GEOLOGICAL AND ENGINEERING CHARACTERISTICS OF SOME KENT BRICKEARTHS A thesis submitted to the University of London (Imperial College of Science and Technology) for the degree of Doctor of Philosophy in the Faculty of Science. by Terence Walter Mellors June 1977 ABSTRACT This thesis presents a study of some geological and engineering characteristics of selected brickearths from Kent in the south- east of England. A morphological and stratigraphic similarity is pointed out between brickearth and the Pleistocene deposit known as loess. Some of the brickearths studied have been described previously as loess. Literature relating to the nature and origin of loess and brickearth is reviewed. In subsequent discussion, a significant engineering characteristic of loess is introduced, namely Lhat it has a meta- stable structure which is susceptible to rapid collapse when wetted under load. The phenomenon of metastability is discussed in terms of soil structure and engineering concepts. It is shown that the collapse in this type of material arises from microshearing at intergranular contacts and that the collapse behaviour apparently contradicts that predicted from consideration of the 'principal of effective stress'. In the light of the above discussion, laboratory tests were carried out to observe the properties of the selected brickearths. Following a description of sampling methods and the sites in Kent from which samples were taken, the laboratory tests are described. Engineering laboratory tests were conducted on samples of brick- earth to establish their index properties, to observe their shear strength characteristics and to observe their compressibility characteristics with particular reference to their susceptibility to collapse. In addition, a series of tests was carried out in which specimens of brickearth were saturated with various fluids while under applied loads in order to investigate the influence of the clay fraction in the soil on the collapse process. Geological studies involved mineralogical analysis and soil fabric observations using thin-section and Scanning Electron Microscope techniques. The test results and observations are presented and discussed. Conclusions are drawn relating to the determined engineering and geological characteristics of the materials and, where appropriate, comparisons are made with published results for loess. ACKNOWLEDGEMENTS The author would like to express his sincere thanks to the following for their advice and assistance during this research project: To Dr. P.G. Fookes and Professor J.L. Knill, Supervisors of the author's research; to the Natural Environment Research Council for financial support; to Dr. D. Shearman, Dr. P. Bush, Dr. H. Shaw and Dr. M.P. Kerney of the Geology Department at Imperial College and to his colleagues in the Engineering Geology Division of Imperial College for many helpful discussions; to Helen Pike for typing this thesis; to Kee for collating this thesis; to many other persons within Imperial College and outside who were connected with various aspects of the study and the production of this thesis. Finally, the author would like to express his gratitude to his wife, Branwen, for her patience and encouragement during the period of the research and production of the thesis. TABLE OF CONTENTS Page No. ABSTRACT ACKNOWLEDGEMENTS CHAPTER 1 INTRODUCTION 2 CHAPTER 2 LOESS 7 2.1 The Nature and Origin of Loess 7 2.2 Mechanism of Loess Formation 14 CHAPTER 3 THE NATURE AND ORIGIN OF BRICKEARTH 28 CHAPTER 4 METASTABLE SOILS - THEIR STRUCTURAL CONCEPT 35 AND ENGINEERING BEHAVIOUR 4.1 Metastable Soils 35 4.2 Soil Structure 36 4.3 The Structure of Metastable Soils, with Particular Reference to Loess 38 4.4 The Engineering Implications of the Collapse Process 40 4.4.1 The Prediction of Soil-Structure Collapse 45 CHAPTER 5 SAMPLING PROCEDURE AND SITES 57 5.1 Sampling Procedures 57 5.2 The Sites 58 5.2.1 Pegwell Bay 58 5.2.2 Pegwell Bay - Buried Channel 61 5.2.3 Ford 63 5.2.4 Sturry 63 5.2.5 Pine Farm Quarry 66 5.2.6 Reculver 67 5.2.7 Northfleet 67 Page No. CHAPTER 6 LABORATORY EXPERIMENTS AND PROCEDURES 75 6.1 Purposes of the Laboratory Work 75 6.2 Engineering Tests and Procedures 76 6.2.1 Tests on Disturbed Samples 76 6.2.2 Tests on Undisturbed Samples 77 6.3 Geological Studies CHAPTER 7 RESULTS OF THE LABORATORY TESTING 89 7.1 Natural Moisture Content 89 7.2 Specific Gravity 89 7.3 Dry Densities 90 7.4 Liquid and Plastic Limits 90 7.5 Mechanical Composition 91 7.6 Oedometer Test Results 94 7.6.1 Pine Farm Quarry 95 7.6.2 Ford 96 7.6.3 Northfleet 97 7.6.4 Pegwell Bay 98 7.6.5 Reculver 99 7.6.6 Sturry 100 7.6.7 Pegwell Bay - Buried Channel 102 7.7 Special Tests in Oedometers 102 7.8 Triaxial Test Results 104 7.8.1 Results from tests on air-dry specimens 106 7.8.2 Results from tests on specimens at field 107 moisture contents 7.9 Direct Shear Test Results 109 7.10 X-Ray Diffraction Studies 110 7.10.1 Whole Rock 110 7.10.2 Clay Fraction 111 Page No. Results of the Fabric Study using the Scanning 7.11 115 Electron Microscope 7.12 Thin Sections 123 CHAPTER 8 DISCUSSION - PART ONE 263 8.1 Introduction 263 8.2 Natural Moisture Content and Saturation State 263 8.3 Density and Porosity 264 8.4 Plasticity 265 8.5 Particle Size Distributions 266 8.6 Compressibility Characteristics 267 8.7 Special Tests in Oedometers 276 8.8 Shear Strength Characteristics 281 8.8.1 Triaxial Test Results 281 8.8.2 Direct Shear Test Results 287 CHAPTER 9 DISCUSSION - PART TWO 302 CHAPTER 10 SUMMARY AND CONCLUSIONS 308 CHAPTER 11 POSTSCRIPT 311 APPENDIX I COMPUTER PROGRAM FOR REDUCING DATA FROM 315 DRAINED TRIAXIAL TESTS REFERENCES 321 The Loess There is a deposit, yclept the Loess, That's puzzled the brains of the savants I guess, It's found in the vale, and a-top of the hill, 'Tis scarcely a clay, and it is not a Till; No gravelly bed do its sections unfold, Nor boulder subangular, flattened or rolled; Strange and unstratified, made to distress, Is this great deposit we call the Loess. Would you make its acquaintance, unwilling to lag Behindhand in knowledge? consult Geo. Mag., Where Howorth has marshalled his facts in a train, This 'rummy' deposit for once to explain, By one great debacle tremenjus of floods. That tore up the soils, and stirred up the muds, Making and mixing a liquefied mess, Which settled and dried, and became the Loess. Bold Baron Richthofen now stalks on the scene, To polish off Howorth, so fresh and so green; The steppes of Mongolia resound to his wain, And he kicks up a whorlwind of dust in his train, Which settles on all things, to prove, if you please, The agent of change is the air and the breeze; 'Tis a dusty deposit, no more and no less, A windy formation this funny Loess. Should it prove due to the flood or the gale, Or the Champions dusty or muddy prevail, To either result we may well be resigned, When with instruction amusements combined, So ye keep well your temper and never grow cross, Though hit with the bones of the Mammoth or Bos, We'll fail not to cheer and cease not to bless That pregnant deposit yclept the Loess. A. CONIFER. Hardwicke's Science-Gossip. 1882 Vol. 18. p.199-200. 2 CHAPTER ONE INTRODUCTION In northern Europe the concept of multiple glaciation during the Pleistocene became accepted by most geologists in the latter half of the nineteenth century to explain the intercalation of the various deposits associated with this period. Since then, a great deal of research has been carried out into the effects of glaciation, much of which has been devoted to studies of the chronology of the Pleistocene as revealed in the stratigraphy of glacial deposits on land and in the sedimentary record in oceanic areas as well as the effects of glacial action on the morphology of the landscape. Discussions of these studies can be found in Charlesworth (1957), Embleton and King (1968) and, with particular reference to the British Isles, West (1968). There are areas of the world which were never covered by ice during the Pleistocene but, nevertheless, were affected by the climatic conditions and processes induced by the presence of near-by ice-sheets. These "extra-glacial" areas are referred to as "periglacial" areas. The term periglacial, first proposed by Lozinski (1912), refers in the strictest sense to the zone peripheral to an ice-sheet or glacier but it is more usual to apply the term to the processes and, more generally, to the environment associated with the climatic conditions of such a zone. It is, perhaps, less acceptable to use the term (as some authors have done) to describe climatic conditions in the peripheral zone unless they are arbitrarily defined since they could vary significantly from place to place. For instance, the severely dry, cold climate of the periglacial zone during the Weichsel maximum (the last glacial stage of the Pleistocene) in European Russia must have been very different from the more maritime climate in southern Britain at the edge of the continental ice mass (Embleton and King, 1968). The essential requirement of a periglacial zone is the presence of frozen ground, either as permafrost or as a seasonally occurring phenomenon, so that the extent of the zone can be considered as being between an inner boundary defined by the current margin of the ice and outer edge which is gradational but mainly influenced by the extent of the frozen ground.