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ClRlA C504
London, 1999
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Engineering in glacial tills
N A Trenter
sharing knowledge
building best practice
6 Storey’s Gate, Westminster, London SW1P 3AU TELEPHONE 0171 222 8891 FAX 0171 222 1708 EMAlL
[email protected] WEBSITE www.ciria.0rg.uk
Summary
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Some 60% of the land mass of the UK was covered by Devensian ice, leaving on its retreat a surface largely mantled by glacial tills. Thus, a considerable proportion of construction is built on and in these highly variable materials, which are difficult to sample and test. This CIRIA report draws together current understanding of the origins and formation processes of tills, the landsystems which they create and in which they are found, their distribution within the UK glacial stratigraphy, and how they are classified. This geological background of tills is then linked to their engineering description and classification. The nature of tills as engineering materials is described with reference to typical ranges of their properties. Guidance for engineering in tills covers site investigation, earthworks, shallow and piled foundations, dewatering, tunnelling and landslides. Supported by fourteen case studies, numerous figures and tables, a substantial reference list and glossary, this report provides a practical review of the geology and geotechnics of tills and guidance for engineering practice. N A Trenter Engineering in glacial tills CIRIA Construction Industry Research and Information Association Report C504, 1999 ISBN 0 86017 504 9
0CIRIA 1999
Construction Industry Research and Information Association 6 Storey’s Gate, Westminster, London, S W l P 3AU Telephone 0 171 -222 8891 Facsimile: 0 171 -222 1708 E-mail
[email protected]
Keywords Glacial tills, glacial landsystems, deposition, glacial stratigraphy, site investigation, earthworks, foundations, dewatering, tunnelling, landslides, case studies.
Reader interest
Classification
Geotechnical engineers, engineering geologists, civil and structural engineers, tunnelling engineers, highway engineers, construction professionals
AVAILABILITY CONTENT STATUS USER
Unrestricted Review of available guidance Committee guided Construction professionals
I Published by CIRIA. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, including photocopying or recording, without the written permission of the copyright holder, application for which should be addressed to the publisher. Such written permission must also be obtained before any part of this publication is stored in a retrieval system of any nature.
2
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CIRIA Report C504
Foreword This report is the outcome of CIRlA Research Project 514 Engineering in glacial tills, which was carried out by Sir William Halcrow and Partners Ltd under contract to CIRIA. The report was written by N A Trenter of Halcrow. Dr S L S Wilson was responsible for Section 2 and Dr F S Stewart for Section 3 of the report.
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Following CIRIA's usual practice the research was guided by a steering group which comprised: Mr D P McNicholl Dr J Apted Professor J H Atkinson Dr B G Clarke Mr R J Hutchison Mr Q J Leiper Dr A J Pitchford Mr V Troughton Mr S Walthall Mr P E Wilson Dr M G Winter
Wardell Armstrong Hyder Consulting Limited City University University of Newcastle-upon-Tyne Exploration Associates Ltd Tarmac Construction Ltd CIRIA Stent Foundations Ltd Bechtel Water Technology Highways Agency TRL Scotland
CIRIA's research manager for this project was Mr F M Jardine.
Acknowledgements The project was funded under the Partners in Technology programme of the Construction Directorate of the Department of Environment, Transport and the Regions (formerly Department of the Environment) and by the Highways Agency. CIRIA and Halcrow are grateful for the help given to this project by the funders, by the members of the steering group, and by the many individuals and organisations who were consulted and provided information and material, including: C H Adam J H Atkinson P B Attewell F G Bell R Boorman B G Clarke C P Chiverell D C Curtis C S Eccles R L Edwards C D Eldred J D Findlay R Gardener S Guest D W Hight J M W Holden M T Hutchinson D R Illingworth J A Little CIRIA Report C504
Fugre Scotland Limited City University, London Consulting Engineer University of Natal Wimpey Construction Ltd University of Newcastle-upon-Tyne LTG Ltd Ove Amp and Partners formerly of Soil Mechanics Ltd AMEC Civil Engineering Sir Alexander Gibb and Partners Ltd Stent Foundations Ltd Engineering Geophysicist Rendel Geotechnics Geotechnical Consulting Group Scott Wilson Kirkpatrick and CO Ltd Trafalgar House Technology Westpile Limited University of Paisley 3
J A Lord R J Mair P L Martin W D C Murray J J M Powell M Preene T Roberts E A Snedker L Spasic-Gril J Thompson W A Wallace D Ward I L Whyte R E Williams M G Winter
Ove Arup and Partners Geotechnical Consulting Group Rendel Geotechnics Balfour Beatty Projects and Engineering Ltd. Building Research Establishment W J Associates Limited W J Associates Limited Ove Arup and Partners Sir Alexander Gibb and Partners Ltd Owen Williams Geotechnical Ltd Babtie Group Limited Ove Arup and Partners UMIST Mott MacDonald TRL Scotland
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Professor J A Little of the University of Paisley made available material given in Appendix A under contract to Halcrow. The following organisations kindly provided the case studies outlined in Appendix B: AMEC Civil Engineering Limited Department of the Environment for Northern Ireland Sir Alexander Gibb and Partners Ltd Halcrow
Mott MacDonald Group Ove Arup and Partners Rendel Geotechnics Scott Wilson Kirkpatrick and CO Ltd The University of Newcastle-upon-Tyne
The author is indebted to his colleagues Dr D H Beasley, Dr N J Burt and Mr A S Pycroft for their assistance at various times during the project. Miss B J Hall typed the manuscript and Mr T J Skull drafted the figures. The final report was assembled with the assistance of Mr T D Good of Halcrow Fox. The patient help of Mr I Macey and Ms J Cordrey, librarians at Halcrow, is gratefully acknowledged. Acknowledgement is made to the following organisations who provided and gave CIRIA permission to use the following illustrative material: HAMBREY, M. J. (1994) Glacial environments, UCL Press (London) EYLES, N. (1983) Glacial Geology: an introductionfor engineers and earth scientists, Pergammon (Oxford) EYLES, N. and DEARMAN, W. R. (1981) A glacial terrain map of Figure 2.7 Britain for engineering purposes, Bulletin of the International Association of Engineering Geology, Vol24, pp 173 to 184 LEEDER, M. R. (1982) Sedimentology process and product, Figure 2.8 George Allen and Unwin (London) DEPARTMENT OF THE ENVIRONMENT (DOE) (1994) Landsliding Figure 3.1 in Great Britain, Her Majesty’s Stationery Office (London) Figures 3.2,3.4 and 3.5 JONES, R. L. and KEEN, D. H. (1993) Pleistocene environments in the British Isles, Chapman and Hall (London) DAS, B. M. (1990) Principles of geotechnical engineering (2nd edition), Figure 10.1 PWS - Kent Publishing Company (Boston) TAYLOR, D. W. (1948) Fundamentals of soil mechanics, Figure 10.3 John Wiley and Sons (New York) TOMLINSON, M. J. (1 994) Pile design and construction practice Figure 11.4 (4th edition), E and F N Spon (London) CLOUGH, G. W. and SCHMIDT, B. (1981) So8 clay engineering, Figure 13.1 edited E. W. Brand and R. P. Bremner, Chapter 8, Elsevier (Amsterdam)
Figures 2.2,2.9 to 2.1 1 Figures 2.4 to 2.6
The cover photograph and plates 1 to 6 were reproduced with permission from Glacial deposits in Great Britain and Ireland (1991) edited by J. Ehlers, S. Kozarski, P.L. Gibbard and J. Rose, 589pp, Hfl255K95.00, A.A. Balkema, PO Box 1675, Rotterdam, Netherlands. 4 CIRIA Report C504
Contents LIST OFTABLES. LIST OF FIGURES LIST OF PLATES ABBREVIATIONS NOTATl0N GLOSSARY 1
INTRODUCTION
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1.1 1.2 1.3 1.4 1.5 1.6
2
22 22 22 23 23 24
GEOLOGY OFTILLS: PHYSICAL PROCESSES
25
Glacigenic environment Glacial landsystems 2.2.1 Subglacial landsystem 2.2.2 Supraglacial landsystem 2.2.3 Glaciated valley landsystem Glacial processes 2.3.1 Glacier flow 2.3.2 Debris capture and entrainment 2.3.3 Debris deposition Till deposition 2.4.1 Lodgement tills 2.4.2 Melt-out tills 2.4.3 Flow tills 2.4.4 Deformation tills Till-related landforms Other glacigenic sediments 2.6.1 Glaciofluvial system 2.6.2 Glaciolacustrine system 2.6.3 Glaciomarine system Summary of Section 2
25 28 29 30 31 33 33 33 34 34 34 37 37 38 38 39 40 41 42 44
GEOLOGY OF TILLS: REVIEW OF GLACIAL STRATIGRAPHY IN THE UK
45
2.4
2.5 2.6
2.7
3.1
3.2 3.3 3.4 3.5 3.6
4
21
2.1 2.2
2.3
3
Geological history Techniques of investigation Engineering property characterisation Applicability of methods of analysis and design Parameter selection Difficulties during construction
9 10 13 14 15 17
Anglian 3.1.1 North Sea drift 3. I .2 Lowestoft tills Wolstonian/Paviland Early Devensian Late Devensian Late Devensian - Loch Lomond Summary of Section 3
46 48 48 50 52 52 54 54
ENGINEERING CLASSIFICATION OF TILLS
66
4.1
66 66 68 68 68 68
Till fabric 4.1.1 Depositional fabric - subglacial 4.1.2 Depositional fabric - supraglacial Depositional fabric - glaciated valley 4.1.3 4.1.4 Post-depositional fabric 4.1.5 McGown and Derbyshire classification
CIRIA Report C504
5
4.2
4.3 4.4
4.5 4.6
5
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5.2
5.3
5.4
5.5
5.6
5.7
6.3
6.4
6.5
6.6
a2 a4
a5 85 a7
Drained peak and residual shear strength Drained peak shear strength 5.1 .I 5.1.2 Residual shear strength Effects of depositional processes on tills 5.2.1 Depositional factors Post depositional factors (all landsystems) 5.2.2 5.2.3 Pre-consolidation pressures Permeability and coefficient of consolidation 5.3.1 Permeability 5.3.2 Coefficient of consolidation Compressibility and deformation 5.4.1 Compressibility 5.4.2 Deformation modulus Remoulded and reconstituted samples 5.5.1 Remoulded tests 5.5.2 Reconstituted tests 5.5.3 Other properties 5.5.4 Limitations Properties of till fills 5.6.1 Strength and compressibility 5.6.2 Compaction and permeability Summary of Section 5
aa a9 91 91 92 92 94 97 97 97 102 102 103 104 104 105 105 110 112
113
SITE INVESTIGATION 6.1 6.2
70 70 70 70 70 72 73 76 77 78 a0 81
85
ENGINEERING PROPERTIES OF TILLS 5.1
6
Plasticity and particle size 4.2.1 Subglacial tills 4.2.2 Supraglacial melt-out and flow tills 4.2.3 Glaciated valley lodgement tills Plasticity and the ‘T-line’ concept 4.2.4 Grading and the ‘dominant soil fraction’ concept 4.2.5 Plasticity and grading characteristics of some British tills 4.2.6 Effects of weathering Undrained shear strength 4.4.1 BRE test bed sites Scatter of undrained shear strength results 4.4.2 4.4.3 Representative strength of tills Use of correlations between undrained shear strength and SPT N value Summary of Section 4
113 115 116 116 117
Preliminary investigations Strata definition and investigation methods 6.2.1 Trial pits (trial excavations) 6.2.2 Cable percussion methods 6.2.3 Rotary core methods Sampling 6.3.1 Tube sampling 6.3.2 Rotary methods 6.3.3 Bulk and block sampling In-situ testing 6.4.1 Standard penetration test 6.4.2 Cone penetration test 6.4.3 Pressuremeters Groundwater and permeability Permeability determination from grading tests 6.5.1 6.5.2 In-situ permeability tests 6.5.3 Pumping tests Summary of Section 6
iia
119 120 121 122 123 124 129 131 132 132 132 134
6
CIRIA Report C504 ,
7
7.1
7.2
7.3
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7.4
7.5
8
8.2 8.3
8.4
\
148 148 149 151 151 152 152 152
EMBANKMENTS
153 153 153 155 155 155
Stability analysis 9.1.1 Precedent 9.1.2 Effective stress analysis Failure surfaces Summary of Section 9
SHALLOW FOUNDATIONS 10.1 10.2 10.3 10.4
11
Drainage Till fabric and cutting orientation Stability analysis 8.3.1 Precedent 8.3.2 Total stress analysis 8.3.3 Effective stress analysis Summary of Section 8
9. I
9.2 9.3
10
135 135 135 135 136 136 138 138 138 138 140 141 142 143 143 144 144 145 145 146 146 146 147
Excavation 7.1.1 Misidentification of rockhead 7.1.2 Presence of large boulders 7.1.3 Water-bearing soils and bedrock 7.1.4 Selection of plant Fill acceptability and control 7.2.1 Particle size distribution Water content and plastic limit 7.2.2 7.2.3 Undrained shear strength 7.2.4 Compaction test 7.2.5 Moisture condition value 7.2.6 CBR 7.2.7 Discussion Placement 7.3.1 Handling till mixtures 7.3.2 Handling wet tills 7.3.3 Trafficability Compaction 7.4.1 Nature of till 7.4.2 Type of plant 7.4.3 Layer thickness and number of passes 7.4.4 Water content Summary of Section 7
CUTTINGS 8.1
9
135
EARTHWORKS
Variation in soil type and effects on bearing capacity Variation in soil type and effects on settlement Construction difficulties Summary of Section 10
PILE FOUNDATIONS 1 1.1
11.2 1 1.3
11.4
11.5
Pile selection and design considerations Design depth Mixed successions 11.3.1 Subglacial conditions 11.3.2 Supraglacial conditions 11.3.3 Glaciated valley conditions 11.3.4 End bearing in mixed successions Shaft resistance characteristics in clay tills 11.4.1 Shaft adhesion 1 1.4.2 Shear strength Shaft resistance characteristics in granular tills
CIRIA Report C504
156 157 158 160 160
161 161 162 162 162 165 165 165 166 166 167 167 7
Construction considerations 1 1.6.1 Bored piles 11.6.2 Driven piles 1 1.6.3 CFA piles Summary of Section 11
168 168 169 169 170
GROUNDWATER LOWERING
171
12.1 12.2 12.3 12.4 12.5
171 172 172 173 173
1 1.6
11.7
12
13
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14
TUNNELLING
174
13.1 13.2 13.3 13.4 13.5
174 177 177 178 179
More than one soil type in the face Varying soil thicknesses and rockhead depths Nests of cobbles and boulders Varying groundwater conditions Summary of Section 13
LANDSLIDING
180
14.1
180 182 182 182 182 182 183 184 185 185 186 187
14.2 14.3 14.4
14.5
15
Sump pumping Wellpoints Pumping wells Ejector systems Summary of Section 12
Inland landslides 14.1.1 Complex landslides 14.1.2 Debris flows 14.1.3 Planar slides 14.1.4 Rotational and multiple rotational slides Coastal landslides Contribution of glaciolacustrine deposits to instability of tills Landslide remedial measures 14.4.1 Drainage 14.4.2 Modification of slope profile 14.4.3 Retaining and restraining structures Summary of Section 14
CURRENT CAPABILITIES AND UNCERTAINTIES
188
15.1 15.2 15.3 15.4 15.5
188 188 189 189 189
Observational method and contract procedure Database requirements Sampling methods Laboratory test procedures: remoulding and reconstituting samples Laboratory test procedures: coarse clast-dominant tills
REFERENCES
191
APPENDIX A Site location and sources of plasticity and particle size
209
distribution data (University of Paisley)
APPENDIX B Case Studies: 3
INDEX 8
1 Site Investigation, Chapelcross, Dumfriesshire 2 Site Investigation, north west England Site Investigation, Cumbria 4 Cardiff Gate business park, Cardiff, south Wales Road widening, east midlands area 5 Cuttings for major road projects, north east England 6 7 Bored piling in glacial till deposits, Tyneside 8 Power station, north east England 9 Shaft, north west England 10 Fylde coastal water improvement scheme, Lancashire I I Whitby cliff stabilisation and coast protection, NE England 12 St Dogmaels landslide, Pembrokeshire 13 Site investigations in tills in Northern Ireland 14 The St Clair river tunnel, Ontario to Michigan
213 214 216 218 222 223 224 225 226 232 234 237 239 243 249 252
CIRIA Report C504
Tables Table 2.1
Characteristics of genetic till types
35
Table 2.2
Summary of erosional and depositional landforms
39
Table 3.1
Schematic representation of main glaciations in the UK for Middle to Late Pleistocene
47
Summary of Anglian and equivalent tills in the UK and on the UK continental shelf
57
Summary of Post AngliadSaalian (‘Wolstonian’/Paviland) tills in the UK and on the UK continental shelf
60
Summary Table of Early/Middle Devensian tills in the UK and on the UK continental shelf
62
Summary of Late Devensian tills in the UK and on the UK continental shelf
63
Table 3.6
Summary of Loch Lomond GlaciatiodReadvance tills in the UK
65
Table 4.1
Characteristics and geotechnical properties of glacial tills
69
Table 4.2
Gradational series of till textures
73
Table 4.3
Variation of plasticity data, Kielder dam till
76
Table 4.4
Variation of average till properties with weathering, NE England
77
Table 4.5
f , values for some British tills
83
Table 5.1
Results of large triaxial tests on rockfill
87
Table 5.2
Summary of pre-consolidation pressures measured in oedometer
92
Table 6.1
Guide to selection of sampling methods in glacial tills
119
Table 6.2
Guide to selection of in-situ test methods in glacial tills
122
Table 6.3
Some factors influencing accuracy of SPT results in tills
123
Table 6.4
Characteristics of some pressuremeters in current use in the UK
129
Table 7.1
Advantages and disadvantages of various engineering properties for acceptability and control purposes in tills
137
Table 8.1
Preliminary cutting side slopes in tills
151
Table 9.1
Preliminary embankment side slopes in tills
153
Table 14.1
Types of inland landslides in glacial soils
180
Table 14.2
Types of coastal landslide in glacial soils
183
Table CS8/1
Results of pile tests on north east England tills
227
Table CS 10/1
Ground classification system (GCS) for conditions at tunnel horizon
235
Table CS10/2
Likely behaviour of water-bearing strata in tunnel face identified in Table CSlO/l
236
c,l/N relationships for tills from Northern Ireland
243
Table 3.2 Table 3.3 Table 3.4
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Table 3.5
Table CS13/1
CIRIA Report C504
Figures Figure 2.1 Figure 2.2 Figure 2.3 Figure 2.4 Figure 2.5 Figure 2.6 Figure 2.7 Figure 2.8 Figure 2.9
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Figure 2.10 Figure 2. I 1 Figure 3.1 Figure 3.2 Figure 3.3 Figure 3.4 Figure 3.5 Figure 3.6 Figure 3.7 Figure 4.1 Figure 4.2 Figure 4.3 Figure 4.4 Figure 4 3 a ) Figure 4 3 b ) Figure 4.6 Figure 4.7 Figure 4.8 Figure 4.9 Figure 4.10 Figure 4.1 1 Figure 4.12 Figure 4. I3
10
Evolution of glacigenic deposits Cross-section of typical temperate glacier illustrating the various glacigenic sediments Diagram illustrating examples of the three main components of a landsystem The subglacial landsystem The supraglacial landsystem The glaciated valley landsystem Distribution of glacial landsystems in mainland Britain Example of glaciojluvial deposition (Scott glacier, Alaska) Processes and sedimentary products in glaciolacustrine setting Sediment sources and processes operating in a fjord influenced by grounded tidewater glacier Ice dynamics, sediment sources and sedimentary processes, products and their interpretation at the margin of the Antarctica ice sheet Climate over the past million years based on analysis of oxygen-isotope ratios from deep sea sediment cores Maximum extent and possible southern limit of Anglian Glaciation WolstonianlPaviland GlaciationlMunsterian (Ireland) possible ice coverage and extent Possible EarlylMiddle Devensian Glaciation Maximum limit of Devensian Glaciation and dominant ice flow directions (not all contemporaneous) Model of extent and thickness of the Late Devensian ice sheet in the British Isles Extent and probable general ice direction of Loch Lomond Glaciation in Scotland General description of fabric type in glacial tills Idealised plasticity and grading characteristics of some British lodgement tills Illustration of T-line concept Plasticity data for British tills Ternary textural diagram showing contposition of various British tills using the McGown and Derbyshire (1977)scheme Some typical till gradings Variation of plasticity ( a )and particle size distribution ( b )for tills at Holderness Variation of geotechnical properties in one metre square test section Garston site: undrained shear strength, cu,and liquidity index v depth for till Redcar site: undrained shear strength, cu,and liquidity index v depth for till and associated glacial soil Cowden site: undrained shear strength, cu,and liquidity index v depth for till Effect offailure dejnition on scatter size Effect of specimen size on strength offissured material SPT ‘ N v undrained shear strength, cu,at corresponding depth for glacial till at Chapelcross
26 27 28 29 30 31 32 40 41 42 43 45 49 51 53 55 56 56 67 71 72 72 74 74 75 75 79 79 80 81 82 83
,
CIRIA Report C504
I
i Figure 5.1 Figure 5.2 Figure 5.3 Figure 5.4 Figure 5.5 Figure 5.6 Figure 5.7 Figure 5.8 Figure 5.9 Figure 5.10
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Figure 5.1 1 Figure 5.12 Figure 5.13 Figure 5.14 Figure 5.15 Figure 5.16
Figure 5.17 Figure 5.18 Figure 5.19 Figure 5.20 Figure 5.21
Figure 5.22 Figure 5.23
Figure 6.1 Figure 6.2 Figure 6.3 Figure 6.4 Figure 6.5 Figure 6.6 Figure 6.7 Figure 6.8 Figure 6.9 Figure 6.10
Peak angle of shearing resistance,$:,, v plasticity index for tills Impact of density and percentage fines on angle of shearing resistance,@:, Residual angle of shearing resistance,@',,v plasticity index relationship for tills Diagrammatic representation of depositional and post-depositional processes for glacial tills Diagrammatic representation of some of the factors affecting the consolidation of a till layer (subglacial and glaciated valley landsystem) Stress paths illustrating process offlow till formation Freeze-thaw process and influence on soil consolidation Relationship between specijc volume and permeability for tills Variation of permeability, k , with effective stress, of ,forjssured Scottish till Variation of coeflcient of consolidation, c,, with effective stress, o',for Cow Green and other tills
85 86 88 89 89 90 91 93 94 95 98
Garston site: shear modulus, G , and liquidity index v depth for till Redcar site: shear modulus, G. and liquidity index v depth for till 99 and associated glacial soil Cowden site: shear modulus, G , and liquidity index v depth for till 99 Secant shear modulus, G , normalised with respect to effective overburden pressure a:(, derived from triaxial and plate loading tests on glacial till 100 Resonant column secant shear modulus, G, normalised with respect to Gma,v shear strain results for glacial till 101 Secant shear modulus, G, nornialised with respect to effective v overconsolidation ratio (OCR)for overburden pressure a:,, 102 undisturbed and reconstituted specimens Variation of undrained shear strength, cu,and water content, w, for 103 remoulded Kielder tills Variation of undrained shear strength, cu,with total water content, w, for remoulded glacial soil containing various proportions of granular materials 106 Relationship between undrained shear strength, cu,water content, w, for 107 Northumberland glacial tills Grain packing in coarse granular materials 108 Relationship between relative compaction value (vertical axes) and number of passes (horizontal axes)for three soils and various items of compaction equipment 109 Influence of soil clods on permeability of compacted clay 110 Variation of coefficient of permeability, k, of compacted clay soils (tills and recent alluvium) with compaction water content, w, and plasticity index, PI 111 Illustration of the relationships between landsystems, land facets and land 114 elements used in land surface evaluation Use of terrain evaluation to assist site investigation procedures 115 Some typical groundwater table conditions arising from glacial till successions 117 Stages in void formation during drive sampling 120 Block sampling techniques 121 Relationships for the interpretation of static cone penetration tests 128 Results of pressuremeter tests on glacial tills 130 Undrained shear strength, cu,determined from 865mm plate tests and Menard pressuremeter tests on the Cowden till 131 Effect of variable ground conditions on permeability test results 133 Interpretation of pumping tests in glacial tills 133
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11
Figure 7.1 Figure 7.2 Figure 7.3 Figure 7.4 Figure 7.5 Figure 7.6 Figure 7.7 Figure 8.1 Figure 8.2
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Figure 8.3 Figure 9.1 Figure 10.1 Figure 10.2 Figure 10.3 Figure 11.1 Figure 1 I .2 Figure 1.3 Figure 1.4 Figure 1.5 Figure Figure Figure Figure
2.1 13.1 13.2 13.3
Figure 13.4 Figure 14.1 Figure 14.2 Figure 14.3 Figure 14.4 Figure CS2/ 1 Figure CS212 Figure CS2l3 Figure CS3/ 1 Figure CS3/2 Figure CS313 Figure CS3/4 Figure CS3l5 Figure CS8/ 1 Figure CS8/2 Figure CS813 Figure CS8/4 Figure CS8/5 Figure CS8/6
Distribution of undrained shear strength test results during construction of the Kidder Dam Compaction test properties as a basis for design Methods of presenting MCV data Application potential of moisture condition apparatus based on results of ground investigations Correlations between CBR, MCV and undrained strength for glacial soils from Northumberland Results of laboratory tests on till from which limits of acceptability may be set Problems encountered in placing jills of different soil types Drainage of cuttings: mixed succession, predominantly clay Drainage of cuttings: mixed succession, predominantly granular Stability of intact till slopes in north east England Typical downstream slopes of British embankment dams with low plasticity clay$lls Bearing capacity of layered clay: stronger over weaker Bearing capacity of rigid foundation on thin soji clay layer Kogler method for determining vertical stresses in buried strata Importance of achieving design depth with respect to load cases General description of pile types and pile requirements with respect to landsystems in the UK Ground conditions typically associated with the three landsystems in the UK and their consequences for piling End bearing resistance of piles in layered soils Suggested relationship between the adhesion factor, a, and undrained shear strength, cu,for clay tills Range of application of dewatering techniques Classijication of tunnel stability Effect of tunnel face pressure on face stability and ground movements Face pressures required for stability and for control of ground movements Range of ground conditions for slurry and earth pressure balance TBMs Types of landslide Schematic diagram of the failure mechanism for Holderness coastal tills Forces acting on restraining and retaining structures Examples of use of restraining and retaining structures Plot of undrained strength v depth for glacial till (NW England) Plot of SPT ‘ N value v depth for glacial till (NW England) Plasticity chart for glacial till (NW England) Plot of undrained strength v depth for glacial till Plot of SPT ‘N’ value v depth for glacial till Plasticity Chart: upper glacial till Plasticity Chart: lower glacial till Effective stress tests: glacial till Results of pile test PL4 Results of pile test PL5 Results of pile test PL6 Results of pile test HP2 Results of pile test HP4 Results of pile test HP5
139 139 140 141 142 142 143 148 149 150 154 157 157 159 162 163 164 166 167 171 175 175 176 176 181 184 186 187 216 216 217 21 9 220 22 1 22 1 22 1 228 228 229 229 230 230
CIRIA Report C504
12 .
,
Results of pile test HP6 Sketch indicating damage to shaft
Figure CS I I / 1 Figure CS 12/1
Typical cut and fill slopes showing drainage features Particle size distribution for glacial till
Figure CS 12/2
Plasticity chart for glacial till
Figure CS 12/3
Peak shear strength for glacial till
240 241
Figure CS 12/4
Residual shear strength for glacial till
24 1
Figure CS 1Z5
SPT ‘N’ v depth for glacial till
242
Figure CS 12/6
242
Figure CS 14/2
Pernieability test data for glacial till Results of undrained shear strength and SPT tests for glacial tills at Dunmore, Antrim Results of undrained shear strength and SPT tests for glacial tills at Wattstown, Coleraine Results of undrained shear strength and SPT tests for glacial tills at Doogary, Omagh Results of undrained shear strength and SPT tests for glacial tills at Dungannon, Tyrone Plot of vertical effective stress and pre-consolidation pressure o’,( v elevation (Port Huron Bank) Plot of undrained shear strength v elevation (Port Huron Bank)
Figure CS 14/3
Plot of stability number N v elevation (Port Huron Bank)
Figure CS 1 3/ 1 Figure CS 13/2 Figure CS I3/3 Figure CS 13/4
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23 1 233 238 240
Figure CS8/7 Figure CS9/ 1
Figure CS 1 41 1
Cover Plate Plate 1 Plate 2
Banded tills, Cadeby, western Leicestershire. Each band reveals distinct matrix type and erratic content (Photograph: T. Douglas, 1974). Aber-mawr, Irish Sea till overlying locally derived head. Note jointing and paucity of clasts in till. Scale 50 cm (Photograph: C. Harris, 1986). Ffos-las. Upper till (FL5). Note strongly developed fabric and high erratic content (Photograph: R . Donnelly, 1985).
245 246 247 248 250 250 251
Cover 125 125
Stratified till with chalk lenses and flow structures near the base of the Third Cromer Till, West Runton, Norfolk (Photograph: P Gibbard, 1978). Folding and step faulting of laminated till near the base of the Third Cromer Till, West Runton, Norfolk (Photograph: P Gibbard, 1978).
126
Plate 5
Section 6 m high in weakly bedded brown lodgement till with locally imbricated clasts. Bed of gravelly till by hammer. Holm (NB 453307) near Stornoway, north Lewis (Photograph: J.D. Peacock, 1977).
127
Plate 6
Streamlined forms and striae on peridotite. Note open fractures caused by 127 stress relief (glacial unloading?). Locality (NM 373990) west of Loch Bealach Bhic Neill, Rhum (Photograph: J.D. Peacock, 1975).
Plate 3 Plate 4
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Abbreviat ions BP
Before present
BRE
Building Research Establishment
CBR
California bearing ratio
CSL
Critical state line
DMRB
Design Manual for Roads and Bridges (HMSO)
EPB
Earth pressure balance (tunnel boring machine)
FDP
Full displacement pressuremeter
FM
Formation
LL
Liquid limit
MCA
Moisture condition apparatus
MCV
Moisture condition value
MDD
Maximum dry density
OCR
Overconsolidation ratio
OMC
Optimum water (moisture) content
PI
Plasticity index
PL
Plastic limit
PBP
Prebored pressuremeter
PSD
Particle size distribution
SBP
Self-boring pressuremeter
SHW
Specification for Highway Works (HMSO)
SPT
Standard penetration test
TBM
Tunnel boring machine
TRL
Transport Research Laboratory
b -
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Notation Factor relating shear strength of rockfill, t, to normal effective stress, (5' Area of pile base Area of pile shaft Factor relating shear strength of rockfill, t, to normal effective stress, o' Breadth of foundation Undrained adhesion (Figure 10.1) Undrained shear strength Mean undrained shear strength over length of pile shaft Undrained shear strength at pile base
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Undrained shear strength of intact soil Undrained shear strength of stronger (upper) layer Undrained shear strength of weaker (lower) layer Coefficient of consolidation in vertical direction Coefficient of consolidation in horizontal direction Effective cohesion Effective cohesion (constant volume) Effective cohesion (peak) Effective cohesion (residual) Depth from ground surface to tunnel axis Compression index Permeability index Swelling index Sieve size upon which 10%of sample is retained Depth from ground surface to underside of foundation Diameter of tunnel Void ratio Void ratio at start of test Young's modulus Young's modulus (drained) Young's modulus (undrained) Percentage of soil finer than 6 pm Ratio of mass of dry matrix to total dry mass Shear modulus Particle density Distance from underside of foundation to top of weak layer Thickness of weak (soft) layer
-e Relative density emax emax
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k
Coefficient of permeability
k0
Coefficient of permeability at void ratio eo
KS
Coefficient of earth pressure
L
Length of foundation
m
Constrained modulus Coefficient of volume compressibility Standard penetration test (SPT) blowcount Bearing capacity factor Cone factor Bearing capacity factor Imposed load due to surface construction (Figure 13.1) Cone resistance
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Ultimate bearing capacity of pile base Ultimate bearing capacity of pile base a distance 10B above top of weak layer Ultimate bearing capacity of pile in weak layer Ultimate bearing capacity of pile shaft Overconsolidation ratio W
Total water content (matrix plus clasts)
wm
Water content of matrix
ws
Average water content of clasts
a
Shaft adhesion factor Factor relating constrained modulus, m, to cone resistance, q, Angle of shearing resistance Angle of shearing resistance (constant volume) Angle of shearing resistance (peak) Angle of shearing resistance (residual) Unit weight Total dry density (matrix plus clasts) Matrix dry density Total overburden pressure Vertical stress applied at ground surface Vertical stress acting on top of weak layer Tunnel face pressure Normal effective stress Vertical effective stress at pile base Pre-consolidation pressure Effective overburden pressure Mean vertical effective stress over length of pile shaft
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Glossary of geological terms aeolian
Wind-borne, wind-blown or wind-deposited.
biogenic
A term applied to material produced by the action of living organisms.
bioturbation
The breakdown and reworking of sediment by the action of its contained organisms.
boulder clay
A term for till, no longer favoured by glacial geologists.
calving
Ice wastage by shedding of large ice blocks from a glacier’s edge usually into a body of water.
cold stage
See glacial periodglaciation.
corrie (coire)
Deep steep-sided hollow formed by glacial erosion, typical of the glaciated valley landsystem.
crag-and-tail
A glacially eroded rock-cored hill with a tail of till formed downglacier.
comminution
The gradual breakdown of rocky materials by weathering and erosion to form progressively smaller particles.
dead ice topography
Complex of eskers, kames and kettle holes formed when ice wastes in situ.
deformation till
Till comprising weak rock or unconsolidated sediment detached by the glacier from its source, the primary sedimentary structures distorted or destroyed and some foreign material admixed.
diachronous
Development of the same facies at different places and at different times.
diamict
A poorly-sorted terrigenous sediment containing a wide range of particle sizes. Embraces both diamictite (lithified) and diamicton (nonlithified).
diamicton
See diamict.
drumlin
A streamlined hillock, commonly elongated parallel to the former iceflow direction, composed of glacial debris, and sometimes having a bedrock core; formed beneath an actively flowing glacier.
englacial debris
Debris dispersed throughout a glacier, derived either from the surface through burial or crevasses, or from the uplifting of basal debris by thrusting processes.
esker
A long, commonly sinuous ridge of sand and gravel generally aligned parallel to the ice flow, deposited by a meltwater within or below the ice.
facies
A sediment type characterised by an assemblage of features, including lithology, texture, sedimentary structures, fossil content, geometry, bounding relations. Lithofacies refers to a particular lithological type.
fjord
A long, narrow arm of the sea, formed in part as a result of erosion by a valley glacier.
flow till
Till that has been transported and emplaced by debris flow.
fluvial
Transported and deposited by rivers.
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gelifluction
Down slope movement of part frozen soil or rock debris in periglacial environment.
glacial debris
Material in the process of being transported by a glacier in contact with glacier ice.
glacial period/ glaciation
A period of time when large areas (including present temperate latitudes) were ice-covered. Many glacial periods have occurred within the past few million years, and are separated by interglacial periods.
glaciated
The character of land that was once covered by glacier ice.
glacier
A mass of ice, irrespective of size, derived largely from snow, and continuously moving from higher to lower ground, or spreading over the sea.
glacier bed
Surface on which a glacier rests, may comprise bedrock or previously deposited glacial or non-glacial sediments.
glacier ice
Any ice in, or originating from, a glacier, whether on land or floating on the sea as icebergs.
glacigenic sediment
Sediment of glacial origin. The term is used in a broad sense to embrace sediments with a greater or lesser component derived from glacier ice.
glaciofluvial sediment
Glacial debris reworked by running water.
glaciolacustrine sediment
Glacial debris deposited in a lake.
glaciomarine sediment
A mixture of glacigenic and marine sediment, deposited more or less contemporaneously.
glaciotectonite
Deformed glacier bed sediments created as a result of glaciotectonic deformation.
glaciotectonic deformation (glaciotectonism)
The process whereby subglacial and proglacial sediment and bedrock is disrupted by ice-flow. Results in the formation of deformation till or glaciotectonite. It is usually manifested in the form of distinct topographic features in which folds and thrusts are commonplace.
glacioterrestrial sediment
Glacial debris deposited directly on land.
gravity flow
The process of transport of unconsolidated sediment down a slope, under its own weight either subaerially or subaquatically.
grounding-line
The line or zone at which an ice mass enters the sea or a lake and begins to float.
hummocky (ground) moraine
Groups of steep-sided hillocks, comprising glacigenic sediment, formed by dead-ice-wastage processes. Some hummocky moraines may be arranged in a crude transverse-to-valley orientation and may reflect thrusting processes in the glacier snout. Both terrestrial and marine types occur.
iceberg
A piece of ice of the order of tens of metres or more, that has been shed by a glacier into a lake or the sea.
ice cliff
A vertical face of ice, normally formed where a glacier terminates in the sea, or is undercut by streams. The term is also used more specifically for the face that forms at the seaward margin of an ice sheet and which rests on bedrock at or below sea level.
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ice sheet
A mass of ice and snow of considerable thickness and covering an area of more than 50,000km2.
ice shelves
Large slabs of ice floating on the sea, but remaining attached to, and partly fed by, land-based ice.
ice streams
Part of an ice sheet or ice cap in which the ice flows more rapidly, and not necessarily in the same direction as the surrounding ice. The margins are often defined by zones of strongly sheared, crevassed ice and are affected by the topography of the underlying landform.
kame
A steep-sided mound, hummock or hill of sand and gravel deposited by glacial streams adjacent to a lateral glacier margin.
kame terrace
A flat or gently sloping feature, deposited by streams that flowed towards or along the margin of a glacier.
kettle (or kettlehole)
A self-contained bowl-shaped depression formed within an area covered by glaciofluvial deposits. A kettlehole forms as a result of the burial of a mass of glacier ice by stream sediment followed by the subsequent melting of the ice.
landform
Morphologically distinct land surface features.
landsystem
A characteristic association of landforms and glacial sediments controlled by the nature of the glacier and the surface over which it moves.
lithofacies
See facies.
lodgement
The process whereby basal glacial debris is ‘plastered’ on to the substrate beneath an actively moving glacier.
lodgement till
Glacigenic sediments deposited by plastering of glacial debris from the sliding base of a moving glacier, by pressure melting andor other mechanical processes.
mClange
Jumbled and incoherent mass of rock fragments of various sizes and angularities.
melt-out till
Glacigenic sediments deposited by a slow release of glacial debris from melting ice that is not sliding or deforming internally.
meltwater channel
A form of channel cut in to solid rock or drift by the erosive action of glacial meltwater.
moraine
Distinct ridges or mounds of debris laid down directly by a glacier or pushed up by it. Moraines include a lateral moraine which forms along the side of a glacier; a medial moraine occurring on the surface where two streams of ice merge; and a fluted moraine which forms a series of ridges beneath the ice, parallel to flow. Transverse moraines include a terminal moraine which forms at the farthest limit reached by the ice.
nivation
Erosion due to frost shattering with the formation of nivation hollows.
outwash plain
A flat spread of debris deposited by meltwater streams emanating from a glacier.
palaeosol
An ancient soil horizon or buried fossil soil.
proglacial
Area lying adjacent to and usually in front of a glacier.
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regelation
The process of refreezing of ice subsequent to melting caused by pressure within an ice mass.
rhythrnite
A sedimentary unit comprising a repetitive succession of sedimentary types, e.g. mudsand. A varvite is one specific type of rhythmite.
roche rnoutonnke
A rocky hillock with a gently inclined, smooth slope facing up-valley resulting from glacial abrasion, and a steep, rough slope facing downvalley resulting from glacial plucking.
sandur plain
Extensive flat or gently sloping fan-like accumulation of glaciofluvial sediments.
shield
Large area of old crystalline rock, usually largely Precambrian in age, forming the core of a continent.
slickenside
A polished or scratched planar surface.
stadia1
Time represented by glacial deposits.
stria (striae)
One of a series of parallel straight lines scoured on to rock or rock fragments by the glacial process.
su bglacial
The area beneath a glacier.
subglacial debris
Debris that has been released from ice at the base of a glacier.
subglacial gorge
A steep, often vertically sided gorge cut into bedrock by a subglacial stream under high pressure.
superglacial
The area on the top surface of a glacier.
supraglacial debris
Debris that is carried on the surface of a glacier. Normally derived from rockfalls and usually angular.
talus cone
A cone of debris formed by the slow downslope movement of a superficial mass of rock fragments.
till
A sediment transported and subsequently deposited by or from glacier ice, with little or no sorting by water.
till plains
A wide area of low relief created by till deposition.
tunnel valley
Large valley or trough cut into drift or bedrock by a subglacial stream flowing in an ice tunnel.
varvite (varved clay)
A thin laminar bed of sediment divided into a thicker, lower, lightercoloured band of sand grading upwards into a thinner, upper, darkercoloured band of silt or clay.
wastage
The process of overall shrinkage of an ice body, including loss by melting, evaporation, wind erosion and calving.
whaleback
Smooth, glacially sculptured bedrock knob of modest size resembling the back of a whale.
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CIRIA Report C504
1
Introduction
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This report on engineering in tills is the second in a series to be published by CIRIA on the engineering properties of major UK soils and rocks. There is still debate as to the precise definition of glacial tills but it is generally accepted that they are only part of a whole suite of glacigenic sediments. Soils of glacial origin, such as glaciolacustrine laminated silty clay and glaciofluvial sand and gravel, frequently exist in intimate contact with tills and are often responsible for problems in investigation, design and construction. For this reason they are also considered in the report where appropriate. Glacial deposits are amongst the most widespread in the world. In Great Britain, the area believed covered by Devensian ice amounts to some 60% of the total land area and glacial deposits are particularly well represented in the colder, wetter, upland parts of the country. As well as being of widespread occurrence, tills and associated glacial soils are amongst the most difficult to engineer, due to their marked variation both in thickness and in engineering characteristics. Because of their widespread occurrence, tills are of much relevance to British civil engineering. Linear structures such as roads and railways can traverse kilometres of tills and the design and construction of the works can be at risk from their spatial variability and complex groundwater conditions. Variation in depth and morphology of underlying rockhead presents a similar and related challenge. Work of fundamental importance to the engineering study of tills in the UK commenced in the early 1970s at the University of Strathclyde and valuable publications largely devoted to fissured Scottish tills continued until 1985. Three ‘test bed’ sites, each underlain by glacial tills, were established by the Building Research Establishment in the mid 1970s in connection with the design and construction of North Sea offshore structures. The sites were to provide excellent opportunities to compare the results of various testing methods in largely matrix-supported tills over some fifteen years. Materials taken from these sites were tested at various British universities, including City University, London, where there were developments in the measurement of fundamental soil properties using reconstituted sample procedures. Significant work was also undertaken by the Transport Research Laboratory on the stability of cuts and embankments in a variety of soils, including glacial tills. More recently a programme of research into north east England tills was initiated by the University of Newcastle. It will include studies into engineering properties as well as the creation of a database to incorporate the results of investigations of tills revealed when working some sixty of the region’s opencast coal sites.
.
CIRIA report PG5 (1978) was concerned with piling in glacial tills, but the present report is wider in scope. It summarises some of the available information on tills to provide essential background, together with design and construction guidance, for the engineer and engineering geologist working in British glacial terrains. The geology of tills is summarised in Sections 2 and 3 and their engineering classification is outlined in Section 4. A description of the more commonly used engineering properties follows in Section 5 and an account of frequently adopted site investigation techniques is presented in Section 6, emphasis being placed on the special requirements tills generate. Sections 7 to 14 describe engineering in tills and associated glaciolacustrine and glaciofluvial deposits; again emphasis is placed on the particular properties of tills which create difficulties and delays. Section 15 outlines some current capabilities and uncertainties. Features requiring special consideration when engineering in tills include: geological conditions of deposition techniques of investigation engineering property characterisation applicability of methods of analysis and design parameter selection difficulties during construction.
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1.l GEOLOGICAL HISTORY Tills are quite unlike other soils encountered in this country. They are normally defined as being transported and subsequently deposited by or from glacier ice (or ice sheets) with little or no sorting by water. Unlike a marine sediment, subject to one-dimensional consolidation during the sedimentation process, the conditions of deposition of a till are particularly complex. The nature of a till depends on the regional topography, the ground surface over which the ice traversed, and the form of the glacier (or ice sheet) itself. Tills are often associated with other soils of glacigenic origin and glaciofluvial and glaciolacustrine deposits are well known. Although they may not attain the same thicknesses as tills, glaciofluvial sands and gravels and glaciolacustrine silty clays can have a controlling influence on the engineering behaviour of a succession of glacial soils. Developments in terrain evaluation over the past two decades permitted a rationalisation of the glacigenic environment into one of three landsystems. Each landsystem has certain features in common which permit a level of prediction to be made of some of the essential engineering characteristics. The landsystem concept also permits explanation of some of the reasons for the variations inherent in tills and associated glacial deposits.
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1.2
TECHNIQUES OF INVESTIGATION
Cable percussion methods are widely used in the UK. Penetrating large till thicknesses with gravel and cobble layers can be difficult using these techniques unless large casing sizes are employed. The results of Standard Penetration Tests (SPTs) made in large diameter boreholes are often suspect and BS1377: 1990 advises against their use in boreholes larger than 150mm diameter. There are clearly difficulties in the use of cable tool methods and SPTs in deep glacial successions. Rotary methods, particularly wireline, with modem bits and circulating fluids are promising, but the equipment is expensive and costly to manoeuvre in the often rugged glacial terrains. Static cone penetration testing has been widely studied and progress made in its interpretation, but in coarse granular soils, cobbles or boulders can prevent deep soundings. Pressuremeter tests may not always be possible if suitable size test pockets cannot be secured, or if large clasts obstruct selfboring equipment. Given the difficulties of retrieving undisturbed samples of tills, much effort has been devoted to exploring the use of remoulded or reconstituted samples for the measurement of strength and deformation characteristics. The largest specimen diameter which can be tested in most laboratories is 100 mm, so there is uncertainty as to the effects that the removal of large particle sizes could have on the results. Moreover, remoulding or reconstituting inevitably destroys till fabric and any cementing which could be present. Correlations are available between shear strength and plasticity characteristics (e.g. liquidity index) or water content. These correlations provide a guide to undisturbed shear strength selection, but it should be remembered that plasticity and water content values are determined on fine particle sizes (< 425 pm), usually unrepresentative of the till sample as a whole. Studies demonstrate the importance of granular particles on the undrained shear strength and MCV values of remoulded tills. Use of correlations based on fine particle sizes should be treated with great care and cannot usually be recommended except on a site-specific basis. <
1.3
ENGINEERING PROPERTY CHARACTERISATION
The variety of depositional processes which characterise tills produces a correspondingly wide variation in their engineering properties. Undrained shear strength and deformation properties are notoriously variable. Lodgement tills are usually considered to be heavily overconsolidated by the weight of the ice, but this is often not so. Recent work on modem glaciers has demonstrated high pore pressures beneath the ice, so that effective stresses during lodgement can be low and quite unrelated to ice thickness.
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Till fabric reflects unloading due to ice melt and to post-depositional processes such as freeze-thaw activity and groundwater change. Consequently fabric, upon which engineering performance depends, is often multi-directional. Concepts such as representative strengths developed for the overconsolidated fissured clays of southern England may also be relevant to tills, possibly not only to the drumlinised tills studied by McGown and co-workers (e.g. McGown et al., 1977).
1.4
APPLICABILITY OF METHODS OF ANALYSIS AND DESIGN
As with other soils, ground engineering design in glacial tills is based upon: precedent analysis observation.
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Precedent suggests that tills present a wide range of engineering problems reflecting the variety of till types, groundwater conditions, and associated soils in the glaciogenic environment. Temporary works design is at least as difficult as permanent works design, with groundwater and its management a major factor. Site investigation should provide an accurate description of the ground and groundwater so that variations in conditions may be taken into account during design. Analysis in geotechnical engineering, whether for permanent or temporary conditions, invariably assumes a degree of homogeneity: for example, the soil is assumed to be either cohesive or granular; the shear strength or deformation modulus is assumed to be either uniform or to vary in some stated manner with depth. Such sweeping and simplifying assumptions can seldom be made for tills; indeed, their identifying characteristic is their heterogeneity. Analytical models are often used in design but, because the models cannot normally incorporate the wide range of ground and groundwater variations inherent in tills, parameter selection (below) assumes the utmost importance.
1.5
PARAMETER SELECTION
The nature, size and geometry of the structure should be considered in parameter selection. A linear structure provides the opportunity for observation and, to some extent, performance assessment depending upon the construction programme. A cutting, an excavation for a retaining wall, a pipeline or a tunnel drive all provide an opportunity to inspect often large exposures of soil (usually larger than available for inspection at site investigation stage) and this permits an element of iteration not possible with, say, a foundation. Unless man-access is available, little detailed soil inspection is possible when constructing a bored pile foundation and soil descriptions are usually limited to what may be inferred from arisings; in the case of a driven pile foundation, no soil inspection is possible at all. Excavation for spread foundations provides opportunities to inspect the sides and base, but there is little opportunity to inspect the bearing soils beneath. Structure size is important in another context: relatively small granular layers or lenses could threaten the progress of a pipe jack, but would present no major problem for the progress of a fullface road tunnel. Thus, it is essential to link the scale of potentially troublesome features of the tills discovered during the site investigation stage with that of the structure itself. This is particularly important where groundwater is concerned. Parameters such as shear strength should also be selected in the knowledge that discontinuities in the tills may also play their part in stability assessment. The number of test methods available is also a determinant in parameter selection. For example, SPT and unconsolidated undrained triaxial test results may give better security of choice than the results of one test method on its own. Likewise, the difficulty in conducting and interpreting SpTs in clast-dominant materials points to a conservative interpretation of results in the absence of other methods of testing.
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1.6
DIFFICULTIES DURING CONSTRUCTION
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Groundwater and run-off play a significant part in earthworks and, as already noted, tills are often present in the wetter parts of upland Britain. It is a matter of observation that a small change in water content can make all the difference between acceptable and unacceptable till fills. Drainage, both during and after construction, is of the utmost importance. Granular soils within tills are often water-bearing and they may occur in an unpredictable and haphazard fashion. This makes the need for dewatering methods particularly difficult to forecast and the variations in grading often exhibited within any one granular body introduce major uncertainties in permeability. Small lenticular bodies disclosed during site investigation may be representative of much larger and more continuous water-bearing bodies during main works construction with the potential for consequential difficulties and delays.
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CIRIA Report C504
2
Geology of tills: physical processes
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The formation, transportation and deposition of a till is a complex process operating in space and time. Most of the northern British Isles was covered by ice in the middle and late Pleistocene and at any one site there may be tills of more than one age, separated vertically by material of a different genesis. Further complexity may be introduced by weathering processes. Eyles and Sladen (1981) suggested that the apparent differences in geotechnical properties of some Northumberland tills was not due to different genesis, but to weathering of the same till (Section 4.3). Interpretation of a glacial succession is made easier by recognising the close genetic association between the nature of the deposit and the corresponding geomorphological landform (Boulton and Paul, 1976). They are controlled by both the nature of the glacier (or ice sheet) and the surface over which it moves (the glacier bed), such that the association of landforms and glacial sediments is represented by well defined three-dimensional patterns termed ‘landsystems’ (Boulton, 1972). Recognition of the landsystems, together with an understanding of the glacial environment which produced them, assist in determining the character and configuration of the deposits themselves (Eyles and Dearman, 198I). The systematic approach to the interpretation of glacigenic sediment sequences, with particular reference to tills, is followed throughout this section. The glacigenic environment is first described and the landsystem approach outlined. The processes operating to produce glacial tills are detailed and the sediments and landforms characterising tills described. Other glacigenic sediments are then outlined briefly to assist in the interpretation of complete glacial sequences. Having reviewed their genetic origin and classification, section 3 deals with the distribution and stratigraphy of tills in mainland United Kingdom. For the geological terms used in this report see the glossary.
2.1
GLACIGENIC ENVIRONMENT
The glacigenic environment is a broad descriptive term covering any environment affected by direct or indirect glacial activity. Debris entrained in, on or beneath the ice, may be deposited within one of four environments: glacioterrestrial glaciofluvial glaciolacustrine and glaciomarine. The associated depositional and erosional processes produce a characteristic suite of sediment types and geomorphological forms. The glacioterrestrial environment is responsible for the deposition of till, primarily defined as a sediment transported and subsequently deposited by or from glacier ice (or ice sheet), with little or no sorting by water (Dreimanis and Lundquist, 1984). The evolution of glacigenic deposits is illustrated in Figure 2.1. Several environments may operate within one glacier or ice sheet, depending on its size and geographical location. For example, a terrestrial glacier dominated by subglacial lodgement till deposition will usually include some active glaciofluvial processes (Figure 2.2).
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T
Figure 2.1
26
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T 1
Evolution of glacigenic deposits
CIRIA Report C504
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Figure 2.2
Cross-section of typical temperate glacier illustrating the various glacigenic sediments (from Hambrey, 1994)
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GLACIAL LANDSYSTEMS
2.2
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To simplify the interpretation of complex glacial sediment sequences, Fookes er al. (1975a) applied terrain analysis to the glacigenic environment and proposed three simplified landsystems. These comprised three main groups of glacigenic sediments and geomorphological landforms, with different engineering characteristics. However, Boulton and Paul ( 1 976) suggested a modification of the approach such that landsystems represented patterns of associated glacial elements. This idea was developed further and a landsystem was defined as a recurrent pattern of genetically linked glacial features related to the type of glacier and the form of the glacier bed (Eyles and Dearman, 1981; Eyles, 1983). These features include characteristic geomorphological expressions (landforms), sediment sequences, and glacier bed topography related to glacial erosion (Figure 2.3).
........ . .. . . . .
/J::::::\
(i)
Surface landforms used to identify landsystems eg
(ii)
Sediment sequences from one or more ice advances eg
(a) (b) (c)
Lateral moraine Hummocky moraine Drumlin
n
Supraglacial melt-out till Subglacial lodgement till Glacier bed
(iii)
Bedrock topography, exposed or buried eg
Figure 2.3
(d) (e)
buried meltwater channel buried roche moutonnee
Diagram illustrating examples of the three main components of a landsystem
The glacigenic environment may be generalised into three distinct landsystems: subglacial supraglacial glaciated valley. Subglacial landsystems are typical of glaciated lowlands, such as the English lowlands, where sediments were deposited by large ice sheets (as opposed to glaciers) leading to extensive and sometimes deep cover of glacial soils. Debris can gather on, or in, ice sheets or glaciers and when deposited by melting or wasting ice they constitute part of the supraglacial landsystem. The glaciated valley landsystem occurs in highland areas where ice lobes are broken into valley glaciers; the Scottish highlands, the Lake District and parts of Wales are examples. The main characteristics of these three landsystems are described below and are illustrated in Figures 2.4 to 2.6. A map showing the distribution of landsystems in the UK is given in Figure 2.7. 28
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CIRIA Report C504
2.2.1 Subglacial landsystem
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The subglacial landsystem (Figure 2.4) evolves where landforms are created and sediments are deposited at the ice base. In shield areas of low to moderate relief underlain by hard igneous and metamorphic rocks such as Canada and Scandinavia, the subglacial landsystem is characterised by wide exposures of scoured, but not deeply eroded, bedrock with thin sediment cover comprising coarse grained variably consolidated tills and glaciofluvial gravels and sands. In areas of lower pre-glacial relief with flat-lying or gently dipping sedimentary strata such as most of lowland England and parts of east and central Scotland, the subglacial landsystem comprises thick, finer grained, sediments often overlying a glaciotectonised glacier bed.
Rockhead
(8) cut and fill fluvial sediments deposited as sand and gravels in interconnected subglacial channels or as laminated clays in subglacial ponds. Lenses of resedimented till may be incorporated into fluvial sediments
(1)
striated rockhead surface locally overdeepened by subglacial erosion
(2)
buried channel over-steepened by subglacial meltwaters and filled with subglacially derived sediments
(9)
(3)
rock rafts, glaciotectonised rockhead and deformation till depending on bedrock lithology
(10) slickensided bedding plane resulting from
(4)
bouldery unit of scree-like debris filling lee-side cavities in rockhead surface
(5)
bedrock
Glacigenic sediments (6)
preferentially orientated clast long axis
(7)
distinct flat iron shaping of clasts composed of fine-grained lithologies; coarse grained lithologies produce clasts of higher sphericity, frequently found as boulder pavements
fluvial sediments reworked, deformed and incorporated in subsequent tills subglacial shear
(11) near vertical en-echelon joints orientated with respect to glacier flow direction
Landform (12) drumlinised, streamlined, low-relief surface. Where rockhead is close to the surface, rock-core drumlins and crag and tail landforms may develop (13) esker ridge; a subglacial channel fill that survives as a positive topographic feature not having been sheared off and buried by till
NB: Nature of rockhead strongly influenced by rock type. Rock rafts and boulder units expected to be less common in weak rock terrains than in areas underlain by Palaeozoic sediments, igneous and metamorphic rocks
Figure 2.4
The subglacial landsystem (after Eyles, 1983 and Eyles and Dearman, 1981) ~
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2.2.2 Supraglacial landsystem
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Where the glaciated surface produced by an ice sheet or glacier is obscured by sediments deposited during wasting, the sediments and landforms are collectively referred to as the supraglacial landsystem (Figure 2.5). Large debris accumulations develop on the surface of the ice sheets or glaciers. Where the ice margin is thin, slow moving and under strong compression, englacial (i.e. carried or entrained within the glacier) and subglacial debris may migrate from their respective positions to accumulate on the glacier surface and combine with supraglacially derived debris. As the glacier wastes, melt-out and flow processes result in the formation of characteristic hummocky supraglacial topography. Ground conditions are dominated by rapid vertical and horizontal lithological variations in the glacial sediments, together with a large component of glaciofluvial deposits and the common presence of lodgement tills at depth deposited during the ice advance (Eyles and Dearman, 1981).
Rockhead (1)
subglacially cut buried channel, glacigenic debris filled
Glacigenic sedirnents (2) (3) (4) (5) (6) (7)
crudely stratified melt-out till formed by meltdown of alternating debris-rich and debris-poor basal ice with variable preservation of englacial clast orientation; cobbles and boulders frequent flow tills strata deforming as a result of meltdown of adjacent ice-cores drumlins buried lodgement till supraglacial melt-out and flow tills
Landforrn (8)
hummocky moraine obscuring streamlined surface of lodgement till
Figure 2.5
The supraglacial landsystem (after Eyles, 1983 and Eyles and Dearman, 1981)
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2.2.3 Glaciated valley landsystem
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This landsystem (Figure 2.6) is encountered in highland areas of strong pre-glacier relief. Ice sheets are broken up by mountains into separate glaciers that often coalesce on surrounding lowland margins. Coarse angular debris derived supraglacially from the valley sides is transported in large volumes on the glaci.er surface. Thinning of the ice sheet results in accumulation and deposition of this debris by melt-out and flow processes. The glaciated valley landsystem is, therefore, characterised usually by thin tills and hummocky moraine topography along the valley floor, in association with complex lateral moraine accumulations deposited between bedrock walls of the valley and the former valley glacier (Eyles and Dearman, 1981). Downslope movements, including landsliding in historical time, can obliterate much of the original glaciated valley topography, as occurred in the south Wales coalfield (Wright, 1991).
Glacigenic sediments (3)
(4)
lodgement till often hard or dense with streamlined drumlinised surface containing cobbles and boulders thick hummocky sequences of supraglacial melt-out straddle valley floor and overlie lodgement tills in places; coarse debris including far travelled clasts, cobbles and boulders
Figure 2.6
(5)
complex glaciofluvial sediments and flowed tills deposited in kettle holes or against lateral moraines
(6) valleyside fans discharging large quantities of coarse debris to lateral moraines
Landform (7)
medial moraine
(8)
lateral moraine ridge
The glaciated valley landsystem (after Eyles, 1983 and Eyles and Dearrnan, 1981)
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SOUTHERN
G LACIAT10N
PERIGLACIAL
SOUTHERN LIMIT OF ANGLIAN GLAClATlO
(a) Subglacial landsystem
..........
ml
Lodgement till dominant Drumlinised surface (lines indicate flow direction) Scoured bedrock surface with little glacigenic cover
(b) Supraglacial landsystem
(c) Glaciated valley landsystem
(d) Other
Figure 2.7
32
I
I
Areas with no preserved glacial deposits
Distribution of glacial landsystems in mainland Britain (after Eyles and Dearrnan, 1981)
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2.3
GLACIAL PROCESSES
2.3.1 Glacier flow Internal deformation in glaciers in mountain regions occurs by a process of gravity-induced creep (Drewry, 1986). Flow may be extensional or compressional depending upon the position within the glacier and the morphology of the underlying glacier bed. The second mechanism of glacier movement, basal sliding, is responsible for the bulk of the erosion, transport and deposition of glacial debris. The concept of basal sliding is firmly established but the mechanics of the process are the subject of debate. In addition to gravity-induced movement there are two contributory factors (Hambrey, 1994): pressure melting and regelation (re-freezing) at the ice/glacier bed interface sliding on a basal water layer which lubricates the glacier bed surface.
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Boulton (1991) suggested that the mechanism of rapid movement of surging ice sheets flowing over sediments in relatively lowland areas is quite different, with over 90% of the forward movement due to shear in the sediment rather than flow in the ice or slip at the ice/sediment interface.
2.3.2 Debris capture and entrainment As already remarked, glacial debris is generally derived from erosion of the underlying glacier bed or from material which falls on to the glacier surface from flanking valley sides. The mode of derivation has implications for the lithology and engineering characteristics of the subsequent glacial deposits:
(a)
Subglacial debris
This forms the largest component of glacially transported material and is primarily derived by erosion of the glacier bed. Debris is transported in a basal traction zone where the processes of crushing, fracturing and abrasion are common (Hambrey, 1994). Debris in the traction zone can be incorporated into the ice by pressure melting and regelation (Section 2.4.1). As re-freezing occurs, debris ranging from clay to boulders may become attached to the glacier base. Debris layers can be several metres thick depending on the thermal regime of the glacier, i.e. warm or cold based (Hambrey, 1994). Boulton (1991) considered that ice sheets moving over weak rocks, such as occur in most of the English lowlands (of Triassic to Cretaceous ages) caused shear deformation to depths of several metres and produced tills which were a mixture of the subglacial lithologies traversed by the ice.
(b)
Supraglacial and englacial debris
Supraglacial debris is derived from rockfalls, avalanches, debris flows and aeolian dust and accumulates at the margins of the glaciers or at the junction where ice flows combine. As snow accumulates on the glacier, some of this debris may become incorporated in the ice body and will be englacially transported (Lundquist, 1988a). Supraglacial debris may also be transferred to an englacial position by opening of crevasses in the glacier surface (Hambrey, 1994) and movement may continue through the ice by gravity and differential flow processes. Thrusting and folding in deforming ice under compressive flow may also be responsible for the movement of some subglacial debris to an englacial or even supraglacial position, i.e. to within or on top of the glacier. Sediments picked up by a glacier are mobile and they may migrate to positions within the glacier distant from their original points of capture.
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2.3.3 Debris deposition As with debris capture and entrainment, debris deposition can be complex. Glacially transported debris is released before or during ice wastage by a number of different mechanisms which control the type of glacigenic deposit formed. Deposition may occur directly from the ice to form a till (Section 2.4.1) or may be carried away by some agent (Lundquist, 1988a) to be deposited in a glaciofluvial, glaciolacustrine or glaciomarine system (Section 2.6).
2.4
TILL DEPOSITION
As already noted, tills are essentially glacioterrestrial deposits which meet the following conditions:
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they consist of debris that has been transported by a glacier (or ice sheet) they have a close spacial relationship to, and are deposited from or by, a glacier sorting by water should be absent or minimal. Till is the most widespread and variable of all glacigenic deposits and is probably one of the most widespread sediments on earth (Dreimanis, 1988). It is usually bimodal or polymodal in particle size and ranges from clast-supported to rock flour or clay with occasional clasts. Tills may be structureless or stratified and may comprise any local or exotic material picked up and transported by a glacier. The term ‘till’ gives no indication of composition but it is commonly taken to consist of a diamicton, an unlithified admixture of un- or poorly-sorted fine to coarse sediments often containing boulders (Flint er al., 1960). The processes operating within glacial environments are still not fully understood, despite the study of modem glaciers. Information on the origin of tills is therefore under constant review and this complicates our understanding of their geotechnical properties. However, it is generally accepted that the main depositional processes which produce tills are lodgement, melt-out, gravity flow and deformation, giving rise to a four-fold genetic classification. The following describes the depositional characteristics and relevant geotechnical properties of the four till types, summarised in Table 2.1 .
2.4.1 Lodgement tills /
The characteristics of lodgement tills are determined by the landsystem. Subglacial lodgements in lowland areas formed by deep shearing of weak rock terrains reflect the nature of the deposits which the ice sheets traversed. In the English lowlands, they may contain rock types of more than one (weak rock) origin, perhaps with occasional hard rock erratics from further afield. Boulton (1991) suggested that material in the deforming layer is normally at high water content, close to the liquid limit. Transport in the fluid state, together with consolidation under thin ice at the ice sheet margins, explains the surprisingly low pre-consolidation pressures demonstrated by some tills, much lower than would be anticipated by considering the full weight of the ice sheet. Evidence to support highly fluid transport comes from delicate and intact shells identified in contemporary tills in Spitzbergen (Boulton, 1991) and from intact and well preserved fauna within older Anglian tills (Little, 1991). Such delicate features would not be preserved in the more robust depositional process described below. The colours exhibited by tills are determihed largely by the nature of the terrain traversed. The cover plate illustrates a banded till with each band reportedly reflecting a different matrix type and erratic content. Debris from different sources were brought together for deposition in intimate contact.
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Table 2.1 Characteristics of genetic till types (from Dreirnanis. 1988 and Elson, 1988) Criterion
Lodgement till
Melt-out till
R o w till
Deformation till
Deposition
Deposited by plastering of glacial debris from the sliding base of a moving glacier, by pressure melting andor other mechanical processes (Hanibrey, 1994).
Deposited by a slow release of glacial debris from ice neither sliding nor deforming internally (Dreimanis, 1988).
Deposition accomplished by gravitational slope processes and may occur supraglacially, subglacially or at the ice-magin (Dreimanis, 1988).
Comprises rock or unconsolidated sediment detached by the glacier from its source; primary sedimentary structures distorted or destroyed and some foreign material admixed (Elson, 1988).
Position and sequence
Lodged over older glacial Usually deposited sediments or on bedrock during glacial retreat.
Most commonly the uppermost glacigenic deposit.
Formed and deposited subglacially, often where the glacier moves upslope.
Variable basal contact but seldom conformable over longer distances. Tills may fill shallow channels or depressions.
Variable basal contact.
Associated with most ice-marginal landforms.
Landforms rarely diagnostic.
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Basal contact
Lodgement and melt-out tills formed and deposited at glacier base. Contact with the substratum (bedrock or unconsolidated sediments) generally erosional and sharp. Glacial erosion-marks and clast alignment have same orientation. Supraglacial melt-out tills may have variable basal contact.
Land forms
Mainly ground moraines, Those ice-marginal drumlins, flutes and othei landforms where subglacial landforms. glacier ice stagnated.
nickness
Typically one to a few metres thick but may attain substantial lhickness in the English lowlands; relative lateral :onsistency.
Very variable. Single units usually a few centimetres to a Individual flows few metres thick. Units usually a few tens o f centimetres to metres may stack to much greater accumulated thick. Units may thickness. stack to accumulated thickness of many metres.
Varies u p to many metres depending upon nature of glacier bed.
Structure
Usually massive but may :ontain various :onsistently oriented macro- and micro;tructures. Subhorizontal jointing :ommon and vertical and :ransverse joints may ilso be present. 3rientation of leformation structures -elated to stress applied ,y moving glacier and nay be laterally :onsistent.
Either massive, or with faint structures partially preserved from debris stratification in basal debris-rich ice. Loss of volume with melting leads to draping of sorted sediments over large clasts.
Either massive or displaying various flow structures depending on type of flow and water content.
Primary structure may be preserved but usually deformed, especially in upper part of the sequence which may blend into other massive tills.
h i n size :omposition
4brasion in traction zone Juring lodgement Jroduces silt-size ,articles typical of odgement tills. Most iave relatively consistent ;rain-size composition :xcept for the basal part which may contain Ioulders of local glacier xd.
Winnowing of silt and clay-size particles occurs during melt-out. Some particle size variability inherited from debris bands in ice. Supraglacial meltout tills of valley glaciers contain characteristic coarse grained debris.
Usually diamicton with polymodal particle size distribution. Some particle size redistribution and sorting may occur during flow. Inverse or normal grading may develop.
Deformation tills derived From weak rocks contain :lasts separated by minor mounts of finer matrix. Zlast size reflects bedding .hichess of original naterial.
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--~ - - - --
__
Table 2.1 Characteristics of genetic till types (from Dreimanis. 1988 and Elson, 1988) (Continued) kiterion
Lodgement till
Melt-out till
ilow till
Ieformation till
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~
.ithology of lasts and natrix
,ithological composition )ften more consistent han other tills. lomposition of matrix mticularly uniform. Materials of local ierivation increase in ibundance towards basal :ontact.
Supraglacial melt-out .ill more variable in :omposition with increased possibility of :xotic material.
,i thological ,omposition generally ame as source naterial. May include ncorporated glacier )ed or exotic naterials depending )n debris source, ransport and Ieposition.
)eformation tills ;enerally have same ithological composition i s underlying sediments. 3ccasional erratics present iarticularly in upper part )f the sequence.
:last shapes md their iurface narks
Subangular to iubrounded clasts. Bullet-shaped, faceted, :rushed, sheared and streaked-out clasts more :ommon in lodgement ;han other tills. Lodged :lasts striated parallel to direction of the lodging movement.
Variable degree of roundness but angular clasts occur where supraglacial melt-out debris is englacially or supraglacially derived.
f present, soft iediment clasts may )e rounded or ieformed by shear. More resistant rock :lasts will retain their xiginal shape.
,last shape and surface narks generally inherited From original material and iot diagnostic. Clasts p e r a l l y transported passively and not significantly modified.
3bric
Strong macro fabric with Fabric inherited from clast long axes parallel to glacier transport. Meltout process may local direction of weaken fabric, movement. Transverse particularly microorientation possible, fabric. associated with folding and shearing.
Fabric may be random or strongly ieveloped and_ parallel or transverse to flow direction. Fabric may vary laterally over short distances.
Preferred orientation rare and generally reflects shearing deformation.
Consolidatior permeability density
Most lodgement tills over-consolidated if adequately drained. Bulk density, penetration resistance, and seismic velocity usually high, whilst permeability low, relative to other till types
Melt-out tills less overconsolidated than those formed subglacially. Bulk density and penetration resistance lower and more variable than lodgement till. Permeability more variable.
Usually normally consolidated and relatively permeable. Density lower than in lodgement tills.
Variably consolidated. Low densities reflect dilatancy due to continuous glacial shear stress.
Relevant references for summaries of diagnostic properties
Goldthwait (1971). Boulton (l976b), Dreimanis (1976). Boulton and Deynoux (198 I), McGown and Derbyshire (1977). Eyles er ul. (1982).
Boulton (1976b), Dreimanis (1976). McGown and Derbyshire (1977), Lawson (1979), Boulton and Deynoux (1981). Shaw (1985).
Boulton (1976b). Lawson (1979, 1982) Boulton and Deynoux (I98 I), Lutenegger ei ul. (1983). Gravenor er (11. (1984), Rappol (1985), Drewry (1986).
Elson (1988), Boulton (1979).
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In hard rock terrains and in the glaciated valley system, lodgement tills result from the ‘plastering’ of glacial debris from the sliding base of a moving glacier, by pressure melting a n d o r by other mechanical processes (Hambrey, 1994). Debris is deposited as individual clasts or as sheets or slabs. In the dynamic pressure and temperature environment beneath a glacier it may be reincorporated and redeposited repeatedly. Debris is released by pressure melting ice and is lodged as the result of collision or frictional resistance exerted by the glacier bed. Lodgement may occur on both bedrock and older till surfaces and may fill existing cracks or hollows and form oval or cigar shaped drumlins (McGown, 1985). Plates 1 and 2 illustrate the range of particle sizes which may be demonstrated by individual tills. The hard rocks traversed in highland Britain often contain only a small proportion of clay minerals, and clay- and silt-sizes in the resulting tills may contain ‘rock flour’, usually particles of quartz or feldspar, typical of the rock-forming minerals of the host terrain. Deposition of glaciated valley tills involved coarse clasts which backed-up behind cobbles and boulders. Finer sand- and gravel-sizes were forced into them, creating a hard and dense deposit; this occurred when tills were lodged against ridges of glacier bed or were thrust into hollows or crevasses. Downstream deposition could be patchy and poorly sorted.
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2.4.2 Melt-out tills Melt-out is the slow release of debris from glacier ice which is neither sliding nor deforming internally (Hambrey, 1994). It is particularly common at the margins of glaciers where there is stagnation. Subglacial melt-out occurs primarily in response to geothermal heat where the ice is thick enough to insulate the deposit from freezing ambient temperatures. If the water escapes without disturbing the sediments, traces of the original fabric may be preserved. Supraglacial melt out occurs mainly in the summer and, as the debris is released, there may be sliding and reorientation which destroy the original fabric. Fine grained particles will have been winnowed out during deposition, often to be concentrated in pockets or lenses elsewhere. Clasts may be widely variable in rock type, origin and size. Gap grading is common. Conditions of deposition of melt-out tills are such that they are ‘let-down’ out of a disappearing ice sheet and may be normally or only lightly over-consolidated and softer or looser than the corresponding lodgement till. There is often a wider variety and frequency of larger and more angular clasts. Melt-out tills from glaciated valley landsystems may demonstrate particularly coarse and angular particle sizes.
2.4.3 Flow tills These tills originated from englacially transported material. They are derived from debris exposed at the surface of the ice during wasting which flowed down-glacier to congregate at the retreating glacier snout. In the English lowlands, the rocks traversed were typically weak Triassic, Jurassic and Cretaceous sediments and the products were typically clay- to sand-sized, with numerous gravel-sized clasts of harder sandstones and cherts. In the highlands, coarser cobble- and bouldersized clasts of hard rocks were transported englacially and also occurred as lateral and medial moraines on the glacier surface. In the great ice sheets which covered much of lowland Britain, relatively little was transported on the glacier surface itself, apart from wind-blown silts and fine sands. Because of their depositional environment, melt-outs are usually of low density, coarser and more varied in grain size than the lodgements with little or no orientation of the clasts. Flow tills were deposited at water contents varying from their liquid limit (or above) to their plastic limit (or below) and show a wide range of sedimentary structures including banding, laminations, slumping or folding, all with strong orientation. Under the influence of water, winnowing and sorting were common with some horizons depleted in clay sizes and others correspondingly enriched. Laminated clays formed in pools bounded by ice and by older deposits.
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2.4.4 Deformation tills Glaciotectonic deformation takes place when the stress imposed by the glacier or ice sheet on the glacier bed exceeds the bed strength and the material undergoes brittle or ductile deformation. The results may range from a ‘glaciotectonite’ comprising deformed glacier bed sediments, to the incorporation of glacier bed sediments in the basal traction zone or elsewhere in the ice mass and the subsequent deposition of the sediments as deformation till.
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These tills can also comprise large slabs of original bedrock, either intact or more usually broken and fragmented, but traces of the bedrock’s primary structure may persist. Clasts are angular and their size may reflect the original bedding or joint spacing of the bedrock. Pockets or lenses of unconsolidated sediment may be randomly dispersed within the deposit. Large rafted blocks may be seen. An example of a raft of chalk picked up bodily within an otherwise matrix dominant till is shown in Plate 3. Note how the long chalk raft has survived intact, suggesting that the surrounding till was fluidised at the time of deposition. More structural features are illustrated in Plate 4. In highland Britain, the hard igneous, metamorphic and ancient sedimentary rocks traversed by valley glaciers produced a wide range of particle sizes from clay to boulders (Plate 5). Sometimes, whole rafts of rock were ripped up by the sole of the glacier and re-deposited with the original rock structure still evident. Harris (1991) describes a sheared zone at least 2 m thick between relatively undisturbed metamorphic rock and locally derived lodgement till at Wylfa Head, Anglesey, and Money (1983) reports large rafts of bedrock within tills at dam sites in the UK. These bedrock rafts and sheared bedrock zones can cause substantial problems when determining rockhead (see Appendix B, Case Study I ) .
2.5
TILL RELATED LANDFORMS
Glacial erosion and deposition produced characteristic geomorphological features or landforms. Their recognition may aid identification of landsystems and hence prediction and interpretation of ground conditions (Section 2.2). A description of some of the more common landform types is given in Table 2.2. The term ‘landform’ is restricted to the feature when exposed at the surface. However, glacial successions may include ancient landforms buried by younger materials. They have considerable engineering implications as they may lead to unexpected lateral variations in sediment thickness or uneven rockhead surfaces. Buried erosional landforms formed by ice or meltwater can produce significant problems for the engineer, particularly in environments where surface topography is subdued and wide variations in rockhead elevation are unexpected. Whalebacks and roches moutonnkes are above surrounding rockhead elevation, whilst glaciated valleys and meltwater erosion features are below rockhead elevation. A good example of whalebacks is illustrated in Plate 6; the rocks themselves are striated and fractured, possibly by stress relief; The typical U-shaped lateral profile of glaciated valleys is well understood by engineers, but less well known is the variation in longitudinal profile which may be a result of over-deepening due (for example) to the entry of a tributary glacier. In these circumstances it is unwise to assume that depth to rockhead along the long axis of a glaciated valley will be of uniform gradient. Depositional landforms buried by younger deposits may lead to unforeseen variations in soil profile. Where preserved on the ground surface, their presence can usually be established during the desk study or walk-over phase of the site investigation (Section 6.1). Drumlins are probably the best known depositional landform and are commonly elongated parallel to the ice-flow direction. Some drumlins have a bedrock core which can be confused with larger boulders at the site investigation stage. Moraines are the other main type of depositional landform, the most common of which are described in Table 2.2.
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Table 2.2 Summary of erosional and depositional landforms ?ormat ion Land form irocess tY Pe
Approx scale'
Landform description
Irosion by lacier
10 m 100 m
Smooth, scratched, glacially eroded bedrock knoll with whale-shaped profile.
Roche moutonnte
1 m100 m
Rocky hillock, with a gently inclined, smooth slope facing up-valley and a steep rough slope facing down-valley.
Glaciated valley (fjord)
100 m 100 km
Glacially eroded bedrock often U-shaped and usually with sediment infill. Known as fjord where valley floor below sea level.
10 m 50 km
Steep, often vertically sided gorge, cut into bedrock, often along lines of weakness.
I m100 km
A term to encompass a variety of channels created by glacial meltwater.
Crosion by ieltwater
Whaleback
Subglacial gorge
'
Meltwater channel
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Meltwater pothole kposition
Engineering implication
Erosional features particularly well formed in crystalline rocks. If buried by younger sediments they may produce unexpected variations in rockhead depth.
May be produced in preglacial bedrock or in glacigenic sediments. When filled and buried by younger sediments they may produce unexpected variations in rockhead depth.
Lateral moraine Features representing different geomorphologica arrangements of till Non-orientated landforms generated by valley deposition. Where glaciers during retreat or stagnation. Debris preserved, their recognitioi deposited as a chaotic arrangement of hummocks. aids landsystem Streamlined hillock, commonly elongated parallel to identification leading to a ice-flow direction and sometimes having a bedrock better three-dimensional core. Can occur singly or in swarms of hundreds, understanding of the orientated with the blunt end facing upstream. glacial sequence. Where buried by later sediments, Rounded, strongly linear ridges often extending they may contribute to downstream of large boulders embedded in complex, laterally lodgement till. inconsistent glacial sequence. Belts of glacial debris formed perpendicular to ice motion at the front of a glacier; often eroded by meltwater during glacier retreat. from supraglacial and englacial debris.
'
2.6
Hummocky moraine
100 m 100 km
Drumlins
IOm10 km
Fluted moraine
10 m I km
End moraine
10 m 100 km
Scale indicates the full range encountered in all glacial environments and does not necessarily reflect the range likely to be encountered in Britain.
OTHER GLACIGENIC SEDIMENTS
The previous sections described the characteristics and classification of tills which were defined as glacial debris deposited from or by a glacier with little or no sorting by water. However, further water-borne transport of glacial debris is common and the resulting sediments are frequently associated with tills in preserved glacial sequences. A knowledge of these sediments and characteristic landforms is essential for the identification of landsystems. Their genesis and characteristics merit discussion because, as will be seen, they can have a marked effect on the engineering behaviour of a till-dominated succession. Water-borne transport is usually by meltwater which may be produced supraglacially, englacially or subglacially by one of the following mechanisms: surface melting supplemented by rainfall and run-off from melting snow along the valley sides subglacial melting due to geothermal and frictional heat melting due to the pressures produced by mass flow.
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Transport may also occur as a result of non-glacial streams eroding the glacier margin or from wave action where a glacier terminates in water (Lundquist, 1988b). Once mobilised, debris is transported and then deposited as waterlain sediments and classified as glaciofluvial, glaciolacustrine or glaciomarine, depending on the environment of deposition. Waterlain glacial deposits demonstrate similar structures and characteristics to other waterlain sediments. Distinguishing them as glacigenic may not be straightforward. High flow regime, cyclic sedimentation and dropstones may give an indication of glacial origin, but frequently the most reliable indication of their origin is their proximity to tills, microfossil and mineral content.
2.6.1 Glaciofluvial system
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The glaciofluvial system is the drainage for a glacier and, in many glaciated areas, is the predominant pathway for sediment deposition (Figure 2.1). Glaciofluvial sediments are therefore widely distributed in formerly glaciated areas and are most commonly found as sand-gravel-cobble deposits occurring as mounds or extensive plains with extremely variable form and texture (Jurgaitis and Juozapavicius, 1988). Glaciofluvial sediments are deposited primarily in three main settings: (a)
subglacially in channels cut into till or into solid bedrock beneath the ice to form eskers. They are usually parallel to the direction of ice movement and may be sinuous or straight, bifurcated or beaded. They typically range from 10 m to 100 km in extent and are often preserved as ridges on a sheet-deposit of till.
(b)
ice-marginally where streams of both glacial and non-glacial origin are forced to flow along the ice margins. Kame deposits occur as terraces or isolated hummocks. They have no specific orientation and may range from 100 m to 10 km in extent.
(c)
proglacially in an ice frontal position or down valley as the braided meltwater streams spread across the valley floor to form low relief debris sheets known as outwash plains (Figure 2.8). These outwash plains can vary up to 50 km in extent.
coarse gravel (>loo mm size) sand
fine gravel (cl00 mm) vegetated outwash, marsh, moraine
mudflats glacier ice
Figure 2.8
40
bedrock
Example of glaciofluvial deposition, Scott glacier, Alaska (Leeder, 1982)
CIRIA Report C504
Outwash plains and kames may contain bowl-shaped depressions called kettleholes. These collapse features were created by the post-depositional melting of ice masses originally incorporated in the glaciofluvial deposit. Kettleholes were frequently filled with water and sediments so that the features may be preserved as silt or clay filled depressions. The engineering significance of soft clays or silts in otherwise dense coarse granular deposits should be clear. Kettleholes can also form in moraines where ice blocks melted in situ. The above gives rise to a tripartite genetic classification of subglacial, ice-marginal and proglacial deposits within which further subdivision is based on the geometry of the deposit (Jurgaitis and Juozapavicius, 1988). Where preserved, the geometry of the deposit may be recognised as a landform which will aid early identification and interpretation of the local landsystem (Section 2.2).
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2.6.2 Glaciolacustrine system The glaciolacustrine system is an extension of the glaciofluvial system and may occur subglacially, supraglacially or proglacially, wherever meltwater becomes ponded into lakes (Figure 2. I). Sediments deposited within this system are more widespread than suggested by the number of reported sites and they may reach considerable thicknesses (over 20 m) in glacially over-deepened troughs (Derbyshire, 1975). Depositional processes within the glaciolacustrine environment are varied and give rise to complex assemblages of facies depending on the following interrelated factors (Ashley, 1988):
. proximity to ice lake basin geometry slope stability nature of the sediment source position of the meltwater inflow within the lake water column relative densities of stream and lake water nature of the lake density stratification seasonal and non-seasonal effects influencing ice cover and run-off. .I
U
Of these various factors, the density differences within the water mass and between the lake water and meltwater streams supplying the debris are particularly important in controlling sedimentation patterns. Higher density sediment-laden meltwater will sink to the bottom and act as an underflow whilst a low-density meltwater will rise and act as an overflow (Figure 2.9). Identical or similar densities lead to interflow or mixing. The density of the lake water is controlled by temperature so the concentration of dissolved salts and the amount of sediment in suspension varies both seasonally and vertically. Seasonal variations in lake water density, and hence in sedimentation, give rise to the annual cycles preserved in characteristic glaciolacustrine varvites (Derbyshire 1975, Fookes et al., 1975a).
-
decreasing sedimentation rates
PROXIMAL
'\.a settling out of suspended sediment diamidton
t A..
cross-bedded sand and gravel
'-
.A ice-ratted debris - .. - - -
stumping
diamicton
(debris-flows)
BOlTOM SETS
/ ./
laminated (rhythmites or varves)
Figure 2.9
-
DISTAL homogeneous clay
Processes and sedimentary products in glaciolacustrine setting (from Hambrey, 1994)
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Damming of meltwater by topographic features, which may have been glacially modified, provides a variety of glacier-fed lakes which differ in size, longevity and sedimentary characteristics (Ashley, 1988). However, all are fed by meltwater and can be classified according to their proximity to the ice as either ice-contact (proximal) lakes or distal lakes. Ice-contact lake sediments are highly variable in lithofacies with a mixture of mass movement and current related deposits and, except for large semi-permanent lakes, they are unlikely to demonstrate temporal or spacial trends in grain-size and sorting (Ashley, 1988). Conversely, distal lakes are beyond this dynamic and variable environment and are characterised by fluctuating sediment input combined with seasonal lacustrine processes.
2.6.3 Glaciomarine system
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Glaciomarine sedimentation is controlled primarily by the climatic setting, the nature of the glacier interface with the marine environment and the water depth of the basin of deposition. Sediments comprise a mixture of glacial detritus and marine sediments deposited more or less contemporaneously (Andrews and Matsch, 1988). The glacier component may be released directly from glaciers or indirectly by gravity (slumping), stream flow or iceberg rafting. Glaciomarine sediments, therefore, vary laterally from ice-proximal diamicton, gravel and sand facies through an intermediate gravelly silt and mud facies, to distal marine environments where the glacial imprint is seen as ice-rafted debris (Andrews and Matsch, 1988). The environment is generally subdivided into two main geographical settings (Hambrey, 1994): Fjord setting: sediments are supplied from glaciers in the form of ice-contact deposits, glaciofluvial deposits and iceberg-rafted debris. In addition, there is marine input of suspended sediments and biogenic material, and terrestrial input comprising fluvial, rockfall, aeolian and gravity-flow debris (Figure 2.10). Continental shelf and deep ocean setting: sedimentation is dominated by grounded icemargins, floating ice tongues, ice-shelves and open marine processes. Sediment is supplied predominantly from subglacial debris exposed at the grounding line with minor inputs from supraglacial debris and debris transferred from the ice to the marine environment, via ice shelves, ice cliffs, outlet glaciers and ice streams (Figure 2.1 1). Where the margin of the ice mass within these two settings ends at the terrestriaVmarine boundary, a distinct set of ice-marginal landforms and deposits develops with clear evidence of glacial origin. Where the sediments are deposited some distance away in an ice-distal glaciomarine environment, the glacial origin is less distinguishable and may be represented by a small proportion of ice-rafted debris, a change in sedimentation rate or even a change in the microfossils present in the sediments.
ciraue
- _\,
It
lodgement
1
4 alacial meltwater
~rarispori
subglacial meltwater
c
isusDensionb
subaqueous outwash deposition
nonglacial
7- 7
rhythmites
bedrock
gladiotectonic deformation
Figure 2.10
Sediment sources and processes operating in a rjord influenced by grounded tidewater glacier (from Hambrex 1994)
.
~-
L
42
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-~
minor supraglacial debris (ablation zone)
ll
minor englacial debris basal debris with diamict apron beneath
minor ice-rafting
.......... .......... .......... .......... .......... ..........
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minimal ooze andlor sedimentation diatomaceous
ICE-FILLED FJORD
OUTER CONTINENTAL SHELF (BANKS)
INNER CONTINENTAL SHELF
(a) Ice shelf in recessed state
grounding line at palaeoshelf break melt-out close to grounding line with steady sediment supply from wetbased glacier
I .
subaqueous gravity flow
minor supraglacial debris
....................................... ...................................... ....................................... ...................................... ....................................... ...................................... ....................................... ......................
limited ice-rafting
\
..............
I
“ 7 -
\ ’
massive diamict (waterlain till)
‘;veakly stratified diamict (waterlain till to proximal glaciomarine sediment)
7
dropstones
slumping I
till or deformation till)
biogenic
............... .... ............ ell stratified
(b) Ice shelf advanced t o edge of continental shelf
Figure 2.1 1
Ice dynamics, sediment sources and sedimentary processes, products and their interpretation at the margin of the Antarctica ice sheet (from Hambrey, 1994)
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2.7
SUMMARY OF SECTION 2 The glacigenic environment is one affected by direct or indirect glacial activity. Glacioterrestrial, glaciofluvial, glaciolacustrine and glaciomarine environments are identified (see Figure 2. I). Tills are of glacioterrestrial origin, deposited by or from glacier ice (or ice sheets) with little or no sorting by water. The glacigenic environment may be generalised into three distinct landsystems: subglacial, supraglacial and glaciated valley. Each has its characteristic deposits, landforms and rockhead morphologies. Tills are lodgement, melt-out, flow or deformation in origin. All reflect the nature of the rocks traversed by the glacier or ice sheet.
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Glacial erosion and deposition produced characteristic geomorphological features or landforms which aid identification of landsystems. Buried erosional and depositional landforms also exist and may introduce significant engineering problems.
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Geology of tills: review of glacial stratigraphy in the UK
3
Glaciation may have affected the British Isles between eleven and sixteen times in the Pleistocene with other cool or cold phases (DOE, 1994). At least three glaciations occurred in the past 500 000 years, namely the Anglian, the WolstoniadPaviland and the Devensian, with possibly two others (Figure 3.1). Ice sheets frequently reworked or removed deposits of previous cold stages but in some areas it is possible to find older tills preserved. Kirkhill in north east Scotland (Buchan) is a good example of a preserved succession formed as early as the Anglian.
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It is essential to be aware of the dynamism of the glacigenic environment in space and time. Two factors complicate the compilation of a chronostratigraphy of onshore tills. The first is the occurrence of tills of different provenance at the same locality deposited during the same glacial episode (often contemporaneously); during early glacial studies they were often incorrectly attributed to different glacial episodes. The second factor is the incomplete preservation of the sediment record due to syn- and post-depositional processes which often reworked or removed large parts of the glacial record both spatially and temporally.
Oxygen-isotope ratio -1 -2 0
Years (BP) ........ ........
10000
100000 200000
h
E v a,
..........
300000
.-E
..........
400000
3
..........
500000
m a, $10
..........
600000
..........
700000
..........
800000
..........
900000
..........
1000000
g 5 c
U
c
I
.-c
15
Colder More Ice Low sea-level
Figure 3.1
Devensian Glaciation interglacial (Ipswichian)
Glaciation (Upland Britain?) interglacial
Wolstonian Glaciation interglacial (Hoxnian)
Glaciation? interglacial
Anglian Glaciation
Sedimentary evidence for five earlier glacial episodes
v
Warmer Less Ice High sea-level
Climate over the past million years based on analysis of oxygen-isotope ratios from deep sea sediment cores (after DOE, 1994)
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The generally accepted sequence of glacial events and stratigraphies which affected the UK and adjacent continental shelf is shown in Table 3.1. The earliest evidence of glacial conditions comes from deep sea late Pliocene sediments, near Rockall, which are c.2.4 million years old (Shackleton et al., 1984). However, the Pleistocene epoch is recognised as the main period during which ice sheets were present in the UK. Early Pleistocene glacigenic deposits from the Midlands indicate that glacial conditions were probably present in northern England and Wales during this time. The deposits comprise predominantly glaciofluvial sediments and are not included in Table 3.1. Preserved tills date from the middle to late Pleistocene. The Anglian, WolstoniadPaviland and Late Devensian cold stage environments and their respective glacigenic deposits have been studied extensively. The ice masses which developed are recognised as being dynamic and extensive with several source regions. Evidence of the Early Devensian stage is limited and open to question, but generally confines glacial activity to north east Scotland, the Moray Firth Basin, eastern England and parts of the Midlands. Ice may well have covered the whole of the UK north of the proposed maximum, but no evidence remains, presumably having been removed or reworked by the subsequent Late Devensian ice. The Loch Lomond glaciation (also known as the Loch Lomond Readvance) is the most recent glacial stage. It was largely limited in extent to the west highlands of Scotland and eastward to the Grampians, together with the Lake District and parts of Wales. Although an ice dome developed over the west highlands, the Loch Lomond glaciation is characterised by corrie and valley glaciers of the glaciated valley landsystem rather than by extensive ice sheets. Figures 3.2 to 3.7 illustrate the maximum extent of the main glaciations which affected the UK and the continental shelf since the middle Pleistocene. Within any glaciation it is possible to have readvance and retreat stages before the ice reached its maximum extent. These are not represented in these maps as they are within the maximum boundary. Taken together, Figures 3.2 to 3.7 highlight the areas subjected to more than one glaciation. Further details of the till chronostratigraphy and distribution are given in a Tables 3.2 to 3.6 (at the end of this Section) which present information on the characteristics of the main till groups described and classified. They are not exhaustive and are designed to illustrate the variability of the depositional environments. There remains significant controversy over the chronostratigraphy of UK tills; any till encountered in the field should be assessed as a unique lithofacies and described and classified on its own merit before being correlated with other local tills. When referring to localities and in accordance with accepted practice, local till names are used: e.g. Cromer or Withernsea till. Where there is no local name, the tills are identified by their regional context: e.g. Lake District or Northumberland tills.
3.1
ANGLIAN
The extent and descriptions of till units ascribed to the Anglian glaciation are presented in Figure 3.2 and Table 3.2. In England, two main tills are recognised within this glaciation: North Sea drift or Cromer tills Lowestoft tills. The North Sea drift or the Cromer till was deposited by the initial ice advance of the Anglian. Ice originated in southern Norway, spread southwest across the present-day North Sea, and expanded into northeast and east Norfolk. The origin of the ice is evidenced by tills containing erratics and minerals of Norwegian provenance. A further advance occurred when ice originating in northern Britain advanced into the Fenland Basin and then radiated out from the advancing ice stream into Norfolk and the English Midlands. The greater strength of this ice advance deflected the downwasting and retreating Scandinavian ice mass and deposited the Lowestoft tills (previously called chalky boulder clay). These tills frequently incorporated reworked tills of the North Sea drift. L
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rable 3.1 Schematic representation of main glaciations in the UK for Middle to Late Pleistocene
c
C
a,
15
50
n
-1
m-
W
Notes I 2 3 4
m
a-
0.-
VI-
0 0
m c
m
I
>-vu-
m
a - a.- m - o o m c m
Stockport Formation and Irish Sea Tills are equivalent. 'Wolstonian' offshore only. Onshore Wolstonian may be equivalent to Anglian. Lowestoft 'lills and North Sea Drift XIISare contemporaneous. Marine - UK continental shelf (E - North Sea; N - northern North Sea; W - Western Approaches, shelf edge)
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The North Sea drift and Lowestoft tills were contemporaneous in part (Bowen et al., 1986) but the latter are far more extensive. The Lowestoft tills are found in eastern England as far south as the north London area, and can also be traced into the Midlands where variations in lithological composition have led to the use of local till names (Table 3.2). In north Norfolk and around Norwich, the North Sea drift and Lowestoft tills form interdigitating deposits. In north east Norfolk, the Lowestoft tills are absent, suggesting that only Scandinavian ice was present in this area during the Anglian. Recent research has suggested that some Anglian deposits were previously misinterpreted as younger Wolstonian (Sumbler, 1983a, 1983b). The body of evidence is increasing (Bowen et al., 1986; Jones and Keen, 1993) and there may be some redefinition of glacial stratigraphy. Table 3.2 illustrates those tills currently ascribed to the Anglian cold stage.$
3.1.1 North Sea drift
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The succession of tills found at Cromer, Norfolk, represents the type section for the North Sea drift. The sequence has been variously interpreted as glacioterrestrial (Hart, 1988), glaciomarine (Gibbard and Zalasiewicz, 1988; Eyles, Eyles and McCabe, 1988) or a mixture of both (Lunnka, 1988). Three tills have been recognised at Cromer: the First, Second and Third Cromer tills (Plates 3 and 4), in order of decreasing age, interbedded with glaciofluvial deposits (Banham, 1968, 1971). According to Shotton et al. (1977), the first two are lodgement whereas the third would be largely melt-out. They have small characteristic differences in colour and carbonate content but maintain lithological unity being,of the same provenance and containing Scandinavian minerals (Perrin et al., 1979). Some Scandinavian erratics have been found as far south as Bedford, Hitchin, Cambridge and Ipswich, illustrating the southern extent of the ice advance, although the Cromer tills have not been recognised in these areas. The presence of a till possibly contemporaneous with the Cromer tills in the Forth Approaches off the east coast of Scotland has also been proposed (Stoker and Bent, 1985).
3.1.2 Lowestoft tills The Lowestoft tills type site is Corton, Suffolk. The deposits comprise a silt-and-clay-rich diamicton with predominantly chalk clasts. The main local variations of the Lowestoft tills include the following: (a) Marly drift: chalk-rich diamicton which separates the Cromer and Lowestoft tills north of Norwich and includes the Gipping till (b) Contorted drift: Cromer till deformed by weight of overlying Cromer Ridge gravels (Banham, 1971) (c) Triassic till: Triassic-clay-rich diamicton (d) Oadby till: Lowestoft till with sand- and gravel-rich basal units found in Essex and Hertfordshire (e) Thrussington till: interdigitates of Marly drift and Triassic till found in Leicestershire. The Oadby and Thrussington tills have also been attributed to the younger Wolstonian (Section 3.2), although recent work suggests they may belong to the Anglian. The evidence remains equivocal and further research may reinstate the onshore Wolstonian as defined by Shotton (1953, 1983).
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?
Southern limit of Anglian/Munsterian Glaciation (after Bowen et al., 1986)
l_... IIIIp ..... .... ..... ......... .... ..... ?
Margin ice-dammed lake at maximum glaciation
0,
Ice cover Unknown extent Note: The Anglian cold stage included the most extensive ice cover within the Middle to Late Pleistocene. The western and northern extent is poorly constrained but probably consists of the morainal banks present at or near the present shelf break, in association with probable ice stream channels.
Figure 3.2
Maximum extent and possible southem limit of Anglian Glaciation (adapted from Jones and Keen, 1993)
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Other Anglian deposits identified in the UK and its continental shelf are shown in Table 3.2 and include the Nurseries till and the Bubbershall clay of the West Midlands and Birmingham area. In the North Sea Basin, the main evidence for the Anglian stage is the presence of deeply incised valleys infilled with glacigenic sediments of the Swarte formation (Cameron et al., 1987).They may be interpreted as tunnel valleys or subaerial drainage channels incised by enormous quantities of meltwater and developed in an area without ice cover, although this hypothesis conflicts with onshore evidence (Ehlers and Gibbard, 1991).There is further evidence that tills were deposited in the north and south of the North Sea Basin, but little evidence of tills outwith the channels developed in the central North Sea Basin (Cameron et al., 1987;Holmes et al., 1993,1994).
3.2
WOLSTONIANlPAVlLAND
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The possible extent of the WolstoniadPaviland ice mass and associated tills is presented in Figure 3.3 and Table 3.3. This cold stage consists of the separate Wolstonian and Paviland glaciations and can be correlated with the Saalian identified in the Netherlands. As noted (Section 3.l),some controversy exists over the age of some of the WolstoniadPaviland tills which may eventually be redefined as Anglian. Evidence for the WolstoniadPaviland glaciations is limited. The type succession was established at Wolston and described by Mitchell er al. (1973).Two tills, the Lower Wolston Clay (containing Triassic sediments) and the Upper Wolston Clay, were deposited between periglacial glaciolacustrine and outwash deposits and are indicative of two separate ice streams. The first originated in the north/northwest and the second in the northeast of the type locality. Tills attributed to this cold stage are also preserved in east Yorkshire (Basement till), east Lincolnshire (Welton till) and Durham (Warren House till). In addition, possible Wolstonian tills have been identified in north east Scotland, the midland valley of Scotland, Speyside (Sutherland, 1984)and in the English Midlands. However, neither the southern limit nor the full extent of the ice cover is well established (Shotton, 1986). As already noted, Shotton (1953,1983),working in the Coventry-Leicester area, placed the Oadby and Thrussington tills in the Wolstonian. The younger Oadby is subdivided into upper and lower and is separated from the Thrussington till by a sequence of sands and silty clays. The tills possess distinct mineralogical (Scandinavian) erratics and biostratigraphic and lithostratigraphic successions which allow them to be distinguished from the older Anglian and younger Devensian tills found in the same areas of north east England, Yorkshire and Lincolnshire (Bowen et al.,
1986). Evidence from Wales suggests that there was an independent glaciation (Paviland) active during this cold stage (Bowen er al., 1986),and evidence from Ireland also indicates that localised ice sheets and corrie glaciers developed during the Munsterian cold stage (Mitchell et al., 1973). In the North Sea, the sediment succession suggests that ice incursion into the basin was limited. It is unlikely that ice from continental Europe or Scandinavia was in contact with ice from the UK, or that ice extended from mainland Europe on to the UK continental shelf (Cameron et al., 1987).
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Ireland
,+++-Ice axes or domes
---- e VG
U
Irish inland ice Irish Sea and North Channel Ice Valley glaciers and mountain ice-caps
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heast Scotland
Ice limits 1 2 3
Gipping ice front (Baden-Powell, 1948) Gipping maximum glacial advance (West, 1968) Wolstonian ice advance (West, 1977)
4
5 6
Wolstonian ice margin (Straw, 1983) Wolstonian ice margin (Catt, 1981) Southern limit of the Welton / Paviland / Saale Glaciation (Bowen et a/., 1986)
Major Wolstonian ice flow (Straw, 1983)
Figure 3.3
Wolstonian / Paviland Glaciation /Munsterian (Ireland) possible ice coverage and extent (Adapted from various references cited in the key and in Section 3.2)
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3.3
EARLY DEVENSIAN
The concept of an Early Devensian cold stage remains as yet unproven; some evidence exists which suggests that ice sheets may have been present during this stage. The possible extent and list of tills tentatively attributed to this cold stage are presented in Figure 3.4 and Table 3.4. There is little to support an Early Devensian glaciation, although Straw (1 979b) and others suggested stratigraphic and geomorphological evidence from the east of England. Elsewhere in the north and west of England, any Early Devensian tills would almost certainly have been reworked during the subsequent Late Devensian.
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In Scotland, the concept of an Early Devensian cold stage is also unproven. The two upper tills of the succession at Kirkhill (Buchan) have been attributed to this stage, and contemporaneous sediments in the North Sea have been interpreted as ice proximal and ice distal glaciomarine deposits (Chesher and Lawson, 1983). Similarly, tills in Ulster have been attributed to the Early Devensian cold stage but the evidence is not conclusive (Colhoun et al., 1972; McCabe, 1987; McCabe et al., 1987).
3.4
LATE DEVENSIAN
The tills attributed to the Late Devensian stage are the best documented of all tills in the UK. They were formed by the last major ice mass and are often the materials from which present soils developed and were least affected by post-depositional processes. The extent of ice cover and main ice-stream directions are presented in Figure 3.5. Table 3.5 contains information about the main till groups. The thickness of the ice predicted to have covered the country in late Devensian times is illustrated in Figure 3.6. According to this model, an ice thickness in excess of 1800 m covered the western highlands of Scotland and of between 1400 and 1800 m much of the rest of the British Isles. The Late Devensian tills preserved onshore are generally attributed to the Dimlington glaciation, whilst those present offshore are attributed to the Late Weichselian glaciation. This differentiation arises because of the traditional incorporation of the offshore tills into the European stratigraphy. For the purpose of this work the stratigraphic differences can be considered secondary. The type locality for the onshore tills of the Late Devensian is Dimlington, Holderness, east Yorkshire. At this location, the Withernsea till (Derbyshire et al., 1984) and the Skipsea till are identified. At Chelford near Cheshire, tills of the Stockport formation have also been attributed to the Late Devensian stage (Worsley, 1985). Most of the Scottish mainland and Western Isles were glaciated during the Late Devensian, although it has been proposed that parts of north east Scotland were ice free (Sutherland, 1984). It is more probable that these areas were covered by cold-based ice which was inactive and did not alter the underlying landforms and sediments significantly (Stewart, 1991). At least two ice accumulation centres were present at Rannoch Moor and the Southern Uplands. The interaction of the two ice masses was crucial to the depositional processes. It has been suggested that Shetland maintained an independent ice sheet that extended up to 60 km east but was not in contact with the contemporaneous Scandinavian ice mass (Flinn, 1967). Extensive deposits of till relating to the Late Devensian stage are found on the UK continental shelf. These deposits are up to 20 m thick and suggest that almost 50 km of ice extended eastward from the present-day coast (Stewart, 1991). The position and type of the terminus of the eastern ice margin is currently open to discussion (Stewart, 1991). Similarly the extent of the ice mass in the west of the UK is also open to debate (Holmes et al., 1993, 1994).
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Figure 3.4
Possible Early/Middle Devensian Glaciation (adapted from Jones and Keen, 1993)
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3.5
LATE DEVENSIAN - LOCH LOMOND
The ice extent and description of common attributes of tills likely to relate to the Loch Lomond glaciation (readvance) are presented in Figure 3.7 (Scotland) and in Table 3.6. From Figure 3.7 it is clear that the ice mass which developed at this time was far more restricted in extent than that of the previous glaciation. In addition, the development of isolated corrie and valley glaciers was more prevalent than at the maxima of the previous cold stages. The most extensive ice cover in the UK at this time was in Scotland with the main ice accumulation at Rannoch Moor and the western Highlands, with a small outlier developed in the Grampian mountains. However, with the exception of the accumulation area in the west, most ice cover was as valley or corrie glaciers. In Wales, evidence of a readvance of corrie glaciers is also documented from Snowdonia and the Brecon Beacons (Jones and Keen, 1993). It is probable that where conditions were appropriate, the ice accumulated in other upland areas of Wales and extended as far south as the south Wales coalfield (Wright, 1991).
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In England, evidence of the Loch Lomond stage is limited to the Lake District, where valley and corrie glaciers developed but were restricted in extent. Tills associated with localised ice sheets, corrie and valley glaciers usually have a very localised provenance. Sediments were supplied by reworking existing deposits with the addition of coarse material, supplied as talus from exposed rockwalls, to the surface and margins of the glacier. The local provenance of the tills is maintained by ice flow, itself dominated by local topographic controls. This would prevent far-travelled erratics from inclusion in the tills, except from reworking.
3.6
SUMMARY OF SECTION 3 At least three glaciations affected the UK in the past 500 000 years: the Anglian, the WolstoniadPaviland and the Devensian. The extent of ice cover and till deposition for a particular glaciation does not always coincide because of later reworking. Two main tills are recognised within the Anglian: the North Sea drift and the Lowestoft tills. They are largely confined to eastern England but the North Sea drift also extends patchily into the Midlands and the north London area. They comprise lodgements of weak rock origin with Scandinavian erratics (North Sea drift) and chalk (Lowestoft till). The WolstoniadPaviland are separate glaciations. The Wolstonian tills occur in east and north east England, and the English and (possibly) the Scottish Midlands. However, the extent of ice cover is not well established and there is controversy as to the age of some tills. The Paviland is confined to south east Wales. According to some estimates, up to 1800 m of ice covered parts of the UK during the Late Devensian, the best documented of the British glaciations. Areas affected include north east and north west England, Scotland, Wales and much of Ireland. Extensive deposits of Devensian tills are found on the UK continental shelf. The Late Devensian Loch Lomond glaciation (or readvance) was restricted to valley and corrie glaciers located in parts of Scotland and possibly the Lake District and Wales.
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-
Dominant ice flow direction (not contemporaneous)
cold Cold-based (inactive) ice cover
?
Figure 3.5
Uncertain ice cover
Maximum limit of Devensian Glaciation and dominant ice flow directions (not all contemporaneous) (adapted from Jones and Keen, 1993 and Stewart, 1991)
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a
Ice thickness, metres
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I/
//
“Ice free Buchan”
> 1800 1400 - 1800 1000 - 1399
600 - 999 0-599
200
Figure 3.6
Model of extent and thickness of the Late Devensian ice sheet in the British lsles (from DOE, 1994). Compare Figure 3.5 for current interpretation of “ice free Buchan”.
Loch Lomond ice masses
L.
Areas of corrie glaciers Localised ice movement
---- Probable ice shed Note: Ice terminating in sealochs would have been coupled to submarine moraines and would have fluctuated in position with time. Valley/ outlet glaciers would have been dominant.
Figure 3.7
56
Extent and probable general ice direction of Loch Lomond Glaciation in Scotland (adapted from Sissons, 1980)
CIRIA Report C504
Table 3.2
Summary of Anglian and equivalent tills in the UK and on the UK continental shelf
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57
Licensed copy:Jacobs UK Limited, 06/04/2009, Uncontrolled Copy, © CIRIA
Table 3.2
Summary of Anglian and equivalent tills in the UK and on the UK continental shelf (continued)
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CIRIA Report C504
Table 3.2
Summary of Anglian and equivalent tills in the UK and on the UK continental shelf (continued)
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Summary of Post AnglianlSaalian ('Wolstonian'/Paviland) tills in the UK and on the UK continental shelf
Table 3.3
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Summary Table of Early/Middle Devensian tills in the UK and on the UK continental shelf
Table 3.4
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Table 3.5
Summary of Late Devensian tills in the UK and on the UK continental shelf
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Summary of Late Devensian tills in the UK and on the UK continental shelf (continued)
Table 3.5
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CIRIA Report C504
Licensed copy:Jacobs UK Limited, 06/04/2009, Uncontrolled Copy, © CIRIA
Table 3.6
Summary of Loch Lomond GlaciatiorVReadvance tills in the UK
c
h
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CIRIA Report C504
65
4
Engineering classification of tills
The type of landsystem, and hence the nature of the associated tills, is strongly dependent upon whether deposition was within lowland or highland terrains. Those parts of lowland Britain which were glaciated are largely occupied by weak rocks of Triassic to Cretaceous ages, while the corresponding parts of highland Britain~areunderlain by hard rocks of igneous and metamorphic, Pre-Cambrian and Palaeozoic origins. The type of rock from which the debris was derived contributes substantially to the geotechnical properties of the tills because it determines mineralogical content and strongly influences particle sizes. However, this is a simple picture and i t should not be forgotten that the ebb and flow of ice movements will mean that the products of one glaciation may be picked up and incorporated in later glacial advances.
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4.1
TILL FABRIC
Fabric was defined by Derbyshire et al. (1976) as the summation of all the directional properties of a till, including solids and voids. It includes clasts, layers, lenses, fissures, cracks and joints, varying in scale from less than a millimetre to tens of metres in extent. There have been frequent references to its effect on engineering properties of tills: e.g. Rowe, 1972; Grisak et al., 1976; McGown et al., 1974; Marsland et al., 1982; McGown, 1985; Hossain and McKinlay, 1991; Hossain, 1996. Work by Marsland (1977) demonstrated a clear link between amount of ‘fabric’ existing in the test specimen (in that case foliation fabric) and its undrained shear strength (Section 4.4.3). Derbyshire et al. (1985) distinguished between primary fabric (features produced during the depositional process) and secondary fabric (features induced by post-depositional factors). As a genetic classification it is logical, but the application of the terms primary and secondary is unfortunate because they suggest that ‘primary’ structure may in some sense be more important than ‘secondary’. As is well known, the discontinuities induced by ‘secondary’ post-depositional processes can exert more control over engineering behaviour than some primary processes. The classification is retained here, but the terms ‘primary’ and ‘secondary’ are removed in favour of ‘depositional’ and ‘post-depositional’ fabric, respectively. The topographic region (lowland or highland Britain), the nature of the bedrock and the landsystem have a controlling influence on fabric, as illustrated in Figure 4.1. A more complete interpretation of fabric in tills is given in Table 2.1.
-
4.1 .IDepositional fabric subglacial The massive shearing action of the ice sheets as they overrode the weak bedrocks of the English lowlands produced a relatively fine grained and homogeneous fabric with clasts varying from a mClange to orientated weak rock fragments. Orientated harder Scottish and Scandinavian erratics were also incorporated, depending upon the provenance of the ice. In the Scottish lowlands, the harder rocks produced a coarser, more gap-graded till with many more orientated hard rock clasts, in some cases similar to the glaciated valley tills. Thrusting and shearing during transport and deposition sometimes produced shear planes orientated sub-horizontally or as near-vertical conjugate shears, orientated parallel with the direction of glacier flow. Usually slickensided, these shear planes are quite unlike those produced by post-depositional processes, described below (Boulton, 1970; McGown, 1985). McGown (1985) reports thin clay, silt or sand coatings to the sub-horizontal shear surfaces formed in drumlinised tills in west central Scotland.
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66
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CIRIA Report C504
1 I Melt-out
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and flows
vcrtical fissures stress r e l i d on ice rctrent
Horicontal to near vertical fissiires freezetliaw. groundwater table
Figure 4.1
vcrticnl liswres s t r s s rclicf on ice retreat
vertical titsiires freczcthaw, groundwriter rnble fltictuarions
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General description of fabric type in glacial tills (see also Table 2.1)
CIRIA Report C504
57
4.1.2 Depositional fabric - supraglacial The fabric of melt-out tills is more variable in particle size, with clasts often disorientated. Particularly varied conditions exist in highland Britain: melt-outs formed from englacial debris originating many kilometres away may reside with products of lateral and medial moraines of local provenance. Where deposited in isolation, some englacial debris may demonstrate orientation parallel to the ice flow and the clasts vary from fine gravel- to cobble- and boulder-sizes. Angularity may vary too, but more rounded clasts are unusual, unless they result from the reworking of earlier lodgements. Depositional structure of melt-out tills is not readily observed, although conglomerations of finer particles may drape themselves over larger clasts. Deposited from the same englacial debris as melt-outs, flow tills may show more sedimentary features, including bands, lenses and layers, with normal or inverse grading developed. Particle sizes are varied but they may exhibit more fines than the melt-outs. Much depends upon the water content at deposition. This determines whether the debris will flow, slump or slide, with the fabric altered accordingly.
-
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4.1.3 Depositional fabric glaciated valley The forces generated during the lodgement process produce highly orientated and frequently deformed clasts, often bulleted, crushed or streaked-out. Orientation may be parallel to glacier flow or at right angles where debris was thrusted or forced. Striae are present, parallel to the direction of lodging movement. Particle sizes are usually bi- or multi-modal and low clay contents are common, the majority of fines being silt-size, with rock flour frequent. Shear during deposition may produce sub-horizontal to near vertical jointing, as discussed for lodgement tills in Section 4.1.1.
4.1.4 Post-depositionalfabric McGown (1985) found two distinctive types of fabric in drumlinised west central Scotland tills, believed to result from stress relief low-angle features of low asperity (from 1 to 4 mm) without any infill two or four sets of generally planar near-vertical features of medium to high asperity (2 to 8 mm), clean below 2 to 3 m depth and stained by weathering above. Average spacings of the features varied between 30 mm near ground surface and 140 mm at 10 m depth. Stress relief mechanisms such as the above would not affect melt-out and flow tills and would only be expected in matrix-supported subglacial and glaciated valley tills. Other effective stress changes which influence the development of fabric, including supraglacial tills, are post-depositional groundwater changes and freeze-thaw activity. In the latter, large suctions and pore water migration caused by advancing ice fronts give rise to vertical fissures. Ice lenses parallel to the advancing ice front are also produced, normally creating sub-horizontal fissures or joints.
4.1.5 McGown and Derbyshire classification A genetic classification helps to understand depositional processes, but it is of limited assistance to the engineering geologist in the field, who wishes to impart the maximum geotechnical information. A more appropriate approach would be to adopt the classification due to McGown and Derbyshire (1 977), who superimpose the ‘dominant soil fraction’ concept (Section 4.2.5) upon the genetic scheme. Each till type is subdivided into four dominant soil fractions and, for each till type, information is given on the fabric, thus aiding identification in the field and laboratory. The McGown and Derbyshire scheme is given in Table 4.1.
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. ..
The strength of the McGown and Derbyshire approach is that engineering information can be readily conveyed, albeit in relative terms; furthermore, the right hand side of Table 4. I quotes useful information on common geotechnical properties including relative density and compressibility. The addition of a soil description to BS5930 to this form of classification, supplemented if necessary by one of the suggested modifications (e.g. Norbury, Child and Spink, 1986), augments the engineering information to be obtained from visual inspection in the field or from the study of samples in the laboratory. Table 4.1
Characteristics and geotechnical properties of glacial tills (from McGown and Derbyshire, 1977) Relative scales ( 1 (low) to 9 (high) )
Till
Lodgement
DSF'
G
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till
Melt-out till
Mc
Overconsolidation ratio
Fabric features
MACRO: Interlayering of glaciofluvials, joints, fissures, contortions. Consistent preferred clast orientation. MESO: Fissuring. Contortion. Moderate to very high consistency of preferred orientation of clasts. MICRO: Moderate to high degree of parallelism of fines in sympathy with clast surfaces.
MACRO: Occasional interlayering with glaciofluvials. Clast preferred orientation often retained from englacial state. MESO: Moderate to high preservation of preferred clast orientation from englacial state, especially in subglacial type. MICRO: Open to moderately closed arrangements of fines with many englacial arrangements retained, especially in subglacial type. ~
Flow till
G
W
Deformation till
I
4-7
I
1
Permeability
Anisotropy
5-6
7
2-3
1-2
I
4-5
3
2
2-4
7-9
2-6
3-5
4-5
2-6
3-6
5-8
2-7
4-7
3-4
3
2
7
6-8
2-4
1
1-2
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7
4
5
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1
5
TOTAL FABRIC: Deformed bedrock structures related to ice movement direction.
DSF = Dominant Soil Fraction (Section 4.2.5) where:
CIRIA Report C504
CompressDensity ibility
~~~
MACRO: Interlayering with glaciofluvials common. Segregation, contortions, layering and fissuring in upper section and nose of flow. MESO: Aligned low angle orientation of clasts conforming to flow direction rather than ice direction. MICRO: Rather compact parallel arrangement of fines related to flow rather than direction of ice movement.
G W
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G = Granular or clastic till W = Well graded till
6 2-5
3
3
5-8
No value given
Mg = Granular matrix till Mc Cohesive matrix till
69
4.2
PLASTICITY AND PARTICLE SIZE
Till plasticity and particle size distribution are functions of mode of deposition and the nature of the host terrain over which the ice sheet or glacier was active. This is discussed in the following and illustrated in Figure 4.2 for UK lodgement tills.
4.2.1 Subglacial tills The clays, shales, chalks and mudstones which formed the glacier bed of much of the English lowlands produced tills ranging typically from CL to CI on the plasticity scale. They also produced reasonably homogeneous matrix-supported materials whose mineral suite was similar to source bedrock. As already noted, the clasts in these lodgements were often harder fragments of the same rocks, e.g. harder chalks, mudstones and sandstones. In Scotland, where harder igneous and high grade metamorphic rocks gave rise to a more resistant glacier bed, till thicknesses may be smaller and the tills more akin to the glaciated valley materials discussed below. Plasticities are usually low (CL) and the tills more heterogeneous in grain size.
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4.2.2 Supraglacial melt-out and flow tills In the English weak rock terrains, melt-outs are usually more varied in grain size than the lodgement tills which may underlie them, and the plasticity properties more variable. In hard rock areas, clay soils in this landsystem are usually localised in pockets, lenses and layers of varying thicknesses, possibly exhibiting a seasonal varve component.
4.2.3 Glaciated valley lodgement tills The glaciated valley landsystem which controlled till formation in the highlands of Scotland, parts of Wales and the Lake District produced a more heterogeneous till than its subglacial counterpart. Downslope processes of post-glacial solifluction and landslipping mantled the tills with debris which cannot always be distinguished from the underlying glacial deposits. Wider ranges in particle sizes may be expected with tills produced in the glaciated valley environment. In the Scottish highlands, the grinding action of lodgement on igneous and high grade metamorphics produced the rock flour of quartz and feldspar mentioned in Section 2.4.1. Plasticities varying from non-plastic to CL are typical of these tills, with a varied and frequently gap-graded grain size distribution.
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4.2.4 Plasticity and the ‘T-line’ concept Plasticity indices are measured on soil particles finer than 425 pm. Dunibleton and West (1966) showed that measured values of liquid and plastic limits, and hence plasticity index, depended upon the amount of coarse granular material contained within the sample. Their results for kaolinite mixed with various proportions of natural quartz sand (grading 0.05 to 0.20 mm, with 10% less than 0.075 mm) are shown in Figure 4.3(a). Roughly linear relationships between clay content and liquid limit and plasticity index are demonstrated, with both increasing as clay content increases. If liquid limit and plastic index are plotted on a standard plasticity chart (Figure 4.3(b)), it will be seen that they plot above or below the Casagrande A-line depending upon the proportion of clay present. Unlike marine clays which have had ample opportunity for segregation of the fine and coarse fractions, glacial soils are poorly sorted and the proportion of granular material less than 425 pm is substantial. The liquid limits and plasticity indices are correspondingly smaller and plot above the A-line close to the T-line (as shown by Boulton and Paul, 1976).
L--
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9 Glaciated valley
7
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Lodgements
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Idealised plasticity and grading characteristics of some British lodgement tills
The example given in Figure 4.3(b) relates to a mixture of a particular clay mineral and a particular quartz sand. The behaviour of real glacial soils with respect to the A- and T-lines will depend upon the nature of the clay mineral and on the nature, size, shape and grading of the included granular material. For example, if the kaolinite used in the Figure 4.3(b) were replaced by the more active illite, the plasticity data would plot further upwards to the right for a given clay content; likewise, if the kaolinite were replaced by rock flour or from only partly altered mica, as would be typical of the hard rock terrains in northern Britain, the plasticity data would plot further to the left. Plasticity data from more than forty UK tills are shown in Figure 4.4. In most cases they plot on or above the T-line and in the CL to CI ranges. CIRIA Report C504
71
. _
(80) Denotes kaolin
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Relation between liquid limit, plasticity index and clay content for mixtures of kaolinite with different coarse fractions of natural quartz sand.
Figure 4.3
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Liquid Limit (%)
6o
Variation of plasticity index with kaolin content with respect to Casagrande A-line and Boulton and Paul T-line.
(b).
Illustration of T-lineconcept (after Boulton and Paul, 1976; data interpolated from Dumbleton and West, 1966)
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North of England Tills Southern English Tills
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Plasticity data for British tills (from University of Paisley, 1996: key as in Appendix A)
4.2.5 Grading and the ‘dominant soil fraction’ concept McGown and Derbyshire (1977) stated that of the various identifying features of tills ‘their particle size distribution and fabric (stone orientation, layering, fissuring and jointing) are the most informative in terms of their engineering behaviour ...’. The importance of particle size was recognised in the Specificationfor Highway Works (1991), where it was used as the basis for soil classification for earthworks purposes. Direct reference is made to fines content (less than 63 pm) and to uniformity coefficient (d,,,/d,,). Many tills are bi- or multi-modal and McGown and McArthur (1971) and McGown and Derbyshire (1977) showed how geotechnical properties like shear strength and maximum dry density varied with the percentage weight of fines. McGown and Derbyshire (1977) provided the classification given in Table 4.2, based on work on Scottish lodgement tills:
72
CIRIA Report C504
Table 4.2
Gradational series of till textures (from McGown and Derbyshire, 1977)
Dominant soil fraction Clasts
Nature'of dominant fraction
Approx percentages of fines
Textural description
0 - 15
Granular (G)
I5 - 45
Well graded (W)
Granular
45 - 70
Granular matrix (Mg)
Cohesive
70- 100
Cohesive matrix (Mc)
Granular
No dominant fraction Matrix
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Between about 15 and 45% fines, i.e. well graded tills, the fine and coarse particles interfere with each other to give the optimum engineering performance in terms of strength, compressibility and density. Outside these approximate limits, tills behave as coarse soils with fines interspersed (granular) or fine soils with clasts interspersed (matrix). This work was expanded by Barnes (1987) and by Winter and Suhardi (1993). Using glacial till, Barnes investigated the variation in matrix (20 mm) on sand and gravel mixtures, i.e. much coarser soils than tested by Barnes. A similar pattern emerged with matrix dry density reducing relatively slowly to about 45% stone content, then decreasing sharply as the stone content increased further. Similar mechanisms were invoked for the matrix dry density decrease. The results by Barnes can be interpreted in terms of strength and deformation properties. They suggest that small granular contents make little, if any, contribution to sample strength but at roughly 40% granular content, particle interference causes a substantial increase in shear strength. From Table 4.2 it could be inferred that this behaviour should commence at 55% granular content, not 40%. However, the work reported in Table 4.2 is based on proportion of fines (
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Relationship between relative compaction value (vertical axes) and number of passes (horizontal axes) for three soils and various items of compaction equipment. Relative compaction expressed as percentage of maximum dry density achieved by BS 2.5kg rammer method (from Parsons, 1992). All layer thicknesses 150mm unless otherwise stated
CIRIA Report (2.504
109
5.6.2 Compaction and permeability Coefficient of permeability is frequently specified as a control for the construction of clay landfill liners; a value of 10-9mm/s(maximum permeability) is often quoted (National Rivers Authority, 1989).
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Lambe (1958) proposed that the permeability of a compacted clay fill depended upon its water content with respect to optimum: dry of OMC, clay particles flocculated during compaction and permeability was relatively high; wet of OMC, particles dispersed, drainage paths lengthened and permeability was relatively low. This explanation fitted the observation that permeability of compacted clays decreased dramatically at water contents at, or just above OMC, but evidence for flocculation and dispersal has not always been found (Benson and Daniel, 1990). An alternative ‘clod theory’ was proposed by Olsen (1962) and supported more recently by Elsbury er al. (1 990), Benson and Daniel (1 990), Murray er al. (1 992) and Wright et al. ( I 996). Permeability is considered to be controlled by the macro-structure of the clay, rather than by its micro-structure, and the majority of the water (or permeant) migrates through the ‘inter-clod’ spaces rather than through the denser or harder clods themselves (Figure 5.22). With further compaction and with increasing water content, the clods are broken down and remoulded, producing a more uniform but lower permeability clay soil.
Remoulded clods
Unbonded clods Figure 5.22
Influence of soil clods on permeability of compacted clay (from Elsbury et al., 1990) (Arrows indicate direction of permeant flow)
The clod theory was investigated by Benson and Daniel ( 1 990) who performed permeability tests on plastic (non-glacial) clay having clod sizes of 19 and 5 mm. The results demonstrated that dry of optimum (2.5 kg rammer method) the clay with 5 mm clod size was a million times less permeable than the clay with the 19 mm clods. Wet of optimum the difference fell to a factor of about ten. Clay with 19 mm clods compacted with the 4.5 kg rammer was also some ten times less permeable than the same material compacted with a 2.5 kg rammer. Photographs accompanying Benson and Daniel’s paper show the ability of both higher water contents and increased compactive efforts to break down the soil and remould the clods to obtain a more uniform and lower permeability material.
110’
CIRIA Report C504
Some clay tills, when worked, could readily produce clod-like structure. Work by Murray et al. ( 1 992) demonstrated the relationship between dry density, water content and permeability for four
tills and two alluvial clays. Permeability at about OMC was measured for the six materials tested and decreased as water content increased (Figure 5.23(a)). Permeability also decreased as PI increased (Figure 5.23(b)). The results are interesting and point to a limitation on the use of an equation such as 5.5 (Section 5.3.1) which relates permeability to void ratio (and hence to dry density). For a given compactive effort permeability of compacted fill decreases as water content increases at water contents close to OMC (Mitchell et al., 1965). If the compactive effort increases, permeability will also decrease. When used with compacted soils, equation 5.5 predicts a change of permeability with dry density irrespective of the compactive effort employed.
n
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W
0
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Kl
Y
w (%) (a) Figure 5.23
PI (%)
(b)
Variation of coefficient of permeability, k, of compacted clay soils (tills and recent alluvium) with compaction water content, w, and plasticity index, PI (from Murray et al., 1992). Compaction by MC apparatus
Work on the permeability of compacted soils was also conducted by Wright et al. (1996). They showed that the permeability of a clayey silty sandy gravel, such as would be typical of some Scottish glaciated valley and subglacial lodgements, also decreased as the water content increased and was minimum at OMC; there was little change in permeability as water content increased above OMC. It was considered that at low water contents the silty clay particles would have behaved as clods with the majority of flow through the granular ‘inter-clod’ soils; as water contents increased, the clods and granular soils would have been remoulded achieving a uniformly low permeability at water contents at or above OMC.
CIRIA Report C504
111
5.7
SUMMARY OF SECTION 5 Tills exhibit a wide range of effective strength properties. The effective cohesion intercept c’is often assumed to be zero for clay tills and the peak angle of shearing resistance @; correlates roughly with plasticity index (Figure 5.1). For granular tills, at a given relative density, particle shape, strength and grading control drained strength properties. There is a broad correlation between residual angle of shearing resistance @‘, and plasticity index for clay tills, but with a marked reduction at a plasticity index of about 20%.
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The consequences of depositional processes for the engineering properties of tills are complex, but broadly there are fundamental differences between subglacial and glaciated valley lodgement tills on the one hand and supraglacial melt-out tills on the other. Depositional features of relevance to engineering properties include sub-ice temperature, inter-ice water, ice thickness and sub-ice drainage; post-depositional features include groundwater and freeze-thaw effects. No clear picture emerges as to the behaviour of coefficient of consolidation, measured on English tills, with changes in effective stress. More consistent behaviour is demonstrated by fissured Scottish tills where a decrease in cv with increasing effective stress is usually reported.
Relationships have been established between compression and swelling index and liquid limit or plasticity index for a number of tills. Such relationships are probably best used as preliminary guides and on a site-specific basis. Limited data suggest that the large strain shear modulus of Devensian tills may be of the order of 20 MN/m2but substantially stiffer values have been reported for some materials. Till stiffness is strongly dependent upon shear strain and has also been shown to vary with overconsolidation ratio. When tills cannot be successfully sampled using conventional U1 00 or block sampling techniques, consideration may be given to testing material remoulded or reconstituted in the laboratory. For representative results it may be necessary to check that where coarse particles have to be excluded, the till’s original dominant soil fraction (Section 4.2.5) is not changed and that the maximum particle size is not more than one sixth of the specimen diameter. Laboratory work using glacial soils has shown the importance of granular content on engineering properties of tills. In particular, undrained shear strength becomes more sensitive to the addition of water as the granular content of the fill increases. The importance of handling granular and clay soils separately in earthworks is underlined. The permeability of clay tills is usually considered to be controlled by the macro structure with the majority of water (or permeant) passing through ‘inter-clod’ spaces. Tests on remoulded clay tills show that permeability at optimum water content decreases as both water content and plasticity increase. For a given compactive effort, permeability decreases as water content increases, to a minimum at about optimum water content.
r
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The wide variation in the ground and groundwater conditions associated with glacial terrains frequently introduces problems for site investigations. This Section concentrates on the special aspects of site investigations for glacial tills; a description of the more routine aspects may be found in Weltman and Head ( 1 983).
6.1
PRELIMINARY INVESTIGATIONS
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Attention should be paid to the preliminary desk study and walkover survey phases of the site investigation. Time spent during these phases should help to identify the correct scope of work and select the right equipment before mobilisation, avoiding expensive delays. Guidance on essential techniques can be obtained from the Code of Practice for Site Investigations (BS 5930 : 198 1) and from Perry and West (1 996). As noted in Section 2, Fookes et al. (1975a) recommended that for sites underlain by tills, the area should be classified using terrain evaluation techniques (Dowling and Beavan, 1969). Landsystems for the glacigenic environment were proposed which were later modified and developed by Boulton and Paul (1976) and Eyles and Dearman (1981). The report of a Geological Society Working Party (Anon, 1982) subsequently rejected the term ‘terrain evaluation’ in favour of the term ‘land surface evaluation’ and, in a state-of-the-art review, proposed a series of recommendations for the practice of these techniques. Land surface evaluation involves the identification of the following landscape units (Anon, 1982; Eyles, 1983), as illustrated in Figure 6.1 : (a) Land element: the smallest landform unit of the landscape which is itself indivisible on a landform unit basis. It is generally consistent in form and material and can be mapped at scales larger than 1 :600. For the glacigenic environment typical land elements include drumlins and kames. (b) Land facet: the basic landform unit of mapping. It comprises one or more land elements grouped as being reasonably homogeneous and distinct from the surrounding terrain. For the glacigenic environment it includes drumlin fields and outwash plains and is suited to mapping at scales up to 1 :100 000. (c) Landsystem: recurring genetically linked land facets defined by soils, vegetation and topography, which create a recognisable pattern, typically mapped at scales between 1:250 000 and 1 :1 000 000. For the glacigenic environment a typical landsystem may include a drumlin field flanked by an outwash plain and esker. Together these features form a set of linked land facets deposited at the base of an ice sheet. Land surface evaluation in any terrain requires that the landsystem containing the site is identified usually with the aid of aerial photographs, together with the land facets and land elements within the site area (Anon, 1982). Smaller sites may be contained wholly within a land element or land facet while linear structures could traverse whole landsystems. The identification of these genetically linked topographic features during the preliminary phases of the site investigation can give an indication of the subsurface conditions. It should aid the overall understanding of the site and assist in the optimum location of expensive investigatory techniques. This is illustrated diagrammatically in Figure 6.2 where the site is taken to be in terrain associated with the supraglacial landsystem (Figure 6.2(a)). The geology comprises a hummocky moraine of melt-out till, associated flow till with drumlinised lodgement till beneath. This is shown in plan on Figure 6.2(b). Bedrock underlies the lodgement till and is cut by a channel subsequently infilled with glacial sediments.
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Figure 6.1
Illustration of the relationships between landsystems, land facets and land elements used in land surface evaluation (see Table 2.2 and Glossary)
Without prior recognition of the glacial environment, the site investigation may have been undertaken using a regular grid of boreholes as illustrated in Figure 6.2(c). Important subsurface features would have been missed if the grid had been too coarse. However, with the use of geomorphological mapping and aerial photographs the hummocky moraine land elements and facets should be recognised and the site identified as supraglacial. A knowledge of the glacial environment would then indicate the potential for subsurface features and a geophysical survey could be commissioned. With the channel identified, further investigations could take the form of targeted geophysical surveys and a borehole programme. The latter could comprise a regular grid supplemented with deep boreholes to investigate the depth of the channel and the characteristics of its infill (Figure 6.2(d)). The use of geophysical methods in site investigations in glacial terrains is described in Appendix B, Case Study 1.
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(See Figure 2.5 for legend)
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(a) Supraglacial landsystem
Drumlins on buried lodgement till surface Limits of buried channel I
Surface comprises hummocky moraine of melt-out tills with associated flow tills (b) Plan view of site
(c) Regular grid borehole investigation
Stage 1 Geomorphological mapping and air photography Stage 2 Geophysical survey across whole site
Zone of shallow seismic survey Boreholes to prove drumlins and bedrock. Deep boreholes to prove depth of buried channel
(d) Phased investigationfollowing terrain evaluation
Figure 6.2
6.2
Use of terrain evaluation to assist site investigation procedures
STRATA DEFINITION AND INVESTIGATION METHODS
The identification of the strata succession at a site is a major concern in any ground investigation. Methods of strata definition will vary, depending upon the landsystem. Cable percussion boring and rotary drilling may be satisfactory for the subglacial tills often found in the English lowlands, but penetration and representative sampling using these techniques may prove difficult, even impossible, in the coarser deposits typical of glaciated valley terrains, where deep trial pits or trial excavations may be necessary.
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With glacial tills, problems include: penetration of a mixed sequence which may comprise clays, cobbles and boulders possibility of mis-identification of boulder(s) as bedrock identification of glaciofluvial layers or lenses and glaciolacustrine laminated silty clays obtaining representative samples suitable for identification and testing purposes. The choice of investigation method depends in part on the expected depth to bedrock, a factor which should be assessed by the preliminary investigation. Layers or lenses of water-bearing granular soils can have a controlling influence on construction performance in glacial terrains; likewise, glaciolacustrine laminated silty clay can control cut-slope stability and have a major bearing on landslide activity. Their identification during site investigation is essential and techniques should be adopted with this in mind.
6.2.1 Trial pits (trial excavations)
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A strategy for accurate identification of rockhead is an essential prerequisite for site investigation in tills, noting that abrupt changes in rockhead level may be expected, whatever the nature of the bedrock. Trial pits using a mechanical excavator provide good opportunities for visual inspection and may penetrate 1.2 m depth unsupported; greater depths require support. All pits require stringent safety precautions for access. Depths to 15 m have been reported (Matheson and Keir, 1978), but in selecting mechanical excavation to investigate depths in excess of about 5 m, consideration must be given to safety, dewatering, spoil disposal and to the possible effects on adjacent properties. Deep trial pits should be used with care in small confined sites in built-up areas where waterbearing granular tills are present. Heavy groundwater abstraction and their possible collapse could threaten adjoining structures. Nevertheless, trial pits (or trial excavations, as the larger and deeper forms should be called) are an excellent way of investigating tills, especially the coarser clast-supported materials, because they provide an opportunity carefully to sample, log and photograph. A good understanding may be obtained of the particle sizes, the nature and extent of the matrix, the presence and orientation of any fissures, and the presence of fabric both at the micro and macro scales. Experience indicates that the size and frequency of coarser till particles (i.e. the cobble-and boulder-sizes) are often the cause of earthworks claims. Consequently, trial excavations can play an important part in avoiding, or at least limiting, costly delays. Provided that shoring is not excessive, a trial pit approach is low cost and, in addition to the visual inspection it affords, there are opportunities to take large numbers of bulk and block samples (Section 6.3.3). For investigations in coarse clast-supported tills, consideration should be given to trial pits rather than boreholes as the primary means of strata identification. Boreholes may be used for the purpose of locating and describing underlying rockhead (where out of range of the excavator) and for standpipe or piezometer installation purposes.
6.2.2 Cable percussion methods Cable percussion boring in glacial tills produces satisfactory results over a wide range of depths although, unlike a trial pit investigation, the strata succession is usually inferred from samples taken at discrete depth intervals. Should preliminary investigations suggest coarse granular soils above rockhead, then the smallest usable casing diameter would probably be 200 mm in order to penetrate the coarser material. If several water-bearing strata are present, separated by clay soils, more than one casing size will be necessary in order to effect a seal. This could mean commencing the borehole with 300 mm diameter casing or larger, and successively reducing casing size with depth.
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Somerville (1983) describes the use of piling rigs capable of boring in 500 mm diameter for the purpose of penetrating cobble and boulder beds, typical of some glacial terrains. Good preliminary information is essential if correct decisions are to be taken on casing sizes at project commencement (Section 6.1). Starting the borehole with too small a diameter lining tube may mean incomplete information is obtained on groundwater conditions or, in extreme cases, that the full depth of boring will not be achieved. Typical groundwater conditions in glacial terrains are illustrated in Figure 6.3. Borehole 1
Borehole 2
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(a) Water strikes registered in borehole 2 not necessarily representative of those for borehole 1
(b) Sub-artesian groundwater conditions caused by interlayering of cohesive and granular tills. Granular tills shown dotted
Figure 6.3
Some typical groundwater table conditions arising from glacial till successions
Indirect information on the nature of granular soils can be obtained from chiselling, and the time spent and depth penetrated in the borehole should always be recorded, along with chisel size. For reasons of expense, some engineers prefer to limit the time spent chiselling to a given period. The progress made can be compared with that made in the same time period in other granular soils at the site, permitting a crude but often useful indication of relative density. Bulk samples of chiselled materials (see below) should always be taken. While chiselling inevitably fragments some of the particles, it is normally possible to identify rock type and to infer particle size and shape.
6.2.3 Rotary core methods Rotary core drilling provides the opportunity for continuous recovery of soils and rocks, but the equipment is less manoeuvrable and substantially more expensive than cable percussion. For these reasons, it is less often used. A compromise is the adoption of a ‘pendant head attachment’, a hydraulic power unit which may be suspended from the cable tool tripod and used to drill through the existing cable percussion casing to ‘prove’ bedrock or penetrate larger boulders. The size of these units is such that vibration during rotary coring is often sufficient to produce drilling induced fractures in the core and a clear idea of natural discontinuities and their spacing cannot always be achieved.
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Conventional rotary core drilling is usually performed in 76 mm core size or larger, with double tube barrel and a form of plastic liner. With the correct combination of drilling bit, rate of bit penetration, drilling speed, flushing medium and pump pressures, recovery even in tills can be good. As explained in Section 6.3.2, there are substantial changes in effective stresses during coring. Even though the amount of disturbance caused by mechanical straining is low, the resulting core sample may be too disturbed for some laboratory tests. Nevertheless, cores can be representative and provide a good opportunity for a continuous description of the soil and rock types penetrated.
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Colback (1977) described the results of rotary core drilling in the tills for the Brenig dam investigation. Most core loss was reported at the top and bottom of the runs with bottom loss due to slipping from the core catcher and top loss due to spinning clasts. (The angle at which the bit first comes into contact with the clast can cause it to dislodge, ‘spin’ and ultimately destroy the core.) The occurrence of core loss was reported as being most frequent when casing was advanced, because of the debris which collects at the bottom of the hole as a consequence. At Brenig, duplicate and even triplicate holes were drilled adjoining the first borehole in order to recover zones of core loss. Great care is necessary to measure sample depths accurately if information due to core loss is to be retrieved in subsequent boreholes. Improved results are reported in many soils with the wireline system. Here the outer barrel forms the casing and the inner core barrel is manoeuvred within the outer barrel with the aid of an overshot device, latching into the outer barrel when coring is required. Advantages claimed include more rapid sample retrieval, which reduces the opportunity for sample swelling in contact with the flushing medium. Furthermore, the volume and the velocity of the flushing fluid are reduced which reduces potential for core erosion. The stiffer drill string also reduces the risk of fracturing the core by rocking. The major disadvantage is that, unlike conventional rotary drilling, the bit is not brought to the surface during sample retrieval and cannot be routinely inspected for damage and wear. Since damage is likely with coarse gravel or cobbles, wireline methods may not always be suited to the coarser clast-dominant tills. The flushing medium is fundamental to the success of rotary core drilling. Good results in tills were reported with air by Somerville (1983), provided that up-hole velocities were sufficiently high. Hepton (1995) believed that drilling muds were preferable to water because they served three essential functions in addition to cooling and cleaning the bit: their increased viscosity facilitated transport of cuttings at lower up-hole velocities, thus reducing erosion drilling muds formed a gel-like filter cake adding stability to the borehole wall weighting agents (such as barytes) could be incorporated to resist artesian pressures. Glacial tills are amongst the most difficult materials to sample, so properly designed drilling muds appear to offer distinct advantages when investigating these terrains. The competence of the driller is also a most important factor, particularly with drilling bit selection and choice of penetration rates. Too much ‘crowd’ on the bit can cause a cobble or boulder to be pushed ahead in a weak succession with consequences for core recovery.
6.3
SAMPLING
Depending upon till type, the following methods may be used: U100 drive sampling rotary core sampling bulk and block sampling (from trial pits).
A guide to their selection is given in Table 6.1. BS 5930 : 1981 should be consulted for quality classification of soil samples.
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Table 6.1
Guide to selection of sampling methods in glacial tills
Glacial soil type (typical description)
Equipment/Method of Sampling
Matrix-dominant (sandy clay with gravel)
U100 drive
Generally suitable depending upon clast content: maintain frequent checks for circularity of sampler; change cutting shoes regularly and keep inside of sampler lightly oiled
Clast-dominant (clayey sandy gravel with occasional cobbles)
U100 drive
Generally unsuitable: using double tubes may make it possible to recover a (disturbed) sample sufficient for lithological description
Matrix-dominant (sandy clay with gravel)
Block
Suitable: adjust block size to cope with largest expected clast dimension
Clast-dominant (clayey sandy gravel with occasional cobbles)
Block
Depends upon amount and plasticity of matrix: where sufficient to form a coherent block during excavation, the procedure may be used; adjust block size to cope with largest expected clast dimension
Matrix-dominant (sandy clay with gravel)
Rotary core double tube plus liner
Suitable for strata definition, but swelling on contact with flushing medium may preclude use for laboratory test purposes
Clast-dominant (clayey sandy gravel with occasional cobbles)
Rotary core double tube plus liner
Sometimes suitable for strata definition, but core retention may be difficult; bit wear may be high; usually unsuitable for laboratory test purposes
Matrix-dominant (sandy clay with gravel)
Wireline
Generally suitable. Note that the bit is not routinely brought to the surface to permit inspection for wear, as with the conventional system
Clast-dominant (clayey sandy gravel with occasional cobbles)
Wireline
May be suitable, depending on amount, nature and angularity of clasts. Note that the bit is not routinely brought to the surface to permit inspection for wear, as with the conventional system
Comment
On certain projects, special procedures including freezing and chemical injection have been employed to extract samples of granular soils. Such procedures are not normally part of British site investigation practice and are not considered further.
6.3.1 Tube sampling The conventional 100 mm diameter U100 drive sampler, used routinely in British site investigation practice, can be successful in extracting representative samples of clay tills. In some cases, they are suitable for laboratory test purposes (Class 1, BS 5930 : 1981). The success of these samplers will depend entirely on the tills: matrix-dominant tills can often be sampled, provided that the cutting shoes are regularly changed and that the inside of the sampler is always lightly oiled. Sample quality decreases rapidly as the coarse granular component increases and, even in matrixdominant tills, recovery with UlOO samplers may be poor if the liquidity index of the matrix falls below zero. Thin-wall push samplers can also be employed in some tills (see below). The area ratio of the push sampler is typically some 10%whereas the UlOO sampler is about 25 to 30% (Weltman and Head, 1983); hence a much less disturbed sample can be retrieved. Thin-wall samplers can easily buckle if, for example, the cutting edge meets a stone or if the bottom of the borehole is uneven. Most undisturbed samplers are driven in a dry or relatively dry borehole. Rowe (1972) describes how better results may be obtained if a full head of water is maintained during cable percussion boring, thus minimising the reduction in effective stresses during the sampling process. All forms of drive samples suffer, to a greater or lesser extent, from the following disadvantages: strains are induced into the material being sampled (see below) there is water content redistribution voids may be created where large clasts are pushed aside (Figure 6.4).
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(a) Drive sampler clear of large clast
Figure 6.4
(b) Drive sampler displaces and commences rotation of clast
(c) Drive sampler further displaces and fully rotates clast out of ifs way, leaving void shown; soil adjoining void compacted
Stages in void formation during drive sampling
Thin-wall push sampling (referred to above) was extensively used during investigations in the largely clay tills at the various BRE test bed sites (Sections 4.4.1 and 5.4.2). Their use was often successful and a full sample retrieved, but the tills sampled had few clasts of hard (igneous or metamorphic) rocks, which could buckle or at least badly distort a thin-wall tube. Aside from this limitation, Hight ( 1995) notes that thin-wall samplers have substantial benefits over conventional U100 drive samples: shear strains and hence mechanical damage are reduced, as is the thickness of the disturbed zone around the sample periphery, so limiting the increase in mean effective stress which would otherwise occur. Much lower strengths were recorded for the U100 samples of Cowden till than for the thin-wall counterparts (Marsland and Powell, 1985) (Section 4.4.1).
6.3.2 Rotary methods Shear strains (and mechanical disturbance) are lower for samples extracted by rotary core methods than by tube sampling procedures. A rotary core sample may come into contact with the drilling fluid and swell, especially during the lengthy period involved in bringing it to the surface. As a consequence, there is a reduction in the sample's mean effective stress. This was demonstrated for London Clay by Hight et al. (1 993), where mean effective stresses measured on rotary core samples were some three times smaller than those measured on thin-wall tube samples. Undrained shear strength measurements made on rotary core samples were also normally significantly lower than thin-wall tube derived strengths. This was not reported by Vaughan et al. (1975) when rotary core drilling in the Cow Green clay tills, possibly because they were not heavily overconsolidated and hence less likely to swell. Working with stiff to hard Woolwich and Reading Beds clay, Hight and Jardine (1993) showed that efective strengths obtained from tests on rotary core samples were substantially greater (c'= 200 kN/m2; $'= 27") than those from tests on conventional U100 drive samples (c'= 0 kN/m2; $'= 22"). Results for thin-wall samples were not available, but work by Hight (1 995) implies that effective strengths derived from these samples would have been greater than the U100 values, although less than the rotary cored material. The water contents and plasticity characteristics of most clay tills are substantially lower than those of the Lower London Tertiaries and experience in tills may differ (Vaughan et al., 1993). Nevertheless, it seems reasonable to expect that the reduction in shear strain (mechanical damage) caused by thin-wall and rotary core sampling can only be beneficial as far as testing stiff clay tills is concerned. They would appear suitable for many English lowland tills but much less so for the coarse granular clast-supported lodgements from the glaciated valleys or for the lodgements characteristic of some upland areas.
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6.3.3 Bulk and block sampling Bulk samples are the normal method of sampling coarse granular soils if only disturbed material is wanted. This method of sampling in fine grained granular soils is unsatisfactory because the fines are often washed out. A disturbed but representative sample is more likely to be obtained using two U 100 tubes screwed together. Minimum sample quantities are quoted in BS 1377 : 1990, up to coarse gravel size. For coarser soils, Head (1988) suggests the use of the simple relationship: Mass of sample = 100 x mass of largest particle
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Sample quantities calculated in this way can be substantial, with in excess of 100 kg required for coarse gravel and cobbles, such as would be common for some tills. Only a trial pit or trial excavation could yield such quantities. Coarser particles obtained from cable percussion boreholes are often unrepresentative of the true grading of the deposit and may be fractured by chiselling. Care is needed in assessing the particle shape, size and distribution of the actual as opposed to the sampled material. Block sampling may be adopted to retrieve samples of matrix- and some clast-dominant tills which could otherwise be disturbed by tube sampling. The operation is performed in a trial pit (Figure 6.5), first by excavating the top and sides ‘over-size’, then by trimming sufficiently to allow a steel box to be lowered, thus retaining the sample in position. The base of the sample is then carefully undercut and removed. Layers of muslin and wax may be substituted for the steel box. The technique may be adapted for the direct abstraction of test specimens, i.e. without the intervening sampling process. Little (1 988) describes obtaining horizontally and vertically orientated oedometer specimens of a matrix-dominant till from St Albans by tapping 75 mm diameter stainless steel rings directly into the soil from a bench cut into the trial pit and then carefully excavating using a trowel. The specimens were sealed, dispatched to the laboratory and maintained in a humidifier until required for testing.
ground level
pit supports omitted
soil carefully trimmed as box is pushed over sample
Figure 6.5
/
8lock.sampling techniques (from Weltman and Head, 7983)
The technique, also widely used in stiff clays and weak rocks, will succeed if the till matrix is sufficiently plastic to hold the sample together. As already noted, matrix-dominant tills can be friable (liquidity index well below zero) and in such cases block samples may also be disturbed. The technique should only be used at depths where excavation by a trial pit is both safe and practicable. CIRIA Report C504
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6.4
IN-SITUTESTING
The difficulties inherent in sampling glacial soils point to in-situ testing as a worthwhile alternative approach to establish engineering properties. In the following, the appropriateness of some commonly used test methods is discussed and suggestions made as to the glacial soil types in which they may be employed. The suggestions are summarised in Table 6.2.
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Table 6.2
Guide to selection of in-situ test methods in glacial tills
Glacial soil type (typical description)
in-situ test
Comment
Matrix-dominant (sandy clay with gravel)
SPT
Generally suitable: may need correlation to convert N-value to shear strength depending on nature and amount of matrix (see Section 4.5)
Clast-dominant (clayey sandy gravel with occasional cobbles)
SPT
Generally suitable, but may fail on coarser clasts
Matrix-dominant (sandy clay with gravel)
CPT
Generally suitable, but may fail on coarser clasts
Clast-dominant (clayey sandy gravel with occasional cobbles)
CPT
Generally suitable, but may fail on coarser clasts
Matrix-dominant (sandy clay with gravel)
Pressuremeter (PBP and FDP)
Suitable only where matrix is sufficiently cohesive to allow good quality borehole to be formed
Clast-dominant (clayey sandy gravel with occasional cobbles)
Pressuremeter (PBP)
Expert assessment required
Matrix-dominant (sandy clay with gravel)
Pressuremeter 6BP)
Suitable, only where clasts are occasional or soft (e.g. chalk) and (or) highly weathered. Will fail on the occasional erratic
Clast-dominant (clayey sandy gravel with occasional cobbles)
Pressuremeter
Expert assessment required
Matrix-dominant (sandy clay with gravel)
Permeability (Pumping)
Unsuitable
Clast-dominant (clayey sandy gravel with occasional cobbles)
Permeability (Pumping)
Generally suitable, but variation in thickness of permeable bodies may require many observation wells for a complete characterisation
Matrix-dominant (sandy clay with gravel)
Permeability * (constant head)
Suitable
Clast-dominant (clayey sandy gravel with occasional cobbles)
Permeability * (constant head)
Suitable but outflow rates may be large requiring substantial water supply
Matrix-dominant (sandy clay with gravel)
Permeability * (variable head)
Generally suitable
Clast-dominant (clayey sandy gravel with occasional cobbles)
Permeability * (variable head)
Suitable, but disturbed base of borehole could affect results.
* in piezomerers or in boreholes Coarse granular particles within tills have the same effect on the in-situ test as the corresponding sampling process: the soil is disturbed during insertion of the sampler or the in-situ test apparatus. The soil is arguably less disturbed during in-situ than laboratory testing, because the potentially disturbing operations of extracting the sample and forming a test specimen are both avoided.
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6.4.1 Standard penetration test In British practice, the SPT is conducted in pre-formed cable percussion or rotary boreholes by measuring the number of blows (the N-value) necessary for a ‘split-spoon’ to penetrate 300 mm after a 150 mm seating penetration. The SPT is probably the widest used in-situ test for investigating glacial tills. Work summarised by Clayton (1 995) demonstrated major pitfalls in obtaining accurate results in all soils. Some are related to the method of boring (especially cable percussion boring), to the SPT equipment employed, and to the particular procedures adopted. Some of these disadvantages are set out in Table 6.3. Table 6.3
Some factors influencing accuracy of SPT results in tills (see Clayton, 1995)
Factor Boring Method
Comment Percussion techniques can disturb base of borehole for a depth up to 3 x borehole diameter (see Clayton, 1995) Failure to keep borehole topped up with water above groundwater level
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Casing driven too close to (or below) base of borehole Failure to use small enough equipment when boring in granular soil. BS1377: 1990 requires boring tools to have diameter no greater than 90% of borehole size and to limit the test to I50 mm diameter boreholes Equipment
Use of undersize rods leading to whipping and inadequate energy transfer to split-spoon device Use of poorly functioning trip hammer leading to inadequate energy transfer to split-spoon device
Procedure
Inadequate or inappropriate reporting of results
BS 1377 : 1990 and Clayton (1 995) state that the N-value may be affected if the borehole size is significantly larger than 150 mm. This presents difficulties when conducting tests at shallow depth in a borehole programmed to penetrate large thicknesses of glacial soil. Starting casing sizes may have to be large if penetration to rockhead is to be assured. Clayton suggests that where large diameter boreholes are necessary because of gravel or cobbles (typical of some tills), the method of boring or drilling immediately above the test section should be changed to the small diameter required by BS 1377. Reporting the results is a well known problem with the SPT in coarse granular tills. Full test penetration cannot always be achieved and procedures vary for reporting the results. They include: (a) reporting the number of blows to achieve the quoted test penetration (e.g. 45 blows for 225 mm) (b) as above, but calculating the number of blows pro rata to the actual penetration achieved (e.g. if 45 blows were needed to penetrate 225 mm, then the ‘N-value’ is interpreted as 45 x 300/225 or 60) (c) reporting the test penetration achieved for 50 blows (e.g. 50 blows for 65 mm). Refusal frequently occurs, not in the test but in the seating penetration, i.e. the first 150 mm drive. Because this is in the disturbed zone beneath the base of the borehole, it makes comparison with the results obtained in the 300 mm test drive particularly difficult. In practice, there seems no alternative but to report the number of blows for the seating as opposed to the test penetration, e.g. 45 blows for 140 mm (seating). Proposed relationships between undrained shear strength and SPT N value were discussed in Section 4.5. More information on these relationships is given in Appendix B, Case Studies 1 to 3 and 13.
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6.4.2 Cone penetration test The CPT may be employed both as a ‘profiling’ device and for assessing geotechnical parameters. Experience has shown that soils of a given particle size demonstrate a relatively narrow range of cone resistance qc and friction ratio R, values and this forms the basis of the classification scheme such as shown in Figure 6.6(a). Experience with clay tills indicates that the R, value often falls within 4 to 7%, close to the ‘peat’ range. Evidently tills are ‘non-textbook’ materials and the use of the CPT as a profiling device should always be accompanied by borehole control so that apparently anomalous behaviour may be clarified. The size of the cone and its rate of penetration mean that the device may fail to identify very thin layers or lenses contained within otherwise homogeneous materials. Cones of different sensitivity are available and, in glacial tills, where a wide range of soil types and consistencies are present, there may be difficulty in selecting a cone with the desired sensitivity for all likely soils present. This is particularly relevant where a dense granular till is overlain by (say) soft to firm alluvium.
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Cobbles or boulders can deflect the tip and set up severe eccentricities during loading. Experience suggests that clasts from 100 to 115 mm size are pushed aside during the test, but this must depend upon the strength and compressibility of the surrounding matrix. Modern cones are fitted with inclinometer devices to monitor deviation from the vertical during testing. For clays, cone resistance q, and undrained shear strength cL,are normally related by;
where N , is the cone factor and o:, is the vertical effective stress. It is necessary to obtain an indication of N , if cu is to be determined and case histories in the literature indicate a wide variation, depending upon OCR, soil sensitivity and plasticity index. Adam (1 985) plotted undrained shear strength against cone resistance for both onshore and offshore sites and superimposed lines corresponding to N , factors of 10, 15 and 20 (Figure 6.6(b)). The cone probably measures strengths closer to the intact than the representative strength because of its small size with respect to fissure spacing (Section 4.4.3); consequently it would be logical to calculate N , in equation 6.1 using the results of shear strength tests made on 38 mm diameter specimens. As noted elsewhere, extracting a 38 mm diameter specimen is often impracticable in tills and 100 mm sizes are normally preferred. Use of lower 100 mm specimen diameter shear strengths would produce higher N , values than would 38 mm specimen diameter strengths. In Figure 6.6(c) N , values obtained by analysis of static cone penetration tests from the three BRE test bed sites are shown (Sections 4.4.1 and 5.4.2). The analyses were made in terms of 100 mm diameter triaxial test results and fissure spacings are taken into account (Terzaghi, Peck and Mesri, 1996). Also shown are results of tests made on other insensitive clays. Constrained modulus, m, for glacial clays may be obtained from the relationship:
Adam (1 985) quotes the following amfor low plasticity soils like tills, for the stated q, values:
q, < 0.7 MN/m2; 3 < am< 8 0.7 < q,< 2.0MN/m2; 2 2.0 MN/m2; 1 < am< 2.5 According to Adam, it is often found that q, for most Scottish tills is >2.0 MN/m2 and that amis usually close to 2.5.
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Plates 1 and 2 1.
Aber-mawr, Irish Sea till overlying locally derived head. Note jointing and paucity of clasts in till. Scale 50 cm (Photograph: C. Harris, 1986).
2.
Ffos-/as. Upper till (FL5). Note strongly developed fabric and high erratic content (Photograph: R. Donnelly, 1985).
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Plates 3 and 4
3.
Stratified till with chalk lenses and flow structures near the base of the Third Crorner Till, West Runton, Norfolk (Photograph: P Gibbard, 1978).
4.
Folding and step faulting of laminated till near the base of the Third Crorner Till, West Runton, Norfolk (Photograph:P Gibbard, 1978).
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CIRIA Report C504
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Plates 5 and 6
5.
Section 6 m high in weakly bedded brown lodgement till with locally imbricated clasts. Bed of gravelly till by hammer. Holm (NB 453307) near Stornoway, north Lewis (Photograph: J.D. Peacock, 1977).
6.
Streamlined forms and striae on peridotite. Note open fractures caused by stress relief (glacial unloading?). Locality (NM 373990) west of Loch Bealach Bhic Neill, Rhum (Photograph: J.D. Peacock, 1975).
CIRIA Report C504
127
I
100-
lFin:andI lmedium Isand \ very
40-
I I
I
Clay, sill. sand mixlures
I
I\ dense I \I \ \ I
.
I\Denshl
I I \I
I\
\ \
Mediu:\ dense
I\,
I
I
I
I
I s"ty sand
I 1
'
1. Expect some overlap in the type of zones noted. Local correlations desirable 2. Friction ratio values decrease in accuracy with low values of qc and in desiccated soils
Sllty
I
clay Sandy
1
//
*ay
Insensitive inorganic day
/
I
I
Notes
-
a-"6
4- ,
c 2-2 v)
2-
0
.-v) ?!a
,
c
1 --
6 0.4 -0.2 I
0
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I
I
I
I
1
I
t I I
1 2 3 4 5 6 7 Friction ratio Rf (local side frictionlcone end resistance x 100)
(a) Classification scheme used with cone penetration tests (from Adam, 1985)
z-1 z
30 -
10
E
e
d
a Garston Till A Cowden Till RedcarTill
/ NK=20
-
25 E E 0 20 -
/P *' / /
E
/
15 5 10
a,
c
6
-
' // /-
-
A A
I
I
I
I
I
I
I
I
I
2-4
,q=/~ /A-'
bv
1 1
e* e .
:- _ - - -,"---8 - 4
,-*--
0 London Clay, Brent Cross
0 London Clay, Canons Park A Gault Clay v Boom Clay
5 I
.
Fissure spacing Cone diameter
I
I
I
I
I
I
250 500 Undrained shear strength, c, (kN/m2) 0
(b) Relationship between cone resistance, qc,and undrained shear strength, c, (after Adam, 1985)
Figure 6.6
(c) Cone factors for stiff fissured clays as a function of fissure spacing and plasticity index (Terzaghi, Peck and Mesri, 1996; data from Powell and Quarterman, 1988)
Relationships for the interpretation of static cone penetration tests
L--/
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CIRIA Report C504
6.4.3 Pressuremeters The pressuremeter is available in three forms: PBP
prebored pressuremeter
a pressuremeter lowered into a test pocket created by rotary drilling
FDP
full-displacement pressuremeter
a pressuremeter connected to a 15 cm* electric cone and pushed into place thus creating its own pocket by displacement
SBP
self-boring pressuremeter
a pressuremeter with an integral drilling head creating its own pocket by replacement
Characteristics of some of the PBP and SBP pressuremeters currently in use in the UK are given in Table 6.4.
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Table 6.4
Device
Characteristics of some pressuremeters in current use in the UK (after Mair and Wood, 1987) Installation Method (Test Category)
Measurement Systems Pressure
Deformation
Capacity
Control Method
Maximum Maximum Pressure Cavity Strain (MN/m2) (o/.)
MCnard Pressuremeter (Type GB)
Installed in preformed test pocket in borehole (PBP)
Water: surface pressure gauge
Indirect volume measurement
10
27
Stress Control
Oyo (Elastmeter 100)
Installed in preformed test pocket in borehole (PBP)
Water, oil or gas: surface pressure gauge
LVDT averaging displacement of two points
10
12
Stress control
Self-boring pressuremeter (Camkometer)
Self-boring (SBP)
Gas: diaphragm transducers in probe
Three independent strain gauged feeler arms
4
20
Strain
Cambridge Insitu high pressure dilatometer
Installed in preformed test pocket in borehole (PBP)
Oil: diaphragm transducer in probe
Six independent strain gauged feeler arms
20
or stress control
25
Stress control
All three types of pressuremeter have been used in glacial soils. The NX size (76 mm diameter) PBP, the most widely used of any pressuremeter, can be employed in glacial till provided a stable NX test pocket can be formed. Preboring inevitably disturbs the ground to be tested and, in tills, the amount of disturbance increases as the gravel content increases. It is important to minimise the disturbance when forming the pocket by: (a) careful drilling at low penetration rates, with mud or foam flush circulated at the lowest practicable velocity to reduce potential for erosion or scour of the test pocket sides (b) creating a special test pocket that is cylindrical, with a diameter no greater than 110% of the diameter of the probe (c) inserting the pressuremeter rapidly into the test pocket once formed to reduce the potential for swelling or collapse of the pocket sides. __
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The FDP can be used in any glacial soil in which it is possible to push a 15 cm2 cone. It is usual to restrict its use to relatively gravel-free clay and matrix-dominant tills. The probe is installed with the same equipment as used for electric static cone penetrometers. The probe displaces the soil so that there is disturbance of the ground to be tested during installation. The amount of disturbance is controlled by the diameter of the probe and should be similar for all tests in a uniform till deposit. The SBP is restricted to tills containing little or no gravel. It is installed with a rotary rig and the drilling has to be done carefully to minimise disturbance.
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It is normal and recommended UK practice to carry out an unloadreload cycle during a pressuremeter test. Typical test results in tills are shown in Figure 6.7(a) and 6.7(b), in which the pressure applied to the till is plotted against the cavity strain defined as the ratio of the change in radius of the cavity with respect to the initial probe radius. The shape of a good quality SBP test curve is closest to an ideal test; that is a test which starts at the in-situ horizontal stress. There is always some disturbance during installation of PBP and FDP probes so the starting pressure is either less than (PBP) or greater than (FDP) the in-situ total horizontal stress. Figure 6.7(c) is an example of normalised elemental secant shear moduli derived from several unload-reload cycles carried out at different strain levels in different tests. Note that shear modulus, G, is independent of drainage, but it increases as the effective stress level at which the test is carried out increases. For that reason it is standard practice to take into account the increasing effective stress during a test in granular clast-dominant tills when producing the G value for design. The effective stress in clay matrix-dominant tills is very nearly constant during a test once yield commences. 3000
-
3000 -
- 1200
Silty sand till
%
- 800
2000 - Clay till with significant gravel and cobbles U)
-400
g p- 1000a
2
,
I
Average cavity strain (membrane displacemenffradiusof the probe) (%)
Average cavity strain (membrane displacemenffradiusof the probe) (%)
(a) Self boring pressuremeter tests in glacial till
(b) High pressure dilatometer tests in
glacial till
I000
100
~-
...
'oo.ool
I
... ...
0.01
01
1
Shear strain (%)
(c) Variation in elemental secant shear modulus with shear strain taken from reload curves carried out in self boring pressuremeter tests in glacial tills (Tests included up to three unload-reloadcycles)
Figure 6.7
130
Results of pressuremeter tests on glacial tills (results courtesy of University of Newcastle)
._
CIRIA Report C504
Installation disturbance is inevitable with pressuremeter tests in most tills so judgement has to be used in their interpretation. Engineers experienced in analysing pressuremeter tests should be consulted at an early stage when the need for pressuremeter testing is foreseen. All pressuremeter testing should be carried out by skilled and experienced operators and discussions should be held in advance of the site operations to decide on the most suitable probe for the information required and the ground conditions expected.
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As noted in Section 4.4.1, there are substantial differences in the values of undrained strength calculated using the Gibson and Anderson (1961) and Palmer (1972) procedures for PBP and SBP equipment. This was demonstrated at the BRE test bed sites where the Gibson and Anderson approach, modified by the method due to Marsland and Randolph (1977), gave results closer to plate loading test values than Palmer's method. In Figure 6.8, the results of undrained shear strengths calculated using the two quoted methods for the Cowden till are plotted against strengths determined from 865 mm diameter plate loading tests. While generally higher, the modified Gibson and Anderson procedure yields strengths markedly closer to the plate strength than the alternative method of calculation. Houlsby and Withers (1988) provide a method for the analysis of the results of the FDP device. Further information on test interpretation is given by Mair and Wood ( 1 987) and Clarke ( 1 994). Undrained shear strength, c, (kN/rn2)
Undrained shear strength, c, (kN/rnz)
-
-
4 -
4 -
-
-
8 -
-
-
h
E
' 5
12 -
8 -
h
A
16 -
20 24
-
-
-
24 A
Pressuremeter test C, values calculated using Gibson and Anderson (1961) Figure 6.8
6.5
Plate loading test resuIts
A
Results using Gibson and Anderson (1961)
W 0
0
0
O0
E
A
-
a
12 5 P O" 16 20 -
A
0
o Results using
Palmer (1972)
*loo
-
0
Pressuremeter test c, values calculated using Palmer (1972)
Undrained shear strength, c,, determined from 865mm plate tests and Menard pressuremeter fests on the Cowden till, using two calculation procedures (after Marsland and Po well, 1985)
GROUNDWATER AND PERMEABILITY
Groundwater conditions are likely to be complex at sites underlain by glacial tills for the following reasons: interdigitation of granular lenses in clay tills, producing local perched groundwater tables interlayering of granular and clay tills producing artesian or sub-artesian groundwater blanketing of water-bearing bedrock by clay tills, to produce artesian or sub-artesian conditions. These various conditions are illustrated in Figure 6.3. !
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A major difficulty with glacial tills is that lenses (i.e. discontinuous layers) can vary in area1 extent from a few square centimetres (pockets) to tens of square metres, and can vary in thickness in the same manner. This makes their recognition in borehole investigations particularly difficult because it is never certain whether a small pocket is not in fact the tip of a much larger lens. Moreover, because of their disposition, groundwater flow in discontinuous granular bodies may be highly directional, a factor not always taken into account in the analysis of pumping tests.
6.5.1 Permeability determination from grading tests The most common method of determining the coefficient of permeability k from granular samples extracted from boreholes or trial pits is by use of Hazen’s relationship:
k (d~ = 0.01 ) x&,,
,,
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where d is the 10%particle size (mm) obtained from grading analyses. Using data collected from dewatering projects at thirty sites (not all in glacial soils), Preene and Powrie (1993) concluded that where soils contained more than 20% fines, calculations based on the mean permeability obtained from application of Hazen’s rule underestimated the steady state flow rate by up to 100 times. For less than 20% fines, flow rates were generally over- or underestimated by a factor of not more than three. Difficulties in retrieving representative samples of granular soils during cable percussion boring because of loss of fines are well known. As already noted, double U100 samplers (i.e. two samplers screwed together) may be able to retrieve a representative (although disturbed) sample of granular glacial soil, and this form of sampling permits an indication of its structure on extrusion. Layers, laminations or pockets of clay contained within glacial soil can be observed readily and allowed for in permeability assessment. Grading analysis to produce the d , , size inevitably destroys structure. Consequently, it is essential to inspect a sample on extrusion, before grading analysis, so that its structure can be identified and allowed for in interpretation.
6.5.2 In-situ permeability tests Field or in-situ permeability tests normally performed in a borehole or in a piezometer are generally classified as variable or constant head tests. For a description, see Weltman and Head (1983). It is often difficult to determine how continuous a granular body within a till will be from borehole information, yet its size is critical in interpretation of the results. Figure 6.9(a) illustrates the conditions assumed in the interpretation of a standpipe constant head test, while Figure 6.9(b) shows a possible actual case. In Figure 6.9(b) it is not the permeability properties of the granular soil which would be measured, but the surrounding cohesive material; the granular body would simply be the test section having a ‘shape factor’ corresponding to its own particular geometry. Thus, in the case described, far from measuring the permeability of the granular body, the test would actually measure the permeability of the surrounding cohesive material, inaccurately, since the shape factor would not be correctly known. When forming the test section, disturbance of the sides will occur. Fines from the formation, carried in suspension during boring or drilling, will settle out, clogging the filter pack when installed. These factors which will tend to reduce the measured permeability are normally regarded as being an inevitable limitation of the test (Powrie and Preene, 1994).
6.5.3 Pumping tests For granular till acquifers, analysis of drawdown v distance and drawdown v time data for observation wells situated a known distance from the pumping well provides much valuable information on hydraulic characteristics. In particular, analysis of these data yields probably the best estimates possible of the coefficient of transmissivity (the rate of flow under unit hydraulic gradient through a cross section of unit width over the whole saturated thickness of the aquifer) and the coefficient of storage (the volume of water released from storage per unit surface area of the aquifer per unit decline of head).
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CIRIA Report C504
(a) Conditions assumed in falling head
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test : k measured
Figure 6.9
- k gravel
(b) Actual conditions being tested: k measured k clay
-
Effect of variable ground conditions on permeability test results
Analysis of these data also provides indications of aquifer geometry and of groundwater flow. For example, Figure 6.10(a) indicates that there is strongly directional flow because the gradients of the drawdown v distance curve for observation wells (a) and (c) are significantly different from wells (b) and (d). Figure 6.10(b) illustrates a drawdown v time curve for an aquifer affected by recharge and Figure 6.10(c) demonstrates the characteristics of a limited aquifer, with the break in the slope of the drawdown v time curve indicating the point at which the expanding cone of depression reached the aquifer boundary. Log distance
b
Direction of flow
1 1 1
(a) Idealised drawdown v distance plots for observation wells at right angles, indicating effect of direction of groundwater flow
Log time I
U
3
b
Log time I
time at which source of recharge is reached by expanding cone of depression
(b) Idealised drawdown v time plot indicating aquifer recharge
b
time at which impermeable boundary reached by expanding cone of depression
(c) Idealised drawdown v time plot indicating limited aquifer extent
Figure 6.10 Interpretation of pumping tests in glacial tills CIRIA Report C504
133
Observation wells should be carefully positioned if maximum value is to be achieved from the pumping test: four wells in a right angle array, penetrating the full aquifer thickness, are essential if the hydraulic characteristics are to be understood and for proper drawdown v distance relationships. In complicated aquifers an array of eight wells is preferred.
6.6
SUMMARY OF SECTION 6 Time invested in the preliminary desk study and walkover survey should mean that the correct scope of site investigation work is identified and that the right equipment is selected before mobilisation. The concept of landsystems (Section 2.2) may be introduced at this stage.
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Layers or lenses of water-bearing granular soils of glaciofluvial origin can have a controlling influence on construction performance in glacial terrains and can be critical to programme and cost; likewise, glaciolacustrine laminated silty clays can control cut-slope stability and have a major bearing on landslide activity. Their identification during site investigation is essential and techniques should be adopted with this in mind. Methods of strata definition will vary, depending upon the landsystem. Cable percussion and rotary drilling may be satisfactory for the subglacial tills often found in the English lowlands, but penetration and representative sampling using these techniques may prove difficult in coarser deposits exhibited in some glaciated valley terrains, where deep trial pits or trial excavations may be necessary. The Standard Penetration Test remains one of the most popular in-situ tests in tills. While best practice requires the SPT to be performed in small casing sizes, large casing diameters may be necessary if cable percussion boreholes are to penetrate large glacial soil thicknesses. The method of boring immediately above the test sections may have to be changed in such circumstances. Cone Penetration Tests are often used in glacial soils but penetration may be prevented by cobbles and boulders. They may also fail accurately to detect the depth and thickness of very thin layers or lenses contained within otherwise homogeneous materials. In glacial terrains, where wide ranges of soil type and consistency are present, there may also be difficulty in selecting a cone with the desired sensitivity for all soils present. All three types of pressuremeter test may be made in clay tills given suitable conditions, but the PBP (pre-bored pressuremeter) is probably the most widely used (Section 6.4.3). Results obtained are critically dependent upon minimising the disturbance created when forming the test pocket. Results also vary substantially with test procedures and calculation methods. Groundwater conditions are likely to be complex in glacial terrains. Interdigitation of sand or gravel layers or lenses in otherwise clay tills may produce local perched groundwater tables and artesian or sub-artesian conditions may also be encountered. Such conditions could also be met when penetrating bedrock beneath a blanket of clay tills.
. .134
CIRIA Report C504
7
Earthworks
Under this heading, excavation, acceptability, placement and compaction are considered. Cuttings and embankments are treated in Sections 8 and 9.
7.1
EXCAVATION
Problems in excavating glacial tills include: misidentification of rockhead presence of large boulders water-bearing silts and sands water-bearing bedrock selection of appropriate plant.
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7.1.1 Misidentification of rockhead Rockhead, as distinct from overlying cobbles and boulders, should be established during the site investigation so that estimates of the excavated volume of rock can be made. This is needed for progress planning and costing purposes. Misidentification of boulders for bedrock is quite common in British glacial terrains. The practice of ‘coring one metre to prove bedrock’ at the base of a cable percussion borehole is a contributory factor, because boulders can be many times this size. Depending upon the borehole spacing selected, rotary core drilling should penetrate at least 3 m below presumed rockhead, more depending upon the geological relationships identified at preliminary investigation stage (Section 6. I). The use of geophysical methods as well as boreholes to identify rockhead beneath a deformation till are described in Appendix B, Case Study 1.
7.1.2 Presence of large boulders Even if rockhead is correctly identified, excavating large boulders in basal tills can sometimes present as much difficulty as excavation in bedrock itself. This leads to delays and, depending upon contract conditions, claims, if their presence was not established pre-tender. Boulders contained within a till may have fractured along pre-existing discontinuity surfaces during transport and deposition, so they may be less fissured and more difficult to break than the bedrock from which they were derived. Care should be taken in extrapolating the excavatability of large boulders from data obtained from underlying bedrock (of the same type). During excavation of till for placement in the Kielder dam embankment, Anderson and McNicol (1 989) described how logs were kept of the numerous boulders encountered; each log included such data as rock type, shape, size and estimated crushing strength. Such records could be of use for payment purposes at stages during earthworks construction. Failure correctly to assess boulder sizes during site investigation can lead to wrong choice of plant during the main works, with delays and disruption as a result (Section 7.1.4).
7.1.3 Water-bearing soils and bedrock Low plasticity tills can rapidly turn into slurry in the presence of water. Excavation in such soils can frequently be interrupted if water-bearing granular layers or lenses are penetrated. Attention is necessary during site investigation to assess the likely extent of such features. This will influence the choice of excavation technique and the need or otherwise for pre-drainage.
i
CIRIA Report C504
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Similar problems can arise on removing a basal layer of relatively impermeable lodgement till overlying water-bearing bedrock. As with silts and sands, local drainage can do much to improve excavation performance.
7.1.4 Selection of plant Hight and Green (1978) and Cocksedge (1983) reviewed earthworks in relation to construction method. Cocksedge isolated the particular factors important to plant selection in glacial terrains, especially in wetter upland regions. It was suggested that, in such areas, loaders and dump trucks were more flexible than scrapers in excavation and were better able to cope in conditions where rutting could occur. It was acknowledged that in drier parts, output using such plant would be substantially less than with scrapers, especially in flatter regions.
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As far as possible, granular and cohesive soils should be placed separately (Arrowsmith, 1978). The correct choice of plant at the excavation stage will assist this procedure. Arrowsmith suggested that scrapers were more successful in excavating distinct layers than face shovels but, if the clay till beneath the granular layer were weak, excavation closer than 500 mm to the interface might not be possible because the clay might not support the scraper. Excavation by face shovel was also specified for the Cow Green embankment dam (Vaughan et al., 1975) but was eventually replaced by bulldozers, excavating thin layers and feeding dump trucks. This procedure may have been determined by the reported need to remove stones larger than 75 mm. For particularly dense tills, it may be necessary to blast to loosen. Johnston (1975) describes the use of these methods for the Bradan dam. The lower till (bulk density 2.45 Mg/m3) was loosened by blasting and excavated by face shovel. Other factors affecting excavation include: topography; weather; haul length; availability of tips and borrow areas; drainage of borrow areas and haul roads; and environmental considerations.
7.2
FILL ACCEPTABILITY AND CONTROL
Most British glacial soils meet the requirements for acceptable fill, but high plasticity clays (liquid limit in excess of 90%) and frozen or water-logged materials do not. High plasticity clays are unusual in British glacial tills; most are of low or medium plasticity (Figure 4.4). Frozen or waterlogged soils are quite common, especially in colder and wetter northern Britain, and their use would not be acceptable for any purpose.
Apart from embankment dams, most engineered fills support structures such as roads, buildings and hard standing areas, and a fill’s most important engineering property is its ability to resist volume change, either settlement or heave. Given adequate compaction, considerations of bearing capacity or ‘failure’ d o not normally arise. Acceptability criteria are usually set in relation to those engineering properties which affect or control the volume change characteristics of the compacted material. The most important are: particle size distribution water content and plastic limit CBR value undrained shear strength compaction characteristics moisture condition value (MCV). Acceptability is normally assessed at the site investigation and design stages of a project, while control of those materials found acceptable is undertaken later, during earthworks. However, control is of equal importance and reference is made to both aspects in the following discussion. The advantages and disadvantages of the above listed properties from the acceptability and control viewpoints are set out in Table 7.1.
‘_
136
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~-
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Table 7.1
Advantages and disadvantages of various engineering properties for acceptability and control purposes in tills
0
0
E
d E
0 0
5 :
Z
m
I
B
.-m B
-.-
x
P e
.-x m C
s M
-a
E
2
e,
D U
-x .-c
-e! *E .-m .-U
e!
a
CIRIA Report C504
m
c I
C
-s8 L
e,
137
7.2.1 Particle size distribution Acceptable fill in the current edition of the Specificationfor Highway Works (1991) is classified according to particle size (Section 7.4.1). The classification is coarse and use of the particle size distribution test is necessary for fine tuning for control purposes. Given the variability of tills, frequent checks are necessary both at acceptability and control stages.
7.2.2 Water content and plastic limit
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The behaviour of a till in earthworks depends critically upon its water content, particularly the relationship between its water content at the time of placement and its plastic limit (PL). The 1976 edition of the Specificationfor Highway Works required acceptable fill to have a water content less than 1.2 x PL because tills with water contents much in excess of this value would be difficult to traffic (Section 5.6.1 and Figure 5.19). High water contents would also produce unacceptable pore pressures in clay tills which could threaten embankment stability. Variation in water content in some clay tills may be large because of included layers or lenses of granular soils. In any case, judging till acceptability and subsequently exercising site control using this approach suffers from the poor precision of the plastic limit test itself (Sherwood, 1970) and is probably the main reason for discontinuing this acceptability criterion.
7.2.3 Undrained shear strength Undrained shear strength as a control has been widely used (Kennard and Kennard, 1962; Buchanan, 1970; Vaughan e f a l . , 1975; Kennard and Reader, 1975; Kennard et al., 1978). It has the advantage that strength can be related directly to till trafficability. Parsons ( 1 978) published details of the strength required to support various items of plant (Section 7.3.3). The normal method of measuring shear strength is by undrained triaxial tests on 38 or 100 mm diameter specimens but Millmore and McNicol (1983) report the use of 250 mm diameter plate tests for the Kielder embankment dam project where till was used as fill. They point to the difficulties of employing this equipment in congested parts of the site. The undrained triaxial test machine is bulky and expensive and requires a power supply. Its use is therefore restricted to larger sites with access to well equipped and reasonably permanent laboratories. Even then, extracting representative till samples and subsequently preparing specimens for the test itself may present a challenge in the coarser tills. Over 4500 triaxial tests were performed on the embankment core and shoulder of the Kielder dam (Millmore and McNicol, 1983). More than 60% were made on remoulded samples, because of the difficulty of obtaining satisfactory undisturbed specimens (Figure 7.1 and Section 5.5.1). Vaughan et al. (1975) and Kennard and Reader (1 975) also describe the use of undrained strengths measured on remoulded specimens as a control. Earthworks control using undrained shear strength can be considered for some of the English subglacial tills where clasts are usually of weak rock origin and where larger size hard rock clasts are infrequent. This should be determined during the site investigation stage if strength testing as a control is in contention.
7.2.4 Compaction test The compaction test is lengthy and expensive to perform but the precision of the optimum water content and maximum dry density of the BS 2.5 kg rammer method is substantially better than that of the plastic limit test (Sherwood, 1970). The test is also performed on a significantly coarser soil fraction (5.0m
South West North West
1.2 1 :2 1:1.75
1 :3
1.2 I : I .75
1:3 1:2.5* 1 : 1.75
Not quoted
1.2
I :3
I :3
Reported as being an extrapolated result.
As with cuttings, the report also quotes embankment slopes for a mixture of materials and the gradients are usually roughly as steep as those reported for single soils.
CIRIA Report C504
153
The downstream slopes of a number of British dams raised from low plasticity clays (such as tills) were plotted by Vaughan er al. ( 1 978) and are reproduced in Figure 9.1. Slopes of up to I in 2 were suggested as being suitable by these authors for embankments up to 10 m high. This is somewhat less conservative than most of the slopes given in Table 9. I , above.
5
a
B BL BW CG H K SG SL U WW
4
UI
c
m a
n
0 v)
%
3
C (U
UI
c m
1
Black Esk (Lucks, 1966) Blackton Backwater (Wilkinson et al, 1970) Cow Green (Vaughan et al, 1975) Hury Knockendon (Banks, 1952) Seagahan (Lucks, 1966) Selset (Bishop and Vaughan, 1962a) Usk (Sheppard and Aylen, 1957) West Water (Lucks, 1966)
0
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O
2
1
10
‘0
Figure 9.1
20 30 Height of Slope (m)
40
Typical downstream slopes of British embankment dams with low plasticity clay fills (after Vaughan et al., 1978)
Carter er al. (1985) maintain that for embankments less than 10 m high constructed from Scottish tills, side slopes of 1 in 2 may be adopted, similar to the Vaughan er al. recommendations and also close to the 1 in 1.75 figure given in Table 9.1 for northwest England tills. Wet Scottish tills may provide problems during the construction stage and pore pressure build-up may threaten stability of high embankments. Carter et al. suggest that for embankments higher than 10 m, stability can be improved by: incorporating granular and rockfill layers controlling rate of construction slackening side slopes. They describe how alternating 1 m thick layers of rockfill and glacial till, during construction of the 10 m high Canonbie (A7) embankment, allowed construction at planned rates of progress without any reduction in side slope. Likewise a 30 m high embankment for the M90 was built with a 10 m rockfill base followed by alternating layers of till (2 m) and rockfill (1 m), again with no interruption of planned progress. In this case, side slope variations were incorporated only for aesthetic reasons. Such construction strategies depend upon the availability of rockfill or other course granular fill. These materials are more likely to be found in the glaciated valley landsystems of Scotland and north Wales than in the subglacial landsystems which characterise much of England south of the Lake District. The approach described by Carter et al. appears suitable for most embankments constructed in tills in the wetter north British and Welsh glaciated terrains. As noted elsewhere, use of drainage layers in a trial embankment built in the course of constructing the M6 was described by McLaren (1968). The technique was successful and wet till could be employed which otherwise would have been run to spoil.
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9.1.2 Effective stress analysis Shear strength parameters may be obtained from effective stress triaxial tests on samples remoulded or reconstituted to their expected placement water content. It is suggested that the critical state or fully softened effective shear strength be adopted for clay tills, assuming c i Vis zero. Parameters for preliminary design purposes may be obtained from Figure 5.1, noting that it is not always clear how @‘wasmeasured and consequently a conservative interpretation of the data is preferred (Section 5.1. I). Note should also be taken of the contribution of the foundation to embankment stability. The coefficient of consolidation of various UK tills was discussed in Section 5.3.2 and it was noted that for English tills, while generally low, cv showed no consistent trend with changing effective stress. This means that the generally high pore pressures produced by low plasticity clays during construction (Vaughan et al., 1978), dissipate slowly. To maintain appropriate factors of safety, drainage should be introduced. Hutchinson (1977) and Bromhead (1992) should be consulted for details.
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9.2
FAILURE SURFACES
Various authors have shown that failures in stiff overconsolidated clays can be slab-like and relatively shallow (Chandler 1970; Greenwood et al., 1985; Perry 1989; Crabb and Atkinson 199 1). Perry records that the failures in the motorways surveyed in his study varied from slab to shallow circular but the majority demonstrated a combination of translational and circular movement. Vertical depth to failure surface rarely exceeded 1.5 m. Bearing in mind that glacial soils were the largest group investigated (37 km of tills), it seems likely that this form of failure surface is of significance for cuts and embankments formed from most tills. This form of surface should be checked as routine in all stability analyses.
9.3
SUMMARY OF SECTION 9 The sensitivity of low plasticity tills to water content change, incurring from rainfall, or from contact with water from granular layers or water-bearing bedrock, is emphasised and should be taken into account when programming embankment construction. Post-construction stability will be enhanced and maintenance costs reduced if the embankment is raised by ‘over-building’, which permits compaction to the embankment edge. Side slopes of I in 2 may usually be adopted for banks less than 10 m high, for preliminary purposes, although the use of slacker slopes is reported for south English tills (Table 9.1). Wet north British clay tills may be used with coarse granular drainage layers. Final embankment design is normally conducted in terms of effective stress with due regard to the stability (and settlement) of the foundation soils. Shallow ‘slab-like’ failure surfaces in the embankment fill should always be examined. Appropriate drainage may be essential to achieve the required factor of safety.
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10 Shallow foundations
The following characteristics of glacial soils may influence both foundation design and construction: spatial variation in soil type, exhibited as changes in strength, compressibility and consolidation properties variation in depth to rockhead, with consequential variation in thickness of overlying compressible soils the presence of groundwater.
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The current approach to designing shallow foundations in clay soils in the UK (for example Tomlinson, 1995) is to calculate the allowable bearing pressure using shear strength data and to confirm that both total and differential settlements are within tolerable limits by using oedometer test results. With granular soils, a common approach is to select a bearing pressure on the basis of a certain settlement value, taking into account the depth to groundwater with respect to foundation depth (e.g. Burland and Burbidge, 1985). A useful summary of foundation design methods in granular soils is given by Clayton (1995).
10.1 VARIATION IN SOILTYPE AND EFFECTS ON BEARING CAPACITY Variations in soil type affect both allowable bearing pressure and settlement characteristics of a foundation. Where such conditions are present, the usual approach to parameter selection is a conservative assessment of the strength data over a depth of about two thirds of the foundation width below the underside of the foundation, provided that the shear strength does not deviate by more than 50% from the mean value (Simons and Menzies, 1977). A common failing is the misidentification of so-called ‘soft pockets’ during site investigation. Soft pockets are often water-bearing bands or lenses of granular material within an otherwise stiff clay till and the failure to recover a U100 sample is usually taken as an indication of soft compressible conditions. In fact, the only reason for its apparently ‘softer’ consistency is the inability of the granular soil to maintain sufficient suction to be sampled successfully using a U 100 sampler.
Glaciolacustrine clays are often present as part of a glacial sequence, frequently attaining significant thicknesses. They may be softer and more compressible than the tills. An approach to the design of spread foundations for the case where a stiff clay overlies a softer clay layer was given by Meyerhof and Hanna (1 978). They showed that for a rectangular foundation placed at a depth D in a stiff clay of thickness D+h resting on a softer layer, the ultimate bearing capacity quis given by: 4,
where
= =
c”(2)
ca
y, L Nc
=
= = =
[+ 1
0.2
(3
+;)(%)+
c,,~,N, + (1
y,D
(10.1)
undrained shear strength of the underlying soft layer undrained adhesion along assumed plane of punching shear op (Figure 10.1(a)) unit weight of upper stiff layer foundation length bearing capacity factor (5.14)
The other terms are defined in Figure 10.I(a). A feature of the solution is the assumed punching shear mechanism which transfers failure through the upper stiff layer to the underlying soft layer. Das (1 990) gives a relationship between the adhesion c, along the assumed plane of punching shear, and cut,, and cUc2, the undrained shear strengths of the upper and lower clay layers, respectively (Figure lO.l(b)). Ultimate bearing capacity is minimum when h is zero and the footing is placed directly upon the softer layer; bearing capacity increases to a maximum when h increases such that the failure mechanism is contained wholly within the upper stiff layer. A factor of safety, F , is normally applied to calculated ultimate values (see, for example, Tomlinson, 1995). 156
-
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B (foundation breadth)
I
I
/ 1.o
0.9
-.-
0.8
U"
0.7
0.6
0 I
0.2
Weaker clay y2,
&=o'
0.4
0.6
0.8
1.0
C"(*~C"(l,
C"(2)
(a) Definitionof terns
(b) Van'ation ofca/cu(,), with cU /cU(,) according to Meyehof and hanna's theory
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Figure 10.1 Bearing capacity of layered clay: stronger over weaker (after Das, 1990) For the maximum case, there may be some difference between the results obtained using equation 10.1 and those obtained using the Skempton (1951) approach. This is because of the differences in the bearing capacity factors employed in formulating the various equations. Settlements derived from bearing pressures calculated using equation 10.1 should always be checked against allowable values (see below). Stiff tills can also occur beneath layers of soft clay, e.g. an alluvial deposit. Solutions for a rigid foundation of breadth B resting on the surface of a uniform soft clay of thickness H overlying stiff soils are given by Hird and Jewell ( I 990), illustrated in Figure 10.2. Results for rough and smooth foundations are given. This method may also be useful for preliminary estimates of embankment stability, especially where they are reinforced and therefore better approximate to the 'rigid' condition required by the solution. I
I
Rough footing
I
I
i
Smooth footing
I
I i I
I
111
I
I
I
I
I
2
4
6
8
10
Geometric ratio BIH
Figure 10.2 Bearing capacity of rigid foundation on thin soft clay layer(after Hird and Jewell, 7990) CIRIA Report (2.504
157
10.2 VARIATION IN SOILTYPE AND EFFECTS ON SETTLEMENT Compressibility of tills and associated granular soils varies substantially. Furthermore, depths to rockhead below glacial soils can vary markedly across a site. Because settlement is directly proportional both to compressibility and to compressible layer thickness, differential settlement should always be expected in glacial terrains. Differential settlement may also occur if there are major fluctuations in drainage conditions: for example, if a glaciofluvial granular layer overlays impermeable bedrock beneath one part of the structure, but were absent elsewhere. Differential settlement should be evaluated systematically because of the inherent variability of glacial soils. This requires investigation of:
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ground conditions (thickness and depths) and their variation across the plan area of the structure compressibility and coefficient of consolidation properties and their variation groundwater, drainage and consequential effective stress conditions. The impact of the resulting differential settlements on the structure should then be assessed. Settlement magnitudes in tills may be obtained using the methods given in the standard texts (e.g. Tomlinson, 1995). For the case where a surface foundation rests on a stiff clay overlying a softer layer, such as a glaciolacustrine deposit, one of the solutions presented by Taylor (1948) may be used to check the vertical stress acting on the softer soil. Figure 10.3(a) illustrates the method due to Kogler for estimating the vertical stress, acting on a soft layer situated a distance h below the centre of a surface foundation of breadth (or diameter) B. The vertical stress ovfapplied at the surface of the upper stronger stratum is related to Gvr,by:
zVr,
ovr’ovr
(B/h)* . . =
(B//I)~ + 2 (B/h)tanp +4tan2(j 3
( 1 0.2)
Where p is the angle of spread of the ‘pressure bulb’ beneath the foundation, taken to be 55”. For a long strip foundation of breadth B, the relationship is: B/h
-
ovr’ovr
=
B/h + tan (j
( 10.3)
The above equations have been solved for circular, square and for strip foundations in Figure 10.3(b). In both cases, the corresponding curves for the Boussinesq centre-stress relationships are shown. The latter give substantially larger values of GVrthan the Kogler method which attempts to ‘smooth out’ the central peak of the Boussinesq curve and may be more realistic as a result. The above equations and Figure 10.3(b) provide a rapid means of estimating the vertical stress at the top (or centre) of a soft compressible layer from which preliminary settlement calculations may be made. Such calculations would be sufficient for indicating whether or not spread foundations should be used to support a given structure or whether pile foundations should be adopted. Stroud and Butler (1975) suggested that the conventional Skempton and Bjerrum ( I 957) approach to settlement analysis, while satisfactory for clays of high plasticity, probably overestimates settlements for low plasticity clays like most clay tills. This together with the conservative assessment often made of compressibility properties (Section 5.4) can lead to overconservative settlement prediction in glacial tills.
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(a) Definition of terms
1 .o
J
squar
1
2
0.8
0.6 0.4
0.2 0
3
4
5
6
1 .o
0.8
0.6 a: Kogler method b: Boussinesq centre stress
0.4 0.2
V
0
(ii) Strip footings
1
2
3
4
5
6
Breadth-depth ratio B/h (b) Relationship between 5Jq, and B h Figure 10.3
Kogler method for determining vertical stresses in buried strata (from Taylor, 1948)
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-10.3 CONSTRUCTION DIFFICULTIES Difficulties in constructing shallow foundations and basements in tills may be caused by the presence of water-bearing granular layers or lenses at shallow depth cobbles, boulders and irregular rockhead topography artesian pressures glaciolacustrine laminated silty clays.
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Water-bearing granular layers or lenses at shallow depth may slow the progress of the excavation significantly, particularly if they are at founding level and there is a danger of loosening the sides and base. Pumping from sumps will usually be necessary in order to blind the foundation base in the dry. Cobbles and boulders may also hinder progress, especially if their presence was not disclosed at the site investigation stage and there is insufficient heavy equipment to cope with their removal. Varying depth to bedrock sometimes means that part of the foundation is underlain by till and the remainder by bedrock. In such circumstances, the till should be removed and replaced with lean-mix concrete in order to obtain a roughly equal compressible thickness. Artesian pressures can exist in fluvioglacial granular soils between otherwise clay tills, in bedrock beneath clay tills, and in some siltier glaciolacustrine deposits (see Appendix B, Case Study 12). As an excavation for a basement or large foundation approaches the water-bearing body, it is possible that the artesian water pressure would exceed the total overburden pressure and the base of the excavation will ‘blow’. Artesian water pressures should also be taken into account in estimating effective stresses for settlement purposes and for assessing uplift pressures on basement slabs. Information from piezometers installed during the site investigation is essential. See Section 12 for a fuller discussion on groundwater lowering.
10.4 SUMMARY OF SECTION 10 Foundation design and construction in tills should take account of the spatial variability of soil type, the variation in depth to rockhead (and hence compressible thickness) and the presence of groundwater. Solutions exist for the estimation of bearing pressures in layered soils such as are typical of glacial successions (equation 10.1). There are also solutions for the estimation of stresses in soft layers where they underlie stiffer material (equations 10.2 and 10.3). They may assist in determining foundation type. Groundwater and the unforeseen presence of cobbles and boulders account for many of the problems experienced when constructing shallow foundations in glacial terrains.
5,56,63-5 extensive, thick till deposits 52 type locality for onshore tills, Dimlington, Holderness 52 Late Midlandian tills 64 Late Weichselian glaciation 52.64 lateral loads 162 lateral moraine 31,3/,39 leaching, of calcium carbonate in tills 76,77 Liffey tunnel, excavation problems 178 lime stabilisation 144 Ling Bank Formation 47.61 liquid limit 75 liquidity index 75,146 Llanddulas landslides, nonh Wales, underlain by laminated clay I84 Llanhilleth landslide, south Wales, underlain by laminated clay 184 loaders 136 Loch Lomond glaciation (readvance)
46.47,5456,65. I80 Loch Lomond Readvance Tills 47 lodgement process 34.68 lodgement tills 22J1,58-65,67.I62,165.
243-8 British, idealised plasticity and grading characteristics 71 buried 3OJ0 characteristics35-6 and geotechnical properties 69 deposition of 34.37 draped, deep-seated shear surfaces I5 I drumlinised I13,114,150 failure in 149 glaciated valley 70 gradational series of till textures 73 granular interbeds and effect on pile design 165 importance of fabric in cuttings 149 weakly bedded PI27 longitudinal profile. glaciated valleys 38
Lower Cretaceous rocks. tectonised 60 Lower Marsh Till 62 Lower Wolston Clay 50 Lowestoft Glaciation 47.57 Lowestoft tills 46,48,57 including Gipping till 47 incorporate reworked North Sea drift tills 46 type site at Cotton, Suffolk 48 Lowland tills 64 McGown and Derbyshire classification,till fabric 68-9.69 Mariner Formation 47 Marly Drift 47,4857 matrix dry density 73,108 matrix water content 73,108 maximum dry density 72,137,138,139 MCA see moisture condition apparatus MCV see moisture condition value melt-out 3 I .31,42 defined 37 from englacial debris 68 subglacial and supraglacial 37 melt-out lills30,67,90,113,165,190,243-8 characteristics35-6 and geotechnical properties 69 deposition 37 depositionavpost-depositionalchanges
88.89 fabrics 68 from glaciated valley landsystems 37 sandy clay, variability of 75 melt water damming of 42 densities of 41 landforms from erosion by 39 meltwater channels 39 meltwater potholes 39 meltwater streams, braided 40 Menard pressuremeter 129,131 measuring deformation modulus
98,98,99 measuring undrained shear strengths at BRE sites 79 mixed successions drainage of cuttings in 148,148,149 end bearing piles in 165-6 pile requirements in 162-6. moisture condition apparatus 111,140 application potential 141,141 moisture condition value calibration line 140 moisture condition values (MCVs)
22,137,140-1,142,222 and trafficability 144 moraine 38 end 27,39.65 fluted 39 hummocky 30,303 I31,39,59,63,64,1I3,I I4 lateral 31,31,39 low relief 57.59 medial 31 Swedish 97 Moray Fitth 62 motorway earthworks slope condition 149 slope failures 155 Moy Till 62 Munsterian 47,5039 north east England power station case study 226-3I tills perched groundwater tables. till cliffs I83
255
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pile tests 166.226-31 research into 2 I slope stability 150,150 slopes in open-cast workings 15 I variation of till properties with weathering 77,77 Whitby cliff stabilisation and coastal protection, case study 237-8 North Sea Basin 59 evidence for Anglian stage 50 North Sea Drift 47.59 Cromer tills 46,4837 Forth Estuary 47 North Sea glacial clays, variation in undrained shear strength 80 North Sea Glaciation 47.57 north west England shaft case study 232-3 site investigation case study 216-7 till, incidence of failure in cuts I51 undrained triaxial test results and SPT N values 82,83,2 16-7.2 18-2 I Northern Ireland, low plasticity melt-out and lodgement tills 83,243-8 Northumberland tills 63 undrained shear strength and ratio of water content to plastic limit 106,107 weathering in upper tills 76 Nurseries till 50.58 Oadby Till 47.48.60 possibly Wolstonian 50 two facies 58 Oakwood Till 47.61.62 observational method, and contract procedure 188 oedometer measurement. pre-consolidation pressures 92,92 open ditch drains 153 optimum water (moisture) content (OMC) 110,137,138 Otter Bank Formation 47 Otter Bank Till 64 outwash plains 27,40,41 as land facets 113,114 ‘over-building’ 153 overburden pressure 92 overconsolidation ratio 88 effect on shear modulus 101,102 overdeepening, of glaciated valleys 38 overflow plume 41 oxidation, and weathering of tills 76 parameter selection, importance of 23 panicle density 97 particle size 66,76 effect of in relation to compaction properties 105,106,108 effects in relation to shear strength properties 86-7.86 large, plant needed to break down 146 largest particle size 105 melt-out and flow tills 68 seen in trial pits 116 particle size distribution 137,138 data 210-2 pavement design 141-2 Paviland Glaciation 47,50,61 percentage fines, influence of 86,X6 permeability 92-4 of clayey silty sandy gravel I I I coefficient of 110,111,111,132 and compaction 110-1 controlled by macro-structure of clay
I10 and groundwater 131-4 in siru tests for 132
magnitude dependent on test type 94,94 major uncertainties 24 relationship to specific volume 93,93 permeability index 92 permeability testing 122 pile foundation strategies 165-6 preparation in advance of construction at glacial sites 166 pile foundations 161 -70 construction considerations 168-9 bored piles 168,224 CFA piles 169 driven piles 169, 225-30 design depth 162,162 mixed successions 162-6 end bearing in 165-6,166 glaciated valley conditions 163,164, I65 subglacial conditions 162,163,164 supraglacial conditions 163,164.165 preparation in advance of construction I66 selection and design considerations 82, 161-2 shaft resistance characteristics inclay tills 166-7 in granular tills 167-8 no densification in bored piles 167 densification in driven piles 167 strategies for 165-6 trials (tests) in advance for 166 placement handling till mixtures 143-4 handling wet tills 144 trafficability 144-5 planar slides, debris and slab 181,182 plant for breaking down large particles 108,146 compaction 146 movement of on wet till 144 selection of 136 and undrained shear strength 144 plasticity data British tills 71,72 site location and sources of 210-2 plasticity indexlindices 70,76,97 relationship with residual shear strength 87 plasticity and particle size 70-76 glaciated valley lodgement tills 70 grading and the dominant soil fraction concept 72-3 plasticity and grading characteristics of some British tills 73-5 plasticity and the T-line concept 7071,72 subglacial tills 70,71 supraglacial melt-out and flow tills 70 plate load tests BRE sites, deformation modulus 98-100 Kielder embankment dam project 138.139 for small strain deformation moduli 100,100 Pleistocene 45.46 pore fluid migration 89,90 pore water pressures conservative assessment of 152 in embankment fill 95,155 high beneath ice 22 and rotational slides 182 post-construction movement, embankments, avoidance of 153 post-depositional factors freeze-thaw 91 groundwater 91
power station case study, north east England 226-3 I pre-consolidation pressures 88.91 -2 low, lodgement tills 34 some Anglian age tills 92 some Saskatchewan age tills 91.92 ‘pre-failure’ deformation modulus 104 prebored pressuremeter 129 preboring, disturbs ground to be tested 129 pressure melting and debris release 37 and regelation (re-freezing) 33 pressuremeter tests 22,122 at BRE sites, to measure undrained shear strength 79 installation disturbance 129,131 pressuremeters 129-3 I carrying out of unloadheload cycle during testing 130 prebored, full-displacement and selfboring 129 pumping tests 132-4 interpretation of in tills 132-3.133 observation wells 134 pumping wells 172 correct array for good groundwater lowering 172 for Storebaelt crossing (project Moses) I78 tunnel in chalky till, Stanstead Airport I78 punching shear mechanism 156 pyrite, disseminated, effects of oxidation of 76 Ramberg and O s g d model 101,101 reconstitution, destroying interstitial cement 97,101 Red Series 4764 Redcar test bed site shear strengths at 78,79 variation of shear modulus with depth 99 relative compaction-passes relationship 108,109 remouldindreconstituting of t i l l 102-5,189 destroys fabric and cement 22 representative strengths concept 23 residual shear strength 87-8.150,152 relationship with plasticity index 87 resilient strain, time dependent, t i l l fills 145,146 retaining and restraining structures 186-7. 186.1X7 Rhondda Beds 182 rhythmites 41.42 road widening case study, east Midlands 223 roches moutonnees 31.38.39 rock flour 37.70 rockhead at varying depths 177 depth to and dip of 180 glaciotectonised 29 identified using geophysical methods 214-5 misidentification of 135 striations 2 9 j l rotary core methods 115,117-18 pendant head attachment, problem with I I7 shear strains lower in samples 120 wireline systems 22 advantages and disadvantages I18 rotational and multiple rotational slides 181,182 Rowe cell tests 94,96 rutting I44
_J
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Saalian glaciation 47,61 safety precautions. necessity for trial pits/ excavations I16 St AlbansTill93,121 compression index values 97 St Clair railway (river) tunnel boulder problems 178 case study 249-5I SI Dogmaels, Dyfed, failure in glaciolacustrine clays 183,239-42 St Maughans Group 222 sample quantities, minimum 121 sample recovery 2 14-5 samples, remoulded and reconstituted
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102-5 cementing 105 limitations 104-5 largest particle size 105 proportion of gravel 104-5 other properties 104 reconstituted tests 103-4 remoulded tests 102-3 sampling methods glacial tills 118-9,119 bulkand block sampling 121,121 rotary methods 120 tube sampling 119-20 progress in 189 for undrained shear strength 77-9 sand drains 94 sand and gravel 29 glaciofluvial 22,174,225 lenses 2 16-7 Sandy Till, lngham 57 scalping. o f coarser particles, may lead to inaccuracy 106 Scarborough Bluffs shoreline (Lake Ontario), failure of 183 Scotland Early Devensiancold stage unproven 52 Late Devensian glaciation 52 Loch Lomond Readvance 54,56,65 Scottish t i l l s 64,70 embankment stability 154 moisture condition value calibration lines 140,140 scrapers 136,144 secant deformation modulus 104 secant shear modulus
98,100,100,I0 I,101,102 elemental 130,130 secant stiffness values, plate tests 100,100 sedimentsisedimentation biogenic 42 ice-contact lakes 42 seismic refraction 214-5 self-boring pressuremeter 129,129,130.130 Selset embankment dam 94-5.95 Selset Till 104 settlement differential, evaluation of 158 effects of variations in soil type 158 of embankments 153 through loss of fines 17 I see ulso collapse settlement shaft adhesion 166-7,226-31 shaft adhesion factor and undrained shear strength, clay tills 166,167 shafts 232-3,234-6 shallow foundations 156-60 construction difficulties 160 variation in soil type and effects on bearing capacity 156-7 variations in soil type and effects on settlement 158,159 shear deformation 33.89
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shear fissures 149 shear modulus 97.130 geophysical measurements o f 100 variation with depth, B R E test bed sites
98,98,99 shear plains, slickensided, ice sheets 66 shear strain (mechanical damage), reduction in 120 shear strength 23,72.73,167 determined using remoulded specimens
I02 effective, critical state. residual or fully softened 152,155 low plasticity tills, sensitivity of to water content change 103 see ulso drained shear strength; residual shear strength; undrained shear strength shear strength-plasticity characteristics correlations 22 sheared bedrock zones, problems with 38 sheet deposits 57-64 Shelley Till 47 Shelly Till 62 Shetland 59 ice sheet 52 Shetland tills 64 shield areas, subglacial landsystem 29 site investigations 23,11394,216-7,218-
southern England, slope failure in tills 15 1 Southern Upland Tills 47,64 Sprcificufiorijbr H i g h i w y Works ( 1976), water content in acceptable fill 138 Specificufionfbr H i g h w y Works (1991)
146 classification of acceptable fill 138.145 importance o f particle size recognised 72 Spitzbergen, evidence for fluid transport in tills 34 spoil heaps, stabilisation by horizontal bored drains I85 spread foundation excavations 23 SPT SCL' standard penetration test stability controlled by discontinuities 149 may be reduced by sump pumping I71 stability analysis cuttings 151-2 effective stress analysis 152 precedent I5 I total stress analysis 152 embankments 153-5 effective stress analysis 155 precedent 153-4 landslides, before remedial measures 185 stability number 174 Stainmore Ice tills 63 standard penetration test
2 1,223,224,225,239-42
22,23,82,122,123,243-8
and design stages, assessment o f fill acceptability 136,137,I46 groundwater and permeability I3 1-4 identification o f glaciolacustrine laminated silty clay important I16 insifu testing 122-31.128 and piling 170 and piling contracts. hidden information 188 preliminary investigations 113-4,115.
factors influencing accuracy o f 123 N value profile 216-7,218-21,243-48 and pile design 161 refusal, in seating penetration 123
I50 desk studylwalk-over phase 38, I13 sampling 118-21 strata definition and investigation methods 115-18 strict control necessary 78 thorough in mixed successions 162 tills in Northern Ireland 243-8 use o f geophysical methods I 14.214-5 Skipsea Till 47,52,63,73,78 slope drains 148,149,185 slope failures cutting through drumlinised t i l l 149 embankments without drainage 153 motorways 155 tills at Brenig 149,150 slope profile, modification o f for landslide remediation I85 slope stability 223 effects of glaciolacustrine deposits
I5 I , 183-4 effects of sand pockets and lens
151,223 factors of relevance to I80 slumping 4/,42,90,90 slurry shield machines 176,176 smooth wheel rollers 108,146 'soft pockets', misidentification o f 156 soil clods, influence on permeability of compacted clay I10,110 soil types segregation of 136,143-4 variations in effects on bearing capacity 156-7 effects on settlement 158,159 south Wales, large-scale downslope movements I80
Stanstead Aifport, pumping well dewatering of tunnel at I78 static cone penetration testing 22 stockpiling 144 Stockport Formation 47,52,63 stones effects o f on sand and gravel mixtures 73 influence of in fills 106 and matrix water content 108 Storebaelt crossing EPB machines 178 hand mining o f cross-over passages 178 strata definition and investigation methods
115-8 cable percussion methods 116-7 rotary core methods 117-8 trial pitskrial excavations 116 strata succession, identification o f I15 Strathmore tills 64 stress, vertical, estimation o f 158,159 stress path techniques, showing formation of flow till 90-1.90 stress relief P127,149 fissures in t i l l cliffs 82 mechanisms 66-8 and undrained shear strength 78 stress relief joints in rockheads 31 striae 68,P127 structures importance of size 23 linear, give opportunities to inspect large exposures o f soil 23 nature, size and geometry of, to be considered in parameter selection 23 subglacial debris capture and entrainment 33 migration o f 29 movement to englacial or supraglacial position 33 subglacial gorges 39 subglacial landsystems 28.29 ground conditions and consequences for piling 162,164,165
257
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mixed successions 162,165 pile types and pile requirements 163 subglacial shear 29 sump pumping 17 I draws water towards excavations 171 superposition,flow tills 90,91 supraglacial debris 27.42 debris capture and entrainment 33 derivation of 33 transfer to an englacial position 33 supraglacial landsystems 2830,165 ground conditions and consequences for piling 164 pile types and pile requirements 163 supraglacial melt-out 27 hummocky sequence of 31 suspension settling 42 Swarte Formation 5059 swelling after lime stabilisation 144 and compression, isotopic 103 swelling index 97 T-line concept 70-1,72 tamping rollers, compaction success 108 Taren landslide, relief wells drain artesian water I85 TBMs see tunnel boring machines temporary works design 23 tension cracks 149 terrain analysis. applied to glacigenic environment 28 terrain evaluation techniques 1 I3 assisting site investigation procedures 115
Terzaghi filter rules 149 test specimen size, important in relation to fissure spacing 81,82 thin-wall push samplers I19 benefits over U 100 samplers 120 used at BRE test bed sites 120 ThrussingtonTill 47.48,58,60 possibly Wolstonian 50 till chronostratigraphy and distribution 46.57-65 onshore, complicating factors 45 till deposition 34-8 characteristicsof generic till types 35-6 see ulso deformation tills; flow tills; lodgement tills; melt-out tills till fabric 66-9,69,81 and cutting orientation 149-50 defined 66 depositional glaciated valley 67,68 subglacial 66,67 supraglacial67,68 description of 67 important to permeability 93-4 McCown and Derbyshire classification 68-9.69 post-depositional68 reflects unloading 23 relevant to landsliding 180 stress changes influencing fabric development 68 see ulso fissures; foliation till fills, propertiesof 105-11 compaction and number of passes 109 compaction and permeability 110-1 strength and compressibility 105-8, 107,109 till plasticity 73-7.75 till slope stability, factors of relevance to 180 till textures, gradational series of 72,73,74
258
tills 26 acceptability as fill materials 136-43.137 Anglian, contiguous bored pile wall 168 areas prone to inland land-sliding 180 and associated glacial soils, dewatering in 171-3,171 behaving as coarse soil 73 behaviour of in earthworks depends on water content I38 bimodal or polymodal 34.72 changes due to weathering 76 coarse granular close to rock fill 87 problems with reporting the SFT 123 complex conditions of deposition 22 consolidation beneath ice 88-90.89 dense, blasting to loosen 136 difficulties in tunnel construction 174 effects of depositional processes 88-92 depositional factors 89-90 post-depositional factors 91 pre-consolidationpressures 9 1-2 engineering behaviour of database requirements 188 laboratory test procedures 189-90 observation and contract procedure
I88 sampling methods 189 engineering property characterisation223 factors affecting consolidation 88-9,89 granular controls on drained shear strength properties 85 shaft carrying capacity 167 shaft resistance characteristics 167-8 interbedded with glaciofluvial deposits, Cromer 48 low plasticity dependence of undrained shear strength on water content 81 shear strength sensitive to water content changes 103 turning into slurry in presence of water 135 need for properly designed drilling muds 118 Northern Ireland, site investigations 243-8 as part of glacigenic sediment suite 21 peak angle of shearing resistance v.plasticity index 85,85 piles selected to cope with construction problems 161-2 preliminary side slopes cuttings 151,151 embankments 153,153 problems of excavation in 135 problems of strata identification I16 relationship between specific volume and permeability 93,93 representativestrengths 81-2 residual angle of shearing resistance v plasticity index relationship 87.88 retaining and restraining structures 186-7 settlement magnitudes 158 stability of cuttings in 148-52 supraglacial,depositional factors 90-1 tills, engineering classificationof 66-84 effects of weathering 76-7 plasticity and particle size 70-6 till fabric 66-9,69 undrained shear strength 77-82 use of correlations between undrained shear strength and SFT N value 82-4
tills, engineering properties of 85-112 compressibility and deformation 97101,102 drained and residual shear strength 85-8 effects of depositional processes 88-92 permeability and coefficient of consolidation92-6 properties of till fills 105-11 remoulded and reconstituted samples 102-5 total stress analysis 152 trafficability 144-5 and undrained shear strength I06,I38,143 Tranmore sewage scheme, difficult tunnelling conditions 178 Transport Research Laboratory 140 works on stability of cuts and embankments 21 trench drains 185 trial pits/excavations I15 hand-cut samples 121,121.189 sample, log and photograph 116 use of mechanical excavation I16 Triassic Till 47,4833 triaxial tests deformation modulus 98,98,99 dissipation tests 96 effective stress 155 on rockfill 87,87 small strain deformation modulus 100-1 undisturbed and remoulded samples, Kielderdam 103 Black Boulder Clay, Dublin, with small strainmeasurements 101 Cheshire till and pile failure 167 Cowden tills 103-4 tube sampling 119-20 tube-a-manchettegrouting, Kelvin Valley sewer buried channel 177 tunnel boring machines 249-5 I cutterhead problems, mixed face conditions 176 facilities for pattern probe holes 178 see ulso earth pressure balance (EPB) machines; slurry shield machines tunnel construction ground classificationsystem 177,234-6 predicted behaviour of groundwater inflow at tunnel face 171,234,235,236 tunnel face pressure effect on face stability and ground movements 175,175 required for stability and ground movement control 176 tunnel face stability 174,175,175,176,176 tunnel stability, classification of 175 tunnel valleys 50 tunnelling 174-9.249-5 1 hand mining in mixed face conditions I77 more than one soil type in the face 174-7 nests of cobbles and boulders 177-8 varying groundwaterconditions 178-9 varying soil thicknesses and rockhead depths 177 tunnels 232-3,234-6 Tyneside, bored piling in t i l l deposits, case study 225 Ul00drivesampler 119 U100 tubes, double, to retrieve representativethough disturbed samples 121,132
CIRIA Report C504
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U-shape profile. glaciated valleys 38 ultimate base bearing resistance 165 rules when ground conditions not known 165-6 underflow/turbidity current 41 undrained shear strength 137,216-7.2 182 I, 243-8 in engineering classification of tills 7782 BRE test bed sites 78-80 and design purpose 78 factors it depends on 77 representative strength of tills 81-2 scatter of results 80-1 and granular content 105,106,107 importance of granular panicles on 22 influence of fissures and foliation on 80-2,81 related to trafficability 106,138, I43 relationship with cone resistance 124, I2X and S F T N value, correlations between 82-4 tests on Cowden Till 103-4,131,131 use in pile design 167,167 variation of with water content 103,103, 105-6,106.107 uniformity coefficient 92,145 uplift loads 162 Upper Till, Kirkhill 47,62 Upper Wolston Clay 50
valleyside fans 31 varveslvarvitedvarved clays see ul.so glaciolacustrine deposits, laminated silty clays 41,41,70,151,172 vibrating equipment, compaction success
I08 Wales evidence for Late Devensian readvance 54 evidence for Paviland glaciation 50 Irish Sea tills 58 Walton Glaciation 47 Warren House Till 47S0.60 water, migration through 'inter-clod' spaces 110 water content 76,143 and compaction I10,146,153 of debris at deposition 68 importance of with respect to granular content 105-6 with respect to plastic limit 137,138 and shear strength of low plasticity tills I03
and undrained shear strength 106, 106.107 see ulso matrix water content water content migration, a factor controlling sample disturbance 8 I Wear and Blackheath tills 63 Weathered Scottish Tills 47 weathering effects of 76-7,105
weathering mechanisms 76 Wee Bankie Fill 64 Wee Bankie Formation 47 Weichselian 47 wellpoints 178 limitation caused by vacuum 172 suction-lift principle 172 Welsh Till 63 Weltman and Healy relationship 166 Welton Till 47,50,60 west central Scotland, fissured tills 96 drumlinised tills 81 undisturbed, permeability of 94,94 whalebacks 38,39 wheel slip 144 Whitby cliff stabilisation and coastal protection, case study 227-8 wireline systems 189 advantages and disadvantages 1 I8 Withernsea Till 47X63.73.78 Wolf Crag Formation 65 WolstonianlPaviland 45,45,46,47,60- I evidence for glaciations limited 50 possible ice coverage and extent 51 two separate ice streams indicated 50 type section, Wolston 50 Woolwich and Reading Beds clays, effective strengths greater from rotary core samples 120 Wragby Till 60 Wylfa Head, Anglesey 38 Young's modulus 97
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259
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~
Glacial tills are particularly prevalent in the colder, wetter upland parts of Britain and a considerable proportion of construction is built on or in these highly variable materials. Tills and associated glacial soils are among the most difficult to sample, test and engineer because of their marked variations in thickness and engineering characteristics. Engineering in g/acial tiNs draws together understandingof the origins of tills, the related landsystems and their distribution within UK glacial stratigraphy. The book links this geological background t o the engineering and classification of tills, giving the typical ranges of their properties. Guidance for engineering in tills covers site investigation, earthworks, shallow and piled foundations, dewatering, tunnelling and landslides. Supported by 14 case studies and numerous figures and tables, a substantial reference list, glossary and index, this comprehensive ClRlA title provides a practical review of the geology and geotechnics of tills and guidance for engineering practice. This book will be of particular relevance to geotechnical engineers, engineering geologists, civil and structural engineers, and tunnelling and highway engineers.
