,, Quarrying in Sot11erset . · 1 ...... · ··... .S.uppiement nuniber1

. . . . . Hydrology· and Rock Stability- A Review of Existing Knowledge

.,..

NOTE: this is an extract from the document. The fully scanned version is 33MB in size and is available on request ' County Council Planning Department' ,, · County Hall

.... Taunton 1973 Contents Page Pag,;: Page ,.,. 1 Introduction 4

2 Mendip -topography and climate 4 4.2.3. Effects on the Old Red Sandstone aquifer 32 7.3.3. General remarks 57 4.3. Suspended s.ediments in streams and springs 32 2.1 Introduction 4 4.4. Summary of the deficiencies in knowledge of 2.2 Regional division and description 4 8 Conclusions 57 groundwater hydrology of the Mendip Hills 32 2.3 Climate 4 9 Bibliography 57 3 The geology of the Mendip Hills 5 5 Rock slope stability 35 35 3.1 Introduct10n 5 5.1. Introduction 3.2 Lithology 5 5.2. Stability of rock slopes 35 3.2.1. Introduction 5 5.2.1. Height of quarry faces 35 35 3.2.2. The Silurian 5 5.2.2. Na:ture of sedimentary rocks 3.2.3. The Devonian 5 5.2.3. Factors controlling stability 35 3.2.4. The Carboniferous 5 5.2.4. Friction and cohesion 40 3.2.5. Trias 9 5.2.5. Forces acting on a rock mass - sliding and 3.2.6. Jurassic 9 toppling 40 3.2.7. Superficial deposits 9 5.2.6. Sliding along intersecting discontinuity 3.3. Geological structures 9 surfaces 40 3.3.1 Introduction 9 5.2.7.(i) Factors influencing the shear strength of 3.3.2. Fold structures 9 discontinuity surfaces 40 3.3 .3. Faulting 9 5.2. 7 .lil) Testing of shear strength 46 3.3.4. Joints and Bedding plan~s II 5.2.8. Influence of groundwater 46 3.4. Rock Quality Designation II 5.3. Rock quality at depth - geophysics 49 3.5. Summary of the deficiencies in knowledge of 5.4. Stages in the investigation of rock slope the geology of the Mendip Hills 14 stability 49 5.5. Summary of deficiencies of knowledge on 4 Principles of groundwater 14 rock slope stability 49 hydrology and their application Proposals for future research 54 to the Mendip Hills 6 4.1 Principles and problems 14 6.1. Geology and rock mechanics 54 4.1.1. Introduction: groundwater and porosity 14 6.2. Hydrology 54 4.1.2. The Water Table 14 6.2.1. Silurian, Old Red Sandstone and Lower 54 4.1.3. Aquifers and the movement of groundwater 16 Limestone Shale 4.1.4. Permeability 16 6.2.2. 54 4.1.5. The variable nature of permeability in the 6.3. General remarks 55 Carboniferous Limestone 16 4.1.6. Water tracing maps 21 7 Agencies, organisation and 4.1.7. Average values of permeability for the Carboniferous Limestone 21 costs 55 4.1.8. How much groundwater is there? 23 7.1. Agencies 55 4.1.9. Other factors in the hydrology of the 7.1.1. County Planning Committee and others 55 Mendip Hills 23 7.1.2. Consultants 55 4.2. The possible effects of deep quarries on springs 7.1.3 . Universities 55 and the regional groundwater 23 7.2. Organisation 56 4.2.1. The Carboniferous Limestone aquifer 23 7.2.1. Hydrological investigation 56 4.2.2. Possible ~ffects of deep quarries 28 7.2.2. Geology and rock mechanics 56 (i) General effects 7.2.3. General remarks 56 (ii) Complicating factors: disposal of pumpwater 7.3. Costs 56 (iii) Complicating factors: caves and conduits 7.3.1. Hydrology 56 beneath the water table 32 7.3.2. Geology and rock mechanics 56 Authors

T. C. Atkinson B.Sc., Ph.D~. F.G.S. R.Bradshaw M;Sc., PhD~. MJM.M., F.G.S. D. I. Smith B.Sc., M.Sc., F.R.G.S.

D. Denton-Cox, l" .I(T .P J., F .R.I.C.S., D.P .A., (Lond.) County Planning Officer County Hall · TAUNTON Somerset. ' LISt OT Tlgures Page Page

3.1 Simplified geological map of Mendip and adjacent 5.10 Effect of roughness of joints on shearing strength. 47 ,., ~~ . 6 5.11 Shear strength against displacement illustrating peak :~; ~ 3.2. Key to published geological maps of the Mendip strength and residual strength. 48 Hills 7 5.12 Displacement along bedding planes during folding and 3.3. Stratigraphical column for Mendip 8 along faults. 48 3.4. Diagrammatic section showing different types of 5.13 Correlations between measured properties of rocks folds 10 and engineering parameters. SO 3.5. Sections· across three Mendip anticlines 12 (a) Point Load Index and Uniaxial compressive 3.6. · Blopk diagram of a normal fault 13 strength. · 4.1 Diagram to illustrate the differences between primary (b) Frequency of a wave signal and deformation. and secondary pores I 5 5.14a Differences in water pressure between adjacent rock 4.2. Diagram to illustrate the concept of the water table joints. 51 and its relation to water levels in wells 17 5 .14b Groundwater fluctu;itions in high and low porosity 4.3.a Idealised diagrammatic section showing the Chalk systems. 51 aquifer, water table, springs and aquicludes. 18 5.15. Stages in the analysis of slope stability (modified 4.3.b Diagrammatic section across the Mendip Hills, from Hoek and Bray, 1973). 52 showing the principal aquifers and aquicludes 19 5.16. Map showing postion of quarries and the large area of 4.4. Anisotropy and inhomogeneity of permeability 20 4.5. Diagram to illustrate the principles of pumping tests 22 intervening ground. 53 4.6.a Simplified geology and topography of the Mendip Hills . · 24 4.6,b The underground drainage of the Mendip Hills. 25 4. 7. The underground drainage of the East Mendip Area. 27 4.8 Drainage of the Carboniferous Limestone aquifer. 29 4.9 Drainage of a limestone area via caves, closed depress- ions and fissures. · 30 4.10 Flow diagram of the paths taken by water flowing through the Carboniferous Limestone. 31 4.11 Effect of a deep quarry on the water table and hydrology of the Carboniferous Limeston~ 33 4.12 Effect of a deep quarry intercepting a cave qr conduit. .33 4.13 Diagrammatic section showing relative water levels in the Old Red Sandstone and Carboniferous limestone. , 34 5. I. Cross sections of a quarry showing decrease of area · with depth. 36 5.2. Some types of slope failures in quarries. 37 5.3. Slope height versus slope angle relationsbips for hard- rock -slopes. 38 5.4. Block diagram showing types of discontinuities in rocks. ' ' 39 5.5. Relationship between the shear stress T required to cause sliding along a discontinuity to the normal stress C1 acting across it. . 41 5.6. Forces acting on a block resting on an inclined plane. 42 5. 7. Critical height of a vertical slope containing a planar discontinuity dipping at an angle

l t I. 'I

Fn Rhaetic and Jurassic · ~ Lower Coal Measures Old Red Sandstone

0 2 3 4 5 Kilometre• Trias Millstone Grit Silurian

Alluvium R Upper Coal Measures Carboniferons Limestone Dip of Strata (deg~ees) .... 25 ~ Peat Coal Measures Pennant Sandstone · Lower Limestone Shale -- Spot Height (feet) • 421 ~

~; ·, Figure- 3·1 · Simplified geological map of Mendip and adjacent areas 6- ,.,

Bristol 1 inch to 1 mile Bristol District Special Sheet District 1265 1 1 .. Geological Maps l~ttfi!l 2t ·· rsn s ·· ·· ~

..

.;

.Frome

279 Wells.

15 T• • Q T 1p 1 Ki Ia metres Figure 3 · 2 Key to published geological maps of the Mendip Hills ., 0

' 1!;, Jurassic Mainly limestones

Trias Dolomitic Conglomerate Breccia with sandstone matrix 200 Coal measures Shales, sandstones anc thin coal seams

Quartzitic Sandstone Group Ouartzitic sandstones

400

600 Massive limestones, m3inly composed of calcite. Some dolomite locally and thin

Carboniferous Carboniferous Limestone chert and shale bands 800 "'~ ~ <:::

~~"' 1,000 x 2 a. a.

1,200 Lower Limestone Shale Shales with limestone bands

1.400 Mainly quartzitic sandstones and Devonian (Old Red Sandstone) conglomerates with mudstones of variab thickness 1,600

Silurian 1,800

Lavas and tuffs

2,000

It Figure 3 ·3 Stratigraphic-ai column for·Mend occur and may cause minor variations in the manner tion their void space is very different from 1 much such an anticline explains the surface pattegt of the rocks groundwater circulation. Shale bands become more imparL­ greater than that of the massive Carboniferous Limestones. seen on the geological map. Cross-sections illustrating the ant towards the top of the group and may reach two Thus there is a vast difference between the lithology of the nature of the anticlines are given in Figure 3.5. metres in thickness. Jurassic oolites. and those; e.g. the Burrington Oolite, of Carboniferous age. It can be seen therefore that if the folding is simple However it is possible to take a general view of the dips measured at the surface can be extrapolated downwards Carboniferous Limestone and to describe it as pure, massive, For the purpose of this study the thin cover of to give some idea of the structure at depth. The dips must carbonate rock, which is characterized by the development Jurassic rocks, often limestones, are of importance due to change,however.and in areas of complex folding, wltich may of underground drainage. the effects such a cover may have on the underlying ground­ occur for example in the core of the Beacon Hill anticline, water circulation. it may not be possible accurately to forecast what is happ­ (ii) The Quartzitic Sandstone Group ening at depth Without some sub~surface survey being made. 3.2.7. Superficial deposits It is usually considered that the group of beds known In many places the bedrock geology of the area can be The folding at the end of the Ca:boniferous was in the Bristol region as the Quartzitic Sandstone Group are followed by a period of erosion which resU:.ted in the crests of Millstone Grit age. The maximum thickness attained is masked or in some cases completely hidden under unconsol­ of the anticlines being worn away to a near horizontal only about SOm and the localities where it is found are idated (superficial) deposits, sometimes known as 'drift; or surface. Upon this new land surface the Tr::.assic rocks were limited in area. The lithology is dominated by quartzitic 'head'. For example in the to the south of laid down to be followed slightly later by the deposition sandstones in which the quartz grains are strongly cemented Mendip there are peat and clay deposits completely burying of the Jurassic sediments. A second, but less severe, period by silica. the solid geology. On Mendip there are no thick continuous superficial deposits of this kind and the only barriers to of folding occurred some ~e after the Jurassic sedimenta­ (iii) Coal Measures detailed geological mapping are the soil deposits themselves. tion was complete and this affected both the older rocks and those of Triassic and Jurassic age. It imparted a very sight These rocks are only poorly developed on Mendip 3.3 Geological Structures dip to the younger rocks. itself although they attain a considerable thickness to the north. They consist mainly of shales with interbedded 3.3.1. Introduction 3.3.3. F;~ulting sandstones. The above account has outlined the major stratigraph­ Under certain stress conditions rocks may fracture 3.2.5. Trias ical and lithological aspects of the geology of Mendip. along weij defined plane/ or zones along which movement Normally such rocks are originally laid down in horizontal takes place, the movement varying from a few centimetres On Mendip the Triassic strata are dominantly of a layers known as 'beds' each individual bed being separated . to perhaps hundreds of metres or even kilometres. This type known as the Dolomitic Conglomerate which is from the next by a discontinuity called a 'bedding plane'. movement may be a continuing process taking place in extremely variable in thickness and can be thought of as stages over long periods of time. Such planes of fracture infilling former valleys. Normally the Dolomitic Conglomer­ However few of the beds are now horizontal and the and movement are called faults, a simple case of which is ate forms a thin layer masking rocks of earlier age. It is pattern of outcrops displayed on the geological map is a illustrated in Figure 3.6 where one side of the block can be composed of angular rock fragments usually of Carbonifer­ result of the original sequence of beds having been affected seen to have been downthrown with respect to the other. ous Limestone, but sometimes of Old Red Sandstone, set in by later folding or faulting. a matrix of sandstone cemented together by calcite or Fault planes intersect the surface of the earth in dolomite. Despite its name however the rock is frequently straight or sinuous lines depending on the topography and not dolomitic. Away from the Mendip Hills tlie Trias is of 3.3.2. Fold structures the inclination of the plane. They can be seen on geological a very different type and is termed the Keuper Marl,. a fine The Silurian, Old Red Sandstone and Carboniferous maps, or in sections, as firm lines, often running for long gra.ined rock of mudstone or siltstone type. rocks were subjected to qujte severe pressures late in distances, across which different rock groups ~ brought 3.2.6. Jurassic Carboniferous time. This resulted in the formation of large­ into contaat. A major fault on Mendip is the Emborough arch-like structures known as anticlines separated from each Thrust Fault which is indicated on Figure 3.5. On such a Rocks of Jurassic age are only found in the more other by down-fold structures referred to as synclines (see fault older rocks, in this case the Old Red Sandstone, have eastern parts of Mendip. The strata normally form a cover a Fig. 3.4). On Mendip the anticlines and synclines are not been pushed over younger ones, the Carboniferous Lime­ few metres thick overlying Carboniferous or older rocks. symmetrical forms but show evidence that the fold pressures stone, in zones where strong compressional forces were Many of the quarries working the Carboniferous Limestone came from t he south. This gives the anticlines an asymmet­ acting. The fault in Figure 3.6. and the Ebbor Fault in strip off the Jurassic rocks which form an uneconomic rical form as illustrated in Figure 3.4. The angle which the Figure 3.5. are Normal faults where the beds above the overburden. Variations in the lithology of this thin cover of rock makes with the horizontal is known as the 'dip' and is fault plane have moved downwards relative to those below Jurassic material are great but much of the material is measured in degrees. It is -possible for the rocks to be locally i.e. the younger beds are downthrown. limestone. The Jurassic limestones are of a very different overturned by folding of this kind (see Fig. 3.4). type from the older limestones, being softer, less massive Faults can cause problems in quarries in a number of and often oolitic and rubbly. Oolitic limestones are com· The older rocks of Mendip are folded into a series of different ways. In the first place the grinding movement posed of small, near-spherical, grains of calcium carbonate four such anticlines, those of Blackdown, Beacon Hill, North along the fault plane as the rocks slide past each other may and if these have not been subjected to secondary cementa- Hill and . A geological cross-section drawn across break up the rocks to produce fault breccia or even finer ,,.

,.

Recumbent anticline Overturned anticline Asymmetrical anticline Symmetrical anticline

Symmetrical syncline Asymmetrical syncli ne

Figure 3· 4 Diagrammatic section showing different types of fold: 10 'is, grained fault gauge which is obviously unwanted and close relation between the spacing of joints, the lithology minor joints of small extent, to micrlrfoints which may only deleterious material, Secondly, faults which may be contin­ and the thickness of a bed, In some places, for example, a be a centimetre or so in extent. Even where joints are not uous through hundreds of metres act as major planes of 3m - thick bed 6f sandstone will have joints spaced 3m continuous through a rock they may be replaced along discontinuity ~ong which slip may take place. Thirdly faults apart whereas a 30cm thick bed of the same material in the their length,by others which are slightly offset, i.e. they are are zones of easy passage of solutions so that along them same locality will have-joints spaced at 30cm apart, but a en-echelon and such zones while not so important as master the rocks can be weathered or altered in other ways and coal seam 30cm thick may have joints only *em apart, joints may well be ones of potential weakness. thus much reduced in strength. This is particularly true of limestones which are very prone to chemical attack. Finally, Table 3.1 Terms for joint spacing are given in Table 3.2. again pa]:ticularly in limestones, faults can be zones of preferential solution which are of importance in,determining Beds thickness in em. Descriptive term Table 3.2 the position of lines of subterranean flow. Joint spacing The various geological maps of Mendip show the major fault structures and several minor ones but many 100-1000 Very thick bedded Spacing in em. Descriptive term smaller-scale faults are difficult to pick up in the badly exposed ground and may only be revealed when quarrying Very wide 30-100 Thick bedded takes place. 300 3.3.4. Joints and Bedding Planes Wide 10·30 Medium bedded '"! These are planes of potential weakness in the rock 100 which are important in a detailed structural study. Bedding Med, close planes have been described above and are long established Thin bedded features associated with· the original sedimentation of the 3-10 30 rock, Close They represent a break in sedimentation and may just I-3 Very thin bedded s be a plane separating two beds of the same composition, e.g. Very close limestones, or they may separate beds of differing composi­ tion, say shale and limestone. 0·3-1 Thickly laminated Joints and bedding planes play a major part in the Each bed may vary in thickness from a fraction of a development of underground drainage in the Carboniferous centimetre in shales to more than a metre in the Carbonifer­ 0·1- 0·3 Thinly laminated Limestone. The water movement is initially along these . ous Limestone and Table 3.1 gives one set of terms used for planes of weakness which become enlarged by solutional • beds of varying thickness. Joints may develop in response to tensile stresses set processes. For example caves on Mendip are generally close­ up in the crust either due to contraction of sediments or to ly related to the drainage network of former underground folding,or they may be shear fractures as a result of compre­ flow lines and surveys of the caves clearly sliow the control Depending on the conditions of formation they may ssion or they may even be due to torsional forces. On the exerted by the bedding planes and joints. be fairly smooth planar features extending over large areas whole shear joints are markedly planar features which cut or they may be knobbly and rough and discontinuous. through the body of the rock iriespective of the lithology 3.4. Rock Quality Designation The nature of bedding planes must be closely observed when whereas tension joints are more often irregular and tend to the stability of rock faces and correlation between boreholes be deflected by variations in the lithology, This difference The importance of bedding planes and joints is that they break up the intact rock into sub-units whose size de­ is being determined, has an important bearing on the problem of rock slope stability. pends on the frequency of bedding and joints, A much­ Joints are secondary features formed after sedimenta­ broken rock will obviously have quite different properties tion is complete. They are planes of weakness in the rock In any: one area joints may be of two types- systema­ from a massive one and so some attempt ml:st be made to but differ from faults in that no movement has taken place tic and non-systematic In the former they are planar and defme the quality of a rock by reference to the along the plane of the joint. Some rocks are more suscep­ parallel or sub-parallel and occur in sets. whereas in the the spacing of planar discontinuities. tible to joint formation than others and the frequency of latter they are curved and irregular and are little used in any joints can vary widely, The Carboniferous Limestone dis­ analysis though locally they may be important. One such scheme is that called Rock Quality Designa­ plays a bedding and joint pattern that is normally termed tion, which is based on the quality of cores recovered from massive, i.e. the rock is divided into large blocks by the Joint planes vary in scale from through-going or a borehole, the quality depending on joint frequency, zones bedding and joint planes. master joints which may be traced as single planes cutting of softening and rock type, The RQD' value is obtained by through beds over a thickness of hundreds of metres, major counting pieces of core recovered in lengths greater than As a general rule it can be stated that there is a joints of smaller dimensions but still well-defined structures, 10 em and expressing this as a percentage of the total core 0 0 (/) 0c:: :r...

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"'T1 co-· c ro w. (J) OJ 0 ("') 7' a. coor OJ 3 a OJ :::J 0.., 3 m. ·at c ~ length. An RQD value of 0-25 is very poor, 25-. )Oor, 4.1. Principles and problems of the size of individual rock grains and is related im 50-75 fair, 75-90 good and 90-100 very good. to the pattern of folding and faulting iirthe area. Ther 4.1.1. Introduction: groundwater and porosity almost no estimates for the value of secondary porosi· 3.5. Summary of the deficiencies in knowledge any British rocks, partly because this parameter cannc of the geology of the Mendip Hills It is commonly believed that underground water measured in the laboratory but must be . estimated occurs in large. lakes beneath the surface of the earth, studies in the field. However, an indirectly obtained , although the precise location of these lakes is only vaguely of about 1% for the Carboniferous Limestone near Che (a) The succession of rocks is known in some detail for defined · in the popular imagination. This is not true. is probably fairly typical of limestones in the Carbonift those areas which are extensively quarried, e.g. parts Underground water, or groundwater, is normally found· of the Mendip Hills. If anything, this value is probabl of the Silurian and the Carboniferous Limestone. occupying the voids and spaces between the grains which underestimate. In the Carboniferous Limestone espe1 '(he Old Red Sandstone has been drilled in places as at occur in almost all rocks. These voids are often referred to there is a .great contrast between the sizes of intergra B~ngle Farm, Shearmoor Wood and Spring Gardens as pores, and the proportion of the total voltime of the pores,. which are probably only a few hundredths but over much of the area it is unknown. Similarly rock which they occupy is called the porosity of the rock. millimetre in diameter, and the secondary voids which 1 the Lower Limestone Shale, which is of importance Thus, a porosity of 30%, which would be typical for beach in size from less than a millimetre to several metres. because it separates the Old Red Sandstone and sand, means that 70% of the total bulk is occupied by sand great range in the size of secondary voids is due to th Carboniferous Limestone, is poorly known, particular­ and grains and 30% by the spaces between them. If the rock uble nature of the rock. Lines of easy passage of grc ly the thickness and distribution of the limestone is saturated that is if all of the pores are filled with water, water along joints and bedding planes have been enl: bands. then the porosity determines the volume of water contained by solution. Where the resulting solutional conduits are within each unit volume·of rock. For every metre thickness enough to enter they are called caves, but it is clear It is necessary therefore to establish detailed succ­ of beach sand, then, a layer of water 30 em thick could be there is a continuous spectrum of cavity sizes fron essions by means of cored drilling. extracted, provided that the sand was saturated in the first largest caves to the smallest joint or bedding plane pa place. (b) The general distribution of rock types is known and While typical of the Carboniferous Limestone thus the qverall general structure but there are local The example of beach sand is one in which all of the wide range in the sizes of secondary voids is not fou complexities of structure particularly where shales pores are intergranular in nature. They are simply the the Old Red Sandstone. This is because the quartz of' are interfolded with more m'assive beds. It cannot spaces between the irregular grains of the rock. This inter­ the sandstone is largely composed is almost insolub! be assumed therefore that dips at the surface will granular, or primary porosity varies from 30 to 40% in that joints and bedding planes are enlarged very litt necessarily continue to depth and some attempt loose sediments to almost zero in solid rocks. The reason solution, even though they may be wide enough for must be made to establish ·sub-surface structures. for this reduction in porosity is that solid rocks are solid to penetrate along them. because their constituent grains are bound by a mineral (c) Much of the older bedrock in eastern Mendip is cement which occupies the pores, filling them and reducing These concepts of primary and secondary porosi covered with soil, vegetation and younger rocks so the porosity. The primary porosities of rocks vary widely illustrated in Figure 4.1. · that faults are often difficult to determine. They are between these limits, both from _one lithology to another however often revealed in quarrying and the possible and from place within the outcrop of the same lithology. 4.1.2. The Water Table link-up of fault structures across the area should be Some values for the primary porosity of Mendip rocks are investigated. given in Table 4.1. In nature rocks are seldom sat·.uated with water the surface downwards. If a well is dug or a borehole d (d) If quarrying is to extend to the east of Asham Brook Table 4.1: Primary Porosity of Mendip Rock the hole usually passes through "dry" rocks before re< then the nature, extent and thickness of the over­ a depth at which it fills with water. This is because all burden should be determined. (Mean Values) pores at that and greater depth are filled complete!) water. If an adjacent well is sunk through the same : (e) The maps of the Institute of Geological Sciences Old Red Carboniferous Jurassic Oolitic it too will fill with water at about the same depth. : contain a number of recordings of dips but no indica­ Sandstone Limestone Limestone "dry" part of the strata above the water level in th tions of joints. As a basis for considerations of rock some pores contain water, but not all, This water is p( slope stability which will be discussed in Section 5 6.6% 0.18% 10.4% ting more or less vertically downwards from the s many hu:qdreds of measurements of the orientations Other voids besides intergranular pores occur in most under the force of gravity, and dips of joints, bedding planes and fault surfaces solid rocks. These are mainly the linear and planar voids will need to be made over the whole area and be which occur in joints, bedding planes and sometimes faults, The boundary between saturated rock and th( plotted on maps to show regional trends and distri­ where the opposite sides of the fracture do not fit exactly lying rocks in which some of the pores are air-filled is the watertable . In materials with a high primary po: butions. together. In contrast to intergranular pores, whose size is of like beach sand, the water table for:ns a continuous s the same order as that of the grains making up the rock, from one pore to the next. No matte:: where a well int1 these secondary voids may be quite large, up to several 4 Principles of groudwater hydrology and the water table, it will fill with water until the level millimetres or more across. Their size is largely mdependent 14 their application to the Mendip Hills surface in the well is the same as that of the water . ,~

Primary or intergranular ·pores

Area A reduced to scale on area B

A. Primary porosity in an B. Secondary porosity in a unconsolidated material, solid, fractured rock, e.g . beach sand e.g. sandstone, limestone

Figure4·1 The differences between primary and secondary pores This is illustrated in Figure 4.2. fairly well. It is permeable to the flow· of water. Roc... s like where K is permeability, Q is discharge per unTr and tan a is the Chalk are called aquifers (Latin : tJ4UO , water; fe"e , to the slope of the water table. The smaller the value of perm­ Whereas the water table in granular materials is a bear), whereas imperni'eab!e rocks like the containing s ~ra ta eability, the steeper must be the slope of the water table to continuous surface, in materials with a low primary porosity in F igu.re 4.3.a are called aquicfudes. Figure 4.3.b illustrates produce the same discharge of groundwater. but a well developed system of interconnecting secondary the strata of the Mendip Hill in terms of their water-bearing fractures and voids it is a discontinuous surface. A well properties. The Old Red Sandstone is an aquifer, albeit a The permeability is not only affected by pore size but drilled into fractured material will yield no water unless it poor one. The Lower Limestone Shale is an aquiclud e, and also by porosity. If two rocks have the same pore sizes but intersects a water-filled joint or bedding plane. However, dams back the groundwater in the Old Red Sandstone, so one has twice the porosity of the other (i.e. twice as many provided that the fractures are interconnected and all of that the latter overflows to form springs around the edge pores of the same size) then the rock with the higher por­ roughly the same widt h, water levels will be roughly the of the sandstone outcrop. In contrast, the Carboniferous osity will have a permeability twice that of the lower. same in each well. The dotted line on Figure 4.2 .b shows the Limestone is an extremely permeable aquifer; with springs However, the influence of pore size is much greater tfian level to which water would riSe in a well which intersected which overflow around the edge of the hills. this. A rock with the same porosity as another but with such a water-filled fracture at a depth beneath that level. individual pores twice as big, will have a permeability four Tlris level, which is called the piezometric surface is the To return to the Chalk aquifer shown in Figure 4.3.a, times as large. For this reason the secondary voids Uoints equivalent of the water table in Figure 4.2,a, but represents which is not complicated by the presence of caves and and bedding planes) in a fractured rock exert a vastly greater a discontinuous, skeletal surface in which actual interfaces and conduits, it should be noted that the water table fo­ influence on its permeability than the intergranular (prim­ between air and water are only found in the fractures. Note llows the form of the topography in a subdued way, sloping ary) pores. The secondary voids are often 10 - 100 times that c;me of the wells in Figure 4.2.b is shown as being dry, from areas beneath the high ground towards the springs. It wider than the intergranular pores, even in the Old Red even though its bottom is below the piezometric surface. is this slope which provides the driving force, causing the Sandstone. In the Carboniferous Limestone such features as This is because it does not intercept any fractures below the groundwater to flow laterally. Over a long period of time caves and conduits with dimensions of 1 em to 1 metre or piezometric surface, only above it. the outflow from the springs will exactly balance the input more are 100 to 100,000 times larger than the intergranular from rainfall (minus the losses due to evaporation, water voids, while even the narrowest · fractures are 10 to 100 use by plants, etc.). The slope of the water table is adjusted times wider. Thus in the Carboniferous Limestone the The kind of behaviour described in the preceding so that the rate of lateral movement of groundwater towards permeability is largely controlled by the secondary voids - paragraph is probably typical of parts of the Carboniferous the springs just balances the input of water percolating down the joints and bedding planes and the solutionally enlarged Limestone. However, the position is complicated by the fact from the surface. Note that there are springs on both sides caves and conduits which have formed along them. that some fractures are wider than others because of the of the Chalk upland, and that the water table slopes away differential effects of solution, and also that the Carboni­ ip. both directions from a high point beneath the middle. 4.1.5. The variable nature of permeability in the ferous Limestone is traversed by cave passages and conduits, This high point represents the underground watershed. Carboniferous Limestone both above and below the piezometric surface. Almost Note, too, that it is not necessarily coincident with the One of the chief problems in describing the hydrology nothing is known in detail about the numbers of fractures surface watershed, although in some cases it may be. in the Carboniferous Limestone, their spacing, the frequency of the Carboniferous Limestone in terms of its permeability is that the latter is so variable. This variability is of two of different sizes of fractures and solutional openings, or 4.1.4. Permeability preferred directions in which fractures run. These are ques­ kinds. Firstly, the permeability is not isotropic, that is to tions which can only be answered · by means of detailed The slope which the water table in Figure 4.3.a and say, it is not constant in all directions but vanes depending geological mapping and borehole investigations. 4.3.b must have to maintain the average discharge of the upon the direction in which water is being transmitted. It springs is determined by the ability of the rock to transmit is easy to see why this should be so when it is remembered 4.1.3. Aquifers and the movement of ground­ water. This property, which is called the permeability is that the permeability can be chiefly attributed to water water governed by two factors, the porosity and the sizes of the movement along joints and bedding planes. Bedding planes pores. Water moving through the rock from one pore to an form a series of parallel discontinuities of great lateral Groundwater is not normally static. Rain falling on, adjacent one does not flow freely. Its flow is resisted by a extent, and water will be easily transmitted in directions say, the Chalk uplands of southern Britain infiltrates friction, or viscous drag, exerted on the moving water by the parallel to the planes, giving a high permeability in those through the surface of the soil and chalk and percolates walls of the pores. The smaller the pores, the greater is this directions. Water movement at righi angles to the bedding vertically until it reaches the water table. Once at. the level planes is much more restricted by the lack of suitable of the water table, it flows more or less laterally to the edge retarding force. For this reason, rocks such as clays, which have pores of very small size, do not transmit water easily. fractures and the permeability in that direction is corres­ of the Chalk outcrop where springs rise, fed by the ground­ pondingly low. Joints, too, tend to occur in parallel sets, water. Figure 4.3.a shows this situation, with the positions They are aquicludes. On the other hands, those rocks with larger pores, such as unconsolidated sands and gravels, do giving a higher permeability along the trend of the joints of the springs. Note that the latter are located where older and a lower one across it. This kind of variation in permea­ rocks underlie the Chalk or younger rocks overlie it. These transmit water easily. They are aquifers. The permeabilitv is expressed as the ratio of the discharge of groundwater per bility with the direction of water movement at a single site rocks have rather a low porosity and very small pores, and is called anisotropy. It is illustrated in Figur~ 4.4 consequently do not transmit groundwater easily. The unit cross sectional area to the slope of the water table. groundwater therefore backs up behind them until it over­ Unfortunately almost nothing is known about the Q flows at the surface in the form of springs, In contrast the K degree of anisotropy exhibited by the Carboniferous Lime­ Chalk itself has a fairly high porosity and transmits water tan a stone. To acquire this information detailed studies would f6 "'&,

Wells Wells

Air-filled fractures

Water table

·------­------______..:-_-.:-...:-...:-.=_...:--..=-..=------..:-...=----- ..=-:...:-..:--=----=------~...:- ...:---=-----=-=...=---:....=-.:-_-::. Water-filled fractures ~:§:~~:§::§:~~§:f~i=:§:i=~:§:==~:?==~~~~_: ~~~~~~~~~~~~~~~~~~~- - - !:..:::~~-=:-== ~ ~-==--==-=...::::-::::-::::-::-::~ -==~-:::-:: -= ~~-:-- .. ·:..---~-----"'7--=..-=...'"'::...~-=--::...-=:-::...-----=------·-----=--=..-:...-:...-_-:::...-=..-::..-=...""':· Air-filled pores """"'- Water table

Water-filled pores

A. The water table and well levels in granular materials. B. The "water table" and well levels in massive, fractured rock. e.g. Sand e.g. Parts of the Carboniferous Limestone

Figure 4 ·2 The concept of the water table and it's relation to water level in weiJs 17 \.,.,

Surface drainage divide. Underground drainage divide

.• ' / . t t t ,..+------__+ - ~ ..,.... __,. - -:-- ,.,Y - / -Water- table- - _

Chalk Aquifer

Older Rocks (Aquicludes) Younger Rocks IAquicludes)

18 Figure 4· 3a Idealised section showing features of a Chalk aquifer '"""'

Sea Level

Silurian volcanic rocks. Hydrological function not known. 2 - Old Red Sandstone. Poor aquifer. 3 - Lower Limestone Shale. Does not transmit water, except in local limestone beds. Aquiclude, but may leak water from Old Red Sandstone to Carboniferous Limestone. 4 - Carboniferous Limestone. Solutionally widened fractures and secondary pores. Excellent transmission of water. Aquifer. 5 - Triassic rocks. Mainly marls and mudstones, which act as an aquiclude. Calcareous breccias locally at base, functioning as local extensions of the Carboniferous Limestone aquifer. S2 - Springs from Old Red Sandstone. S3 - Sinks in Carboniferous Limestone. S4 - Springs from Carboniferous Limestone.

Figure 4·3b Section across the Mendip Hills showing the principal aquifers and aquicludes 19 ,..,.,.

A. Properties measured at a single point

'f\~·~ _..~;~ ~A~. )f'tA.~ ¥~r-"'(-4 ~ ~ ~ • t.,; ""..~ J .-. ~ I 4-~,.i )~ !_!~ i ).,t ...... 4.c " ._J .....

ISOTROPIC : Permeability equal in all directions, e.g. Sand ANISOTROPIC : High permeability along the dominant or gravel. fractures (longer arrow), lower permeability at right angles to the dominant fractures (shorter arrow). e.g. Bedded, fractured rocks with higher permeability along the con­ tinuous bedding plane fractures and lower permeability •I along the discontinuous joints. II II I

B. Properties meas.ured over a wide area (K =permeability).

Equal K ... High K ..

HOMOGENOUS : Section through a INHOMOGENOUS : Variable joint HIGHLY IN HOMOGENOUS. : Cavern homogenous aquifer with even joint and spacing producing higher permeability lias extremely high permeability, and h~dding plane spacing and porosity, in areas of greater fracture density. remainder of aquifer has more evenly and equal permeability over wide areas .. e.g. Chalk in southern Britain. distributed permeability. e.g. Possibly parts of Carboniferous Limestone.

20 Figure 4·4 Anisotropy and inhomogeneity of permeability :: ~ have to be made of the geological structure, joint and bedd­ ar,~ variability in permeability in the Carboniferous Lime­ springs of the area are monitored for the presence of the ing plane frequency, direction and width. Knowledge of stone is due to the presence of caves and smaller conduits tracers, using instruments designed for the purpose. When these would enable the directions in which permeability formed by solution. Cavers have explored over 23 Km of a tracer introduced at a particular stream sink reappears at was greatest and least to be predicted at a particular site cave passages all over the Mendip Hills. Most of these haye a spring, a direct connection between the two points is and depth below the surface. However, measurements of the been above the water table but whenever a cave has been proved , and the time elapsed before its reappearance is a absolute value of permeability at such a site would require explored from a stream sink· to its bitter end it has been measure of the velocity of flow. By repeating experiments the use of pumping tests. In these tests a central borehole found to disappear into a sump, or waterfiUed passage. Cave for each sink or group of' sinks in turn, maps like that in is drilled at the site under investigation and a network of divers have followed some of these underwater passages Figure 4.6, which shows the drainage of the whole of the smaller observation boreholes drilled around it, as in Figure further. Effectively, they have been swimming or crawling Mendip Hills, can be built up. Note that the connections 4.5.a. Water is pumped from the central hole to artificially below the water table. Divers have also shown that the water between sinks and springs on Figure 4.6 are shown as lower the water table and create a slope towards the central emerging from springs is also flowing in large cave passages, straight lines. The actual cave passages along which the hole. This slope is measured by observing the relative levels some of which may carry very large.volumes of water. The groundwater flows are probably in the vicinity of the posi­ of the water table in the observation holes and the central· entire flow of the Cheddar spring, for instance, is carried tion of these lines on the ground, but the lines are intended hole. ln the direction of greatest permeability the artificially in a single underground conduit about 3m high and several only to indicate the connection, not the position of a cave produced slope of the water table is least and the drawd9wn metres across. The average flow is 81 million litres per day · or conduit. Thus, the best interpretation which can be made of the water table in the observation holes is greatest. This and in times of flood the current is too strong for a diver to from a water tracing map is that large caves beneath the occurs because the higher permeability in that direction swim against it. water table are most likely to be found in areas in which the means that a smaller slope of the water table is required to numbers of established water tracing lines are greatest. This produce the same groundwater discharge as in the direction There can be little doubt, therefore, that much of the does not, of course, preclude the existence of caves :n which of least permeability. In the latter direction the slope of the groundwater of the Carboniferous Limestone is transmitted no lines are shown, either because investigations are incom­ water tab\e is at its steepest and the drawdown in the obser­ to the springs by means of these water-filled cave passages plete there or because caves exist but have no surface vation holes (Nos. 3 and 7 in Fig. 4.5) is corresponG.ingly or by similar conduits which are too small to enter. Because feeders into which tracers can be introduced. the least. Absolute values of the permeability can be cal­ of their large size these conduits have an extremely high culated from a know.ledge of the pumping rate and the slope permeability, whereas fractured limestone only a few tens In this context it is worth pointing out that the water of the water table. of metres away may have a permeability thousands of tracing map in Figure 4.6 was constructed almost entirely times smaller. These large values of permeability mean that from the results of experiments in which the tracers were Measurements of the anistropy of permeability would the slope of the .water table towards a spring will be much injected into a swallet stream, Some success has attended prove important in the design of deep quarries penetrating Teduced in the vicinity of a cave which feeds that spring. efforts to trace water which enters the limestone as percol­ below the water table. If the permeability is very much This lower slope should give rise to local troughs in the ation (see below). While this technique has not yet been greater in one direction than another the ideal shape for water table over the lines of caves. fully developed, it could well prove fruitful in preparing a quarry is to be greatly elongated in the direction of more detailed water tracing maps for small areas in which greatest permeability and comparatively narrow across it. In The presence or absence of caves beneath the water it is proposed to site quarries. this way the area of quarry wall through which water may table is of the greatest importance in the siting of deep enter will be kept to a minimum, and the pumping rate Figure 4.7 shows the detailed drainage of the East quarries, for if a q1;1arry should intercept a cave or conduit required to keep the quarry dry will be kept to a minimum Mendip area, including those sinks whose resurgence is not a great deal more \Vater will entei it than would do so from also. In addition, of course, a knowledge of the absolute known and those springs whose sources are unknown. The fractured but non-cavernous rocks. Unfortunately the de­ value of permeability will. enable the rate of pumping map is far from complete, and it is highly desirable to con­ tection of caves by means other than direct exploration is required for a quarry of given size and depth to be calcu­ duct further experiments in order to finish it. The values of lated. extremely difficult. Geophysical methods have sometimes such experiments would be greatly enhanced if they could proved successful for the estimation of other rock proper­ be combined with borehole observations and pumping tests The second problem in describing the permeability of ties, but for the detection of caves they have almost always at a network of locations across the area, in order to the Carbqniferous Limestone is that it varies not only with given results which are difficult to interpret and often measure the absolute value of permeability, the level> of the the direCtion of water movement but also from place to contradictory. Even trials o.ver known caves have sometimes water table and their geographical variation. place. The Carboniferous Limestone is not homogeneous yielded negative results! The best that can be done is to with respect to its permeability. Part of this inhomogeneity interpret from water tracing maps the areas in which cave 4.1.7. Average values of permeability for the passages are most likely to occur. is due to variations of fracture spacings from place to place Carboniferous Limestone (Fig. 4.4). These variations are related to geological structure and can be mapped. An accurate map of fracture numbers 4.1.6. Water tracing maps · The preceding pages have shown how difficult it is to and spacings in the limestone would enable some areas of Water-tracing maps show the underground drainage establish any reliable estimates for the permeability of the high or low permeability to be predicted. Quarries would, of patterns of cavernous limestones. They are prepared from Carboniferous Limestone. Even a very large number of course, be best sited in areas of low permability. the results of experiments in which dyes, lycopodium measurements made by means of pumping and water tracing spores, bacteriological or radioactive tracers are intrpduced tests would only indicate the likely values and range of However, by far the greater part of the inhomogeneity into swaUet streams disappearing into the limestone. The permeability to be found in a local area. Even such a coarse 21