CIRIA C504 Eng. in Glacial Tills

Licensed copy:Jacobs UK Limited, 06/04/2009, Uncontrolled Copy, CIRIA

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ClRlA C504 London, 1999

Engineering in glacial tills

N A Trenter

sharing knowledge building best practice

6 Storeys Gate, Westminster, London SW1 P 3AU TELEPHONE 0171 222 8891 FAX 0171 222 1708 EMAl L [email protected]. uk WEBSITE www.ciria.0rg.uk


Summary

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

0 CIRIA 1999

Construction Industry Research and Information Association 6 Storeys Gate, Westminster, London, S W l P 3AU Telephone 0 17 1 -222 8891 Facsimile: 0 17 1 -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

Geotechnical engineers, engineering geologists, civil and structural engineers, tunnelling engineers, highway engineers, construction professionals

I

Classification

AVAILABILITY Unrestricted CONTENT Review of

available guidance STATUS Committee guided USER Construction

professionals

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 -

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.

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

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

CIRIA Report C504 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

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

Sir Alexander Gibb and Partners Ltd Halcrow

Northern Ireland

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:

Figures 2.2,2.9 to 2.1 1 Figures 2.4 to 2.6

Figure 2.7

Figure 2.8

Figure 3.1

Figures 3.2,3.4 and 3.5

Figure 10.1

Figure 10.3

Figure 11.4

Figure 13.1

HAMBREY, M. J. (1 994) Glacial environments, UCL Press (London) EYLES, N. (1 983) Glacial Geology: an introduction for engineers and earth scientists, Pergammon (Oxford) EYLES, N. and DEARMAN, W. R. (1981) A glacial terrain map of 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, George Allen and Unwin (London) DEPARTMENT OF THE ENVIRONMENT (DOE) (1 994) Landsliding in Great Britain, Her Majestys Stationery Office (London) 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), PWS - Kent Publishing Company (Boston) TAYLOR, D. W. (1948) Fundamentals of soil mechanics, John Wiley and Sons (New York) TOMLINSON, M. J. (1 994) Pile design and construction practice (4th edition), E and F N Spon (London) CLOUGH, G. W. and SCHMIDT, B. (1981) So8 clay engineering, edited E. W. Brand and R. P. Bremner, Chapter 8, Elsevier (Amsterdam)

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 N OTATl 0 N GLOSSARY

1 INTRODUCTION 1.1 Geological history 1.2 Techniques of investigation 1.3 Engineering property characterisation 1.4 1.5 Parameter selection 1.6 Difficulties during construction

Applicability of methods of analysis and design

2

3

GEOLOGY OFTILLS: PHYSICAL PROCESSES 2.1 Glacigenic environment 2.2 Glacial landsystems

2.2.1 Subglacial landsystem 2.2.2 Supraglacial landsystem 2.2.3 Glaciated valley landsystem

2.3.1 Glacier flow 2.3.2 Debris capture and entrainment 2.3.3 Debris deposition

2.4.1 Lodgement tills 2.4.2 Melt-out tills 2.4.3 Flow tills 2.4.4 Deformation tills

2.3 Glacial processes

2.4 Till deposition

2.5 Till-related landforms 2.6 Other glacigenic sediments

2.6.1 Glaciofluvial system 2.6.2 Glaciolacustrine system 2.6.3 Glaciomarine system

2.7 Summary of Section 2

GEOLOGY OF TILLS: REVIEW OF GLACIAL STRATIGRAPHY IN THE UK 3.1 Anglian

3.1.1 North Sea drift 3. I .2 Lowestoft tills

3.2 Wolstonian/Paviland 3.3 Early Devensian 3.4 Late Devensian 3.5 3.6 Summary of Section 3

Late Devensian - Loch Lomond

4 ENGINEERING CLASSIFICATION OF TILLS 4.1 Till fabric

4.1.1 Depositional fabric - subglacial 4.1.2 Depositional fabric - supraglacial 4.1.3 4.1.4 Post-depositional fabric 4.1.5 McGown and Derbyshire classification

Depositional fabric - glaciated valley

9 10 13 14 15 17

21 22 22 22 23 23 24

25 25 28 29 30 31 33 33 33 34 34 34 37 37 38 38 39 40 41 42 44

45 46 48 48 50 52 52 54 54

66 66 66 68 68 68 68

CIRIA Report C504 5


4.2

4.3 4.4

4.5

4.6

Plasticity and particle size 4.2.1 Subglacial tills 4.2.2 4.2.3 Glaciated valley lodgement tills 4.2.4 4.2.5 4.2.6 Effects of weathering Undrained shear strength 4.4.1 BRE test bed sites 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

Supraglacial melt-out and flow tills

Plasticity and the T-line concept Grading and the dominant soil fraction concept Plasticity and grading characteristics of some British tills

Scatter of undrained shear strength results

5 ENGINEERING PROPERTIES OF TILLS 5.1

5.2

5.3

5.4

5.5

5.6

5.7

Drained peak and residual shear strength 5.1 . I 5.1.2 Residual shear strength Effects of depositional processes on tills 5.2.1 Depositional factors 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

Drained peak shear strength

Post depositional factors (all landsystems)

6 SITE INVESTIGATION 6.1 6.2

6.3

6.4

6.5

6.6

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 6.5.1 6.5.2 In-situ permeability tests 6.5.3 Pumping tests Summary of Section 6

Permeability determination from grading tests

70 70 70 70 70 72 73 76 77 78 a0 81

a2 a4

85

85 a5

a7 aa a9 91 91 92 92 94 97 97 97

102 102 103 104 104 105 105 110 112

113 113 115 116 116 117

119 120 121 122 123 124 129 131 132 132 132 134

i i a

6 CIRIA Report C504

,


7

8

9

10

11

EARTHWORKS 7.1

7.2

7.3

7.4

7.5

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 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 Trafficabil ity Compaction 7.4.1 Nature of till 7.4.2 Type of plant 7.4.3 7.4.4 Water content Summary of Section 7

Water content and plastic limit

Layer thickness and number of passes

CUTTINGS 8.1 Drainage 8.2 8.3 Stability analysis

Till fabric and cutting orientation

8.3.1 Precedent 8.3.2 Total stress analysis 8.3.3 Effective stress analysis


EMBANKMENTS 9. I Stability analysis

9.1.1 Precedent 9.1.2 Effective stress analysis

9.2 Failure surfaces 9.3 Summary of Section 9

SHALLOW FOUNDATIONS 10.1 10.2 10.3 Construction difficulties 10.4 Summary of Section 10

Variation in soil type and effects on bearing capacity Variation in soil type and effects on settlement

PILE FOUNDATIONS 1 1.1 11.2 Design depth 1 1.3 Mixed successions

Pile selection and design considerations

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

11.4

11.5

\

135 135 135 135 135 136 136 138 138 138 138 140 141 142 143 143 144 144 145 145 146 146 146 147

148 148 149 151 151 152 152 152

153 153 153 155 155 155

156 157 158 160 160

161 161 162 162 162 165 165 165 166 166 167 167

CIRIA Report C504 7


12

13

14

15

1 1.6 Construction considerations 1 1.6.1 Bored piles 11.6.2 Driven piles 1 1.6.3 CFA piles


GROUNDWATER LOWERING 12.1 Sump pumping 12.2 Wellpoints 12.3 Pumping wells 12.4 Ejector systems 12.5 Summary of Section 12

TUNNELLING 13.1 13.2 13.3 13.4 Varying groundwater conditions 13.5 Summary of Section 13

LANDSLIDING

More than one soil type in the face Varying soil thicknesses and rockhead depths Nests of cobbles and boulders

14.1 Inland landslides 14.1.1 Complex landslides 14.1.2 Debris flows 14.1.3 Planar slides 14.1.4

Contribution of glaciolacustrine deposits to instability of tills

14.4.1 Drainage 14.4.2 Modification of slope profile 14.4.3 Retaining and restraining structures


CURRENT CAPABILITIES AND UNCERTAINTIES 15.1 Observational method and contract procedure 15.2 Database requirements 15.3 Sampling methods 15.4 15.5

Rotational and multiple rotational slides 14.2 Coastal landslides 14.3 14.4 Landslide remedial measures

Laboratory test procedures: remoulding and reconstituting samples Laboratory test procedures: coarse clast-dominant tills

168 168 169 169 170

171 171 172 172 173 173

174 174 177 177 178 179

180 180 182 182 182 182 182 183 184 185 185 186 187

188 188 188 189 189 189

REFERENCES 191

APPENDIX A Site location and sources of plasticity and particle size 209 distribution data (University of Paisley)

APPENDIX B Case Studies: 1 Site Investigation, Chapelcross, Dumfriesshire 2

4 5 6 7 8 9 Shaft, north west England 10 I I 12 St Dogmaels landslide, Pembrokeshire 13 14

Site Investigation, north west England

Cardiff Gate business park, Cardiff, south Wales Road widening, east midlands area Cuttings for major road projects, north east England Bored piling in glacial till deposits, Tyneside Power station, north east England

Fylde coastal water improvement scheme, Lancashire Whitby cliff stabilisation and coast protection, NE England

Site investigations in tills in Northern Ireland The St Clair river tunnel, Ontario to Michigan

3 Site Investigation, Cumbria

21 3 21 4 21 6 21 8 222 223 224 225 226 232 234 237 239 243 249

INDEX 252

8 CIRIA Report C504


Tables

Table 2.1

Table 2.2

Table 3.1

Characteristics of genetic till types

Summary of erosional and depositional landforms

Schematic representation of main glaciations in the UK for Middle to Late Pleistocene

Table 3.2 Summary of Anglian and equivalent tills in the UK and on the UK continental shelf

Table 3.3 Summary of Post AngliadSaalian (Wolstonian/Paviland) tills in the UK and on the UK continental shelf

Table 3.4 Summary Table of Early/Middle Devensian tills in the UK and on the UK continental shelf

Table 3.5 Summary of Late Devensian tills in the UK and on the UK continental shelf

Table 3.6

Table 4.1

Table 4.2

Table 4.3

Table 4.4

Table 4.5

Table 5.1

Table 5.2

Table 6.1

Table 6.2

Table 6.3

Table 6.4

Table 7.1

Summary of Loch Lomond GlaciatiodReadvance tills in the UK

Characteristics and geotechnical properties of glacial tills

Gradational series of till textures

Variation of plasticity data, Kielder dam till

Variation of average till properties with weathering, NE England

f , values for some British tills

Results of large triaxial tests on rockfill

Summary of pre-consolidation pressures measured in oedometer

Guide to selection of sampling methods in glacial tills

Guide to selection of in-situ test methods in glacial tills

Some factors influencing accuracy of SPT results in tills

Characteristics of some pressuremeters in current use in the UK

Advantages and disadvantages of various engineering properties for acceptability and control purposes in tills

Preliminary cutting side slopes in tills

Preliminary embankment side slopes in tills

Types of inland landslides in glacial soils

Types of coastal landslide in glacial soils

Results of pile tests on north east England tills

Ground classification system (GCS) for conditions at tunnel horizon

Likely behaviour of water-bearing strata in tunnel face identified in Table CSlO/l

c,l/N relationships for tills from Northern Ireland

Table 8.1

Table 9.1

Table 14.1

Table 14.2

Table CS8/1

Table CS 10/1

Table CS10/2

Table CS13/1

35

39

47

57

60

62

63

65

69

73

76

77

83

87

92

119

122

123

129

137

151

153

180

183

227

235

236

243

CIRIA Report C504 Licensed copy:Jacobs UK Limited, 06/04/2009, Uncontrolled Copy, CIRIA

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 Figure 2.10

Figure 2. I 1

Figure 3.1



Figure 3.6

Figure 3.7


Figure 4.3 Figure 4.4 Figure 4 3 a )

Figure 4 3 b ) Figure 4.6


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

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 Cowden site: undrained shear strength, cu, and liquidity index v depth for till 80 Effect of failure dejnition on scatter size 81 Effect of specimen size on strength offissured material 82 SPT N v undrained shear strength, cu, at corresponding depth for glacial till at Chapelcross 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

Figure 5.1 1 Figure 5.12


Figure 5.15

Figure 5.16

Figure 5.17

Figure 5.18

Figure 5.19



Figure 6.1




Peak angle of shearing resistance,$:,, v plasticity index for tills 85 Impact of density and percentage fines on angle of shearing resistance,@:, 86 Residual angle of shearing resistance,@',, v plasticity index relationship for tills 88 Diagrammatic representation of depositional and post-depositional processes for glacial tills 89 Diagrammatic representation of some of the factors affecting the consolidation of a till layer (subglacial and glaciated valley landsystem) 89

90 91 93

Variation of permeability, k , with effective stress, of , forjssured Scottish till 94 Variation of coeflcient of consolidation, c,, with effective stress, o' ,for Cow Green and other tills 95 Garston site: shear modulus, G, and liquidity index v depth for till 98 Redcar site: shear modulus, G. and liquidity index v depth for till and associated glacial soil 99 Cowden site: shear modulus, G, and liquidity index v depth for till 99

100

101

Stress paths illustrating process offlow till formation Freeze-thaw process and influence on soil consolidation Relationship between specijc volume and permeability for tills

Secant shear modulus, G, normalised with respect to effective overburden pressure a:(, derived from triaxial and plate loading tests on glacial till Resonant column secant shear modulus, G, normalised with respect to Gma, v shear strain results for glacial till Secant shear modulus, G, nornialised with respect to effective overburden pressure a:,, v overconsolidation ratio (OCR) for undisturbed and reconstituted specimens 102 Variation of undrained shear strength, cu, and water content, w, for remoulded Kielder tills 103 Variation of undrained shear strength, cu, with total water content, w, for remoulded glacial soil containing various proportions of granular materials 1 06 Relationship between undrained shear strength, cu, water content, w, for Northumberland glacial tills 107 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,

Illustration of the relationships between landsystems, land facets and land elements used in land surface evaluation 114 Use of terrain evaluation to assist site investigation procedures 115 Some typical groundwater table conditions arising from glacial till successions 117

120

and plasticity index, PI 111

Stages in void formation during drive sampling 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

CIRIA Report C504 1 1


Figure 7.1

Figure 7.2 Figure 7.3 Figure 7.4

Figure 7.5

Figure 7.6


Figure 10.1 Figure 10.2 Figure 10.3 Figure 11.1 Figure 1 I .2

Figure

Figure Figure

Figure

1.3

1.4 1.5

2.1 Figure 13.1 Figure 13.2 Figure 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 21 6 21 6 21 7 21 9 220 22 1 22 1 22 1 228 228 229 229 230 230

12 CIRIA Report C504

. ,


Figure CS8/7 Figure CS9/ 1 Figure CS I I / 1 Figure CS 12/ 1 Figure CS 12/2 Figure CS 12/3 Figure CS 12/4 Figure CS 1Z5 Figure CS 12/6 Figure CS 1 3/ 1

Figure CS 13/2

Figure CS I3/3

Figure CS 13/4

Figure CS 1 41 1

Figure CS 14/2 Figure CS 14/3

Results of pile test HP6 Sketch indicating damage to shaft Typical cut and fill slopes showing drainage features Particle size distribution for glacial till Plasticity chart for glacial till Peak shear strength for glacial till Residual shear strength for glacial till SPT N v depth for glacial till 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) Plot of stability number N v elevation (Port Huron Bank)

23 1 233 238 240 240 241 24 1 242 242

245

246

247

248

250 250 251

Cover Plate

Plate 1

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). Stratified till with chalk lenses and flow structures near the base of the Third Cromer Till, West Runton, Norfolk (Photograph: P Gibbard, 1978).

Plate 2

Plate 3

Cover

125

125

126

Plate 4

Plate 5

Folding and step faulting of laminated till near the base of the Third Cromer Till, West Runton, Norfolk (Photograph: P Gibbard, 1978).

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). 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).

126

127

Plate 6 127

CIRIA Report C504 13


Ab brevia t i ons

BP

BRE

CBR

CSL

DMRB

EPB

FDP

FM

LL

MCA

MCV

MDD

OCR

OMC

PI

PL

PBP

PSD

SBP

SHW

SPT

TBM

TRL

Before present

Building Research Establishment

California bearing ratio

Critical state line

Design Manual for Roads and Bridges (HMSO)

Earth pressure balance (tunnel boring machine)

Full displacement pressuremeter

Formation

Liquid limit

Moisture condition apparatus

Moisture condition value

Maximum dry density

Overconsolidation ratio

Optimum water (moisture) content

Plasticity index

Plastic limit

Prebored pressuremeter

Particle size distribution

Self-boring pressuremeter

Specification for Highway Works (HMSO)

Standard penetration test

Tunnel boring machine

Transport Research Laboratory

b- I



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

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

Relative density emax - e emax - emin -

CIRIA Report C504 15 Licensed copy:Jacobs UK Limited, 06/04/2009, Uncontrolled Copy, CIRIA

k

k0

KS L

m

W

w m

ws

a

Coefficient of permeability

Coefficient of permeability at void ratio eo

Coefficient of earth pressure

Length of foundation

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

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

Total water content (matrix plus clasts)

Water content of matrix

Average water content of clasts

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

16 --- /

CIRIA Report C504


Glossary of geological terms

aeolian

biogenic

bioturbation

boulder clay

calving

cold stage

corrie (coire)

crag-and-tail

comminution

dead ice topography

deformation till

diachronous

diamict

diamicton

drumlin

englacial debris

esker

facies

fjord

flow till

fluvial

Wind-borne, wind-blown or wind-deposited.

A term applied to material produced by the action of living organisms.

The breakdown and reworking of sediment by the action of its contained organisms.

A term for till, no longer favoured by glacial geologists.

Ice wastage by shedding of large ice blocks from a glaciers edge usually into a body of water.

See glacial periodglaciation.

Deep steep-sided hollow formed by glacial erosion, typical of the glaciated valley landsystem.

A glacially eroded rock-cored hill with a tail of till formed down- glacier.

The gradual breakdown of rocky materials by weathering and erosion to form progressively smaller particles.

Complex of eskers, kames and kettle holes formed when ice wastes in situ.

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.

Development of the same facies at different places and at different times.

A poorly-sorted terrigenous sediment containing a wide range of particle sizes. Embraces both diamictite (lithified) and diamicton (non- lithified).

See diamict.

A streamlined hillock, commonly elongated parallel to the former ice- flow direction, composed of glacial debris, and sometimes having a bedrock core; formed beneath an actively flowing glacier.

Debris dispersed throughout a glacier, derived either from the surface through burial or crevasses, or from the uplifting of basal debris by thrusting processes.

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.

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.

A long, narrow arm of the sea, formed in part as a result of erosion by a valley glacier.

Till that has been transported and emplaced by debris flow.

Transported and deposited by rivers.

CIRIA Report (2.504 17


gelifluction

glacial debris

glacial period/ glaciation

glaciated

glacier

glacier bed

glacier ice

glacigenic sediment

glaciofluvial sediment

glaciolacustrine sediment

glaciomarine sediment

glaciotectonite

glaciotectonic deformation (glaciotectonism)

glacioterrestrial sediment

gravity flow

grounding-line

hummocky (ground) moraine

iceberg

ice cliff

Down slope movement of part frozen soil or rock debris in periglacial environment.

Material in the process of being transported by a glacier in contact with glacier ice.

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.

The character of land that was once covered by glacier ice.

A mass of ice, irrespective of size, derived largely from snow, and continuously moving from higher to lower ground, or spreading over the sea.

Surface on which a glacier rests, may comprise bedrock or previously deposited glacial or non-glacial sediments.

Any ice in, or originating from, a glacier, whether on land or floating on the sea as icebergs.

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.

Glacial debris reworked by running water.

Glacial debris deposited in a lake.

A mixture of glacigenic and marine sediment, deposited more or less contemporaneously.

Deformed glacier bed sediments created as a result of glaciotectonic deformation.

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.

Glacial debris deposited directly on land.

The process of transport of unconsolidated sediment down a slope, under its own weight either subaerially or subaquatically.

The line or zone at which an ice mass enters the sea or a lake and begins to float.

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.

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.

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.

18 1

CIRIA Report C504


ice sheet

ice shelves

ice streams

kame

kame terrace

kettle (or kettlehole)

landform

landsystem

lithofacies

lodgement

lodgement till

mClange

melt-out till

meltwater channel

moraine

nivation

outwash plain

palaeosol

proglacial

A mass of ice and snow of considerable thickness and covering an area of more than 50,000km2.

Large slabs of ice floating on the sea, but remaining attached to, and partly fed by, land-based ice.

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.

A steep-sided mound, hummock or hill of sand and gravel deposited by glacial streams adjacent to a lateral glacier margin.

A flat or gently sloping feature, deposited by streams that flowed towards or along the margin of a glacier.

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.

Morphologically distinct land surface features.

A characteristic association of landforms and glacial sediments controlled by the nature of the glacier and the surface over which it moves.

See facies.

The process whereby basal glacial debris is plastered on to the substrate beneath an actively moving glacier.

Glacigenic sediments deposited by plastering of glacial debris from the sliding base of a moving glacier, by pressure melting andor other mechanical processes.

Jumbled and incoherent mass of rock fragments of various sizes and angularities.

Glacigenic sediments deposited by a slow release of glacial debris from melting ice that is not sliding or deforming internally.

A form of channel cut in to solid rock or drift by the erosive action of glacial meltwater.

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.

Erosion due to frost shattering with the formation of nivation hollows.

A flat spread of debris deposited by meltwater streams emanating from a glacier.

An ancient soil horizon or buried fossil soil.

Area lying adjacent to and usually in front of a glacier.



regelation

rhythrnite

roche rnoutonnke

sandur plain

shield

slickenside

stadia1

stria (striae)

su bglacial

subglacial debris

subglacial gorge

superglacial

supraglacial debris

talus cone

till

till plains

tunnel valley

varvite (varved clay)

wastage

whaleback

The process of refreezing of ice subsequent to melting caused by pressure within an ice mass.

A sedimentary unit comprising a repetitive succession of sedimentary types, e.g. mudsand. A varvite is one specific type of rhythmite.

A rocky hillock with a gently inclined, smooth slope facing up-valley resulting from glacial abrasion, and a steep, rough slope facing down- valley resulting from glacial plucking.

Extensive flat or gently sloping fan-like accumulation of glaciofluvial sediments.

Large area of old crystalline rock, usually largely Precambrian in age, forming the core of a continent.

A polished or scratched planar surface.

Time represented by glacial deposits.

One of a series of parallel straight lines scoured on to rock or rock fragments by the glacial process.

The area beneath a glacier.

Debris that has been released from ice at the base of a glacier.

A steep, often vertically sided gorge cut into bedrock by a subglacial stream under high pressure.

The area on the top surface of a glacier.

Debris that is carried on the surface of a glacier. Normally derived from rockfalls and usually angular.

A cone of debris formed by the slow downslope movement of a superficial mass of rock fragments.

A sediment transported and subsequently deposited by or from glacier ice, with little or no sorting by water.

A wide area of low relief created by till deposition.

Large valley or trough cut into drift or bedrock by a subglacial stream flowing in an ice tunnel.

A thin laminar bed of sediment divided into a thicker, lower, lighter- coloured band of sand grading upwards into a thinner, upper, darker- coloured band of silt or clay.

The process of overall shrinkage of an ice body, including loss by melting, evaporation, wind erosion and calving.

Smooth, glacially sculptured bedrock knob of modest size resembling the back of a whale.

20

- CIRIA Report C504


1 Introduction

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 regions 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

parameter selection difficulties during construction.

applicability of methods of analysis and design

!

CIRIA Report (2.504 21


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.

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 self- boring 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.

d



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.

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 full- face 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.

CIRIA Report C504 23 Licensed copy:Jacobs UK Limited, 06/04/2009, Uncontrolled Copy, CIRIA

1.6 DIFFICULTIES DURING CONSTRUCTION

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.

24 I

CIRIA Report C504


2 Geology of tills: physical processes

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, 198 I ) .

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).



1- I

Figure 2.1 Evolution of glacigenic deposits

T T 1



Figure 2.2 Cross-section of typical temperate glacier illustrating the various glacigenic sediments (from Hambrey, 1994)



2.2 GLACIAL LANDSYSTEMS

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 (a) Lateral moraine (b) Hummocky moraine (c) Drumlin

(ii) Sediment sequences from one or more ice advances eg Supraglacial melt-out till n Subglacial lodgement till Glacier bed

(iii) Bedrock topography, exposed or buried eg (d) buried meltwater channel (e) buried roche moutonnee

Figure 2.3 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.


. ---


2.2.1 Subglacial landsystem

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

(1) striated rockhead surface locally

(2)

overdeepened by subglacial erosion

buried channel over-steepened by subglacial meltwaters and filled with subglacially derived sediments

rock rafts, glaciotectonised rockhead and deformation till depending on bedrock lithology

bouldery unit of scree-like debris filling lee-side cavities in rockhead surface

(3)

(4)

(5) bedrock

Glacigenic sediments

(6)

(7)

preferentially orientated clast long axis

distinct flat iron shaping of clasts composed of fine-grained lithologies; coarse grained lithologies produce clasts of higher sphericity, frequently found as boulder pavements

(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

(9) fluvial sediments reworked, deformed and incorporated in subsequent tills

(1 0) slickensided bedding plane resulting from subglacial shear

(1 1) 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) ~



2.2.2 Supraglacial landsystem

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) flow tills (4) (5) drumlins (6) buried lodgement till (7)

Landforrn

(8)

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

strata deforming as a result of meltdown of adjacent ice-cores

supraglacial melt-out and flow tills

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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CIRIA Report (2504


2.2.3 Glaciated valley landsystem

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) 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

(4)

(5) complex glaciofluvial sediments and flowed tills deposited in kettle holes or against lateral moraines

valleyside fans discharging large quantities of coarse debris to lateral moraines

(6)

Landform

(7) medial moraine

(8) lateral moraine ridge

Figure 2.6 The glaciated valley landsystem (after Eyles, 1983 and Eyles and Dearrnan, 1981)



SOUTHERN

G LAC I AT1 0 N

SOUTHERN LIMIT OF ANGLIAN GLAClATlO

PE RIG LAC I AL

Lodgement till dominant .......... Drumlinised surface (lines indicate

(a) Subglacial landsystem

ml flow direction) Scoured bedrock surface with little glacigenic cover

(b) Supraglacial landsystem

(c) Glaciated valley landsystem

I Areas with no preserved glacial deposits I (d) Other Figure 2.7


Distribution of glacial landsystems in mainland Britain (after Eyles and Dearrnan, 1981)


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.

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.



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:

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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CIRIA Report C504


Table 2.1 Characteristics of genetic till types (from Dreirnanis. 1988 and Elson, 1988)

Criterion Lodgement till Row till Deformation till Melt-out till

Deposited by a slow release of glacial debris from ice neither sliding nor deforming internally (Dreimanis, 1988).

Deposited by plastering of glacial debris from the sliding base of a moving glacier, by pressure melting andor other mechanical processes (Hanibrey, 1994).

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).

Deposition

Position and sequence

Usually deposited during glacial retreat.

Most commonly the uppermost glacigenic deposit.

Lodged over older glacial sediments or on bedrock

Formed and deposited subglacially, often where the glacier moves upslope.

Variable 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.

Variable basal contact but seldom conformable over longer distances. Tills may f i l l shallow channels or depressions.

Basal contact

Land forms Mainly ground moraines, drumlins, flutes and othei subglacial landforms.

Landforms rarely diagnostic.

Those ice-marginal landforms where glacier ice stagnated.

Single units usually a few centimetres to a few metres thick. Units may stack to much greater accumulated thickness.

Associated with most ice-marginal landforms.

Very variable. Individual flows usually a few tens o f centimetres to metres thick. Units may stack to accumulated thickness of many metres.

nickness Typically one to a few metres thick but may attain substantial lhickness in the English lowlands; relative lateral :onsistency.

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. Sub- horizontal 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 melt- out 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.



- - - - _ _ -~ - -

Table 2.1 Characteristics of genetic till types (from Dreimanis. 1988 and Elson, 1988) (Continued)

kiterion

.ithology of lasts and natrix

:last shapes md their iurface narks

3bric

Consolidatior permeability density

Relevant references for summaries of diagnostic properties

Lodgement till ~

,ithological composition )ften more consistent han other tills. lomposition of matrix mticularly uniform. Materials of local ierivation increase in ibundance towards basal :ontact.

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.

Strong macro fabric with clast long axes parallel to local direction of movement. Transverse orientation possible, associated with folding and shearing.

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

Goldthwait (1971). Boulton (l976b), Dreimanis (1976). Boulton and Deynoux (198 I ) , McGown and Derbyshire (1977). Ey

Documents

CIRIA C504 Eng. in Glacial Tills