.P.) to the Flood Plain (±25 ft. O.D. and 100,000 yrs. B.P.) Under these gravels, thin weathering nays that the zone was probably more extensively developed eroded and overlain. Post gravel weathering is also thought to have taken place.

D. Noak Hill Opportunity was taken of conducting mineralogical analyses on samples from a 1C0 ft. borehole section of clay from Noak Hill, Essex (see fig 7.4). The study was undertaken for two main reasons :- a) The topmost 30 ft. passed from Claygate to London Clay beds - a little known geological boundary and b) engineering parameters for the site and in certain cases even the exact borehole could be obtained. No palaeontological control was obtained for this section but as the lower Claygate boundary was intersected and identified by Dr.Bristow of the Institute of Geological Sciences, no great stratigraphic loss was felt. This investigation provided less engineering data than the foregoing ones but nevertheless its unique geological and geographical situation resulted in some interesting findings which are summarized below. 1) Although an obvious visible change is apparent on examination of the Claygate Beds which tend to contain layers of very silty 110

and then clayey material, no major or abrupt mineralogical change is noted. See ftg. 7.4 and plate 2.1C. 2)General mineral trends may be seen to include very ragged Claygate quartz and clay depth profiles which smooth out on entering London Clay, decreases in pyrite, kaolinite and Tontmorillonite percentages in the Claygates which tend to result in illite increases. 3)The Atterberg Limits show an abrupt and marked change to increased values on entering the London Clay. The respective liquid limit average ±43 to ±78 and the plasticity indices ±23 to ±51. 4)The few available clay fraction figures repeat this displacement (on changing from Claygate to London Clay) from about 28 to 63 percent. The activities of the Claygate and London Clay, however, remain constant at ±0.82 thus indicating a similar mineral suite for both. The above conditions speak for themselves in stressing the importance with respect to property variations, of sedimentological factors such as grain size in an unaltering sedimentation environment with single provenance. 5)The ratio between X-ray clay minerals and sedimentation tests is noticeably lower than the previous sites and here averages 1.1.

E. Herne Bay An intensive investigation was carried out to provide geotechnical data for stability and design calculations on the landslipped coastal cliffs at Miramar, some few miles east of Herne Bay, Kent. Extensive testing was carried out on samples from some selected borehole which penetrated the London Clay and intersected the underlying Oldhaven Sands. The types of testing undertaken included chemical, mineralogical and soil mechanical (index and strength tests). A palaeontological study of the samples provided exact stratigraphical control and sub-zonation of the London Clay. Figure 7.5 shows the pertinent section and profiles. See also plates 7.1W.n.

It is noted that the Lower London Clay as well as part of the Middle zone is intersected. The zonation is here based almost entirely on fossil content and characteristics but also relates to sedimentology very strongly. This is well shown by ‘%4

• 7 ,

/ •I. ,;,..4f ' " 'IP': ,,, s 7 , , _,,,, / , ., • /is, ., , , •11 4 7" ; ',1,4 'y , 41_

- , ,•1 11 '''',- ' e

Plate 7.1. London Clay overlying Cldhaven Dads 1..! miles east of Herne Day.

• _ •

,...... ,„„...... „ ...... „ _

Plate 7.. Burrowed sandy Oldhaven Sands over Woolwich Beds (note the clayey middle section), The steeply standing cliff base comprises Thanet Sands. Bishopstone Gap, aent. 111 the fact that the quartz content is at a high when the zonation boundaries are intersected. Each zone represents a tertiary cycle within two larger but similar cycles which begin with coarse basal sediment, grade into deeper water high clay facies and the transgression ends with a silting up and reappearance of high quartz contents (Refer to Chapter 2-Sedimentology). The major or primary cycle is represented by the Basement Bed- Middle London Clay (fatty) - and the sandy Upper London Clay cum Claygates. The secondary cycle occurs three times and divides the London Clay into Lower, Middle and Upper. The Tertiary cycle is represented by the zones Lower London Clay 1, and 3. It is interesting to note that fatty middle or deeper water portions of each zone (of the primary, secondary and tertiary cycles) are marked by a high portion or high content cusp on both the X-ray and sedimented clay percentages. Although these profiles are the best zonation indications, they are supplemented by certain of the other chemical and mineralogical profile lines, e.g. Pyrite.

It is not only possible to distinguish stratigraphic zones using the profiles but it is also possible to infer factors regarding their environment of deposition. For example, the Lower London Clay 1 zone, stretching from and including the Basement Bed to about 63 ft. (refer to figure 7.5). Fairly high percentages of detrital minerals are present (quartz and felspar) indicating nearshore conditions and/or an active sea coupled with fairly rapid, constant deposition. Sand size percentages are also quite high for London Clay. Clay mineral percentages and clay traCtioss are small. Roughly median but variable pyrite contents probably indicate an Eh of -0.25 and a pH of ±7.6 with about medium salinity (i.e. 35 g/_:;;.) Low siderite and dolomite percentages emphasize these latter two figures. Low chlorite and illite values indicate possibly even lower salinities (as low as 25 g/U;.). This could mean shallow cold open sea conditions with much inflow of freshwater and sediment. The coarser sand and silt fraction being dropped fairly near-shore while all the clay minerals except montmorillonite are carried to deeper water. The montmorillcnite is deposited in relatively enriched proportions because it has floculated on contact with even this low salinity. 90 HERNE BAY

80 00/

70 /x PLASTICITY 0 O / 0 x/ INDEX 0 X /0 x x X/ / x x X 60 0 x /0/ 0 x x x / 0 )/ xx x O • SEDIMENTATION X - X RAY 0 / 0 SO /

40 50 60 70 80 90 CLAY MINERALS (PERCENT)

FIG: 7-6 CLAY MINERAL PERCENTAGE VERSUS PLASTICITY INDEX HERNE bAY O Z o

1. 0 0 0

0 0 00 0 O.9 7 O O 7 ACTIVITY 7

0.B z 0

0.7

1 20 30 40 50 MONTMORILLONITE CPERCENT IN CLAY FRACTION)

FIG.7-7 MONTMORILLONITE PERCENT VERSUS ACTIVITY A fairly good correlation is apparent for Mg, Mn and Sr, the former two of which have low values. These metal ions are often absorbed in the carbonate mineral lattice and their decreased values tend to indicate low salinities.

A ^t-"y of the geotechnical properties reveals that comparison of clay fraction and clay mineral nertentages shows that while the -Su fraction increases reasonably steadily from bottom to top of the section, indicating a continuous fining of the material, the clay mineral and quartz sediment input remains fairly constant except where influenced by the Basement Bed. This interesting fact shows a continuously deepening sea receiving a constant and unaltering source material. The ratio of both types of clay percentages is roughly 1.15, which may be interpreted as meaning that either 15W, of the clay minerals are of silt size or have aggregated tc silt size. The coL.::oser the material, the larger becomes this tendency.

The liquid limit and plasticity index profiles plot as irregular but roughly parallel lines which are not at all similar to the sedimentation clay fraction profile and only vaguely like the clay mineral profile. They do, however, show up the zonation boundaries reasonably well so that at these silty junctions, their values are reduced. See figure 7.6. Both these properties also roughly relate to the whole rock montmorillonite percentage.

The activity thus is anything but constant for this London Clay section and varies from 0.78 to 1.1 within a 40 ft. vertical section! This parameter does not reflect the silty zonation boundaries as it is independent of grading characteristics. Activity relates to montmorillonite percentage in the manner shown in fig.7.7. If activity were to be calculated from mineralogy, it would result in the following : (35 ft. depth figures) Mineral % Act. Product7, Lab.Act. Mont. 21 2.00 0.42 Ill. 4C 1.00 0.40 Kao. & Ill. 18 0.45 0.08 ---- 0.90 U. Calculated values should only be used for very general 13_3

indications as should the cation exchange capacities which are discussed following.

The cation exchange capacity of a clay depends both on the clay mineral (as opposed to quartz) content and the relative proportion of the various clay minerals. As montmorillonite has a heavy weighting effect, the C.E.C. profile strongly resembles that of this mineral. This is effectively demonstrated, A predicted C.E.C. calculation is as follows Mineral % C.E.C. Product% Lab C.E.C. Mont. 21 170 25.00 Ill. 40 30 12.00 Kao. es Chl. 18 10 1.30 33.80 33 The in-situ water content at the site is remarkably constant averaging 28 percent. This value corresponds to the coverage plastic limit, hence the liquidity index is roughly zero, indicating a fairly heavily overconsolidated clay.

As this investigation was concerned with a stability analysis, eight shear box tests were conducted from which effective stress residual friction angle values were obtained. These plotted to provide a profile which increased in value from about 10o near the top to 14o near the base of the borehole. The inverse proportionality of this parameter to clay percent is illustrated in figure 7.0. Both the proportion of clay fraction size and clay mineral content appear roughly related to this parameter. Another interesting plot is that between O'r and montmorillonite percentage both expressed as a whole rock and clay mineral proportion. See fig. 7.9. The latter plot shows good correlation but the relative influence of each of the four plots mentioned is extremely difficult to different- iate.

II, Lateral Trends Following this discussion of individual principal sites, an attempt will be made to relate these by means of regional trends by bringing together the sedimentological principles governing the lithological variations, the stratigraphical control available and the Ltneralogical analyses carried Qut;

14 HERNE BAY

13 X X \

I t' t R

CDEGR E ES)

10 0

0= CF (-2p) X= MINERALS CX RD)

i 1 9 50 60 70 80

CLAY (PERCENT)

FIG.7-8 CLAY MINERAL PERCENT VERSUS EFFECTIVE RESIDUAL STRENGTH 14 HERNE BAY

13 X

12 R

(DEGREES)

10 O. WHOLE ROCK X C F

9 I5 20 25 30 20 30 40 50

(MONTMORILLONITE PERCENT)

FIG. 7-9 PERCENTAGE MONTMORILLONITE VERSUS EFFECTIVE RESIDUAL STRENGTH 114

for this work. A factor „7,2 major impor-rnce wht often leads to confusion, must be dealt with first. This concerns the estimated relative importance of near or off-shore geographic positions versus stratigraphlc variations, or in other words, do the properties or an offshore active fatty cl=ay vary more from its near-shore inactive lean counterpart than a slightly more silty deep water horizon which overlies it?

Analysis of figures from Chapter 4 and values quoted in this Chapter reveal that for Lower London Clay quartz contents decrease away from shorelines particularly on lines of major downwarp. These quartz values range from averages of 39 at Whitecliff Bay, 18 at Sunbury-Ashford, 22 at Streatham, 16 at Foulness and 24 at Walton on the 'laze. This roughly 2 percent variation (within the London Basin) is of the same order of magnitude as is found in values from Lower through to the Upper London Clay. Hence lateral and vertical quartz content variations are of the same magnitude. The vertical variatictins are noted, in each of the principal sites described, to follow lithological and stratigraphical changes in the London Clay sections. The basal portions of each rhythm or zone containing high quartz contents which gradually diminish in the middle portions and again increase in the upper regressive (or silting up) phase. Laterally quartz contents (within the same stratigraphic zone) decrease steadily eastwards, that is away from proposed shorelines and source material entry points. This mineral distribution is what 'amid be expected, and is often found, with sediments entering a large marine basin.

Total clay mineral values show roughly similar patterns but the values of the lateral trends vary by ab out 5 percent more than the vertical variations. These conclusions tend to be confirmed by the few clay fraction (-2u) figures available, i.e. Ashford 52, Bradwell 5C, Central London 55, Hoak Hill ±63, Herne Bay ±65 and Whitecliff Bay 40. The absolute values, however, being roughly two-thirds that of the X-ray analyses.

Montmorillonite lateral variations total about 10 percent whereas its vertical variations at a locality usually amount to less than half this amount. Vertical variations do, however, appear to be controlled or explained by sedimentological principles with montmorillonite showing affinity-for:baS41- BO

/ / z A/ / / / 60 / //G LOCATION - No. OF TESTS

. . . A BRADWELL - 42 B PADDINGTON - 16 PLASTICITY C WATERLOO I FT - 6 D WATER LOO - 106 / E ASHFORD - 6 40 INDEX F VICTORIA - 5 G S. BANK - 9

.., 20 \ .p,

I I I I I I I I I 0 20 40 60 80 100 120

LIQUID LIMIT

FIG:10 CASAGRANDE CHART OF DATA FROM PUBLISHED LITERATURE 80

PLASTICITY LOCATION- No. OF TESTS

40 INDEX A ORFORD NESS - 18 IN HERNE BAY - 30 C NOAK HILL - 104

I I I 0 20 40 60 80 100 120

LIQUID LIMIT

FIG.-II CASAGRANDE CHART OF WRITERS DATA 115

transgressive phases as well as deep water, stronglp marine environments. Thus montmorillonite (whole rock) contents decrease westwards and also from Lower to Upper London Clay.

The Kaolinite distribution is practically the reverse of that for montmorillonite as this mineral displays largest percentages in the Upper London Clay and upper portions of rhythms, that is in regressive phases, and also in nearshore mainly continental environments.

The overall regional mineral distribution is thus one of kaolinite and quartz showing increased contents in a westerly or nearshore directith particularly in the lower and upper rivthmic phases in the case of quartz and upper phase for kaolinite. Montmorillonite and total clay mineral contents increase eastwards particularly in the deep quiet water middle phase for clays and basal transgressive phase for montmorillonite. This relative concentrations of quartz and montmorillonite and quartz and kaolinite occur in the lower and upper portions of a rhythm respectively.

Turning to the geotechnical results available for the five sites, some interesting facts may be gathered with regard to Atterberg Limits from figures 7.10 and 7.11. The former shows a Casagrande Chart plot of the values upon which Bishop and Skempton (loc.cit.) drew many of their conclusions. It shows also that index properties within a 1 ft. sample can vary from between less than one quarter to more than three cuarters of any total site variation. Site values also lay generally parallel to the "A" line showing a constant suite of minerals and activity plots (i.e. Plasticity index versus clay fraction) were linear. Figure 7.11 illustrates the limitation of these statements and brings out the large regional as well as individual site property range and the variation in mineral proportions, the Herne Bay results in particular (see fig. 7.Z) show the non.- linearity of activities. A distinct shallow to deep water • trend of higher indices is obvious from Whitecliff Bay to Orfor(. Ness, for example, but this is clouded by the unexpectedly large site variations encountered. These vertical variations cover an extreme range of values during the primary cycle, i.e. from Lower to Upper London Clay. Secondary and tertiary cycles shOw decreasing index variations. Average Ashford indices are only Plate 7.3. Heavily fissured brown London Clay with septaria nodules.

Plate 7.4. Stiff fissured grey-blue London Clay showing iron staining on fissures. 116

65 percent of Bradwell results whereas ratios of minimum to maximum site index values equal this figure for almost all the sites studied. Hence lateral Atterberg limit variations do not appear to be as large as would be expected or predicted from the mineral distributions and the experimental results described in Chapter 6. When index values of distant sites of similar stratigraphy are compared, however, large variations are noted, e.g. an average Lower London Clay plasticity index at Ashford of 40 as compared to 70 at Orford Ness. The systematic vertical lithologic and stratigraphic variation is reflected in the Atterberg limit profiles at each of the sites and a firmer relationship is often made possible through the use of mineralogical and grading data. For example, at Herne Bay the 0Tr values appear to be strongly tied to the minus 2 micron fraction whereas plasticity values relate well with both montmorillonite and total clay mineral percentages.

Trends of activity are also complicated by single site or stratigraphical variations but in general the former outweigh the latter by some twenty percent resulting in the conclusion that this parameter increases from about 0.00 under nearshore conditions to 1.40 during offshore regimes. This activity range in both a lateral and vertical sense, may be directly attributed to mineral suite variation and not grading characteristics. This large activity range for London Clay was somewhat unexpected until the publication of Skempton (1065) concerning the Bradwell site but the writer s mineralogical analyses helps to explain the property range in what was apparently a regionally homogeneous material. Plates 7.3 and 7.4 illustrate this apparently homogeneous clay by photographs taken of cliff top and cliff base in-situ London Clay. 117

Chapter 8 Regional Trends in the Engineering Index Properties of the Deposit This chapter is an extension of the latter part of Chapter 7 in which a discussion was begun on the regional trends of engineering properties based on data obtained for five individual sites. These results had an advantage that various forms of geological as well as mechanical tests had been carried out on the same samples and hence an immediate and direct comparison of results was possible. The quantity and distribution of this controlled data was, however, rather scanty and hence to obtain a more complete coverage of the whole region, a large bulk of engineering data was gathered from several Site Investigation companies. This data comprised only the geographical borehole positions, C.D. elevations and soil property values. Neither the detailed stratigraphy nor any mineralogical information was available, hence the data was geologically "uncontrolled." Upon collation, the data was plotted both manually and by computer to see whether the resulting maps and trend surfaces agreed with, or were explained by, any of the geological and mineralogical information presented in earlier chapters, e.g. the known lithologic variations, mineralogical trends and experimental results (c.f.Chapter 6). As no direct zonation or mineral information was available, indirect methods of comparison, as described below, were used.

Two forms of plotting were used to create the maps required for this comparison. Firstly, a manual plot was made of available information on the plasticity characteristics and dry density of the clay by tabulating the property and its elevation as near as possible to its location on the map. Thus a qualitative idea was obtained of the vertical variation (over the depth of penetration and testing of the samples), the lateral variation, the vertical homogeniety and also property trends within the deposit.

Interpretation of these maps was kept simple and emperical. Examination of a tabulated set of results at any one locality revealed the vertical property variation from which an estimation of possible rhythmic characteristics, vertical homogeniety and an approximate bean site value could be ascertained. Comparison 118

of these parameters with neighbouring boreholes indicated whether and how rapidly, a lateral trend was taking place and in what direction. The tabulated elevations at each borehole were used to establish whether the same stratigraphy was being dealt with. Comparison of widely separate boreholes produced extreme or limiting values as well as the direction of any trend.

In order to obtain value contours or even a real 3- dimensional appreciation of the property trends as well as to treat all the property value data in a numerical, statistically sound manner, use was made of computer trend surface techniques.

Numerical methods for computing trend surfaces, in general consist of expanding the desired linear regression into a matrix of normal equations, which is then solved by inversion, giving the coefficients of the regression. A variety of schemes exist for inverting matrices, the Dan 8 programme used, invokes simple Gaussian elimination, the matrix, however, being pre- treated by an averaging technique so that exponents of entries are centered around zero, (Eller, Smith and Davis, 1968). By incorporating depth or stratigraphic location as a third geographic variable, least squares approximations can be made of a variable embedded in three-dimensional space. The ependent variable commonly being the mineral percentage or property value.

Three dimensional trend surface maps were produced to show the regional distribution of the liquid limit, plasticity index, liquidity index, undrained shear strength and density of the clay. This last property was recalculated to obtain the dry density and so allow comparison without interference from the degree of saturation of each sample. In addition to these engineering properties, all the major mineralogical data was processed by computer using the same programmes, so the resulting trends could be related by direct visual comparison without further manipulation.

The procedure involved in producing and interpreting these computer maps was to firstly assemble and submit the program., deck (comprising control cards, programme and data deck) for computation. The resulting output, which was individual computer drawn contour sections of a 3-dimensional rectangular volume in space, were assembled to produce a projection map such as figure 119

Interpretation of these trend surface maps was extremely difficult, however, it was usually possible to establish the horizontal direction of a trend, the rapidity or gradient with which trend values changed and the end values or extremes of trends. The contours also sometimes showed a pattern which could be directly related to physiogeographical features such as old shorelines or local troughs or "deeps" in the main basin. Structural forms such as anticlines were also often detected by means of contour patterns. Trend surface contours which displayed a marked horizontal disposition indicated that values were altering much more rapidly in depth than with horizontal distance, hence these properties were probably more strongly related to the nearly horizontal lithology variations than to an east-west (deep-shallow water) lateral trend.

An assessment of the reliability of these trend surfaces could be gained by means of a summary table of statistics. These tables included the following parameters. N 1) Total variation V = E (Z1-2) 2 where Zi is the ith Ala co-ordinate and Z = r Zi/N 2) Mean 1-1 3) Variation nit explained by surface, i.e. S S = r (Zi observed-Zi calculated)2 4) Variationiaplained by surface, i.e. E E = V-S 5) Coefficient of determination, i.e. T T E/V 6) Coefficient of correlation, i.e. L L T.k 7) Standard Deviation, i.e. D D UT) where N number of sets of data 8) F ratio

A. Liquid Limit The 3-dimensional liquid limit trend distribution map, i.e. figure 8.4 is based on some 370 points of data with a coefficient of determination of 0.34, a coefficient of correlation of 0.58 and a standard deviation of 10.4 on a mean value of 77.2. 120

On appraisal of the manual and the computer drawn maps, the following points of interest were noted in connection with the liquid limit distribution in the London Clay. 1)The manual and computerised plots (figures 8.1 and 8.4) were made with the use of some 37C points of information. 2) No information was available for the Hampshire Basin and thus this part of the study concerns only the London Basin. 3)The first general conclusion was that the liquid limit percentages were considerably lower in the west than the east. 4) The values on fig.8.1 varied from less than 70 in the west to over 95 on the Essex coastline. This corresponds to a 25-30% increase in liquid limit over the area. 5) No obvious N-S trend was visible although interpretation directly from the rlgures was difficult. 6)General variations within single profiles were often over 28% in the west but less than n5% in the east. This suggested greater homogeniety to the east (i.e. Essex) in the deeper water more clayey zones of the deposit, 7)The lateral variation Was greater than the vertical one but the latter was often more important on engineering sites because lateral variations would not be noticed over each small area. C) Nearby holes were sometimes seen to show large variations in liquid limit, but examination of the elevations showed this variation to be of stratigraphibAl origin, i.e. different London Clay zones were being compared. 9)In general, a gradual lateral variation from east to west in a single zone was the rule, e.g. values near Orford Ness to around West London (i.e. Lower London Clay). 10)The 2nd degree trend surface for these liquid limit data (See fig.8.4) again showed a marked west to east rise in limit from about 65 to over 95. 11)The contour shape was interesting in that it probably reflects the old shoreline configuration to a certain extent. Both the upper and lower elevation contours displayed this form. 12)The vertical nature of the limit zones (e.g. the 70-75 zone) was interesting in that the computer output suggested that vertical variation was much less important than horizontal variation, thus the significant trend was from west to east. 13)The liquid limit trend (fig. 8.4) was seen to be roughly the inverse of the quartz percentage trend (fig.8.C) which decreased eastwards. On the other hand, it was rather similar to the clay 121

mineral percentage trend as shown in fig.8.9. A fairly prominent NE-SW directional feature being present in both. 14)Examination of the montmorillonite trend surface (fig.8.l0) showed that the contours for this mineral assumed a somewhat horizontal disposition, i.e. contents vary in depth. Its trend surface therefore did not match that of the liquid limit to any marked extent and a relationship was thus difficult to find. 15)The trend surfaces therefore indicate that the liquid limit is largely proportional to the total clay mineral percentage rather than mineral suite proportion variations.

B. Plasticity Index The following points summarize observations or the plasticity index variations within the London Clay. 1)The maps were based on about 37C points of information and only the London Basin was considered. The coef _.:ient of determination was C.34 and the coefficient of correlation C.58. A standard deviation of 10.44 pertains to a mean of 51.2. 2)Values from the west were again low (see fig.8.2) due to the nearness to shoreline and increased quartz content in this direction. 3)The values increased by roughly 30% from ±40 in the west about Reading to ±65 in the east around Harwich. 4)Any north south variation was difficult to assess due to a lack of stratigraphical control, but such a trend, if it exists, was certainly not a major one. 5)The eastern clays (e.g. near Harwich) appeared to be somewhat more homogeneous than their western equivalents (e.g. near Reading) judging from the smaller vertical variations found within each profile in the eastern region. 6)Regional lateral variations are thought to be of a gradual nature, but of constant gradient, so that within one zone little local variation is to be expected. Inspection of the elevation figures again showed that strongly contrasting figures from neighbouring locations were usually at different elevations. 7)Lateral variations between, for example, Reading and Orford Ness were larger than most vertical variations but the latter are more important locally because their effects will be noticed on one site. 8)The rather complex 3-dimensional trend surface map shown in figure 8.5 showed a central mainly E-W trending roughly vertical zone of values ranging between 45 and 55. This direction 122

coincides with the axis of the basin as it extends from about Reading to Fmlness. West of London, the plasticity indices decrease to the north and south of this vertical zone. East of London a zone of high indices is noted toward the Herne Bay and Sheppy regions. It is thus possible that a local trough or "deep" in this region collected exceptionally fatty clays. Palaeontological zonation of the area proved to be difficult and complex as this region was probably an area of driftwood collection, because of the lack of sea currents. 9)The plasticity index trends resemble the liquid limit trends of figure 8.4 in the eastern half of the London Basin region, particularly in the directional and vertical sp acial distribution of the contours. 10)A lateral E-W trend appeared to be the major distribution component of these values and once again the total clay mineral percentage distribution is thought to be more important than the individual mineral suite variations. This conclusion was reached because of the reasonably close likeness of figures 8.9 and 8.5, i.e. clay mineral percentage trends and plasticity index trends.

C. Dry Density and Liquidity Index The following points summarize the observations concerning the 3-dimensional dry density distribution of the London Clay as plotted both manually (fig.8.3) and by computer (fig.8.6). 1)Some information is available in the Hampshire Basin for this property and together with the London Basin some 365 data points were considered. A standard deviation of 4.57 and a mean of 95.60 was calculated for the data. A coefficient of determination of 0.26 and a coefficient of correlation of 0.51 pertained for this surface. 2)The values of dry density varied between about 105 and 80 lbs. per cubic foot. (See fig.8.3) 3)A slight decrease of average values from about 100 to about 80 lbs. per cubic foot was observed on progressing eastwards across the London Basin, i.e. a 20% decrease. The Hampshire Basin values conformed to this general pattern and were all over 100 lbs. per cubic foot. 4)Vertical variatilns at any one locality were not too marked but are probably approximately equal to the total lateral variation over the whole basin. 123

5)The homogeniety, as judged by single locality variations, appeared not to vary significantly between regions in the east and west. 6)An excellent relationship was noted between dry density and lithology. Proof of this was twofdld: firstly, the obviously sandier Hampshire and Western London Basins displayed markedly higher densities than their more fatty eastern region equivalents in Essex. Secondly, if individual values from localities were compared with the plasticity index or liqaid limit values of the same samples, it becomes obvious that the clayey more plastic samples have the lowest densities. 7)Figure 8.6 presents the trend surface block diagram for dry density. It showed, like the plasticity index, a central roughly E-W vertical low density or clayey zone. This zone, with a density of about 94 lbs. per cubic foot, graded to the north and south i.e. near Harefield and Dorking respectively, into more dense zones. The axis of this distribution extended between Reading and Orford Ness. Outer zones had densities of over 100 lbs. per cubic foot. 8)The very similar natures of figures 8.6 and 8.9 bears out the close relationship between clay mineral percentages and dry densities. 9)Very little relationship could be seen between individual mineral trends, such as montmorillonite or kaolinite, and density. Comparison of figures 8.6 and 8.10 and 8.11 leads to this conclusion.

A computer analysis was made of the liquidity index distribution (i.e. water content-Plastic Limit/Plasticity Index) within the London Clay. The trend surface mean was 0.06, the standard deviation 0.11, the coefficient of determination 0.13 and the coefficient of correlation 0.36. Major points of interest were : 1)340 points of data were used in the analysis (see fig.8.12) 2)The values varied about zero from approximately +0.20 to -0.20, i.e. the clay in its in-situ condition always had a water content very close to its plastic limit. 3)The liquidity index distribution pattern, as shown in the trend surface block diagram, varied in a N-S direction but showed strong tendencies to horizontal layering "dipping" southwards. These "dipping" tendencies fall midway between the essentially vertical distribution of the other properties and quartz, and 124 clay minerals and the horizontal distributions of kaolinite and montmorillonite. Both these distributions appeared to exercise some control over this parameter.

D. Undrained Shear Strength The last engineering property to be discussed is that of undrained strength. The three dimensional distribution of this property is difficult to establish because it is not a true index property, i.e. it depends not only on the material itself but also to a certain extent on the surrounding stress situation. Thus at near surface elevations with low overburden pressures, lower strengths are encountered due to the distressed nature of the ground and higher water contents. The following general points may, however, be noted :- (See figure 8.13) 1)Some 365 points of information were used in the analysis. The trend surface had a mean of 3,041, a standard deviation of 144.1, a coefficient of determination of 0.24 and a coefficient of correlation of 0.49. 2)Limited information was available for the Hampshire Basin but on the whole only the London Basin had adequate data to uncover continuous regional trends. 3)Values to the west appeared to be somewhat higher than those to the east. Two apparent anomalies were, however, noticed in that the Hampshire Basin values were rather lower than the western London Basin values. Both of these were sometimes lower than central London sites. This is probably explained by the fact that a very sandy or very clayey material has insufficient particle binder or individual component strength respectively. 4)Unconfirmed strength values of over 5,000 lbs. per sq. foot were not uncommon in the west and central London Basin. Values nearer 3,000 lbs. per sq. foot occurred in Hampshire and in Essex. Where extremely fatty clays are common, strengths often remained below 2,000 lb. per sq. foot even at depth, e.g. near Orford Ness. 5)Variations in one profile were quite marfted, but this is a typical feature of unconfined tests, thus no comment concerning clay homogeniety in depth can be made. 6)No N-S trend was obvious within the London Basin, nor could a trend be seen on approach of an outcrop margin such as near Harefield, N. of London. 7)The lateral variation was very vague and probably does not even 125 have as much effect as local vertical variations which can be quite dramatic, e.g. varying between 700 and 5000 lbs. per sq. ft. These vertical changes were due to both lithology and overburden pressures. Sandy clays, such as the Basement Bed being remarkably strong. 8)The 2nd degree trend surface of strength distribution as illustrated in figure 8.7 was basically a horizontally layered distribution with low strengths at the top and higher ones below. Tbis is probably the pattern of overburden pressures producing low near-surface strengths as opposed to high deep sample strengths. 9)Very little shows through this strong pattern, except in the north west corner where lower strengths were apparently entering the distribution due to the proposed lithology change to very fatty clays in the region of Lowestoft.

E. Discussion In this chapter, a rather difficult task was undertaken. This involved an attempt to find correlations and explanations between "raw" commercially available soil testing results which were uncontrolled stratigraphically, lithologically and mineralogically, and a detailed geological picture of the deposit which had already been presented in Chapter 4. This was accomplished by making use of evidence of relationships discovered both in the detailed study of the indivzdual principal sites in Chapter 7 and the results of the experimentation conducted on selected London Clay samples (See Chapter 6). These studies and experiments, as well as the regional trends discussed in the preceding chapter, showed the strong qualitative relationship between stratigraphy, lithology, mineralogy and soil properties. These were used to relate the regional engineering property trends and explain their connection to geological factors.

The maps illustrating the chapter have shown the 3-dimensional distribution of each parameter both as actual plotted values and statistical trend contours. A purely visual estimate of values could thus be coupled to trend surface contours to provide an idea of absolute value distribution, trend of values, homogeniety of the material, lateral versus vertical relationships and so forth. These in turn were related to various geological parameter distributions, the most important of which appeared to be the total clay mineral percentage and the 3-dimensional J_26 distribution of thD most fatty clay zonez. The individual clay mineral distributions appeared to play only a small part in the trend of these mechanical parameters, with the possible exception of liquidity index. Chapter 9 Engineering Implications of Lonapn Clay Geology Implications of engineering importance which may be drawn from the preceding geological discussions, particularly Chapters 4, 6 and 7, are numerous and range from somewhat basic to more complex in character. Some implications are of a theoretical or hypothetical nature or be in only a partly proven state whereas others are an already acoepted, established fact. With such a large scope of discussion possible, it is only intended here to outline the more general engineering applications from a study of the London Clay geology. Such general applications arise at almost every stage of a well planned geological study, hence it is proposed to use the systematic geological chapter headings as a guide to this- discussion.

A. Implications Drawn from Clay Deposit Literature On reviewing the literature connected ,with clay mineral deposits, it becomeS obvious that the environment of deposition plays an important role and has major engineering implications with respect to the fully consolidated and uplifted sediment. The provenance of the source material is also important in that a relationship between climate and hence type of weathering and potential source material is evident.

The environment of deposition is also of prime importance in establishing the identity of the sediment. Marine environments are usually alkaline with high to very high salinities 30-50g/kg. and hence high ionic concentrations. Temperature is usually cool (because of depth) and Eh varies but is often positive, i.e. oxidizing, till the deposition surface is reached. Illite predominates in these conditions with subsidiary kaclinite, and variable, usually minor, smectite. In lagoonal environments, illite again predominates but smectite or attapulgite occasionally assume major proportions. Kaolinite virtually disappears but chlorite and vermiculite may occur as minor constituents. Aggressive lacustrine environments (IVillot, 1.942), i.e. low pH and high leaching give rise to kaolinitic deposits whereas non-aggressive conditions, i.e. slightly alkaline and poor leaching induce illitic, smectitic and sepiolite type deposits. The preference of these suites 12$ •

for certain environments is important in that they can be roughly predicted and variations correctly anticipated. Hence an idea of the mineralogy and its closely related soil properties may be obtained in advance. The important pore water content and its chemistry is primarily established at this early stage during deposition and its influence on replaceable ions, secondary crystallization, cation exchange and recrystallization or diagenetic effects is established. Pore water is not only geologically important but its characteristics play a part in permeability and consolidation effects by varying viscosity, surface tension and clay flake adherence factors.

Deposition effects are of major importance in that they control the lithology, grain size, mineral distribution and flake and pore space structure of the sediment. Most of the prolerties of the rocks are formed at this time. Assuming that lithology and size effects remain roughly constant, the mineral distribution may be examined. Here the literature is particularly rich (see Chapter 3) but a few clay mineral distribution patterns predominate and these follow simple sedimentological rules. Generally, illite is predominant and ubiquitous, kaolinite is near-shore and syectite off-shore. Variations have been noted in off-shore kaolinite and near shore smectite distributions and these have been explained by the faster differential settling of illite and kaolinite to smectite and also by the flocculation effect of smectite in saline waters (Whitehouse et al., 1960). Normal near-shore (high energy) and tidal effects tend to separate the mineral grain sizes and currents distribute the various fractions about the basin. Once the investigator is aware of these possible variations and finds them in a deposit, the pattern of mineral content and size becomes understandable and hence property variation® may be understood and even roughly predicted.

Diagenesis similarly has been detected in clay mineral deposits. Although the results are often rather subtle with respect to mineralogy, the recrystallization or effects on crystallinity and rock texture may be significant. Chlorite, vermiculite and even montmorillonite have been shown to form during diagenesis (Chapter 3, c.f. Powers, 1953) and although the quantities involved are probably small, a change in properties 129

could result. More important than this are the chemical aspects whereby potassium (and magnesium) are taken up in broken bond positions or replace sodium cations to form an electrically better balanced molecular structure. This structure with its new cations tends to be more stable and has a better crystallinity than the often degraded source material and hence less susceptible to cation exchange capacity, water molecule uptake, and so on. In essence, a more stable mineral suite developes and this reflected in the engineering parameters of plasticity, swelling etc. Recrystallization, often under large overburden pressures, may also lead to stronger mineral bonding with resultant increase in strength and gain in consolidation characteristics.

Weathering of clay minerals has been shown (Chapter 3, c.f. Jackson et al. 1948) to be part of a dynamic equilibrium during which "active" minerals are often formed at the expense of stable ones and good crystallinity and bonding broken down to produce "ragged" poorly bound aggregates of material with somewhat deteriorated properties.

This very brief general review introduces the following discussion which deals in much greater length and detail with the implications of the geological findings of this study.

B. Implications of the Regional Geology The applied sedimentological concepts expressed in Chapter 2 are dealt with now as they explain the lithology and stratigraphy of the region. As S.E.England is entirely covered by sediments, a large portion of which are of Palaeogene age, the importance of understanding the accompanying implications cannot be overstated.

The Palaeogene lithological changes were brought about by relative land and sea movements, which during this time assumed a rough periodicity. The sediments reflected this periodicity as large and small scale rhythms occur in the lithology (See Chapter 2, Stamp, 1921). This larger scale Palaeogene rhythm has been recognised and stressed in this study as it holds the key to a recognition of the vertical boundaries of the thin individual deposit layers which cover mach of S.E.England. 130

Once this major rhythm was recognised, minor ones were sought and found within the framework of the former. The first major implication is that sediments and their properties must only be compared or correlated on an aerial scale if it is certain that exactly the same strata is being dealt with, i.e. more or less one environment of deposition is being considered. This implication is not so obvious when the minor rhythms of the London Clay are being discussed. It would obviously not be wise to compare properties of the Blackheath Beds and the London Clay without recognising the differences in stratigraphy. Nevertheless, comparison has been made many times of different zones within the London Clay which theoretically bear almost as little relation to each other as the first example.

C. Structural Implications It has been shown how two major forces have largely controlled the structural configuration of the London Clay Basin. These forces are the main Oligocene Alpine folding pressures which gave rise to the prominent E-W features through the Weald and Wessex and the more rigid damping effects of the underlying Paleo-Mesc floor rocks of the London Basin where gentle "forelard" flexuming rather than folding was felt and where ancient linos of weakness still predominate through the cover (refer to Chapter 3, Prestwich 1847-1855). These effects coupled with t:1,7, post Pliocene erosion have given rise to the following features :- 1)A Thames valley, i.e. an E-W anticline through the Thanet dome, Cliff dome, Purfleet Grays dome, Charlton dome and Windsor dome. 2) NE-SW lines of folding, flexuring and hence stressing in the Central London Basin area. 3) NE-SW lines of faulting or straining, e.g. near Greenwich, from Surbiton to New Cross and from Dagenham through Hornchurch to Brentwood. 4) NW-SE lines of folding are also noted. These lines of buckling and contortion often interfere with each other giving rise to the sinuous outcrop pattern of the London Clay and the irregular and difficult to predict bottom contours of the clay. These synclines and anticlines produce interference patterns of domes and basins hence distort structural contours considerably 131 and create unexpected thickness variations making depth predictions difficult. They produce c;dd dips and strikes within the clay and make the subtle stratigraphy extremely difficult to follow. All of these structural geology features have obvious engineering implications especially in subsurface construction, where folded (stressed) and faulted (strained) and fractured rock nay be encountered, clay surface on bottom unexpectedly reached, changes in lithology noted requiring alteration of tunnelling technique, etb. The structural geology thus controls the three dimensional configuration of the deposit, which if undisturbed should consist of regular gently dipping strata outcropping in smooth lines.

D. Depositional Implications The section of chapter 4 dealing with the stratigraphical aspects of deposition is of extreme importance in that it brings order and sense to a topic which had at its disposal almost all the facts but to date had not co-ordinated them. This section of Chapter 4 proposes a stratigraphical zonation (after Williams, 197C) for this clay deposit, based essentially on visible sedimentological evidence and backed up by palaeontological observation. This fivefold zonation system is regionally valid and extends from the base to top of the London Clay. Its implications are as follows :- 1)It is heavily based on visible evidence and hence for the first time the engineering geologist can place himself stratigraphically with some reliance within the London Clay. 2)This facilitates a valid aerial comparison of geological and gectechnical properties. That is, cross correlation of properties is largely eliminated. Middle London Clay from Reading or Hampshire may be compared with Middle London Clay from Essex. 3)As the zonation is related to sedimentological rhythms, during which slightly different rock types were laid down, the zonation-lithology variation is more easily understood. 4)As vertical lithological changes help identify the zonation and are thus expected their nature is anticipated. The importance of this factor is enormous. 5)Once the significance of these vertical changes are appreciated and the zones distinguished, a regional aerial variation within each zone may be studied and this again has been shown to be sedimentologically controlled with a grain size and detrital 132

content (or non clay) increase toward the proposed shorelines and source entry points. 6) The London Clay isopachyte maps, although giving an indication of total thickness are also useful when considered in conjunction with the zonal maps as it is then possible to roughly establish the amount of London Clay removed by erosion.

E. Mineralogical Implications The mineralogical results of the testing were presented and discussed in Chapter 4. These results represent the first regional scale mineralogy study of the London Clay ever undertaken. As such, it is of immense v-lue in furthering a knowledge and understanding of the London Clay. Over 300 samples from 22 localities were analysed and these, considering the well known gradual mineral variation within sedimentary deposits, adequately represent the mineralogical distribution pattern. The mineral suite is not an unusual one for a marine sediment and qualitatively it is constant over the region indicating a constant source material during deposition. Its engineering properties may thus also be assumed to be basically constant and this is indeed the case. The clay mineral suite of illite, montmorillonite, kaolinite and chlorite also reuains qualitatively constant over the region but may be differentiated- with some difficulty from the under and overlying deposits.

Calcium h!_.-2 been distinguished several times as the major exchangeable cation in London Clay (Grim, 1951) and this important fact shows that the montmorillonite is of the less active type with an assumed cation exchange capacity of about 80 neg./100g.

A fractionated sample has shown the mineral distribution (See Chapter 4, figure 4.9) as related to grading. This shows that all the minerals in the suite are present in all the size fractions but that the clay minerals predominate below the 2 micron size. The clay minerals are not, however, to be neglected in the plus 2 micron fraction and can even represent 10 percent of the coarse silt fraction. The use of arbitrarily selected, although standard, grading size cutoffs, e.g. 2 microns, should be viewed with caution especially when used in ratios such as activity, i.e. Plasticity Index .i. Clay Fraction. 133

The detailed Alum Bay section as described in Chapter 4 showed the intimate relationship between mineralogy and lithology. Depth profiles of the minerals were shown to relate closely to the geological lo and the palaentological zonation. All these aspects being derived from the sedimentology of the deposit. The causes of the derivations are also dealt with. The fact that the mineralogy varies rhythmically with the stratigraphy, and this has been shown to be true for the clay minerals, as well as the non-ctays, is a significant and important discovery in this apparently homogeneous deposit. This means that if the zonation at a site is established by careful observation, particularly of the lithology, a reasonable estimate may be made of the way in which the mineralogy is varying. A combination of the visual grading characteristics and the estimated mineralogy therefore gives a good idea of the soil properties and the variation which may be expected in depth.

Chapter 4 also indicated how the vertical grading variations merely reflected depth of deposition (and hence distance from shoreline source) variations and that how within one zone, or subzone, the grading became coarser westwards and towards the Hampshire Basin. This coarsening, in an aerial sense, should also be borne in mind as experimentation and experience has shown that even with a constant mineralogy, coarse deposits show lower plasticity characteristics and higher permeability and strength parameters. The sari° principles may be used in dealing with the slightly coarser Lower and Upper London Clay.

Again with respect to the aerial distribution, it has been shown that the clay mineral suites show some variation. taolinite shows continental near-shore affinities and is prevalent in the upper portions of zones whereas montmorillonite concentrates in the deeper water regions and basal portions of the zones. These quantitative variations along with the increasing quartz content in shallow water environments must affect the properties of the clay by making it more active and plastic towards the east. These correlations are to be dealt with shortly.

The relationship between the environment of deposition and the subsequent geological history of the deposit and the properties of the rock are still largely hypothetical. For a 134

theoretical discussion of the influence of these factors on rocft properties, the reader is referred to the first half of Chapter 5 which deals with such discussions in detail.

F. Implications drawn from the Experiments Conducted An interesting phenomena is noted in connection with mineral grain size effects and mineralogy. This creates the interesting situation whereby relatively large proportions of the active clay mineral montmcrillonite is found in the sandy or at least silty, basement beds. The non-clay coarser sized particles acting in opposition to the highly plastic clay minerals. This and many other size and mineral effects were studied in the experiments described in Chapter 5. The engineering implications of these effects are most important and will now be summarised and discussed.

The "size effect" tests, i.e. la and lb, showed that with a constant mineral suite, li e. non-clay and clay mineral proportions, activity remains roughly constant but plasticity increases inversely with size. The decrease in size of the quartz from a medium sand to a clay fraction size has been observed to increase the liquid limit by only about 10 percent. The plasticity index rise is not, however, proportional to this and the trend on a Casagronde Chart plot is toward the "A" line (See figure 6.1), i.e. from a typical CH toward a MH soil classification: the latter having only medium crushing strength and toughness. (Unified Soil Classification).

The experiments conducted by adding varying percentages of minerals to London Clay showed in the case of additional quartz (as would occur on approach of a shoreline) that for every 10 percent of quartz added the liquid limit would decrease by roughly 10 percent in a direction parallel to the "A" line without alteration to the mixtures activity.

The effects of kaolinite addition, again simulating an approach to a shoreline or as occurs during the silting up and shallowing processes, is to reduce the plasticity parallel to the "A" line but only by about half the amount found with quartz. This decrence is effected with an accompanying activity decrease of roughly 50 percent for 50 percent mineral addition. The activity decreasing constantly. Xaolinite thus varies in a 135 most regular manner and like quartz the larger its percentage in the sample, the more inert becomes the soil.

The additiIn of montmorillonite to London Clay activated the clay in a most dramatic and peculiar way. Figures 6.7, 6.8 and 6.9 show these effects. The liquid limit increases dramatically but lic.o parallel to the "A" line whereas the activity is slow to increase at first but then rises very steeply after addition of about 2C percent montmorillonite. This initial amount probably being taken up in creating a state of "lubrication," between particles. Thus, although p lasticity may be expected to increase regionally in an easterly direction due to the decrease of quartz and kaolinite and increase of montmorillonite, the total effect is far from simple to analyse as grading effects must be accounted for as well as this disproportionate plasticity increase of montmorillonite.

An appraisal of the results may be made by use of figure 9.1 which shows the hypothetical representation of both aerial and depth distribution of minerals within a sedimentary basin. The plan distribution shows the hypothetical trend of minerals and grain size from the land through a shallow water zone to deep water. The decreasing size effects (as represented by fine sand and coarse silt) and quartz and kaolinite content toward deep water suggest an increase in plasticity. The effect of increased montmorillonite is only felt after an addition of some 20 percent.

This plan distribution may be extended in a vertical manner to account for a basement bed, a deeper water sequence and a silted up shallower facies. When account is taken of the relative quantities within this rhythm and the experimental results obtained, both a qualitative and rough quantitative estimate of plasticity and activity may be estimated. Figure 9.2 shows the theoretical effect of decreased quartz, kaolinite and grain size and increased montmorillonite on a liquid limit distribution assuming the material in the shoreline region to be the standard. Thus a roughly 50 percent increase is seen in liquid limit from the western shoreline position to a deep water one off the Essex coastline.

DEPTH DISTRIBUTION 9 Rhythm= — B

Rhythm - A

0 0 0 0 CD 0 CD C.) C) 0 0 CJ (.1 CV -3' h ct-t.t

TE NI LO IL

c! R O O

c TM N MO

t's-.) 0 0 0 0 CD 0 .9 0 R CD 0 Q 0 O C,t1 0 J — CD -a t • Cl I I I I I I

Land

0 p. .0 P -0 0 •, - • • • °.

0 — ° Shallow Sea

PLAN DISTRIBUTION

Fig. 9.1 IDEALIZED MII\ ERAL DISTRIBUTION WITHIN A BASIN

Scale : 1/.11,860,000

40 Kilometers 12] 0 20 0

0 0 [1-5] 00 o

0 c,-4/ /7 [4] 0° +25 °

N+30

0 \ —+25 = Increase due to Quartz Decrease \ \ +20 —4451= Mont. Increase (-1-5)0 (+7) -HO), Kai Decrease

Fig. 9.2 THEORETICAL LIQUID LIMIT INCREASE SEAWARD G. Weathering Tests Implications The studies conducted on weathering phenomena in Chapter 5 have shown that on the smaller scale, i.e. hand specimen size, a small increase of clay minerals occur with chlorite (and probably even montrorillonite) forming at the expense of illite. Some leaching is also suspected by virtue of carbonate, pyrite and organic carbon variations in the weathered material. The very slight increase of cation exchange capacity of the weathered material is some confirmation of the proposed mineralogical changes. All these changes are obviously very slight and fall just clear of the experimental error. The conclusi n drawn from these results is thus that weathering on this scale has very little effect on the engineering properties of the material.

Results of tests conducted on the larger weathering features, i.e. samples a few tens of meters apart from cliff top to base for example do, however, show slightly more pronounced effects. The plasticity index increases by roughly 1C percent between the brown and blue London Clay as does the activity. These figures are probably accounted for by reason cf the increased proportion of montmorillonite in the brown clay fraction, aided by the fact that the clay mineral percentage and clay fraction percentage apparently unexplicably decrease slightly on weathering. No change in undrained strength could be found between the weathered and fresh clay at this locality. Grading analyses show that on weathering, the coarse silt and fine sand fractions of fresh samples lose about 10 percent of their content to the medium silt fraction.

These experiments thus show that only a limited effect can be assigned to material weathering. Hence weathering of the London Clay from both mineral and mechanical aspects, can only be called on to modify its properties to a small extent. So that in a vertical section, whereas initial llthological variations can account for up to about a 40 percent variation, weathering only accounts for approximately 10 percent.

H. Conclusions drawn from Single Site Investigations All the above concepts and experimental testing is put to the test in a study of five major sites as described in Chapter 6. These sites all displayed long vertical sections of London Clay on which geotechnical testing as well as mineralogical analysis had been conducted. The sites which are located at Sunbury, Bradwell, Streatham, Noak Hill and Herne Bay all contributed toward a general appreciation of the relationship between sedimentology, mineralogy and soil mechanics. The combined results may be summarized as follows : 1) The clay fraction (-2 micron) was usually proportional to the X-ray clay mineral content but the two figures were seldom identical as the clay mineral content often included silt sized clay mineral particles. This was particularly marked in the coarser grained sediments. 7,) Both clay fraction and clay mineral content showed direct and sensitive relationship to visual geological logging. As the latter formed the basis of the zonation system of Williams, 1970 (backed up by strong palaeontological evidence) the clay fraction and clay mineral content could be successfully used to zone the clay. The other mineral depth profiles were also noted to vary systematically with zonation, i.e. quartz, montmorillonite and kaolinite. 3)At Herne Bay, chemical analyses were shown to relate reasonably well to mineralogy and thus also helped distinguish between zones. 4) The mineralogy and chemistry could also be used to complement various palaeontological suggestions regarding the environment of deposition. This environment is important in many respects, some of which have not been fully explored. For example, shallow water regions of high energy level during deposition, besides usually being characterised by coarse grained sediments, are often rather isotropic on a hand specimen size due to the randomness of deposition and general churning action in these zones. Larger scale features, however, such as washouts, channels, ripplemarks, foreset bedding, etc. may exist. Low energy areas being deeper water, show few large features and are more homogeneous sediments with greatest flake orientation. Bulk of the above cases are linked to the salinity which if high induces parallel orientation of flake by floculation, and the rate of supply of sediment, which if slow allows for differential settling and graded beds under still conditions. 5)The plasticity of the clays (as judged from Liquid Limit and Plasticity Index profiles) was seen to vary primarily as a result of quartz content changes. These were difficult to 138 distinguish from grain size effects but at Noak Hill, for example, small mineral and large grading variations (on entering the Claygate Beds) accounted for large plasticity changes. Varying montmorillonite content has also been noted to induce very little plasticity variation in numerous instances. Thus, the experimental test results of the writer (Chapter 5) seem to hold in nature, with quartz content and grain size effects (and kaolinite content) reducing plasticity while the up to 20 percent in situ montmorillonite variation apparently has little effect. 6)The activity ratio, however, is noted to be sensitive to clay mineral suite proportions and is seen to vary in rough agreement with montmorillonite content. Hence by use of both plasticity index and activity, a good indication is reached of the amount of clay minerals present and the mineral suite proportions respectively. Thus these two parameters when coupled with a general appreciation of the thythmic sedimentological aspects go a long way to creating some order from what was an apparent random property distribution. 7)A parameter found to be most useful in gauging the clay mineral content and mineral suite proportions and hence which provides a most helpful soil index is the cation exchange capacity. Examples have previously been given as to how this figure may bo used to "back calculate" and obtain an indication of the mineral proportions. The cation exchange mechanism is also very close to that which controls swelling and shrinking of clays and as such should presumably give the best index of these tests 8)At Bradwell, Skempton (1965) commented on a little heeded parameter which the writer feels could be used considerably more. This is the bulk density (or better still, the dry density) which is readily available from any strength test data. In London Clay, it correlates strongly with lithology, i.e. the combined effect of mineral specific gravity and pore space, and could be used as a valuable strength cum bearing capacity index test. The more silty the material, the more dense the bulk sample.

I. Regional Trend Implications Following the discussion of these single location profiles, the 139 writer attempted an analysis of the regional soil mechanical parameters in the latter part of Chapter 7 and how these related to regional geological patterns. Upon examination of the rather limited regional pattern produced by combining data from the principal sites, a number of important factors emerge 1) The total lateral variation of the quartz content, i.e. roughly 8 percent appears to be of the same order of magnitude as the vertical variation. This latter change, however, can occur over a matter of a few feet in depth while the former takes many miles to accomplish. The clay mineral percentage distribution follows the same pattern. Thus in any London Clay engineering site the greatest variation can be expected vertically with almost no variation laterally (over this limited extent). 7) Soil parameter profiles may thus also be expected to vary in depth bUt if two or more such profiles are linked laterally over the site little variation in the intermediate ground is to be expected. 3)Lateral montmorillonite variations equal about 18 percent and vertical variations about half this value. As the experimental results (c.f. Chapter 6) have illustrated that some 20 percent of this mineral is required to significantly alter the material properties, little index property change, either laterally or vertically, may be expected to result from such a distribution. 4)A comparison of single location versus total lateral index variation is displayed admirably in figures 7.10 and 7.11 by means of Casagrande Chart plots. The exceptionally high scatter of points from a single sites should be noted to reinforce point 2 5)Activity figures also show these stratigraphical and lateral variations, the latter outweighing the former. These figures are considerably affected by montmorillonite content and thus vertical trends are not as dramatic as for other parameters.

This rather scanty but well controlled regional 3-dimensional distribution is replaced in Chapter 8 by a rather more full set of mechanical data which is virtually uncontrolled from a stratigraphic, lithologic or mineralogic point of view. These uncontrolled distributions are then linked, with several interesting implications, to geological regional trends. The liquid limit distribution showed a gradual and constant increase eastwards. This was equal in magnitude to the vertical variation , particularly in the western half of the London Basin which was noted to be rather less homogeneous than the east. The computer output also suggests that the horizontal variation from west east is more important than, and dominant over, the vertical - rend. This liquid limit variation being related more strongly to total clay mineral percentage and size effects than to individual mineral suite variations.

The plasticity index variations follow similar lines to that of the liquid limit and the 3-dimensional"block diagrams" of Chapter 8 illustrate how the more clayey and fine grained "core" of the Basin is approached the plasticity index increases. Again the percentage clay minerals is thought to be dominant over both decreasing size effects of the particles or increasing montmorillonite content.

Interesting factors concerning the dry density arise on examination of the computer and hand drawn maps :- 1)This property of the soil varies in accordance with the lithology and as such is a useful index parameter. 2) The dry density varies from between 80 and 105 lbs. per cubic foot from west to east and a variation of the same order of magnitude probably occurs vertically at any one locality. It thus changes relatively rapidly in depth but may be used to gauge total clay content in doing so. The lighter the material, the more clayey it is. Porosity almost cert!linly decreases as the density increases but this is not thought to be the primary cause cf the density variation, merely a result of the increased clay mineral content.

The liquidity index distribution varies about, but remains close to zero, indicating a, ubiquitous in situ water content very close to the plastic limit of the clay. This parameter appears from this study to be influenced by both the individual mineral suite proportions and the quartz - clay mineral ratio.

Rather vague trends emerge on consideration of the undrained shear strength, the reasons for this being explained in Chapter 8. Strength in the west appear greater than the east but both display large vertical variation. These variations appear to be somewhat independent of size effects but rather controlled by lithology and individual clay mineral content. The larger the montmorillonite content, the lower the strength. 141

Chapter 10 Summary and Conclusions Chapter 1 of this thesis served to outline and define the scope of study envisaged, concerning the engineering geology of the London Clay. The suitability of this deposit as a topic of study and the need for such an interdisciplinary project relating geology and soil mechanics on a regional scale was emphasized. A detailed work programme was laid out comprising two phases, the first being somewhat philosophical and the second a more rigorous analysis of exactly how the study was to be undertaken. In summary, it appeared necessary to collate and re-analyse available geological data to obtain an accurate appraisal of the structure of the London Clay and its depositional characteristics. New information, in the form of mineralogy and a stratigraphic zonation system, was then apparently needed in order to provide the vital link between the already reason2_bly well established geological and geotechnical properties of the clay. This new information also having to be selectively collated, analysed and plotted.

It was envisaged that this link be accomplished using various -visual, map overlay, graphical and numerical techniques on three separate scales. The first scale on which geological- geotechnical relationships were to be sought was an experimental or one dimensional scale in which laboratory touts simulated in-situ conditions and variations. The second scale involved depth profile diagrams which were limited to a single geological exposure or engineering site, i.e. a two dimensional plot. The third scale was three dimensional, as the deposit was to be examined on a regional basis taking into account properties as they occurred in depth within the London Clay.

This planned programme of study was carried through to completion with consequent discovery of many original and interesting features.

A, Results of the Geological Literature Review In order to establish a base level of geological information from which to buildo a detailed literature review was undertaken. This review began with an historical account of the regional stratigraphy, illustrating the successive subdivision of the 14S

Palaeocene and Eocene sediments.

The extremely important wor% of Stamp in 1921 was stressed, as the applied sedimentology principles which he proposed for the whole of the Lower Tertiary beds were subsequently found to apply in the London Clay itself and were used to explain variations in lithology and depositional environment. Briefly, these basic principles relate the sequence of events to be found during a transgressive-regressive cycle of deposition and explain the variation in lithology (both lateral and vertical) and the timing of these events. The practical application of these hypotheses was then illustrated by means of a detailed discussion of the depositional history of the Lower Tertiary beds beginning with the Thanetian Sea transgression over the planed chalk surface. The sequence of events continued until this marine incursion reached a maximum extension on deposition of the Woolwich and Reading Bottom Bed. Silting up of this basin followed with large amounts of marginal fluviatile deposition occurring during Woolwich and Reading times. While the topmost liading Beds were being deposited, the Oldhaven and Blackheath transgression had already begun cn the same axis as the old Thanet basin. This transgression continued through a shallow (London Clay Basement Bed) and then deep water phase (Middle London Clay) whereafter silting slowly became predominant culminating in Claygate-Lower Bagshot times. A third marine incursion then occurred depositing the marine Bracftlesham and Upper Bagshot Beds.

Temporary !;alts in subsidence during the deposition of the London Clay were of particular importance as they allowed limited silting and shallowing cf the basin giving rise to the rhythmic variations in lithology. This extensive historical geology review of the Lower Tertiary sediments of S.E.England and N.W.Europe, although explaining the mechanism, timing and physical results of such regional rhythmic deposition did not deal with the form of mineralogical and chemical constituents which could be expected to fill such a marine basin, and which indeed were found to be contained in similar basins in other parts of the world. A further literature survey was therefore conducted to review the chemistry and mineralogy of clay deposit genesis. This was divided into four sections 143

which dealt with the four major geologic processes which a deposit must undergo or be subjected to, i.e. depositional environment, deposition, diagenesis and weathering. The chemical and mineralogical aspects of each of these processes were discussed and the following main conclusions reached : 1)Environment of Deposition: Each environment had its own pH, Eh and salinity characteristics which imparted certain distinctive features to the minerals being laid down therein. Conversely, chemical indicators within a mineral suite could be used to help define the conditions of deposition. Mineral suites were also found to vary with environment and it was established that illites predominated in marine environments, with smaller proportions of smectite, kaolinite and chlorite often being present. The converse was also true, that mineral suite characteristics (both qualitative and quantitative) could provide information regarding the environment of deposition. The same principles held true of mineral crystallinity data. 2)Deposition: During deposition, certain chemical adjustments were found to take place, particularly as land derived fluviatile sediments enterod the high electrolyte concentrations of a marine environment. K20 and MgO were often absorbed by the weathered, degraded source minerals. The literature revealed that mineralogical distribution in a marine basin followed two major patterns: kaolinite was almost inevitably strongly related to areas of heavy continental sediment source (particularly in tropical regions) whereas illite was ubiquitous. Smectite (because of its fine size) was usually found in the deep water, quieter regions of the basin (and therefore offshore areas of slow deposition were preferable). The ultternatieo argument has however also been put forward that upon reaching an area of high electrolyte concentration, the smectite could flocculate and settle in nearshore regions. 3)Diagenesis: Fairly substantial chemical evidence has been presented in the literature to support the fact that chemical readjustment occurs in sediments between deposition on the sea bed and their being subject to a maximum overburden pressure. Potash absorption increased crystallinity, smaller expansions (in the case of smectite) and increased chlorite, vermiculite or smectite all result from diagenetic processes. 4)Weathering: Again chemical and mineral changes take place 144 which tend to d‘„lrease crystallinity and alter mineral suites (usually by introduction cf mixed-layer minerals) in such a way as to be more suited to, or more in eauilibriuin with, new conditions.

B. Structural and Sedimentological Developuent of the London Clay Basin The information gathered this far in the study programme indicated from both a theoretical and practical point of view the palaeogeography of the basin, the mechanism of deposition (following sedimentology principles) and the probable chemical and mineral constituents and their distribution within the basin.

The study was further advanced in Chapter 4 by a detailed description of the formation of the deposit as well as a presentation, for the first time in English literature, of individual mineral depth profiles and regional mineral distribution maps of the London Clay.

The structural history and development of the London and Hampshire Basins was discussed at length from the Palaeo-Mezo floor rocks and contact zone through the Albian, Cenornanian, Turonian and Senonian Stages. Chalk structural contours, isopachytes and zone maps were all considered to aid in the reconstruction of the tectonic history of the region. This tectonic reconstruction was furthered by a study of the structural contours and iscpacbytes of the Lower London Tertiary deposits. The pattern of these structural geology events was followed during London Clay times when four main sedimentation "deeps" at Lowestoft, Thames Estuary, Reading and Hampshire were noted. Isopachyte maps and structural contour maps for the London Clay (both hand and computer drawn) were discussed in detail.

The effects of the Oligocene Alpine tectonic movements on the Eocene sediment piles was then examined. Two important features were found : 1) The London Clay formed the deep water or middle part of a large single rhythm of deposition, i.e. the Cldhaven, Blackheath-London Clay-Bagshot (i.e. basal- fine middle-coarse silted upper) sequence, 2) Within this middle section, i.e. the London Clay, smaller tectonic movements must have taken place giving rise to minor lithologic variations. 145

These have been regionally observed and used as the basis of a stratigraphical zonation system for the London Clay. (Williams, 1970) Three rhythms were found and these gave rise to the Lower, Middle and Upper London Clay divisions. Extremely strong palaeontological evidence corroborates this three-fold zonation system which can itself be easily recognised in the western (more coarse grained) section of the London Clay Basin but is more difficult to identify in the eastern regions.

It was thus possible to follow the tectonic movements which had given rise to the mechanics of sedimentation, i.e. the rhythms of deposition. These rhythms governed, by the principles of sedimentology, the lithologic variations and therefore the detailed stratigraphy and zonation of the London Clay. This regional zonation allowed an analysis of regional clay thickness trends, estimates of proximity to shorelines, axes of rapid sinking and fast sediment deposition, etc. (refer to Chapter 4) to be theoretically soundly based. Cross-correlation of clay stratigraphy and clay properties was thus largely eliminated.

C. London Clay Mineralogy In order to study the relationships between lithology and mineralogy and mineralogy and geoteChnical properties of London Clay, a regional 3-dimensional mineralogy survey was undertaken.

Over 300 samples from 22 sections were analysed using specially designed X-Ray diffaction techniques to obtain qualitative and quantitative mineralogical data. The mineral suite of the London Clay wac found to comprise: Quartz, Illite, Montmorillonite, Xaolinite, potash Felspar, Plagioclase, Chlorite, Calcite, Dolomite, Pyrite and Siderite in that approximate order of magnitude. The individual minerals were not found to be restricted to any particular size fraction and were all present throughout the complete range of size fractions. Very little change in mineral crystallinity (particularly of the clay minerals) could be detected in the different size fractions. The mineral suite described above was not found to vary, in a qualitative sense, over the region and minerals neither nppc.t.rod or disappeared with a change from shallow to deep water conditions. 146

The mineralogy, conveniently plotted as depth profiles, could for the first time in a London Clay study be related to the lithology and zonation (or stratigraphy) of each of the 22 sections sampled. The Alum Bay section was chosen for detailed description as an example to show the general form of these relationships. This form was also found to prevail at the other sites.

The quartz contents related strongly to lithology and the sieved sand fraction, each rhythm being reasonably easily recognised by this profile. The tipper zone contained the most qvartz and the lower zone the least. The clay mineral content profile was a mirror image of the quartz profile and hence provided essentially the same information. Interpretation of the kaolinite profiles established this minerals affinity for continental conditions. As the London Clay Basin became silted up in Upper London Clay times, i.e. during a period of regression, kaolinite contemts increased. The same feature is noted, on a reduced scale within each rhythm and by this means zonal boundaries may often be distinguished. The whole rock montmorillonitc content profile tended to be the inverse of kaolinite profile and shows affinity and hence increased proportions during marine transgressive phases. The illite profile presents a subdued image of the total clay mineral line which is not surprisingly since illite usually represents some 50 per cent of the former.

This two dimensional distribution of minerals as represented by 22 depth profile diagrams was then extended throughout the area by interpolation, to establish a regional three dimensional distribution pattern for lithology, zonation and mineralogy. Presentation of these distribution patterns was accomplished using a colour column method, e.g. fig. 4.11, and a numeric averaging technique, e.g. fig. 4.17. The quartz mineral distribution showed a constant regional decrease from about 55% in the west (or nearshore) to 20% in the east (or offshore). Isopleth lines appeared to run roughly parallel to the proposed ancient shoreline. Vertical variations were important in single or neighbouring sections but the total east- west variation of quartz generally exceeded any vertical variation. Hampshire Basin values were consistently higher than London Basin values but no evidence was apparent of any 147 abrupt change between the two basins. Local anomalies, e.g.the Walton on the/Laze section, have been explained by structural features each as anticlinal ridges creating somewhat shallower areas of deposition. The total clay mineral distribution maps showed the reverse of the quartz maps and averages varied between extremes of 40 percent in the west and 85 percent in the east. Occasional "deeps" of sedimentation were again recognised.

The kaolinite distribution showed a strong continental affinity by steadily decreasing seaward (or eastwards) to almost zero content. Vertical variations were, however, more dramatic and extreme, often occurring over only a hundred foot section. In general, however, a kaolinite content of between 10 and 20 percent was found for a large proportion of the London Clay.

Montmorillonite contents increased gradually from west to east with average values ranging from 15 to 3C percent respectively. Vertical variations were also gradual but directly related to lithoiogy and sedimentology as outlined in the Alum Bay example. The overall mineral distribution was thus a basin increasin,; in total clay mineral and montmorillonite content toward the deeper water more typically marine portions and shallow water areas of high quartz and kaolinite content. The montmorA.Uonite and kaolinite showing district affinity to the transgressive (or sinking) and regressive (or silting up) phases of the cycles respectively. Figures 4.11 to 4.f',1, should be examviled as they illustrate this mineral distribution.

A brief and scmcvnat incomplete picture of the land and sea environment during London Clay deposition concludes Chapter 4. This section is necessarily brief as such palaeoenvironmental evidence must be obtained from a detailed examination of the regional flora and fauna and this study did not set out to accomplish this aim.

D. Geotechnical Literature Review and Experiment Results Before introducing geotechnical property distributions into the study or attempting to relate these to the now known lithological and mineralogical trends a literature survey was undertaken to determine the theoretical and practical effects which geological factors could be expected to have on soil 148

properties. This survey formed the bun of Chapter 5. The theoretical result of each pertinent geological process, i.e. deposition, burial and diagenesis, and uplift and weathering was discussed and their effects on clay fabric and mineralogy outlined. Individual soil properties such as v.oid ratio variations within a sedimentary basin were discussed as was the variation of permeability with grain size and mineral composition. Theoretical, experimental and practical aspects and experiences of soil plasticity as related in the world literature were dealt with in an attempt to fully understand the concepts and mechanisms behind this important soil property. London Clay is accepted as an insensitive material of only medium activity, nevertheless these properties were also briefly discussed in the context of their being affected by geological variations within a deposit. Theoretical and experimental factors operating in the alteration of consolidation and shear strength parameters were critically reviewed particularly with respect of grading and mineralogical variation effects.

In order to establish a more firm quantitative "feel" for the direction and amount London Clay properties would vary due to natural in-situ mineral, fabric and grading changes, three sets of experiments were designed to firstly, simulate sedimentation effects, secondly, investigate fabric alterations on removal of overburden and thirdly, ascertain the mechanism and importance of weathering processes on London Clay.

All these experiments had the limitation of being small scale laboratory attempts to simulate regional scale geological processes which had been effective for millions of years. The experiments, however, proved to be successful and extremely useful in providing not only a property trend but also an approximate quantitative order of magnitude for each trend. Thus the relative effect of each variant and process could be gauged. These relative effects have previously only been theoretically discussed, so that the present study, although brief and limited, has advanced this topic quite considerably by providing quantitative results for comparative purposes.

The sedimentation simulation experiments were divided into 140 two sections. The first was designed to establish the effect of grading variations on London Clay. The tests involved altering the size characteristics of silty London Clay samples by grinding, milling and making additions of variously graded quartz. The results indicated that a liquid limit increase of approximately 1C percent may be expected from the known range of in-situ grading variations. The second part of these experiments involved the addition of 10 percent (by weight) portions (up to a maximum of 50 percent) of quartz, kaolinite and montmorillonite to selected London Clay samples which were known to have low percentages in these respective minerals. Addition of 10 percent quartz resulted in a 15 percent liquid limit decrease, a 10 percent kaolinite addition resulted in a 10 percent liquid limit decrease and it was found that a 20 percent montmorillonite addition was necessary to reach a "threshold" value without significant limit alteration, whereafter large limit increases were observed for small montmorillonite additions, Dramatic activity increases also being found after the "threshold" value was reached. These experimental results were su-Dsrimposed on the mineralogical variation maps and a theoretical regional liquid limit distribution map created.

This theoretical distribution bore close resemblance to what was subequently found in practice, hence the basic assumptions, oxperimantal results and conclusions drawn, appear to reasonably sound.

Theoreticl aspects of fabric disturbance, as caused by varying overburden pre-Isures, were briefly discussed and an optical and X-Ray moans of clay mineral orientation measurement advanced in Chapter 6. These new techniques were fully described in a paper by Tchalenko, Burnett and Hung (1971).

A detailed investigation into the mechanism and results of London Clay weathering was also undertaken for the first time by the writer using a systematic work programme. This investigation was conducted on two scales : 1) A hand specimen scale in which weathering was observed to have occurred by means of a "weathered rim" and "fresh core or kernel" effect, and 7) A larger, cliff exposure scale in which the top (or near cliff top) brown weathered material was compared with the bottom 150

(or cliff base) grey blue fresh London Clay. Comparison of the weathered and fresh clay on both scales was undertaken using mineralogical, chemical and geotechnical methods. The conclusions reached were that very mild mineralogical changes were produced (loss of some carbonates, small increase in clay minerals, possible crystalline depredation of illite by K,C loss) mainly by slight leaching and oxidation processes. These resulted in only very minor index property changes, e.g. in the liquid limit and clay fractions, cation exchange increases, and strength decreases, in the weathered material.

The initial characteristics imparted to a rock mass upon its deposition and consolidation (with its mineralogical and grading and fabric variations) thus appear to supersede the changes in properties brought about by subsequent geological processes, i.e. removal of overburden and weathering.

E. Studies at the Principal Sites and Regional Geoteohnical Trends With these theoretical and experimental geotechnical - mineralogical relationships established for the London Clay, it was opportune to investigate in-situ twc dimensional depth profile plots of the clay using data which was available from five major engineering sites in the region. This, it was hoped, would substantiate the experimental and theoretical findings. The five sites discussed in detail were Ashford and Sunbury, Bradwell and Orford Ness, Streatham and Central London, Noak Hill and Heine Bay. For each of these sites the stratigraphy (or zonation) and lithology was known and detailed mineralogical and geotechnical data was available. It was thus possible to study, in oualitative and quantitative terms, the relation between mineralogy, soil (index) properties, lithology and zonation for the five sites.

The methods used to establish relationships from data presented in depth profile form were very simple. The first approach was purely visual with like profiles being described as having like trends. It was also simple to establish inverse relationships by this means. If such a visual trend was to be further investigated, this was accomplished by plotting the one property against the other in graph form and hence establishing linear proportionality or otherwise. No more complex mathematical methods than these appeared to be necessary. 151

The conclusions reached rsinforced the already established relationship between zonation, lithology and mineralogy and also showed the following features : 1)The sedimented clay fraction (-2 micron) profile differed from the X-Ray clay mineral profile (the former bearing little relation to stratigraphy but close relationship to the Orr values) 2)The clay mineral (X-Ray) profile related well to lithology and also appeared to account to a large extent for the plasticity characteristics and cation exchange capacity of the soil 3)The montmorillonite content bore relationship to the soil activity, the cation exchange capacity and to a certain extent the Orr values. Many other relationships were also discussed in Chapter 7.

These two dimensional (depth profile) relationships from the 5 isolated sites were then brought together in a discussion which attempted to establish property trends (in a qualitative rather than quantitative manner) on a more regional basis. This regional analysis was possible as cross-correlation errors, between Upper and ewer London Clay for example, had been largely eliminated by virtue of the zonation system. Casagrande chart plots were used to emphasize the hitherto unstressed variations of index properties of London Clay both from a single site and regional viewpoint. These index properties (namely liquid limit, plastic limit, plasticity index, clay fraction and activity) all increased steadily toward the deeper wr:.ter, eastern sections of the Variations in a vertical direction, which were often quite dramatic, were also discussed. These latter variations were often f=„ald to be directly related to lithological and hence sedimentological characteristics, i.e. the rhythmic deposition, of the London Clay.

As indicated in the introductory sections of this study, a large scale collation of geotechnical data had to be carried out by the writer to allow the regional trends of these parameters firstly to be established and then linked to the regional mineralogical trends. Both the manual and computer drawn plots of these maps were presented and discussed in Chapter 8 which also dealt with the detailed analysis of the resulting trend surfaces. In general, about 380 points of information were used in the plotting of the three dimensional 2nd and 3rd degree polynomial regression 15';•,

trend surfaces.

Although 380 points of information per parameter is a large amount of data, it only provides scanty cover for a regional project and hence the hand drawn contour lines are little more than trends. The computer maps are most certainly only trends as this is their aim. Much use has been made of these property trend maps in this study but their limitations must be borne in mind. They cannot be used to obtain an absolute readout, i.e. values read off them are not accurate as they only form part of a trend surface to which a residual value must be added to calculate the actual property value at that locality. Even using high degree polynomials good "closeness of fit" is difficult to obtain between actual and trend values, i.e. residuals are high. On introduction of three dimensions , trends become weaker and inaccuracies greater but by constant reference to hand drawn contours of the same data and the computer statistics, errors could be eliminatedand highly significant and useful maps produced. These regional trend maps make possible this first attempt at regional property and mineral distribution maps of the London Clay. This quantitative method of analysis is probably of greater value than the qualitative estimation of lateral property trends between major engineering sites, as first described by Bishop et al (1965) and improved upon by the writer in this study.

The liquid limit trend map showed : 1)Values to range between about 5C in the west (nearshore) to 95 in the east, i.e. the Essex coastline (or offshore) 2) No N-S trends were encountered but clay homogeniety appeared to increase eastwards 3)Lateral variations of liquid limit exceeded vertical variations but the latter were often more important and unsuspected on a single engineering site 4)Trend surface contours appeared to run parallel to the suggested ancient shoreline 5)(',uartz content trend showed an inverse relationship to liquid limit trends

Plasticity index maps indicated : 1) A similar eastward increase of values from 4C to 65 percent in what appeared to be a smooth gradient 7) This lateral variation was roughly equal in magnitude to the 153 noted vertical vari9tion 3)Areas of high indices as shown on the trend maps correspended well within the established intrabasinal "deeps" 4)This parameter appeared to relate strongly to the total clay mineral distribution trends.

The regional London Clay dry density distribution 3 1)Varied between about SC and 105 lb/ft 2)Showed a lateral variation which about equalled the vertical variation observed at many localities 3)Excellent relationship was noted between dry density and lithology, thus higher densities were noted in the sandy Hampshire Basin and more locally wherever sandy horizons were noted 4)The trend surface map showed a roughly vertical E-W trending body of low density corresponding to the main clay London Clay mass 5)Densities increased to the north and south of this body as the clay mineral content decreased.

A brief d,..3cription of the liquidity index distribution was entered into and a loose relationship with both the total clay mineral content and the kaolinite and montmorillonite distributions suggested.

The undrained shear strength regional distribution showed higher values in the west than in the east as well as showing in the trend surfaces a marked horizontal distribution component of similar valuer_. This was interpreted as indicating that a vertical variation was more important than a lateral one and was probably explains.; by the vertical lithological variation.

The discussion relating to these regional 3-dimensional property and mineral distributions was necessarily of a general or non-specific nature as trend maps are being dealt with. The writer, however, feels that this In no way detracts from the arguments advanced, conclusions reached or confidence felt in the validity of these conclusions.

By this stage of the study programme, all the relevant available data had been plotted and many direct coLpariscns and relationships established. A discussion of these results and their implications on engineering practice was then presented in Chapter 9. This included an appreciation of the effects of 154 transgressive and regressive cycles and their consequent rhythmic form of deposition. The geotechnical implications related to the possible theoretical, mineralogical and grading distributions were discussed, as well as the engineering implications related to diagenesis and weathering. The important results and advantages gained from the zonation system and isopachyte contours of the London Clay were stressed and it was emphasized that by their appreciation and use, detailed stratigraphic c4fntrol could be gained. The two and three dimensional mineralogical distribution had shown how minerals and properties trend in a systematic and predictable manner and with what results.

Engineering implications of the experimental results were discussed with an aim to obtaining less plastic and less active soils by introducing quartz and/or kaolinite into London Clay material. The interesting fact that montmorillonite content apparently plays only a minor part in London Clay plasticity has also been detected and discussed.

The relat-Lve importance of geological processes acting on a deposit were assessed and their possible benefits and dangers outlined.

F. Analytical Techniques This brings to a close the summary of the planned programme of study as well as a brief description of some of the major conclusions drawn from the theoretical reasoning, analytical results and collation of geotechnical data. Table 10.1 provides a summary lisp, of major work undertaken and results obtained. However, before suggesting topics worthy of further study, a few words to summarize the analytical techniques descgibud in appendix 1 will not be amiss.

More than 300 samples were to be analysed, principally by X-Ray diffraction techniques. It was required to obtain detailed qualitative information of the mineral suites as well as accurate quantitative data with respect to both the whole rock mineral suites and the clay minerals. The method chosen to act as a guide for these quantitative estimates was one used by Schultz in 1964. It involved measuring the heights or areas of mineral peaks on the diffractometer traces, applying an intensity factor to each and converting the resulting products to percentages. As the intensity factors played a critical role in these calculations Table 10.1

List of Major Work Undertaken & Results Obtained

1. Geological literature reveiw of region Detail of regional stratigraphy and theory of rhythmic deposition 2. Literature reveiw of clay deposit mineralogy Possible minerall chemical&geological process and chemistry di stributions&effects 3. Study of the development of the London Clay Detailed description of structure,tectonics& Sea Basin deposition of London Clay 4. Mineralogical analysis of over 300 London cualitative&quantitative data l plotted as f:T, depth Clay samples pro2ile sections 5. Regional London Clay mineralogy study 3-Dimensional interpolation of deith profiles using hand & computer techniques 6. Relate stratigraphy,lithology&mineralogy, Rhythmic deposition&sedimentology ?rinciples control lithology&mineralogy whosc variations govern stratigraphic zones 7. Geotechnical literature reveiw Effects of geological factors on soil mechanical properties 8. Experiments to simulate &/or study clay Gauge importance of each procass&gain estimate of clay sedimentation, erosion&weathering their theoretical effect on clay properties 9. Detailed geotechnical studies at the Geotechnical depth profiles related to mineral, principal sites lithologic&zonation profiles 10.Regional geotechnical studies 3-Dimensional depth profile ilterpolation and computer trend surfaces related to the regional mineralogy 11.Impact of study on civil engineering Engineering implications of new data and concepts practice discussed 1%Development of analytical techniques Accurate,reliable x-ray diffraction technique designed specifically for London Clay a large amount cf cross checking was carried out so that the factors could be verified or if need be, slightly altered to suit the London Clay mineralogy. To establish accurate quartz percentages, four separate experiments were conducted, two involved the use of internal standards, one used an infra-red spectro- photometric method and the last technique invoked mixtures of known additives. Total clay mineral percentages were accurately determined using back calculations based on wet chemical Al203 contents. Illite percentages were similarly back-calculated. Wet chemical carbonate determinations were also used to adjust the Schultz (1964) factors such that X-Ray and chemical carbonate results tallied. Further checks on the mineralogy, particularly with resvect to the clay mineral suites were carried out by means of back calculation techniques using cation exchange capacity determination.

Extensive series of tests to estimate reproducibility errors involved in the sampling, sample preparation, machine running and interpretation stages of analysis were conducted and the results statistically handled. These tests showed the nrobable coefficient of variation to be expected in analyses of mineral suites containing varying proportions of minerals.

It is felt by the writer that the analytical system, sc carefully and specially developed for use on London Clay, provided results for which the accuracy and reproducibility characteristics are reliably known. Therefore, it is suggested that the mineralogical data obtained by its in have a high order of confidence and ray be used with reliance provided the limits of error are borne in mind. By virtue of the fact that X-Ray diffraction was chosen and used as the analytical method, certain minor omissions to qualitative interpretations were necessary. These included an indication of the presence of amorphous material and organic carbon within the samp le and a record of the loss of interlayer water as provided by differential thermal analysis for example,

With regard to topics worthwhile of further study in connection with the engineering geology of the London Clay, the writer recommends an even more closely controlled mineralogical study of the deposit coupled with an extension of the experimental programme to produce a detailed theoretical and practical under- 156 standing of the parameter variations. This could even result in accurate prediction of index properties at least, becoming possible.

The means of studying in a quantitative manner, clay fabric and mineral orientation are slowly being used. These need to be perfected and a complete regional fabric study initiated. This, when linked to the already established mineral distribution, would contribute enormously to a complete understanding of the engineering geology of the London Clay. i57

APT:ENDIZ 1 A. Chsice cf tnalytical Techniques The sub.'ect cf this thesis is a regicnal gesr-ical study ef the Lskidn Clay, unertFen in .i dertc F-sess its influence cm erineerirg 7:eha7i:ur cf the clay. The :Aethc:1 :f invectizati:n chccen vac t: relate '-apped cbangPs win ,:- apPed 117-)S. detailed inf:rJlatien was required with respect tc T:sth and quantitative datr rezarding this Fnparently very hci:!:•genecus senclt4 7e analyt:.ca7 "tcl" was needed fsr this regisnal study and Z. ray :.11f:Zracti:m- vac chcsen as it ceuld previde a fast and reac:naly tcourate result.

As the sulects :f ,ineralogy and X-rry diffractisn are la,nge and cc...-1pler, it is clear that c=pletely rige=s apPrsach .,mo uld nct ac :gyp' in either case, 7,:ut it is siderad that sufPicient detni7 :asp7 intraduced ts :btain a clear and accurate .,:iineralogical picture. The li7litinz analytical 7;enditins ware : large nuar co—Plec (these totalled sver the the required f ',reparationreparaticn cf this nuber sainples, the =. chine ti .erequired fer sapie running, the intricacy ;alculation, and 7aaintenFnce sf the required renr::ducnility (i.e. nr..cicicn) and ac7,uracy standards cn the tl-v IT-rry Llacbiaas in-felved. What was required f the technique Fn accurate whc.le and clay fra2ticn qualitative interpretaticn ef each sr7:ple, a gccd,

:-elati7e- and abslu.ta, vh:le quantitatiye interprataticn, a clFy fracticn qurntitatie interpretFti - n, reprcduility and nr,,cisi.n data, f.7Ja use cf a fairly well established diffracti:n technique thus av:iding tftr:2, ccns1=i7ig 158

7ntr,o,drotiin

A review all literntrl-a was undel-tall'en with the ab,- ve fact-rs in :.in_ tc seek :ut the desired X-ray technieue. ?our raa cubclivisicns are noted in difracti-n techniou.es (17.rr:wn, 1C4fl).

(1) The -71eth071 1r117-wn additives 7:17 addinq a /mrin weight cf pure c=pcnent t c saJtple containing this c,o,:ip.nent, and -leasurin- the refieted intensity Tle - re aTtar the wair-77t 7 .qtat 1-:.-lynent in the ,-,1-i,zina7 Ga n 1e dtected.

(17) The _ e::ternal standards - wherel7 the perk intensity and , :ass r!:„s-rrti-n co2ficient :77 a pure a -2p_nent 7= ,wn to be oJntained within a viztrre are - ensured as well as the pealr: intensfty tht, ^^":::p neatin the so.ple and tha average saTlole -lass als.:rnti:n c,:effioients. her w al"ter, the wei7ht prop-rtiln the n. neat within the sa,.Inle e. an be calculated.

'Me use internal standards wharaT:y a standard substance not c-:mtrined in the ..lizturs is added and convenient re.'103tions- it are o--Ipared separately with c.n-nent reflectns.

(4) 2sti-:ation -,..inerals with :..7.7treely irregular and pcirly r3rystallina strr:turos - where ;y, wf_th n. additi-n standa.rds, Tlentages -ainly -;lny 7ay he esti-_nted. A brie:" rtline and •fiscussin s:.'2.1e. .T the teohniques

rrish T71:r(li".7,7) used an adyano,ed erternnl - 0 ;an•larec te-;/-m4.rfue with. ;ass a's easure.e: ;s 1 • t flptensity 7-7t4- c nc l rr .s 7) rn, -ttained rThey used integrated pea'- !ntensi':7 ane, results 7y o- orrin7 the retical Jersts weight peroentages anr: a-ttual Jerm.s Irn'wn wefF;ht per:entages at dieren s. They -lis-ussed the i:p rtanoe :rientrti n e-^Te:c sh wed 11-,w these 1 r1J /-e cheoed.

Mitcthell use :.7 an internal standard te,.ohnicue with 7C1 then orixydu-: ,?.a7.-;rlate weight neroentages wn n-cly t- an ri--turacy pal,-oe,e)t. The

159

line xca 'einpr :a'en Ft an intensity with Fll J.ther t;. it.

D17.7.I7etcn rnd added 17,f tc- the salnle in Tz:n:2wn pxprtintc, r,:tt an internal sr,.:. 7ntensites Anerals in the sa -,pe were Tcred t - th7se reference -thierFls Fr.,73 use=d, rs a 1:is f-r estintinq the rer:nt,a73s T 7nri-vs 'inerF7..s present in the sa"pie. They thus eztands FitAsells M!:1 T.7.r7-. in d9 th3 175y h1crite, sepi_7its Fntl pmaygrL,rsaIte as wn31 as vari-us e7trr ncn, 'clay . Anerals.

Talvenb,--in- and 7qits (i), ,z.esidas in7estigFting thi77-.ness type .7 - ntin and glye,-lati-n -11a.raner ic.*4 nr. VS3'24 the 7ery "stanc7L-rd ir-trres -,T1.ate - irtures illite, V'ST7 in -,IFying pr - r ns p7.-t peT—;entrqs .pnent 7ersus Frep ,graphs. q'his ga73 9Z en': nncl ana.:,1,s-JI ner-..er

, the f.71.Lanti:-.sti7a- N. ray - 737 Anera7 c in —7 sa)pes 17-cet-1 '- n the T.7

the b T3 :h: c, --711.1h h7e the eisad'raTatars ,, starviardso, sl an.1 sa -.17a3 prew.-rai. n (ne addfn i nn intri':!=ta Fre-V-r te-:hniTT- es Fnd 7.t is 7-n."'

,7_f.st'. 121yrntnfi'3s n Tith Iwn/1 thes? 7- 171:3 hs7a 7:refsy: a1 in . et;i - rls n-7 -1.F-:s -.:rne<7.71. Th.9. 11 nn7sly,ESi 1-7

160

7invs7 t'. ::-saxrds . r s.dditi7e3. Gri-nd rs,d/.ey (4), =rid -ut &n-YSes on Rent ants the G7,1-T ' La7rirt-. They lirperse0 the -.:17c in aistille(i 7s.ter, s,Ann.7-stefl Trssti n

snd cedioented thi.s erl^c.c. Three r':ns rierP - t'len ..s.rrf-ed - n the .if3, i.e. untreated., gly•: "Isteel snd bestal t: c. veie-isnt S.ntes•vstel inte.71sits rare s_11.-1 the tssis 7- ' in 7srs y rlbe 1L i1i3 intoilcity 1,,ns ntta, in . rder t- -11psre it with the 17E r;77-: Istz'!! the illlie is us.'=.d n with rr- nr1 -1,.11. rite. The ',flA inensity is Tirct -" v nite h-ve..Ter ref_ rs0,-,c1:7 n n) then 7-s.- linite (i.e. iT'e '711 4'Y: height), this ier-T''S the -thl rS.te 117,r'-er.1 Sntaftsit,7 ',71i -'-"T,=='P'S. P77,ihse'7:7P'17!"?:5 per:ents:ges. rith t1le .:atIsH is ths' the in 7,:n-1 F.A. eThy is - - ra thr,n,the 7inite ,- 7%17.E; ,1 es n t :lissnper sc Ss vem-ire 1. tba

Tr7r1 s rst'ler .7.fferent s= il-r intf,nsity Tr et - treeyvts Ttns7. sras T - r sb .-Zners1- s- tbf t The %eth'd, these , t,7 linite stv.1 h.-7137r, rer'uir the m7.7 - n -111rits re-:711:tt st s.7.„ 7.5A t ense pr porti these Waarsis. MSc ress7'rti n rl-es n .t - a a Clsy :r ;C5 <71n1 henr:te this e-.11n.So7: is sf.'s'

7.intte ia th: nflf- n C1ay •,-!- ...Yr in rn ';j the ra-710--.fti -ns linite s.t st nre n- t 2J3.mad, thus he soIrt'n n - t

7e7m s ress- ns7.17 s- Iutin. he sv. se -1-12*: ritf. vos- 71. n 7,77s neaSssTyii PP T the "P nesk (i.e. 1.1.f'rite ':.'he t77' n ee the "f (Chi!;- Ka - N f."7.1 161

E7nandinp. rrers, tbe 7A rrer. ben"! rr .7 elLer,n,.e in :::'.1_:- .,7rnrs!t7.nf7- p7fTi' Tha

7A r.7.20. 1").A 17. 4- 7.7.7-,s then rti-n:iez int- :1:!f,a g/y -.;a7-1) e.171;?ncljnr7 7t1t_'T (17A :::;.y;ar 1 ,7:,rre,_!tei t() tha The 1!.p - linf..ts--hY rite nr.- p:rtf- ning vhirth has Tzeen -.enti- ase ^h---17a, then 1-11:Iwee.-

17nitee, Ota7ces -4.1.r-rny "r par . C y issue.72 n the 9 .""luFnti%ati-,7e thterpretati-n .iners11 711 n

hfrrl!n- eer_tilee 7n a su2tn7.1a 11-17y eniclre, the resineer this Anpen,liz clas with chs-rt ,^u:7- seltf rs bead7_n7s s‘t::117.ke treate trrle nternret7t1.; !: _th vr/tntjtt-Iti-7,=), snri T f.s h.-nee that these eestpti.- ns will .vt1ine?11:2.1y the pr -;efu'f'ss, tat:hni.cwes antr.talatt- - ns ~..red 77,y the 7,rriter ia the -mrr:rincir :ut his stuey ,ne L- n6I- n :May.

162

p!iiig s.110. 17-t-3mv5-;i

kr aT 111 rr:

7, -7. -3,-1 -7.n "7",!•: 71..• nvy r 1.3.tr TU per e .3.77' • :cc • Li .-r!,12.2 Lce rt 7. V rr'.1 Ths 1: b, :1..1 sr -.Tx .;3r ar.1-!..,E1.7:- ,1.'sbal- rms.! rf..lha•Y•s•-,L1

s.r.F.:-?' -ysis warsuc - b;3ines1 r-1; 17' an 7.ara eir13.1- 71•*; sr.-.111.3 F!, in3 p T e1- fn an& ,...r.t737:-33r. ,3s a.,T. 1.,rslac, "CC r 31C n p7r.r... 1- brf3 7- 3 ra- sa 1--a••7 re crrshl.n7 sr Tliria. avv:vs. TrIsr 771.5 lit. T3 :I 332 v37. .;pla pp3

t n

Tuecit•••,17.- :-,,r :7 - ,rer9ii' :be77T-1- Te r Tr p.7c' ss.

gr. is j:..1.s•r Is! I] ,:- h s • ns -?12.1 r%S. ed sn 1-21.er rpr- i3e iners1 r.s f3!..- rs iare 3 ' r1.3 Tir7',E7 -7 .::e P 1 nif,177 7 r2pr - , sltov1.:- 1c( - • •••;x ,Z, • • .L -;!_. 11 an .2

-T:he ar.ss r i ri.:2).-i1;F:,•1

Tyo - 5 grr was 7.7'.117-!C suspans._n

Z' us yc Lc -1.7423 -7_:- rn -", C,Y -7 c•;m 7 .7y nnr.) -5,sf.s _ r rph - rs ne D IT7 7:!1 1 ss... 'DJ's rs.s r:7:7 77:"In •m.f1 t1" - re r r prap m.

163

- ns?, , : n ruLL-, tbe ' Th p're • ,.

•11, ni n C Un D n r7- :r nei te pr - :!eT:11%.*:re is -, n(2 7 . 1.S27,7 iris • :. 199-e7') vis-a .'37311: 77 t - ec107- .ffsnarerri. t -- rs-- nc im,•:7, tfch 71."77 t - ta-Tery sr . 77,13. tha a.7,fpaltF-rzn t strn-.1 1 • -1Ly .:7ispars n, c7-377. 7 r in the It' - n3 t7n,,,, (n- 31,7 Efr. )3! '3 ra trs'?.es, - n r e"1" " . 4 -• ) • 7":"S ir trr, n11 SCUCca'f;: tar - 77,7S, so.731t3.:7 E2,7.: 117.er, 713r.:,') rif-CranSfi C127 r11.7 dLstiT'rz:r:7 -77rtsz' t 77,9 - n ?D.1 r.1..3crrte-.1 ' Y w renr err :n ntli- T:sel 7.1

- n ri - ns ry - r

7'29 trerTt or:t -c the sr._ -n9C - n 477 - r'S ?Fi 71s:17.(-tf-t-17 - p 1-.7ar 7r 1.!,!..7.1 rirs rtt2' - ant :Ls " stEn,1-7rfi ou - rt,- s =,r,T tar n repr- d‘LT -i7:1.7.1t7, it is --.3%.t• t7F. irfy

- *::112 7sEr narl!:!1 ts. - c -- '.' s

vara n17,. t rcs;.! - n L177 '117 . q-7A1,7 P. ,74t: 0-1". '777 r'.0,"1 El 77 --79r - na r .ts.1 164 tweak height nv.7:nrtins, whereas a sa'1311ng internretati,7n --r saaple prerarati:7n err-nr qight vastly r,ner the Deal: nr7.p:. rtl.-ns.

The Tinely rrderad th,-,le r cln sa:mies- pv7Aed in the retal ...„ bnlders were us:-a 3rrun -la ,,:le 7achine using 1r-7'n Tiltarad a.7balt rnliatin at P. raiIliapares and '"." *rig.? 7;-•1 I bea Slis, a '7.‘.1 detectr slit, and s7;annin7 at 7' per - filute. A tie -; ns ,m::- Y v, .7.:_ml- 1 pr dITce n su!.,tr7,:a ,ra.f.l.

'.The cs,-, y!„-I el.:r;-).. -.7-te•,. rn,s a:111 7er3 - 1:7),, ,101 c,7 -7rall:-n runs t!El!ite n:.,:qe.al-r.s...1, 1 rner rt- in.:i-n reenerated at 77. 7172islieres pn,21 4fl kil-7 lts ,litT) I hen:, Slii:s, 7:.1 act,.=--.'t27' slit nrd scpnnin7 1,-% 7 per - inT:te. A tie .7,:1,nsant -.: f.', 7.7r1s n?,ed.ed --n this 'rn.Y7s -11.-:;)::.ne rlr'euf';e a reas nably s:.!,.-:. n an-1 r.:gular- bacgr. TInd vitlr- nt 1.r.,Ipinf)2 dcrn the rer. intensiies t• ,- -f.T:,,7th. V'T c.717Tra3tf::n tres varen. run -n ealb :i'" ;iie :/.1entec.I. afTren:ntes in tbe 7- 1-.,7in!T: :r1,,,- • (1) .73-rin,d at r--.- .':e1.11eratare, C;:) -7*-NY-=,,m -I treaec:, (F) rented r. -. - C 7e-r hal:7 ari b - .:r nn ,1 (4) beatel rt 5n7.5- c .2.z- lr. ha: aa h r.

165

Tnterpratati-n and C,:,!antifl .1.7.t1sn- Datn.

Lc rre7i .anti-nee, tha intarprat-T. - n, fies olantf_ficnti n -7 -;ay data was dealt wit) in tw. sanarnta stages, wh:ce ,!1ay frarlti-n Ith rh 7.e r Yt wsa;yary :inere in T-).a iflentif.fad and McIlStrtes a earth. sh -,-11 1 n-ta,1 sn7.-7 tT.1 .7-72y .1.1.nerP,1 pefl 9 is es.;a7.7ished Ft t!,is s-';age an n t the 4 nrli7fl-n1 ivkar;: ntaac -7th r v-;h

'has. :av tjraaely 77S sins these s . urrn..3y re,r,uires ffur- r . re tra7ss T.a -n %lay frantin disc bere any ac:uzate interrratati.- n (mn T7a "Zt as-- be nted that In seraratitr: the Ainus -02y S20 the larer ?.11 flakes (177121_,;b w,ers in:7.0uded in t11 wh_lz (..1%y ast1',7..ate) r re-lain in the silt si7,e fra-Ai-n and hence bu ..unted in the rtlay n ast2zatas. rmarslay, quantties f n-n 1ay .I%nerals y be ;Iny :ran sire c.n.71 are ths stied th pr us tiles ha-r'-ii-'3: a se,771)1, -n the -Any slirtht an ay gsp eriats 13ntween the tw- stages where verlar -f sr.a

inerar int 33,-,th J agc

(1) Wt- le cualttative interpretati:m was first ,3nrriad - ut %s rar sn.- nd.nrd nrarttice. & neat:. anf?:les wera , -n-Terted d srnnin7s than A.S.'r7.M. - r iaT yss f -inerals idantif 1.1 use. The - inarr. ls tbus c ri-se, enresen, ter c; :f the ca,-- xtle and al7. that is th, s nenessar7 is th,N sCali%risi- n 2 this quan4-it7 e.%ween Hp-nents. This 7as -tn.rried w' e- a n. The pr:p ninT ;r :.cc'; he-, ax.A the Tein:htinr 1.1y :--arc :a .s 1 - 4 A 1-7t sgritly altsred as result f tasts ut t- ap7,-.i_in:Ality in - Y3' T1 -9- n (ae -n Trnsis f !?2 9 rifz tat the c7iners is imtarrratal

:7.11.• the 'Ma str nc-ast f r 1.125-raA - n the

7.ay 1:3:1 - TientP,e1 n rider. r,711a -r 166 intensity fact= ts necessary besause each 7:;.ner..1 is a different structrxe, c;_p-citi n an,:a abundrmse and diffrP_cts 3:. absorbs tha iipingin% :i:-*:3.Lys t? a different =tent z'ver the wide range -f '7,,e used in the analysis. The Ochultm (le.1.) fact.ro have been built up using staadard curves ,--.! :Aix:tures ff different ,Ainerals in t clay 2.t.tri;:. These fractions give easily calculated and. reprcducible results and their application t: 7J- nd.:31 Clay has als,:7 been .chented by the writer •using nuAer.us :Aethds. The esti-211ti,:.n .2. -7 the A.ners1 content is rlade by ,ultiplying the strena'est peat height of the !nineral, :inasured in any units (the tenth :.!7- an inch rraph paper squnres is e-,n7enient), by :7,0") - ntansity Pantcr and prp. rtinin!,' this prd',:lt t' .71LI:e a percentage yf the whle relr. The figure ,,-2. ",'.':1 f.s. the larfrest intensity far, representin!T quartz, s.7.;- by actabltshinfr this and :_lultiplying it by the peak it weights the other prAucts by usirt?: the intensity facta,r e--2 quarto as a standard. The '::eth.f.%d described here U28S the adsjusted 2chultn (1tM) factrs in a different ::,anner fr::A that -eutlined in his paper. :n fact, he did ,.:-.t rati::, tha -2inerals int per ;engages but calculated the parsentagas by Teans 72 the relatin peak ht. (cunts/sec) intensity fact::,r (c:.)unts/see) per 1 —) percent. Thus by -,- cuing percentages, the t,:,tal ohol:ld equal 1.17). Thera seex,s, liwevar, little tc chtse between the methc.ds as both start with the nre'ldse that as all the --itnerals are qualitatively established already, they Alst ttal 7,_n perc:ent. E slight advantage ::,l the 3chults :12athed is that i2 the St.' -•• of all '.pine acs present is ,iltside the ;.~Saga N. range :1? :: t7. lk'5 percent, this either indcatas an incrrect f::tr._.r _.: the presence p.7:71'phs ,:atel-ial. This salculatf:p is illustrated by t r:r7::ed el- e,ple.

Tt ._=ptt ta 11.1-ted tr.”t althugh the intensity factors (ufter sli-ht ad.:ustilent) appar.r t_ gi7:a sensible resrlts -;:inin3 -2;r 7,- nden IlLay they _igl-It Le very 'icleac!ing wl)en ens 7the:' cays and jchultm .1). 4.n *7act rec-nd,o slightly altered '_act... 2:74,- T'iarre Chale wfth high b,entnite c ntepts. The 1:-.sni.:n c,- f the T3ncslina often :muses r;A.T.:fusfz,n but it is defined here as the trace p=siti:n :f the 2.inemIs causinq the pear: were absent. The vr;,r7.7.ed ezaple il7ustrates the 167

writer's interpretation. Tables 11.1 gives the factors used for quantitative interpretation of minerals in London Clay. All these minerals as well as several others are found in the Pierre Shale so that the tabulation of Schultt (19C4) is more than necessary. The writer has accepted the findings of Schultz (1904) that absorption of X-Rays plays little part in influencing intensities, so that for the concentration ranges dealt with a reasonably linear relation exists hitigeen mineral percent and diffraction intensities. This does not hold for one of the X-Ray machines used but suitable corrections have been established for it (see section on accuracy).

(7) Clay Fraction Clay fraction (i.e. minus 2 2icron) analysis is carried out on traces obtained using oriented aggregates on porous tiles after the four treatments of the discs as mentioned above have been -lade. '7,ua1itative. interpretation of clay mineralogy is relatively well established. One of the major reference works frequently used is that of Drown (151). The writer found, as in the case of the whole rock minerals, that the London Clay minus 2 uicron fraction suite was remarkably similar to that found in the Pierre Shales and that the well known and exhaustively described qualitative criteria used throughout the literature applies in this case as well. The following criteria were used to identify the clay minerals (Schultz 1004):

1. Naolinite is characterised by a series of basal X-Ray diffraction peaks at about 7A, 3.5A, and others. Judged fro: the shape of the 7A peak (Schultz 10,7,0, fig.2), kaolinite appears to be of a rather poorly crystallized variety.

2. Chlorite is characterized by a series of basal X-Ray diffraction peaks at about 14A, 7A, 4. CA, 3.5A and others. Glyceration does not appear to alter the chlorite peaks. Heating to 020°C and especially to 550°C changes the chlorite structure slightly, hence the 14A reflection increases in size and the other basal orders aLaost disappear.

Z. Mite is characterized by a series of diffraction peaks at 10A, 5A, 2.5A and others that are not appreciably affected Table 11.1 Factors for X-Ray uantitative Interpretation of Whole Rock Minerals (lifter Schultm 1204)

Mineral 2e Peak Position 2e Peak Position Intensity Radiation Cu Radiation Factor ruartz trhe. 21.3 2000 Phagioclase 20.0 22.S 1000 X. Felspar 27.5 22.1 1000 Pure Calcite 2

by glycerol or heat treatnent. Illite is used as a general or group tern for the nicalike clay of partly uncertain species and undeterminable poly morph that generally gives fewer and broader X-Ray reflections than ideal mica. The illite in the London Clay is predoLinantly a dioctahedral aluninous variety judging from the lattice reflections. However, materials fitting the above description of illite may include some nicalike clay other than the usually dominant disordered muscovitelike clay, and may include small amounts of un- detected mixed layers and degraded hydrated mica. For example, optical examination has revealed small amounts of glauconite and biotite that were included with illite in the X--Ray analysis.

4. Montmorillonite is characterized by its expanding lattice. When dried under ordinary room conditions, Aontmorillonite with sodium as the exchangeable ion frequently has one molecular layer of water and a basal spacing of about 12.5A, when calcium or magnesium is the exchangeable ion, it frequently has two molecular water layers and a c-axis spacing of about lE A (Grim 1053, p.57). After glycerol treatment, montmoriilonite exhibits a series of basal X-Ray peaks at 17A, 0.5A, 5.7A, 4.2A, 3.4A, 2.0A and others. When heated at '300°C for half an hour and not :300°C as recommended by Schultz (10:4), volatilization of all absorbed water or glycerol causes a decrease in the basal spacing to about 10A. This slight increase in thermal stability necessitating the higher temperature for complete collapse may be used to distinguish more stable varieties of nontnorillonite within the samples examined. The montnorillionite in the London Clay gives the X--Ray pattern of a dioctahedral aluninous variety.

5.Mixed-layer clay in the London Clay is characterized by a broad basal X--Ray peak between 17A and 15A on the glycerated trace, indicating that montmorillonite layers are usually most abundant. Complete collapse of the c--axis spacing to about 10A after heat treatment of most samples indicates that the only other connon layers are illite, because only illite and nontlorillonite give c-axis spacings of about 10A after heating to 300°C. In a few samples, a 0-axis spacing greater 169

than 10A after heating indicates the presence of a brucitelike interlayered component of moderate thermal stability that prevents collapse to 10A at 230°C but that will collapse at 550°C.

With respect to the quantitative analysis, the method proposed by Schultz (1964) was the only generally recognised one that fitted the mineralogical characteristics displayed by the London Clay minerals. It is a logical, easily calculable method which gives realistic, reproducible and sensible results and appears to give accuracies comparable with any other calculation method or indeed technique. Parts of the technique are taken verbatum from the paper in question (Schultz 1964).

"The relative sizes of the X-Ray diffraction peaks from the basal planes of the clay minerals in the 7A to 17A range provide the basis for calculating the relative amounts of the different clay uinerals. For some calculations, peak area is used and for others, peak height. Methods for measuring peak size and comparing intensities, which were previously derived (Schultz 10(30), are :- /. The peak area is considered to be the sum of five measure- wents of the height above the baseline. One measurement is at the peak position (called peak height) and the other four are at ?;-° intervals on either side of the peak position. The size of the divisions is immaterial, because all measure- rents of basal clay peaks are used on a relative basis only. In practice, the one-tenth-inch grid on the recorder paper is used as convenient division. For notably assymetrical peaks one peak flank measurement is made on the steep side of the Peak and three are made on the less steep side. The determination of peak area thus requires consideration of peak shape. 7. The baseline indicates where the trace would be if the mineral producing the diffraction peak were absent. The position of the baseline depends upon the intensity of background radiation reflected froTsi. other minerals having closely similar lattice spacings. The exact position of the a baseline below initial clay mineral peaks is, to some extent, subject to the personal :judguant of the interpreter. 1.70 a. After heating at 3'000C (3'3C°C for London Clay), all the clay !Anemia co,mon in Pierre Shale (and London Clay) give reflections from their basal planes at about either 7A (kaolinite and chlorite) or ICA (illite, mixed layer and montmorillonite). Therefore, with suitable correction (for 7A peak shape and position, Schultz 13W), an initial division into these two groups of clay minerals can be made by comparing the relative areas of the 7A and the 10A peaks after the sample has been heated to 300°C (300°C for London Clay). 4. In their collapsed state after nO°C heating, aluninous illite, montmorillonite and mixed layer illite montnorillonite give 10A X-flay peaks of about the same area for eaual weights of material. 5. The ratio of peak areas of the 7A and 10A reflections for equal amounts of kaolinite and collapsed illite, montmorillonite or mixed layer illite montmorillonite ranges from 1.1 for poorly crystallised kaolinite to 2:1 for well crystallized kaolinite; the ratio for kaolinite of an intermediate degree of crystallinity is intermediate between 1:1 and 2:1. The sharpness of the 7A peak as expressed by its area: height ratio can be used to evaluate the crystallinity of kaolinite. C. The ratio of heights of the 17A peak of glycerated mont12orillonite to the 0.C-10A peak of collapsed montuorill- onite generally is between 4 and 5 for the sample mounts used (between 1 and 2.5 for London Clay).''

With the''above points in mind, an illustrated worked example may help the reader to follow the method in every detail. The whole rock mineral analysis will be followed by the clay fraction analysis. 171

3) Notes on the calculation of Whole Rock Mineral Percentages 1) All the ma,or peaks, which represent the nine mineral groups tabulated, Table 11.2, are accounted for so that these minerals are the only ones on the trace and the sum of their percentages must equal 100.

2) The smooth background (see fig.11.1) is drawn in by hand and may be seen to pass through the lowest intensities reached throughout the trace.

3) A qualitative interpretation may already be made of the clay minerals present but only a total clay percentage is calculated at this stage.

4) The 21026 quartz peak is used to measure the peak height of this mineral as the intensity of the recommended 20.6° peak is always too large. Special tests were carried out (see section on accuracy) to establish the relation between these two peaks with increasing contents of quartz.

5) The third general clay peak at 01.9°20 as mentioned by Schultz (1954) was usually neglected, because after numerous test traces, it failed to improve the accuracy of quartz determinations and required a considerably longer time to run.

n) Peak heights are easily measured using the one-tenth- inch graph recorder paper. Table 11.2 Working Chart for the Calculation of Whole Rock kineral Percentages Mineral Intensity Quotient: Peak Product: Percentage Factor 2000 Height Quotient x Intensity Peak Height Vector •••••••• •

Quartz 2000 1 374 374 54.5 K.Felspar 1000 2 ,.)C 13 2.5 Pla gioclase 1000 2 5.5 13 2.0 Calcite moo 1 13 16 2.5 Mixed Calcite 2000 1 4 4 0.5 Dolomite 2000 1 4.5 4.5 0.5 Siderite 2000 1 2.5 2.5 20.5 Pyrite 250 0C q 16 2.5 General Clay 100 20 12 740 35.0 General Clay 30 33 7 (230) Totals ,0O 100.0 172

4) Notes on the Calculation of Clay Fraction Mineral Percentages Before proceeding with a detailed description of the clay fraction calculations, a few general points of interest with regard to the traces should be noted. Qualitative interpretation of the traces is reasonably straightforward in that trace 1 (fig.11.2) shows a poorlSrserystalline kaolinite and reasonably pure illite with some montmorillonite mineral also present. Trace 2 (fig.11.2) confirms the presence of illite but shows it to be structurally imperfect or poorly crystalline by its low height: area ratio. The slight asymmetry to the high angle side is common and merely confirms the previous statement regarding structural imperfection: No low angle "tail" is present so that hydro-illite is not present in significant amounts. A reasonably symmetrical but low height: area 17A peak confirms the presence of fairly pure (not mixed layer) montmorillonite, but the mineral is in a poorly crystalline state. A broader less symmetrical peak probably of smaller d spacing would have suggested the presence of a mixed layer mineral. Trace 3 (fig.11.2) shows the total collapse of the montmorillonite to a 1CA spacing as predicted. The kaolinite remains unaltered showing reasonable thermal stability. However, a small 6.50 26 peak makes its appearance now that the overlying montmorillonite peak is removed and this may be interpreted either as pure chlorite or as the remnant of a collapsed montmorillonite-chlorite mixture, i.e. a brucite interlayered component. Trace 4 (fig.11.2) shows the dramatic and total collapse of the kaolinitic structure at 55C°C and the slight increase in intensity of the 6.50 peak. This increase is commonly associated with pure chlorite and the previous, whole rock, observation of both basal and non-basal peaks at typical chlorite spacings tends to confirm the existence of the pure mineral. A montmorillonite-chlorite interlayer would display a glycemted peak nearer 15A than the one observed in these traces.

The location of the baseline is fairly important in these calculations but it does not assume a critical role if its construction is standardized and the fundamentals of its positioning understood. The first and most obvious feature of the baseline is the high intensity it assumes at very low 26 angles. This is only to be expected as the X-Rays are being projected almost without diffraction straight into the counter. 173

As the 20 angle increases the intensity decreases dramatically until, in theory, all anomalous low angle effects disappear at around 8°28. Cf course, this effect cannot be seen in full or its endpoint as the counter has picked up diffraction counts from the montmorillonite mineral already. The best "flexicurve" substitute for this background is to use the 55C°C trace and extend both the low angle downward curve and the 1:.° to 14° 206 straight line portion to meet each other in a smooth curve. The downward curve should just about grade into the straight line section at about 8° 2e. This produces a curve which separates from the trace at about 4° to 5° 26 usually with a small but noticeable "kick" in intensity. The 55C°C trace is chosen for the initial curve construction, before fitting the same curve to the other traces, as it presents the shortest unknown distance of background, roughly from 120 to 4° which has to be drawn in. This is because the kaolinite peak at about 12.5° is missing in this high temperature trace. See figure 11.2.

As measurements are sometimes necessary for both peak heights and areas, on closely adjacent and interfering peaks, or if it is necessary to extend peak flanks until they intersect the background line a knowledge of peak interference patterns is essential. This comprises a complete subject in itself and numerous mathematical solutions have been proposed. (Klug and Alexander,1954). For the purposes of this study, all that is necessary is the application and understanding of the following notes and diagrams. Development of the interference patterns shown (fig.11.3) is based on the definition of background as indicating where a trace would be if the mineral producing the peak were absent. Also it must be remembered that the X-Ray trace is the summation of all pulses registered by the counter, these include background, individual peaks and interference between peaks. Thus when peaks are closely adjacent, the X-Ray trace between these peaks is the sum of the counts received from both the flanks. A study of the hand drawn interference diagrams might prov e interesting and illustrative to the reader and help ellucidate some of the interpretations made herein.

With the main points of the quantitative description complete INTENSITY

1 1 1 1 1 1 12 10 8 6 4 2

20 (DEGREES) STAT RUN $ 2 OR.

FIG. II-3 DIAGRAM SHOWING COMPLETE RESOLUTION FOR A LONDON CLAY MINUS 2 MICRON FRACTION 174 a step by step explanation of the calculations will be followed through, using the figures gained from the traces shown in figure 11.3.

1)Measure the areas (by the technique described above) of the 7A air dried, 10A glycerated and 10A 360°C peaks as well as the heights of the air dried 7A, 10A 360°C, 14A 55C°C and 17A glycerated peaks.

7 ht. 7 area 1C ht. 10 area 14 ht. 17 ht. Nor. 35 60 Gly. 34 90 37 36Co 63 165 55C° 5

2)Calculate the kaolinite peak shape ratio : i.e. 7 area = 60 - 1.7 which indicates a poorly crystalline 7 ht. kaolinite. 3)Using the following table (adapted from Schultz, 1960, fig.2) find the correction factor for this crystallinity of kaolinite to adjust its peak area at 7A to a comparable area at 1CA. This is necessary as the intensity (and therefore peak area) of any diffractometer output varies with 20.

Crystallinity Factor Intensity Correction i.e. 7A Area 'la°. i.e. 7A Area Naos 7A Ht. Itao. 1CA Area 111. 1.2 2.5 1.3 2.0 1.4 1.5 1.5 1.4 1.6 1.3 1.7 1.7 1.8 1.1 1.9 1.1 -.0 1.0 The 7A area is adjusted to the 1CA area as the 10A region of the trace is used as standard and all other regions are adjusted to it to facilitate uniformity of intensity.

Therefore, the corrected 7A area = 60 = 50 T:2 4)The large twofold division in mineralogy as mentioned earlier 175 is now calculated. The corrected 7A area is made a percentage of the total clay minerals present. The corrected 7A area makes up the combined percentages of the kaolinite and chlorite. Kao. + thi. = corrected 7A Area 0 Xao. + Chl. corrected 7A Area + 10A 360 Area (Ill.+ collapsed mont.) 50 =50 =2 50 + 165 215 The figure 215 represents the total clay area, as all the clay minerals contribute to the sum.

5)The proportion of this n3 percent which is chlorite is now calculated. This is done by multiplying the 23 percent by the peak height of the chlorite, i.e. 55C°14A height, and dividing the product by 1.5 x 7A height. This latter factor accounts for the decrease in height of the 7A peak due to removal of chlorite as has been found by numerous acid treatment tests carried out by Schultz (1964, p.8) and the writer. Chl.=(Kao.+ Chl) x 14A55eheight = 2.3 x 5 = -50_ 1.5 x 7A heigHF 1.5 X 17,5* *

6)The kaolinite percentage is thus the remaining proportion of the 23 percent. i.e. Kao. = 23 - 2.5 = 21.5%

7)The illite percentage is found by dividing the isolated illite peak area, i.e. 1CA glycerol, by the total clay area. Ill. = 10A glycerol area 90 44:q corrected 7A area + 1CA 360° area = 176 =

6) As with the 7A area in 3), the 17A montmorillonite peak height on the glycerated trace must be adjusted in intensity to what it would be if it fell in the standard 10A region of the trace. This is accomplished by calculating the ratio of the 17A glycerated montmorillonite height to the collapsed (36C°) 1CA montmorillonite height. Mont011atio= 17A glycerated ht. = 17A Mont. 10A 360 ht.-1CA gly.ht. 1CA(Mont + Il1)-10A(If1) = 37 = 1.27 63-34

9) The montmorillonite percentage is obtained by multiplying the 17A glycerol height by the percentage of mineral not accounted for by the kaolinite plus chlorite figure and dividing the 176 product by the montmorillonite ratio times the 10A 360° height. Mont. = 17A glycerol ht. x 0100 Xao. + Chl.) Mont.Ratio x 10A 360- ht. All this equation achieves is to correct the 17A glycerated peak height to an intensity that would be found at the standard 10A region of the trace so that the 10A 36C° height (i.e. collapsed Mont.ht. + Ill.ht.) can be proportioned between the two mineral components. Mont. = 37 x 77 = 2850 - 1.22 x 63 --Mr 35%

10)The mixed layer clay percentage is defined by Schultz (1964) as the difference between 100 and the sum of all the percentages calculated up to point 9. Mixed layer clay = 100-(Xao + Chl + Ill + Mont) = 100-100 = 0% It should be noted that unless a mixed layer mineral is reasonably positively identified in the qualitative interpretation it is not reasonable to accept this percentage, as the sum of the other minerals often varies between 95 and 105 percent.

11)Although vermiculite has not been found to occur in London Clay, a formula for its rough quantitative estimation, should the mineral be positively aualitatively identified, is as follows Verm.= (14.5A Gly.Area x 1 ) Chl% (- Corr, 7A Area + 16% 360° Area.) -(Verm. Area •. total clay area) - Chl% OIrr. to 10A The already established chlorite percentage is subtracted, as the area measured under the 14.5A Glycerated trace represents both chlorite and vermiculite. The 2-2.5 factor is supplied by Schultz (1960).

12)Hydro Illite, as defined by the Japanese authors already mentioned in the qualitative mineralogy, may be roughly estimated using the equation Hydro Illite = Ill% x 1CA low angle tail area 1CA Gly.area No intensity correction factor is necessary here as the peak areas under consideration fall within about 2020 of each other.

13)As far as the non-clay minerals in the clay fraction are concerned, these are easily identified by their sharp peaks and 20 peak positions. Any quantitative work must only be taken 177 as a rough estimate, but a few methods to accomplish this end are available. Firstly, the method of Schultz (1964) for non-clay or whole rock analysis may be attempted on the clay fraction traces. Secondly, as the 3.3A quartz area is purported to have roughly equal intensities to peaks in the standard 1CA region, the following relation may also be tried: Cu.in clay fraction - 3.3A Cu:Area x Ill% 4.2A Cu.Area x Factor 10A diy.AreW 1CA Gly.Area x The latter of these two relations is probably more favourable as the 3.3A qu.Area may be interfered with by the 3.3A Illite peak. The "factor" mentioned is the same one as dealt with in point 4 of "Notes on the Calculation of Whole Rock Mineral Percentages." 178

D.Reproducibility of X-Ray Results The experimental work carried on this aspect enabled the writer to establish a quantitative indication of the magnitude and source of the major reproducibility errors sustained dyring the X-Ray procedure. Research undertaken to evaluate the absolute accuracy of the method is discussed in the following section. As the X-Ray machines used in this study were "service" department ones, used over a two year period and as the quantitative mineralogy should at least be correct relatively, if not absolutely, the importance of quantitative reproducibility data cannot be understated.

The reproducibility or precision errors invoked during X-Ray may either have been long term or short term ones. Account was taken of the former by running the series of tests to be described over a period of few months, hence incorporating within the short term errors any "time effects" which the machines may have undergone. Short term reproducibility errors were introduced by four variables which accounted for any inconsistencies noted in running a single quantitative analysis on both powdered whole rock and oriented clay fraction. The four general variables were :

1)Interpretation: this includes peak area and height as well as base line errors,

2) Machine: variations in response of X-Ray eouipment including gomometer alignment, power source, receiving and recording components,

3)Sample Preparation: these include unoriented powder packing, dispersing and extraction of the clay fraction and the degree of orientation on the porous discs,

4)Sampling: extracting reproducibly a representative sample for analysis.

Data was collected on each of these four variables by repeated analysis of one bulk sample at various stages of the "standard" diffraction procedure. Examination of the flow diagram (fig.11.4) together with the tables (11.3 a to .) of results may aid in an understanding of the tests carried out. BULK SAMPLE

LONDON CLAY STANDARD MINUS 200 MESH

SPLIT AA SPLIT SPLIT S SPLIT W SPLIT V

UNORIENTED AA UNORIENTED Q UNORIENTED S UN ORIENTED W UNORIENTED V

ORIENTED AA ORIENTED Q ORIENTED S ORIENTED W ORIENTED V

ORIENTED 5I ORIENTED S2 ORIENTED 53 ORIENTED S4 ORIENTED S 5

RERUN S 31 RERUN S 32 RERUN S 33 RERUN 534 RERUN 5 35

INTERPRE. S 331 INTERPRE. 5332 INTERPRE,S 333 INTERPRE. S 334 INTERPRE. S 335

• FIG. 11-4 SUBDIVISION OF SIMPLE SAMPLE FOR STATISTICS TESTS 179

It was thus possible to establish the magnitude and relative importance of each of the variables. Two tests on the Wilk sample were carried out first. Both these tests invoked errors from all four variables. The first test comprised quantitative analysis on five different splits of unoriented powder (Table 11.3a). The second comprised quantitative analysis of oriented aggregate from the five different splits (Table 11.3b). Tats three, four and five were all carried out on the minus 2 micron fraction from one split. Test three, which eliminated sampling errors, was an analysis of five different oriented aggregates from one split (See Table 11.3c). In test four, one oriented aggregate was rerun five times, hence avoiding sampling and sample preparation variations (Table 11.3d) and in test five, one set of diffractometer traces from an oriented aggregate was interpreted five times over a period of time thus invoking only interpretation errors (See Table 11.3e).

The means, standard deviations (JnE;'411(Ex)- where n = no. of determinations) ( n(n-1) x = calculated value ), and coefficients of variation ( (standard deviation x 100) ) ( mean ) from each group are given as well as the totals of the standard deviations and variation coefficients.

It can be seen upon examination of the tables that the standard deviations for test one is generally lower per mineral than tect two although the coefficients of variation reverse this trend. Hence it would appear that although the whole rock minerals are more accurately identified, their increased Table 11.3 a Test 1 - Whole Rock a) 5 splits from one bulk sample

Sample No. ('uartz K.Fels Plag. Cal. Mix.Cal. Dol. Pyr. Clay Total Split AA 23.3 3.8 3.5 3.5 3.8 2.8 3.5 55.7 Split P 28.9 2.2 2.5 2.2 1.2 3.6 3.7 55.7 Split S 28.3 1.6 2.4 1.9 2.6 2.7 2.6 57.8 Split W 31.7 2.2 1.2 2.5 2.5 3.3 3.7 52.8 Split V 30.6 1.8 1.6 1.9 3.2 3.8 3.2 54.0 Mean 28.6 2.3 2.2 2.4 2.5 3.2 3.3 55.2 , t Std.Dev. 0.6 0.3 0.2 0.3 0.3 0.2 C.2 C.4 e •. Coef. of Var. 2.2 11.3 1C.1 11.7 12.4 5.3 5.1 1,C 5e.) Table 11.3 b Test 2 - Oriented Discs a) 5 discs from 1 bulk sample split into 5, i.e., discs corresponding to 1)

Sample No. Xcao. Chl. Ill. Mont. Total Cv.AA 20 50 32 Cv.0 (7n 51 29 Ov.5 24 1 50 29 Ov.W 23 48 28

Cv.V 1 42 34 Mean 21.8 1.6 48.2 30.4 Std.Dev. 0.6 0.1 C.8 C.6 2.1 Coeri. of Var.2.7 7.5 1.7 1.8 13.9 180 number over the clay minerals (Table 11.3 b) reduces their quantitative magnitude and therefore causes a greater error in the reproducibility with which they are detected. This applies especially to minerals with less than about ten per cent content.

Considering tests two, three, four and five, i.e. with decreasing source of error, the individual standard deviation and its total generally decrease consistently showing that each error contributes to a total error as might be expected. A slight exception is noted between tests three and four where the totals are roughly similar indicating that the oriented disc sample preparation did not significantly increase the error. The magnitude of this general decrease is a rough measure of the influence of each of the four previously mentioned variables on the reproducibility of the quantitative values. Each variable except perhaps sample preparation appears to add an error of equal magnitude to the method, although sampling error appears to cause the largest variation. The coefficient of variation totals tend to substantiate these conclusions and again illustrates, with the clay minerals this time, that the smaller the amount of a particular mineral the less reliable its quantitative reproducibility.

Schultz (1964) reached the same general conclusions but finds his interpretation errors the 1-!.rgest. He also fails to reach the reproducibility accuracy of the present work. However, from the diffraction traces illustrated (Schultz 1964) the Pierre Shale appears considerably less homogenous than the London CLmy and so the inconsistencies might be explained.

The problem of the fluctuation of the variation coefficient with individual mineral proportions is summarized in figure 11.5 by plotting the coefficient of the minerals for each test against their mean content determined from X-Ray patterns. This figure shows graphically, the increasing total error of the determination as each variable (interpretation, machine, sample preparation and sampling) is introduced into the method of analysis. Also from the asymtotic nature of the graph it is noted that for minerals composing more than about ten percent of the sample, any one single determination will be within three percent of the average of five repeat determinations. The 12

10 - 0

OEFF 8 OF VAR.

6 0 6

2 @ o El A A\ 4> El No (00 e cc,._) I ,IF) , i , 10 20 30 40 50 0 = I RUNS MEAN MINERAL % A=2 RUNS 0 -3 RUNS 0=4 RUNS FIG. 11-5 SUMMARY PLOT OF X-RAY STATISTICS TESTS C )=5 RUNS Table 11.3 c Test 3 - Oriented Discs a) Five oriented discs from one split

Sample No. Kao. Chi. Ill. Mont. Total Ov. 51 23 1 49 27 Ov. 52 23 2 48 24 0v. 53 23 2 46 27 Cv. 54 21 2 52 23 Ov.55 21 2 49 26 Mean 22.2 1.8 48.8 25.4 Std.Dev. C.1 0.1 0.5 0.4 1.1 Coef. of Var. 0.5 5.0 1.0 3.6 8.7 Table 11.3 d Test 4 - Oriented Discs One disc, each treatment run 5 times

Sample No. !lac. Chl. Ill. Mont. Total 1 22 1 39 39 ,-,i n .-.,. 43 35 3 22 2 42 36 4 18 1 43 38 5 20 1 41 38 Mean 20.6 1.4 41.6 37.2 Std.Dev. 0.4 0.1 0.3 0.4 1.2 Coef. of Var. 1.8 7.v ' 0.7 1.1 10.8 Table 11.3 e Test 5 - Oriented Discs Ond disc, one set of treatments, interpreted 5 times

Sample No. Kao. Chl. Ill. Mont. Total A 23 1 42 38 B 22 1 41 39 C 22 1 4C 38 D 22 1 39 40 E 22 1 42 40 Mean 22.2 1 40.8 39.0 Std.Dev. 0.1 0.0 0.3 0.2 G.6 Coef. of Var. 0.45 C.G 0.7 C.0 1.73 181 reproducibility is smaller for amounts less than ten percent indicating that small quantities of non clay minerals are not consistently detected.

An interesting point raised by Schultz (1964) is that large coefficients may be caused by relatively large mineral particles hence the reproducibility of X-Ray intensity relations may be considerably outside the expected limits unless finer grinding or repeated analysis are carried out. The general uniformity and low magnitude of London Clay coefficients shows that the minerals behave like ideal powders.

In an attempt to verify the above statements, a small grinding experiment was set up, a standard sample was air dried, ground to a powdered state and then further ground in a porcelain end grinder for 5, 10, 15, 20 and 25 minutes. After each grinding period, a subsample was removed, packed in a metal holder and X-Rayed. The variation of mineral percent is shown in figure 11.6. It is thus obvious that, allowing for routine error, no significant variation of calculated mineral percent is obtained on extensive grinding. Hence fairly short grinding times were used and a standard of about 10 minutes adopted for the routine analyses.

Even before this long and short term reproducibility study was carried out, the writer decided to initiate a simple, yet thorough, statistical study to examine machine and sample characteristics.

This study relates to unknown factors concerning the metal sample holders, porous tile discs, scanning speeds, time constants, scale factors, etc. (Talvenheino & White 1959). It being as well to start at the beginning and gather elementary, but important, statistical data on each. The program divided itself into three separate studies :1) Diffractometer characteristics, 2) Metal sample holder characteristics, and 3) Porous tile disc characteristics.

1) Diffractometer Characteristics (a) Test M.E.1 This test was run to assess the machine error that arose when neither the sample position or diffractometer controls were altered. Three traces were obtained by rerunning the sample MINERAL

I0 20 30

MINUTES OF GRINDING THE —200* SAMPLE FIG. 11-6 EFFECT OF GRINDING ON THE ESTIMATED MINERAL PERCENTAGE 182 disc through the same 28 angles three times. To reduce baseline errors to virtually zero, the table below shows the high angle results where almost no baseline guesswork was needed.

Sample 12° ht 12° Area 18° Area 16° ht 26° Area ME1-1 4.22 1.7. 1.00 4.22 1.28 ME1-2 4.18 1.74 0.99 4.65 1.67 ME1-3 4.20 1.73 0.98 4.74 1.70 Mean 4.16 1.73 0.99 4.40 1.55 St.Dev. 0.C1 0.01 0.01 C.07 0.07 Coef. of Var.21 1 21 2 4 To gain some idea of low angle baseline errors as well as the machine errors under consideration, some low angle heights and areas were measured : Sample 6° ht 6° Area 9° ht 9° Area ME1-1 81.5 10.57 50.4 2.75 ME1-2 84.0 11.05 50.4 2.70 ME1-3 83.3 11.74 51.0 2.68 Mean 82.6 11.12 50.6 2.71 Std.Dev. 0.55 0.30 0.20 0.05 Coef. of Var.21 3 21 2 The general conclusions are thus that a machine error of less than, or eaual to 2(coef. of var.) seems applicable. Height measurements are not as affected as areas by machine and base line errors and that low angle baseline errors tend to be larger than high angle ones. (b) Test M.E.2 Machine operators have the option of a number of 2e scanning speeds. The writer chose a 2° per minute speed at the outset as the number of samples to be examined would have precluded slower speeds. With this decision made, the time factor could be adjusted to achieve the smoothness of trace required. This test assesses the error that arises when the scanning speed x time constant factor exceeds the theoretically recommended maximum of 2. Sample TX.16cro ht 14%.° Area 18° Area 26° ht 26°Area ME2-1 1 3.54 1.690.81 5.00 1.40 ME2-2 2 .J.2 .6rT.;, 1.980.78 5.33 1.70 ME2-3 3 3.63 1.90 0.87 4.38 1.65 ME2-4 4 3.08 1.87 2.82 3.74 1.66 183

Mean 3.34 1.86 C.82 4.61 1.60 Std.Dev. 0.08 C.04 0.01 0.19 0.04 Coef. of Var. 1 4 2 The heights seem to decrease and the areas increase slightly with time constant. An average coefficient of variation of 2 is very low for T.C. x scanning speed products which are considerably in excess of the recommended. Hence increasing the time constant to obtain smoother traces seems quite perm- issible in practice. (c)Test I.1 This test merely showed that altering the chart speed either stretched or condensed the trace without altering the basic characteristics or ratios. (d)Test 1.n and 1.4 In order to ascertain whether the choice of 20 per minute scanning speed was not giving erroneous results, two sets of scans were run at 1 per minute using both the gas proportion counter and the scintillorneter counter. Scintillometer Counter

Sample Scan Speed6° Area lf°3 ht 120 Area 6° ht 26°Aren

12-1 lo 1.80 1.35 0.46 5.43 1.14

12-2 20 1.Ein 1.45 0.46 5.91 1.06 Mean 1.81 1.4C 0.46 5.67 1.10 St.Dev. 0.05 0.05 0.00 0.10 0.05 Coef. of Var. 3 3 0 A 4 Sample Scan Speed& Area l_tP ht 12° Area 26° ht 26°Area 12-3 1o 2.30 1.5C 0.49 4.70 1.05 I2-4 ° 1.5C C.52 4.40 1.02 Mean 2.33 1.5C 0.51 4.55 1.04 Std.Dev. 0.01 C.CC 20.01 0.05 20.01 Coef. of Var. 21 0 1 1 21 Gas Proportional Counter Sample Scan Speed12° ht lAo o Area 12° Area 26° ht 26°Area 14-1 1° 1.75' 0.61 0.38 2.51 0.75 14-2 oce 1.55 0.62 0.41 2.04 0.75 Mean 1.35 0.62 0.40 2.28 0.75 Std.Dev. 0.11 20.01 0.02 0.22 0.00 Coef. of Var. 7 1 4 10 184

Sample Scan Speed 12° ht 1S° Area 16° Area S6° ht 26°Area 14-3 270 0.85 036 4.80 1.18 14-4 1° 2.70 0.83 0.335.39 1.15 Mean 2.70 0.84 0.35 5.09 1.17 Std.Dev. C.00 0.02 0.0 0.21 000. Coef. of Var. 0 4 4 1 The coefficients of variation are somewhat larger than the previous tests but still do not assume major proportions. These figures are of rather varied magnitude but average 2.4. For some unknown reason the gas proportional counter seems less steady than the scintillometer counter. With the 2!) runs the heights decrease slightly but the areas increase and the totals thus equal out, (e) Test 3.1 As most of the quantitative work is carried out on very slow runs, it was decided to run two sets of slow runs on the two machines, i.e. gas proportional counter and scintillator counter to see if the heights and areas measured (of course subject to the usual interpretation errors themselves) were any more accurate than the previous tests. Machine settings were : 20 Speed/Minute Collinators Tiie Constant 2,0.4.00 o 2 2 100-200 1 1 1 Sample 35,o ht 250 Area 27° ht V,7o Area 3.40 1.95 5.02 2.96 S1-3 3.70 2.13 5.02 2.97 Mean 3.55 2.04 5.G2 .965 Std.Dev. 0.16 0.16 0.00 20.01

Coefl.of Var.5 21 These coefficients, which average about 3.5 show that it is not really possible to take measurements any more accurately on these slow runs then on the faster runs used by the writer.

2) Metal Holder Characteristics (a) Test H.1 A small single sample was cone and trisected, each third being carefully packed in a separate holder. The test thus indicated the reproducibility of the packing, machine and interpretation techniques. 185

Sample 200ht 200 Area 21° ht 31° ht 35° ht K 1.22 0.80 3.26 1.C5 0.76 L 1.17 0.74 3.59 1.00 0.83 M 1.28 0.76 2.73 0.85 0.93 Mean 1.22 C.77 3.16 0.97 0.81 Std.Dev. 0.04 0.02 0.15 0.01 0.04 Coef.of Var 3 2 5 1 4 These coefficients, which average 3, are reasonably uniform and small considering they represent the three variables mentioned. They are smaller than either the Schultz (1964) or the writers previous coefficients and this is somewhat surprising as only heights and areas were considered here. In general they show that the holder preparation technique gives satisfactory reproducibility. (b) Test 11.7 This test was a repeat of two previous tests, using variable time constants and scale factors, but this time with a whole rock, metal holder sample. Sample T.C. 12°ht 12°Area 20°ht 21°ht :CFAr,s 112-2 1 2.07 0.77 1.82 4.44 1.17 H2-3 2 1.93 0.73 1.70 3.88 1.18 Mean 2.00 0.75 1.76 4.16 1.18 Std.Dev. 0.07 0.12 0.07 0.30 1.01 Coeflof Var. 3 6 4 8 1

Sample Scale 12°ht 12°Area 20°ht 21°ht 20°Area Factor 112-1 16(i.)2 1.80 0.88 1.84 3.72 1.40 112-3 8 1.93 0.73 1.70 3.88 1.18 Mean 1.87 0.82 1.77 3.80 1.29 Std.Dev. 0.04 0.02 0.02 C.03 C.04 Coef.of Var. 2 3 1 1 3 The results tend to corroborate the disc sample results as the time constants variation creates more error than the scale factor variation and a lower time constant gives larger peak heights with about the same peak area. The coefficient of variation averages a low figure of 3.2.

3) Porous Disc Reproducibility (a) Test D.1 The uniformity of the oriented clay surface was put to the test when a single disc was rotated through 1200 three times with 186 traces being run at each position. o Sample &Area 9oArea 12 ht 12°Area 18°Areai D1-1 9.27 2.48 3.50 1.55 0.72 D1-2 3.95 2.21 3.34 1.54 0.75 D1-3 9.81 2.83 3.50 1.67 0.80 Mean 9.34 2.66 3.45 1.59 0.76 Std.Dev. 0.22 0.11 0.11 C.03 0.02 Coef.ofVar. 2 4 3 2 3 These figures represent machine, baseline and orientation or disc surface errors. The coefficient averages 2.8, which is only slightly higher or even equal to previous machine and baseline errors. This indicates that a very high degree of uniformity exists over the surface of the porous disc. (b) Test D.2 In this test the reproducibility of three discs made from one solution was considered. The amount of clay sucked onto each disc was varied from being very thin, to thin, to the correct t hickness. The statistics results are as follows Very Thin Sample 4°-99 Area 12°Area 12°ht 21°Area Disc T 1.51 0.30 C.98 0.60 Disc S 2.67 0.34 0.62 0.46 Mean 2.09 0.32 0.80 0.53 Std.Dev. 0.58 0.02 0.09 0.07 Coef.ofVar. 28 7 11 13 Thin Sample 6°Area 12°ht 12°Area 18°Area 25°Area Disc G 3.65 3.95 1.41 0.83 0.64 Disc H 3.18 3.28 1.20 0.67 0.49 Mean 3.42 3.67 1.3C 0.75 C.57 Std.Dev. 0.50 0.51 0.13 0.09 0.07 Coef.ofVar. 14 14 10 12 12 Correct Thickness Sample 6°Area 12°ht 12°Area 18°Area 25oArea Disc D 1.67 3.38 C.97 0.36 0.40 Disc P 1.55 3.00 0.84 0.39 0.4C Mean 1.61 3.19 0.90 0.38 0.40 Std.Dev. C.09 0.19 0.07 0.02 0.00

Coef.ofVar. 5 6 8 14 0 187

These results demonstrate dramatically the reduction of reproducibility errors as the correct thickness of clay is reached. At the correct thickness, an average coefficient of 4 is quite acceptable for the reproduction of discs from one solution. Test D.3 In this test, it was verified that the blank porous discs had no inherent peaks of their own. lee

E.Accuracy This section deals with the methods employed to establish the accuracy and standardization of the quantitative interpretation of the diffractometer traces which were run following the technique of Schultz (1964). In order to do this, it was necessary to use various independent Quantitative methods to "fix" mineral percentages as accurately as possible for comparison and factor adjustment purposes.

As "typical" London Clay appeared to be made up of roughly 55 percent clay minerals, 30 percent quartz, 5-1C percent carbonates and 5-10 percent "others," it became clear that the quartz and carbonate contents were the most easily checked. Hence if the small percentages of "others" were accepted as correct the total clay mineral percentage could be found fairly accurately by difference. This was the underlying reasoning behind the following experiments, although one or two methods to be described indicate the clay mineral percentage directly themselves. It must be emphasized that the writer considers the quartz and clay mineral percentages to be of extreme importance in other parts of this study, hence the extensive efforts to obtain high degrees of accuracy with respect to these minerals. 1) Cuartz Determination The following four methods were attempted solely to determine the percentage auartz in the whole rock samples. The same natural six samples were used each time for comparison purposes. method 11 In 1969, Till and Spears published "an X-Ray diffraction method for determining quartz in sediments which was both rapid and precise, with a coefficient of variation of 1,9 percent. Samples are ignited at 95C°C prior to X-Ray analysis. This removes the interference of clay peaks, increases the relative intensity of the quartz peaks and reduces the initial matrix variation of samples. The peak ratio of quartz (4.2,'6A) to an added internal standard boehmite (6.18A) is measured. Quartz content is obtained from a working curve constructed using similar rocks of known free silica content" which were produced by mixing weighed amounts of crushed quartz to a matrix of London Clay clay fraction (see figure 11.7). The standard working curves for both the copper and cobalt radiations (used to check each other) are presented as well as the published working curve which corresponds extremely well to the writer's

2.0 1 /MIN,110.1-1, TC I, S F4 PEAK HEIGHT It IF • 400 CPS RATIO iED CURVE

QUARTZ

110C H MITE

1.0

20 40 60

FREE SILICA, IN IGNITED SAMPLE (PERCENT)

FIG. 11-7 QUARTZ PERCENTAGE WORKING CURVE 189 copper radiation line (as it should, because Till and Spears (1969) used only copper radiation). These curves (from both radiations) were used to evaluate the unknown quartz percentages in the six "standakd" samples. Quite a large reliance is placed on this method with its close resemblance to the published curve and even although much fewer working curve points were used a coefficient of variation of twice that quoted is probably accurate, i.e. about 4. Method 2 Small amounts of each of the six "standard" samples were placed in the hands of the Imperial College Analytical Services Laboratory for a free quartz determination. This analytical laboratory decided to use an X-Ray diffractometer to do this, but exactly which of the methods (internal standards, external standards, known additives, multi-component mixtures, etc.) was used is not known. The resulting percentages are quoted to an accuracy of about ±5 percent. A high reliance is placed on these results, within the limits of the variations, of course. Method 3 This method was a complete break from the ones previously described and involved "an infra-red technique which may be used for the rapid, accurate determination of quartz in sediments and sedimentary rocks. The technique utilizes the introduction of samples to the spectrophotometer in a solid form as XBr discs, and is made quantitative by controlling the amount of analysis material in each disc," (Chester and Green 1968). A series of standards were prepared by the writer under the supervision of R.Chester containing various proportions of quartz and montmorillonite clay mineral matrix. The standards were then hand ground for a predetermined time (by use of a guiding curve established by the writer) , and the Br (see fig 11.8) discs prepared under a vacuum at 15 tons pressure for minutes. Spectrograms were prepared and a standard curve (fig 11.9) was drawn showing the relationship between the weight of quartz and the absorbance of the 1,1.52 micron band. The absorbance value (A) being found from the transiLission spectrum percentages X and Y (as shown in figure 11.10) using the relation A-10 log 1/T. The standard error of estimate in the infra-red values is quoted as c.723 and the correlation coefficient as C.gge which means that 99 percent of the results are within 13.2 percent of the estimated value in a 100 percent quartz sample. The coefficient of variation of repeatability figure being 4.7 percent. 1.0

0.9

INFRA RED 0.6 ABSORBENCE

0.4

0 0 8 0 0 0.2

1 I 1 5 10 15 20 25 GRINDING TIME (MINS)

FIG. 11-8 GRINDING CURVE FOR A.DBURNETT O.5

INFRA RED 0.4 ABSORBANCE

•3

0.2

1.0/

5 10 15 20 25 30 35 40 45 50 PERCENT QUARTZ FIG. 11-9 STANDARD CURVE FOR QUARTZ DETERMINATION WAVE LENGTH ( )u )

12.0 13.0 14.0 IS •0 1 I I I

100 Y 97.8 TRANSMISSON 0.00966 ABSORBANCE

ABSORBANCE = X- Y PERCENT 0 .44011 70 TRANSMISSION

X 35 -5 TRANSMISSION

--. 0.44977 ABSORBANCE 30

1 I 1

900 800 700

WAVE NUMBER CC M 1 )

FIG. 11-10 TYPICAL INFRA RED SPECTRUM SHOWING THE 12.5p QUARTZ DOUBLET (USING A HILGER AND WATTS H 900 INFRASCON4 SPECTROPLATOMETER WITH 1:1 TRANSMISSON, SCALE, 2 x ENERGY AND 16 MIN. SCAN) Numerous London Clay samples were then analysed, amongst them the six standard samples. The results of these tests by this method gave extremely anomalous results which bore no reserblance to the two previous and the following method. The figures are without exception vastly lower than the apparently more accurate previous values and do not even bear a linear relationship to these values (see fig 11.11). The reason for this discrepancy is unknown and no feasible explanation can be put forward by R.Chester (senior author of the Chester and Green (1968) paper). The fact that the infra-red figures are not even constantly lower than the more reliable X-Ray values (i.e. not linearly proportional) throws great suspicion on the applicability of this technique for use on London Clay. The only possible explanation for the anomalous figures is that the montmorillonite clay matrix used for the standard curve produced vastly different transmissions then the natural London Clay matrix. Method 4 This method, which entails the use of the X-ray diffractometer again, was specially designed by the writer to establish the characteristics of the machines to be used in the routine analysis of the many hundred London Clay samples, while at the same time providing accuracy data for quartz determinations. The technique falls under the heading mentioned previously of "mixtures of known additives." It entails the addition of known weights of pure crushed quartz to the clay fraction (minus micron) of a standard London Clay sample. The weights are all on a 1C5°C basis and the London Clay-clay fraction has the advantages of containing very little quartz to start with and providing the identical matrix material to the majority of samples to be analysed as it is, of course, London Clay itself. Hence matrix problems, which may have thrown method 3) areigh, are eliminated. To provide full and comprehensive data on all running conditions both available Philips machines were used, the older valve machine on copper radiation and the newer transistor machine on cobalt radiation. Each machine also produced traces of the six standard samples on the two most favourable settings, i.e. with time constand and/or scale factor adjustments. The main object was to produce a working curve for each machine to show quartz peak height (or intensity) against percentage quartz. A second objective was to compare the 21° and 26.5° (cu radiation) "0 auartz peak heights on addition of quartz, i.e. through the C-50 percent quartz, standards range. cl) 7

INFRA 20 RED

PERCENT (1) 8

o 5

I0 06

1 10 20 30 40 50

MEAN X-RAY QU PERCENT 4 7 = SAMPLE No. AND % FIG. II-II INFRA RED VERSUS X-RAY QUARTZ PERCENTAGE VARIATION

50 0

40 0

4.0 4.5 QUARTZ 30 0 X CPERC ENT)

4.0 20 0 X

TRANSISTOR MACHINE CO Kot 27 MIN. r-o.H. TO 400 CPS.

3.8 I0 X 0

0= 24°20 X = 31'20

2.7

20 40 60 80

100 200 300 400 PEAK HEIGHT OR INTENSITY FIG 11-12 A) QUARTZ PERCENT VERSUS X- RAY PEAK HEIGHT QUARTZ

CPERCENT)

TRANSISTOR MACHINE CO K° 2/MIN. I 0.1 I TCI 400 CPS

20 40 60 80 PEAK HEIGHT OR INTENSITY OF 24° 20 QU FIG. II-I2 ED QUARTZ PERCENT VERSUS X-RAY PEAK HEIGHT 191

The first machine to be studied was the transistorized machine. Plots were made of the 24° and 31° (to radiation) Pe peaks versus quartz percentage for the machine run at 2°/minute, time constant and 1000 counts ver second as well as 'O/minute, time constant 1 and 400 counts per second. These graphs (fi3s. 11.12a & 11.13a) show reasonable straight line plots for the '24°28 peak heights but rather poor straight line plots for the more intense 31° heights. This rather favours the use of the lower intensity 240 quartz peas which also have the marked advantage of seldom going off the top of the X-Ray trace paper (as the 31° peak often does). Another unusual, anomalous and extremely perturbing feature is that the 31° heights bear no constant relation to the linear 24° heights (figs. 11.3:13 & 11.13b). Thus it does not always appear possible to merely multiply the 24° height by 3.3 (the well known theoretical factor) to arrive at the 31° intensity. The figures next to the X's on the graphs (figs 11.12a & 11.13a) represent the measured ratios of 31° height to ne height and it is seen that they vary from about 3.0 for low quartz percentages to a high of about 4.7 between 20-30 percent quartz and then diminish again to about 3.5 for around 40-50 percent quartz. As the theoretical ratio is 3.0-3.5 and as only ene section of the graph shows higher values the figure of 3.5 has been used in calculations to obtain a 31° quartz peak intensity for use in the whole rock minerals calculations. This is necessary as Schultz (1964) only quotes an intensity factor for the 31° peak and not the 2e, peak. The writer thus deviates from the Schultz (1964) calculation slightly in using the .7,4° quartz peak height (instead of the 31° one) and multiplies this by 3.5 to get a 31° ouartz peak intensity. This machine, with cobalt radiation, has been used about 8C percent of the time in the routine analysis of whole rock minerals while the older valve machine with copper radiation was used for the bulk of the clay fraction mineralogy.

The same experiments and plots were carried out on the older valve machine with copper radiation at 2° per minute, time constant 2 and scale factor 8, as well as at 2° per minute, time constant 2 and scale factor 16. The plots (figs.11,14a b & 11.15 a & b) differ remarkably from the previous ones in that they are not linear but curved showing that the peak height or intensity tends to increase dramatically with larger

3.3 4.0 50 X X 0

3.6

40 X 0

QUARTZ 4.9

XO (PERCENT) 30

4.8

20 X 0

TRANSISTOR MACHINE CO Kcc TC±1000 CPS

3.2 10 X 0 0 = 24° 2e X = 31° 20

10 20 30 40

100 200 PEAK HEIGHT OR INTENSITY FIG. 11-13 A) QUARTZ PERCENT VERSUS X-RAY PEAK HEIGHT 50

QUARTZ 3 CPERCENT

TRANSISTOR MACHINE CO K.c 2/MIN 110.1 -1- ICI 1000 CPS

10 20 30 PEAK HEIGHT OR INTENSITY OF 24° 20 0U. FIG.11-13 B) QUARTZ PERCENT VERSUS X-RAY PEAK HEIGHT 192 quantities of quartz. This is probably because as quartz assumes important proportions the diluting and damping effect of the clay matrix rapidly decreases and one is left with the very intense quartz peaks showing through and these are far more intense than any of the matrix t_inerals. These observations hold valid for both the 21o and n7o quartz peaks although a better curve may possibly be drawn through 21° peak intensities. This machine is thus not very well suited to whole rock work of this nature as working curves need to be constantly consulted (as opposed to the other machine) and checked, and this, over a period of months, becomes tedious and time consuming. Also the matter of ;"7° to 210 peak height ratios is noted, although this time the variable ratio appears to increase from about '.0 for low quartz percentages to 5.5 for percentages averaging 5C (fi;s.11.14a and 11.15a). Hence if this machine is to be used for quartz determinations a rou3h trial calculation must first be carried out to establish the appropriate quartz content and then use must be made of the correct 27° to `"•1° ratio to obtain the best possible quartz percentage (See figs 11.14b and 11.15b).

Cnce again, as in methods 1), 2) and 3), the six "standard" samples of London clay were analysed and this tire the results appeared to be sensible. It must be stressed that although the analytical method described above is normally used to create working curves from which the unknown sample percentages are ascertained, in this case the curves were merely to show up previously unknown machine characteristics and to establish the extremely important 21° to 27° peak ratios of the quartz. With these factors known the use of the correct height ratio is possible, whereafter the straightforward whole rock calculation (as explained above) may be implemented.

These four methods complete the experiments carried out to establish the accuracy of the quartz determination. Method 3), i.e. the infra-red photospectroscopy produced such different results that the total experiment will be d isregarded and the statistics worked out using the results from the other three methods. (See table 11.4) 6.8 50 X 0

6.0 40 X 0

QUARTZ 5 .8 3 X 0 PERCENT

5 • 0 20 X O VALVE MACHINE CU. Kot2"/MIN. I.- 0-1-1° TC2 SF 8

5.5 0 - 21. 20

10 X 0 X - 2T 20

10 20 30 40 100 200 300 400 PEAK HEIGHT OR INTENSITY FIG. I H4 A) QUARTZ PERCENT VERSUS X-RAY PEAK HEIGHT 50 5.5 0 0

5.6 0 0

QUARTZ 27° QU PEAK 30 4.5 0 0 (PERCENT) 21° QU PEAK

20 4.0 0 0 VALVE MACHINE CU K.c 2./MIN. I- 0.1-1 TC2 SF 8

I0 3.0

2 -0

0 0 10 20 30 40 PEAK HEIGHT OR INTENSITY OF 21' 2 A QU. FIG. 11-14 B) QUARTZ PERCENT VERSUS X-RAY PEAK HEIGHT 50 5.5

40 5.0

QUARTZ 27° OU PEAK

30 4.5 21° QU PEAK (PERCENT)

20 4.0 VALVE MACHINE CU K..c. 2./MIN. 1-01-1 TC2 SF 16

0 0 10 3.0

2.0

10 20 PEAK HEIGHT OR INTENSITY OF 21° 20 QU. FIG. 11-15 B) QUARTZ PERCENT VERSUS PEAK HEIGHT

6.5

50 X 0

6.0 40 X 0

5.8 30 X 0

QUARTZ

(pERCENT)

4.7 20 X 0 VALVE MACHINE CU Koc 27M1N. It0.1-1' TC 2 SF 16

4.5

10 X 0 0= 21 29 X - 27 20

2.5

10 20 100 200 PEAK HEIGHT OR INTENSITY FIG. II- I5 A) QUARTZ PERCENT VERSUS X-RAY PEAK HEIGHT Summary of Methods 1), 2) and 4) for Quartz Determinations

Sample Meth 2 Meth 1 Meth 4 Mean Std.Dev. Coef.of Var. Whitecliff 42 47 40(x5.5)41.3 0.5 1.1 Bay WR Whitecliff 48 43 43(x5.5)44.6 1.2 Bay FC Herne Bay 26 32 23(X4.C)n7.0 1.9 7.0 BH 5-4 Aveley 25 37 32(X4.5)30.6 2.5 6.0

Wraysbury 90 25 74(x4.0) 63.0 1.1 4.7 S2:-16 Orford Ness 15 20 70(x3.5)18.3 1.2 6.5 371v No15

Table 11.4 Sample M. 2, M.1 M.4 Writers Mean Std. Coef: Analysis Dev. of Var. Whitecliff 42 .V 36 4C.0 0.6 2.0 Bay WR Whitecliff 48 43 43 36 47.5 1.4 3.3 Bay FC Herne Bay 26 32 23 21 75.5 1.4 5.5 BH 5-4 Aveley 22 25 37 32 29 30.8 1.5 4.7 Wraysbury SO 25 24 20 22.2 0.8 3.4 Z2-16 Orford Ness 15 20 20 22 19.2 0.9 4.5 ap15 193

Hence it is seen that even on use of the above three well known quartz determination methods, a standard deviation of 2.5 is reached with a coefficient of variation of B.0 (See Table 11.4a) The figures for quartz content as deterwined in the course of the writer's routine analysis was then added to the statistics to see if any noticeable variation took place. (These latter figures were obtained from calculations which included all the new factors established during the tests whose description follows this discussion). Table 11.4b shows a decrease in both standard deviation and coefficient of variation indicating that the writer's quartz values fall well within the sample population and thus are not any more erroneous than the other special methods tried.

2) Chlorite Determination Cne of the interesting factors put forward in the clay fraction mineral percentage calculations dealt with the evaluation of the relative amounts of :.,:aolinite and chlorite present. This numerical factor of 1.5 allows for an adjustment of the 14A 550° peak (i.e.chlorite) to the 7A region of the curve and hence estimates that part of the 7A peak (kaolinite and chlorite) that is attributed to reflections from chlorite. The factor is based on a comparison of X-Ray traces of oriented aggregates before and after they are treated with warm 6N hydrochloric acid for a few hours and after being heated to 36C° and 550°. Because of orientation differences in the two slides peak size comparisons were all made relative to the 1CA 300°C reference peak, No other minerals appeared to be affected by the acid treatment.

Some ten samples were studied by this means and although the total amount of chlorite was rather small, less than about 1C percent, the amount of decrease in height of the 7A peak due to removal of chlorite by acid treatment averaged two-thirds (the reciprocal of which is 1.5, i.e. easier for calculation purposes) (as did those of Schultz, 1964) of the height of the chlorite peaks at 14A after heating at 550°C and before treatment with hydrochloric acid. Although the chlorite is only encountered in small quantities in London Clay and hence its calculation is only of relatively minor significance, it is reassuring to note that this mineral behaves in the same manner as that in the Pierre Shale for which the whole clay fraction calculation was 194 designed, hence reaffirming the writer's opinion that the Schultz (1964) method of quantitative analysis is readily applicable to the London Clay.

3) Cation Exchange Capacity The quantitative clay fraction mineralogy is a most important aspect of this regional London Clay study. Although the technique of Schultz (1964) has been followed very closely and the calculations and factors improved upon in certain respects and although the mineralogy of the Pierre Shale and London Clay seems largely complimentary, this does not mean that accuracy may be taken for granted. Schultz (1964) used a relatively easy chemical calculation to check the clay mineralogy and the writer has also carried out such a check. But these tests of accuracy are founded upon principles with well known drawbacks and limitations as well as requiring a full scale elemental chemical analysis of each sample in order to calculate the results. Such analyses reauire highly skilled personnel and are thus very time consuming and expensive to obtain.

A second technique, with at least as good a chance of providing accuracy data, has been thought up and tried by the writer. This involves the use of the total sample cation exchange capacity in order to proportion the percentages of clay minerals present. The argument being that each mineral has a characteristic cation exchange capacity range and that a collbination of these valves in their correct weight proportions should total the whole rock figure. Again there are numerous drawbacks and limitations of which the viritcr is fully aware, however, it is felt that any reasonable check on the quantitative clay mineralogy is worth the effort. The cation exchange capacity determination is also clasically known to be a very laborious and delicate test but a rapid, easy and apparently accurate Methylene Blue Absorption technique described by Nevins and Weintritt (1967) was used by the writer on samples which had already been submitted for classical cation exchange capacity determination. Thus two methods of cation exchange capacity have been attempted as a check against each other and ultimately for use in checking the clay mineral proportions.

The Methylene Blue Absorption technique consists essentially of an aqueous, well dispersed clay slurry b_ing titrated directly with the C.01N methylene blue dye to an endpoint, which was 195 recognised by spot testing the titrated slurry on Whatmans No.1 filter paper for a sign of free dye. (Jones 1964). The cation exchange capacity being calculated by means of the relation- C.E.C. meg./ 1CC x Vol.titrant x Normality leCg. Sample wt. (dry clay) (dry clay) Full disperson was obtained by the addition of 1C ml. British Standard (1377) Na.Hexametaphosphate dispersing agent. It was found that the agent had no effect on the dye, but that without its use floculation often occurred, vastly reducing the c.e.c. obtained. The values found using this technique were then compared with the Imperial College Analytical Services Laboratory results which were quoted to an accuracy of ±1 percent. The latter results were obtained making use of the method given for "Ca" exchange capacity in M.L.Jackson, "Soil Chemical Analysis," (1958). This method basically consists of washing an acidified sample twice in neutral 1N NaOAc. The sample is then washed five times in 114 Ca Cr?. solution (neutral), excess salt being removed by five washings with CC percent acetone until no chlorine is seen in the AgNC3 test. The Ca is replaced by 5 washings with 1N NH4C Ac (neutral) into the supernatant liquid which is then analysed for Ca using a flame spectrometer or titration method. 196

A comparison of the two sets of results, i.e. frob the full chemical determination and the nethylene Slue method may be seen in table 11.5. The figures quoted for a Whitecliff Bay sample are not compared as the full chemical method was carried out on a separated minus micron fraction of the sample while the methylene blue sample was the total rock. This leaves only the anomalous WCR 50 clay fraction values unexplained. An interesting point to note with regard to the Whitecliff Bay Sample is that if quartz is assumed to possess zero cation exchange capacity the proportion of quartz needed to reduce the clay fraction c.e.c. of 45 to the whole rock c.e.c. of 3 is roughly 4C percent, which ties in very well with the 42, percent quartz content established by X-ray diffraction.

Assuming these cation exchange capacity values to be reasonably accurate, proportioning of the clay mineral percentages using their cation exchange capacities as guides can be undertaken. Table 11.6 shows the calculation of the c.e.c. for each sample assuming an individual c.e.c. for kaolinite, chlorite, illite and montmorillionite of 5, 10, 15 and SO respectively. The new total clay mineral percentages are used in the calculations. As can be seen on comparison of the chemical and X-Ray calculated values reasonable agreement is obtained for all but one sample, i.e. the Whitecliff Day whole rock sample. This correlation tends to show that the mineral proportions are roughly correct. This is especially true in the case of montmorillonite which has a c.e.c. of SC and any error would quickly be noticed.

4) Carbonate Determination To check upon the accuracy with which the carbonate minerals were being determined a wet chemistry gasometric method was used to establish carbonate percentages in some whole rock samples. These percentages were then compared with those produced by X-ray diffraction, the same samples being used, of course. This chemical method was developed by Dr. P.Bush of the Geology Department, Imperial College and has a claimed reproducibility and accuracy of about 1 percent.

The sample to be analysed is reacted with hot concentrated phosphoric acid and the gases produced drawn through scrubbing tubes to remove hudrogen sulphide, acid spray and moisture. Table 11.5 Comparison of Cation Exchange Capacities by the Chemical and Methylene Blue Methods

Sample Chemical Methylene Blue Herne Bay BH5-4 47 47 Wraysbury S`A-16 34 Herne Bay MS 26 43 43 Oxford Ness B 12 No.15 55 61 WCR5C Comp.C.F. 44 62 Mean 44.6 49.0 Std.Dev. 6.78 11,38 r = 0.873 r = Coef.. of Correlation Std.Error of difference = 5.9 Difference between Means = 4.4

As the difference between the means does not exceed three times the standard error of the difference, it appears that there is very little difference between the populations. Table 11.6 Cation Exchange Capacities Calculations

Sample New Factors (5) Xao (1C) Chl (15) Ill (CC) Mont Chem Calc. Clay Mins% c.e.c, c.e.c. Whitecliff ±90 17 3 47 33 45 39 Bay -Su Herne Bay 68 19 .7, 51 28 47 43 13H5-4 Wraysbury 70 19 53 26 33 iS S2-16 Herne Bay 6C 24 2e. 5C 24 43 41 MS26 Crford Ness 7S 10 1 43 46 55 32 all!: No.15 W.C.R.50 ±O© 22 ,-,, 43 33 44 35 Su Whitecliff 58 17 3 47 33 32 Bay Whole Rock

c.e.c. met./100g. of clay = 100 x 1 x Vol.titrant x Normality (mect./m1) Samp'e C.14.7J 197

After this cleaning process, the gas is reacted with a measured amount of Oa N barium hydroxide solution. When the reaction is completed, the excess barium hydroxide is back titrated with C.flq hydrochloric acid using thymolphthalin as an indicator. This gives an end point at pH 9.8, so none of the barium carbonate is attacized.

Two sets of determinations were carried out, one by the writer and one by a skilled technician. The results are plotted in the form of X-Ray versus wet chemistry, carbonate percentages. (See figs. 11.16 & 11.17). The points are rather scattered but a linear relationship seems reasonable showing that the X-Ray carbonates at least increase as the chemistry ones do. Exact linear plots are not really essential as the percentages being dealt with are very small. Examination of the graphs allows for an adjustment of the Schultz (1964) carbonate factors to bring diffraction values into line with the more accurate chemical figures. The "carbonate factors" have, in fact, been adjusted to lower the percentage of carbonates found by diffraction, as approximately three and a half times too much carbonate was being computed when use was made of the original factors.

gence, in conclusion to this set of accuracy tests, the writer feels that the carbonate percentages now calculated are quite acceptable from a relative as well as an absolute standpoint and an accuracy error of about T.1.5 percent seems reasonable.

5) Full Chemical Analysis In addition to the above carbonate cheristry tests, it was decided to submit six whole rock samples for a full chemical analysis. The free quartz determinations also having been undertaken on the same samples. Thus the analytical Services Laboratory of Imperial College carried out the required wet chemistry to a total error of ±C.38 weight percent. The analysis results are shown on Table 11.7.

The first point of interest is the loss on ignition percentages, i.e. from 11C°C to lOCC°C. This ignition loss may be due to three causes • a) OH lattice water escapes from virtually all the clay minerals between these temperatures as is shown in typical dehydration curves (e.g. Grim, 1968, p.r8C-284). The percentage loss in weight for the pure minerals varies individually but ranges bet. 5 83 8. ANALYSIS BY A.D. BURNETT

B 0 0 C V z 0 28 / z V CHEMICAL z z cy, V X 17 02 z' CARBONATES 0D V 08 0A V 018

V 0 6 z z z V 042 V V V F ,X 27 O

2 4 6 B 10

X RAY % CARBONATES 0 - BH5 X = MS

(HERNE BAY LOCALITY) FIG. 11-16 CHEMICAL VERSUS X-RAY CARBONATE PERCENTAGES

ANALYSIS BY SKILLED TECHNICIAN

5 V

4 O D CHEMICAL 0 E V 0/0 CARBONATES O A 3 VGP''

B V 0 1

r".

OH Z 1

2 4 6 8 10 X= RAY cro CARBONATES

FIG. II-17 CHEMICAL VERSUS X-RAY CARBONATE PERCENTAGES Table 11.7 Full Chemical Analysis of 6 Samples

A.S.L.No. 6172 6173 6174 6175 6176 6177 Error Sample 196tOpf 1OrteWf Ufirg24Bay peley payebury SficgrgoBss

• Moisturks 2.43 2.12 2.3C 4.43 2.34 2.99 ,, 105-110'0 Loss on Ign. 6.03 6.52 7.56 7.5r 8.44 7.93 +L.C5.2; 110-1C000 8102 62.83 62.40 54.78 55.79 52.98 55.03 +C.05% Ali" C3 15.19 14.53 19.58 18.23 19.60 1(.28 tZ.D57, Fe2C3 6.05 5.74 7.73 6.'19 7.97 7.1C tC.f15% TiC2 C.lf' C.1C C.C8 C.08 0.08 0.Cei tC.01% CaO 2.',.)6 2.89 2.21 1.45 2.04 1.43 -10.06% MgG 2.49 7.53 2.78 3.19 7.94 3.03 "I0.0.24 X20 2.65 3.23 3.25 3.2C 3.56 3.15 4.-0.091'., Na20 0.05 0.07 0.15 0.05 0.24 0.03 1.-C.1.1 Total 100.10 100.13 100.4 100.23 10C.19 1CC.C3 ±0.38% Free Ouartz 42.0 48.0 26.3 25.0 7C.0 14.9 +57J Eel.

Figures quoted in weight percent 198 b) COn gas, formed from the decomposition of carbonates and the boss of volatile constituents may cause a small weight change in the sample. c) Organic material contained in the clay. Values from this source may be obtained by the difference between total loss on ignition and determination of loss of water, sulphur and other inorganic volatiles.

It is thus seen that as loss is sustained from both a) and b) in London Clay, it leaves only a very small percentage available for c). This postulated low quantity of organic material in the natural clay is borne out by carbon determinations carried out by Weir and Catt (1959) who found only 1-2% carbon present in London Clay. The low percentage of organic material is rather fortuitous as it enables better quality X-ray traces, D.T.A. traces and electronmicrographs to be obtained. It also reduces pretreatment losses and problems considerably as well as reducing extraneous cation exchange absorptions.

In order to provide a rough check on the reliability of that X-Ray determined values for total clay, a graph of total clay versus alumina (i.e. Al203) was plotted. This is shown in figure 11.18 as a superimposition on an original Schultz (1964) figure. The reason for this plot is that nearly all of the alumina occurs in the clay minerals with felspar being the only other alumina containing mineral found in the samples. As the felspar percentage rarely exceeds 5 percent the alumina content from this source is less than 1 percent. The London clay values plot roughly on a straight line which lies well above the average ratio of A1203 to total clay line found by Schultz (1964) for Pierre Shale. The points are also noted to fall between the expected range of Al',C3 in illite and the Ar=4,3 content in kaolinite. As each clay mineral contains a varying amount of AlnC3 in its lattice, a more accurate plot is obtained by calculating the amount of A1n3 in each sample using the X-Ray determined clay proportions and plotting this against the Al2C3 content obtained from chemical methods. This plot is shown in figure 11.19. The points form a rough straight line which lies above the 1.1 ratio line and in fact just falls outside the 10 percent error area. Table 11.8 summarises these calculations. Assuming that the proportions of the clay

/ / 20 / 6176 / 617400 / 0 6177 / \O / 0 bl, / / 6175 9-`, i....t, 44/ / .4,1. 0," ,.•,../.. / \)\/ c,s ' rTh'CV / CY/ A,/ Ass. P : ,S41 •k-- / \ 2 \-\ ((,.. --....,/ -1 / 6172 \-' On'/ 0 Cr q. • 15 v4"/ 6173 P\' ‘r / CHEMICAL 0‹< AL 103

(PERCENT) oc-\\- • •o\-'

40 SO 60 70 80 90

X-RAY DETERMINATION FOR TOTAL CLAY

(PERCC EENT)NT)

FIG. I I- IS AL2 03 VERSUS X-RAY CLAY PERCENTAGE

20 6174 / 6177 0 / 0 0 , • 61 76 <2- / .• •• 0 / 18 6175/

/ / / / / / 16 / AL2O 3 6172 / 0 DETERMINED BY / CHEMICAL METHODS 0 / 6173 (PERCENT) 14 / /

1 1 1 10 14 16 18 20 22 AL 2(4 CALCULATED FROM X-RAY DATA (rERCENT) FIG. II-I9 CHEMICAL VERSUS X- RAY AL203 PERCENTAGES Table 11.8

Mineral Percentages in Samples Analysed

Sample Felspar Naolinite Chlorite Illite Mont. 'lay Mins. 6172 Whitecliff Bay W.R. 5 17 3 47 33 47 6173 Whitecliff Bay F.C. 4 17 1 45 37 47 6174 Herne Bay BHS-4 4 19 2 51 28 52 6175 Aveley 22 3 13 c 41 44 60 6176 Wraysbury S2-16 4 19 "e, 53 26 GO 6177 Crford Ness BR1114o.15 7 10 1 43 46 CO Calculation of Alumina 0.412C2) from X-Ray Data

Sample Felspar Xaolinite Chlorite Illite Mont. Total C.F. 6171' Whitecliff Bay 1.1 3.2 C.3 5.9 13or 47 6172 Weathered Rim 1.1 3.6 C.3 7.3 3.4 15.7 57 6.73 Whitecliff Bay C.9 3.2 C.25.7 3.1 13.1 47 6173 Fresh Core C.9 3.6 C.1 6.2 3.6 14.4 53 6174 Herne Bay 0.9 4.4 C.2 7.8 2.9 16.2 58 6174 BH S-4 0.9 4.8 0.2 8.7 3.2 17.3 63 6175 Aveley C.7 3.2 C.3 6.7 4.7 15.6 6',.. 6.75 °"7, C.7 3.6 0.2 7.8 5.2 17.5 7,. 6176 Wraysbury C.9 4.4 C.3 8.6 2.8 17.0 6C 6176 S2-16 C.9 6.1 0.3 11.4 3.7 2 ),.4 CC 6177 Crford Ness 1.5 2.4 C.1 7.0 5.1 13.1 6C 6177 BH19 No.15 1.5 2.8 0.1 8.1 5.8 18.2 7C

Table 11.8 (Continued) 199 minerals are roughly correct and that the chemically determined A1203 content is also correct, it is obvious that too low a total clay percentage was being estimated from the X-Ray traces. It can be readily seen how these graphs (Figs. 11.18 & 11.19) may be used to check the Schultz (1964) X-Ray total clay factors. Calculations by the writer appear to show that a 5-15 percent clay mineral increase would bring all the points well within the 10 percent error area and on the straight line almost coincident with the 1.1 ratio line. Thus all the Ainc3 would be accounted for and the total clay and probably felspar percentages made reasonably accurate, i.e. ± about 7 percent error. It must be stressed that all these accuracy tests have used the quoted Schultz (1964) factors to estimate their applicability to London Clay, and only after various plots or computations has an adjustment to the above factor been suggested and instituted. Hence it is, in this case, that the extra 5-15 percent clay minerals are to be found on X-Ray trace re- calculation using the new or adjusted factors. It is thus advocated that the Schultz factors for clay minerals are altered but also that the new lower carbonate percentages (already discussed) pyrite percentages and felspar percentages (to be discussed) will help make up the required increased clay mineral content. Suitable clay mineral factors are suggested as being 2 Peak Position (Cu fir) Intensity Factor 19.9 90 34.8 54 61.9 27

With those comments on organic carbon and total clay percenta,N, it is now possible to use the full chemical analysis data to obtain a reasonably accurate estimation of illite content. This is made possible because only felspar and illite contain X20. Thus if allowance is made for the reasonably small quantities of felspar xno all the remaining X20 may be allocated to illite. Hence if a sample contains 5 percent felspar and a mixture of X felspar and plagioclose averages 8 percent X1,70 (Schultz 1964), then C.4 percent 1120 should be subtracted from the chemically determined total X20, Now Schultz (1964) suggests that an average illite contains 7 percent X20 while Walker (194$) suggests 3.5 percent. Using an average of 8 percent (by weight) and multiplying this by the Table 11.9

Illite Percentages as Calculated from Chemical and X-Ray Diffraction Results

Sample Clay Min.% Fels.% Chem Ill. X.R.D.I11. 6172 Whitecliff Bay 47 5 18 6172 Weathered Core 47 2.5 6173 Whitecliff Bay 47 4 23 21 6173 Fresh Core 47 r 25 6174 Herne Bay 58 4 24 29 6174 BH 5-4 58 25 6175 Aveley 60 3 24 25 6175 22 60 1.5 6176 Wraysbury SO 4 27 36 6176 32-15 6C 28 6177 Orford Ness 60 7 20 26 6177 BB No.15 6C 3.5 23 200 remaining T)C content en estimate of the illite percent is obtained. Table 11.9 shows the values obtained using both the chemical and X-Ray techniques. Alternative chemical illite percentages are tabulated as it is believed that this table provides proof that use of the Schultz (1964) factors results in an overestimation of the felspar content. Thus adjusting the intensity factor to 2000, i.e. equal to that of quartz, results in a much closer fit to the X-Ray illite percentages. That these two figures for illite are close to each other is quite remarkable and acts as an excellent check that this X-Ray diffraction technique is giving reasonable results even for the clay fraction calculations.

In conclusion, a table (11.10 of new, adjusted factors for X-Ray quantitative interpretation of London Clay whole rock minerals is presented. This has been used for the routine analysis. These new factors were set up after analysis of the described experiments. Table 11.10 Adjusted Factors for X-Ray Quantitative Interpretation of London Clay Whole Rock Minerals

Mineral se Peak Position -e Peak Position Intensity Cu Radiation Co Radiation Factor Quartz 26.6 31.3 2CCC Plagioclase 28.0 32.6 2C,CO X.Felspar 27.5 32.1 20CC Pure Calcite 29.4 34.c 2CCC Mixed Calcite s".9.6-30.0 34.7 2000 Pure Dolomite 31.0 36.2 2CCC Mixed Siderite 31.7-31.9 37.1 2C0C Rhodochrosite 3C.8-31.5 36.0-36.8 3CC Gypsum 11.6 13.5 15CC Pyrite 33.1 38.6 5C0 Clay Minerals 19.9 P3.1 9C it 34.6 41.0 54 vt 61.9 73.5 27 201

6) Summary and Conclusions The main object of this appendix on X-Ray diffraction has been to set out a full but concise description of the techniques used and the reasons for their choice. Much weight has been given to this aspect of the research as it was considered essential that as accurate a mineralogical picture as possible was esttblished in order to make correlations and investigate relationships between geological and soil mechanical parameters. It is hoped that this section therefore fulfils the twofold purpose of recording the techniques used as well as affording the opportunity for precision, accuracy and factorial analysis and adjustment.

The technique set forth by Schultz in 1964 was closely followed, after slight modification, in preference to numerous others reviewed because it was a rapid, accurate, precise and easy to follow mineralogical method of analysis apparently leading to reliable results within the time restriction imposed for the many samples to be examined.

The sampling, sample preparation and treatment technique is recorded for reference and standardization reasons. The relative importance of qualitative and quantitative, whole rock and clay fraction, analysis is emphasized. The former, with its closely related field of mineralogy, being only briefly discussed as it is a well established realm with relatively few difficulties. The quantitative aspect is covered in greater detail and gives worked examples to illustrate salient points.

The reproducibility characteristics of any technique are of prime importance if use is to be made of the relative accuracy of the results. Hence there would be little use in comparing the results of one sahple to its predecessors if there was no certainty as to the reproducibility of that sample itself. Much effort has therefore been put into this aspect and a full analysis og every variable considered significant including machine characteristics, sampling, preparation and interpretation has been investigated and its reproducibility characteristics tabulated and analyzed, use being made of standard deviation and coefficient of variation statistics. The main conclusions reached indicate that each variable or procedure introduced gives rise to further, roughly equally sized errors, that the SPC2 low percentage components are not detected consistently and that for all minerals a coefficient of variation of greater than 3.0 exists for components comprising less than 1C percent of the sample.

Cnce the precision of a method has been established, its accuracy may be determined. This is done by comparing the results obtained from that method with one or more sets of results from more reliable techniques. Hence the values are compared in an absolute sense and it thus becomes possible to adjust the less accurate results toward the more correct values. This technique was carried out using auartz, carbonate, total clay and illite. The quartz coefficient of variation appears to be about 6 percent, while that of carbonate and total clay is about 2and 7 percent respectively. These coefficients are close estimates and give a good idea of the accuracy of determination of these minerals. The quantitative analysis clay fraction factor for chlorite was also checked and found to be reasonably close to the one suggested by Schultz (1964) hence the chlorite content, and indirectly the whole technique, seems to withstand rigorous inspection.

The numerous attempts to quantify clay mineralogy have met with grave difficulties and the collection of accuracy data with respect to these minerals is equally difficult. The writer, in introducing cation exchange capacity and wet chemistry data to act as a rough check on the clay mineral proportions, has overcome some of these difficulties and succeeded in establishing that his results are, at worst, in the correct order of magnitude. Indeed, it is thought that the total clay, illite and chlorite values all lie within a ±1C percent error. This leaves the important kaolinite and montmorillonite minerals unaccounted for but the cation exchange capacity calculations tend to show that these mineral proportions are also reagbnably accurate: -'_

In conclusion, it is considered that the technique chosen, described and developed herein is the most rapid, accurate and precise available for the examination of many hundred London Clay samples under the conditions imposed. 203

Acknowledgements The writer wishes to acknowledge and thank Dr. P.G.Fookes for suggesting the topic of study and the constant attention, supervision and encouragement he has shown throughout the duration of these studies at the Royal School of Mines.

To the Soil Mechanics Division of Imperial College, particularly Dr. J.Huthinson, Dr.J.Tchalenko, Dr.P.Vaughan and Dr.R.Chandler, the writer expresses his thanks and appreciation for sample sections received and the use of unpublished test data, also to Dr,D.Petley of Portsmouth College of Technology for use of some unpublished test results.

Many of the geotechnical data were obtained and used with the kind co-operation and permission of the following organizat- ions:- Soil Mechanics Ltd., Binnie and Partners, Greater London Council-Soils Dept., McKinney Foundations Ltd. and Nuttall Geo- technical Services Ltd.

Palaeontology knowledge has played an important role in this study and the writer is indebted and grateful to Dr.G.Williams for permission to analyse many of his London Clay samples as well as to use and publish his zonation system of this deposit, albeit in a slightly simp lifted form. Thanks are also expressed to Mr.C.King of the Palaeontology section of the Geology Dept. at Imperial College for palaeontological help.

Appreciation of the co-operation and help received from the service sections of the Geology Dept. is also acknowledged. These sections and personnel include Mr.R.Curtis and D.Bird of the X-Ray laboratory, Dr.P.Bush and R.Edwards of the Sedimentology section, Dr.M.Muir and P.Grant in connection with the scanning electron microscope.

For help and discussion throughout this study, the writer thanks Dr.J.L.Xnill, Head of the Engineering Geology Division and his past and present colleagues in the Division, especially Dr.B.Denness, Messrs. LC.Sanders, I.Watson, T.Mellors, J.Jackson, J.Huntington, M.de Freitas and R.Best. In particular, Dr.J. Ferguson is to be specially thanked for the time spent and interest shown in respect to the computing techniques used in the study. 204

For permission to use 12 official photographs the writer thanks the Director of the Institute of Geological Sciences. The writer lastly acknowledges with thanks the partial sponsorship of both the Council for Scientific and Industrial Research, S.Africa and the Rio Tinto Zinc Corporation, without whose financial help this study would not have been possible. 205

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