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,, Quarrying in Sot11erset . · 1 . .. · ··... .S.uppiement nuniber1 . Hydrology· and Rock Stability-Mendip Hills 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 ' Somerset 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. Carboniferous Limestone 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 <X . 43 5.8a Stable situation for sliding. 44 5.8b Conditions for slidill.g and toppling. 44 5.9 Wedge failure on two intersecting discontinuities'. 45 ~~ I -1 1 Introduction In 1969 the Planning Committee of Somerset Coun._ of the possible financial outlay involved by e. of these dry valley networks culminates in Qleddar Gorge Council instituted a comprehensive survey of the quarrying method. to the south of the region whilst Burrington Combe presents industry in Somerset. The findings of this survey, after a similar pattern to the north. · discussions with all interested parties, were to be used as the This plan of attack was discussed and agreed to at a basis of a policy for the control of quarry development meeting on January 1st, 1973 between Mr. J. Vincent, The other salient feature of the limestone surface is in the county. Chairman of the County Planning Committee, Mr. E.M. the absence of surface water. Surface streams are normally Owens, Lt. Col. A.E.F. Dowse-Benan, Mr. C.L. Keeler and only found draining from the sandstone summits. As soon The report, Quarrying in Somerset, compiled by Mr. the three authors of this report. as tl}e stream courses cross from the sandstone onto the C.L. Keeler was published in 1971 since when it has become limestones of the plateau they immediately disappear under­ clear that there are conflicting interests in land use for ground. •LocaUy the name 'swallet' or 'slocker' is given to agriculture, quarrying, water-gathering grounds and recrea­ 2 Mendip-topography and climate such disappearing streams. Many of the caves of Mendip are tion. 2.1 Introduction associated with .such swallets, the caves following for a limited distance the underground stream courses. Swildon's It seems to be generally agreed that the national need Mendip with its waterless plateau and spectacular Hole, Stoke Lane Slocker and East Twin Swallet are all for crushed stone will continue to increase and that the gorges forms a distinctive landscape region. Its particular examples of such swallet caves. Mendips will be a major source of such stone in view of its charm, history and scenery are all intimately linked to the presence of the particular limestone out of which most of geographical situation. Thus quarrying operations are likely The wate!. draining from the sandstone summits apd to increase in intensity in the future. the region is sculptured. Conflicts of recreational use, groundwater resources, agricultural practice and its value as indeed from the limestone plateau itself re-appears at the large springs, or resurgences, which occur around the One of the possible ways of minimising the impact of a source of good quality stone all stem from the properties of this limestone.

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