SOil MECHANICS 1Br:iacigles and Practice

FOURTH EDITION

Graham Bames

Preface xiii Made ground 17 Aims of the book xiii Organic soils 17 The Eurocode Approach xüi Volcanic soil 19 Changes in the Fourth Edition xiv Particle size fractions - fundamental basis 19 Companion website xiv Very coarse soil 20 Acknow ledgements xiv Coarse soil 20

Acknowledgements XV Fines 21 Case studies xvii Composite soils 21 Videos xviii Fine soil 21 List of symbols xix Mass structure 23 Note on units xxiv Degree of weathering 24

SI units xxiv Geological origin 24 Mass, force and weight XXIV Soil classification 26 Stress and pressure XXIV Particle density 26 Density and unit weight xxiv Particle shape 26

1 Soil formation and nature 1 Particle size distribution 27 Grading characteristics 29

Objectives 1 Density 30 Soil formation 1 Density index 31

Man-made soils 1 Water content 32 Contaminated and polluted soils 2 Atterberg limits 34 Naturally occurring soils 2 Activity index 38 In situ soils - weathered rocks 2 Volumetrie shrinkage 38 In situ soils - peat 3 Shrinkage limit 39 Water-bome soils 3 Soil model 40 Glacial deposits 4 Pore size distribution 42 Wind-blown soils 6 Summary 43

Soil particles 6 Further Reading 44 Nature of particles 6 Worked Examples 45 Clay minerals 7 Exercises 52

Soil structure lO Cohesive soils lO 3 Permeability and seepage 53 Granular soils 14

Objectives 53 Summary 15 Further Reading 15 Penneability 53

Introduction 53 2 Soil description and classification 16 Groundwater 53

Objectives 16 Flow problems 54 Soil description 16 Flow into excavations 54

Soil categories 17 Flow around cofferdams 54

V

vi Contents

Dewatering 55 Summary 86 Flow through earth structures 55 Further Reading 86 Stability problems 'running sand' 55 Case Studies 87 Boiling or heaving in cofferdams 55 Worked Examples 92 Piping 56 Exercises 102 Heaving beneath a clay layer 56 Uplift pressures 56 Soil pores 57 4 Effective stress and pore

Pressure and head 57 pressure 105

Darcy's Law 58 Objectives 105 Effect of temperature 58 Total stress 105 Empirical correlations for k 59 Total and effective stress above the Layered soils 60 water table 105 Laboratory test - constant head Pore pressure below the water table 106

permeameter 61 Total stress below the water table 107 Laboratory test - falling head Effective stress 107

permeameter 62 Effective stress below the water table 108 Laboratory test - hydraulic cell - Stress history 109

vertical permeability 63 Normally consolidated clay 109 Laboratory test - hydraulic cell - Overconsolidated clay 110

horizontal permeability 64 Overconsolidation ratio 111 Laboratory test - triaxial cell - vertical Overconsolidation ratio in the ground 111

permeability 65 Desiccated ernst 111 Borehole tests - open borehole 65 Present state of stress in the ground 113 Borehole tests - packer tests 67 Mohr's circle of stress 113 Borehole tests - piezometers 68 In situ horizontal and vertical stresses 114 Pumping tests 69 Undrained and drained conditions 116 Groundwater lowering and pore Loading 117

pressure control methods 70 Unloading 118 Seepage 72 Pore pressure parameters - theory 118

Seepage theory 72 Pore pressure parameters A and B 120 Flow nets 74 Capillary rise above the water table 121 Flow net construction 75 Capillary break layers 122 Seepage quantities 77 Effective stresses above the Seepage quantities - plan ftow net 78 water table 123 Total head, elevation head and pressure Suctions 124

head 79 Climatic and desiccation effects in the Pore pressure and uplift pressure 80 aeration zone 125 Failure by uplift (buoyancy) 80 Expansive soils 127 Seepage force 80 Frost action in soils 128 Quick condition, boiling and internal Depth of frost penetration 128

erosion 81 Frost susceptibility 130 Critical hydraulic gradient 81 Frost heave test 131 Failure by internal erosion 81 Ground freezing technique 131 Failure by heave - sheet piling 82 Summary 131 Failure by piping 82 Further Reading 132 Seepage through earth dams 83 Case Studies 133 Seepage through flood banks, levees 83 Worked Examples 136 Soil filters 84 Exercises 140

Contents VII

5 Contad pressure and stress Effeet of ageing 167 distribution 142 Compression of residual soils 168

Objectives 142 Isotropie eompression 169

Contact pressure 142 Anisotropie eompression 169

Contaet pressure - uniform loading 142 Consolidation 170

Contaet pressure - point loading 143 Introduetion 170

Stress distribution 144 Terzaghi theory of one-dimensional

Stresses beneath point load and eonsolidation 170

line load 144 Solution of the eonsolidation

Assumptions 144 equation 171

Foundation depth 146 Isochrones 172

Stresses beneath uniformly loaded Average degree of eonsolidation 173

areas 146 Oedometer test 175

Bulbs of pressure 147 Rowe eonsolidation eell 176

Stresses beneath a flexible rectangle Consolidation properties 177

and eirele 147 Coefficient of eonsolidation, c - root V

Principle of superposition 147 time method 178

Stresses beneath flexible area of any Coeffieient of eonsolidation, cv - log

shape 147 timemethod 180

Stresses beneath a flexible rectangle - In situ ev values 181

finite soil thiekness 147 Two- and three-dimensional

Stresses beneath a flexible eirele - eonsolidation 181

finite soil thiekness 151 Correction for eonstruetion period 182

Stresses beneath a rigid rectangle 151 Undereonsolidation - recent sediments

Embankment loading 151 and baekfills 183

Summary 154 Preeompression by sureharging 185

Further Reading 154 Vaeuum preeompression 187

Worked Examples 155 Radial eonsolidation for vertieal

Exercises 159 drains 188 Summary 191

6 Compressibility and Further Reading 191

consolidation 161 Case Studies 192 Worked Examples 197

Objectives 161 Exercises 205 Introduction 161 Compressibility 161

Void ratio/effeetive stress plot 161 7 Shear strength 208 Reloading eurves 163 Objectives 208 Preconsolidation pressure u ', Introduetion 208

eonsolidation yield stress dy' 163 Stresses and strains in soils 209 Overeonsolidation ratio (OCR) 163 Representation of stresses 209 Effect of sampling disturbance 163 Pole 209 Determination of u ' 164 Prineipal stresses 210 In situ eurve for noirnally eonsolidated Axial symmetry 210

clay 165 Plane strain 211 In situ eurve for overeonsolidated clay 165 K

0 eondition 211

Effect of load inerements 166 Normal and shear strains 211 Effect of duration of load 166 Mohr eircle of strain 212 Effect on pore size distribution 166 Volumetrie strains 212

viii Contents

Shear strength 214 Tue elastic wall 249 Effect of strain 214 Real soils 249 Idealised stress-strain relationships 215 Residual strength 249 Idealised yield and plasticity 216 Summary 250 Yield curves 216 Further Reading 251 Failure criterion 217 Case Study 251 Failure of soil in the ground 217 Worked Examples 253 Stress paths 219 Exercises 257 Effects of drainage 220 Test procedures 223 8 Shallow foundations -

Shear strength of sand 224 stability 260 Stress-strain behaviour 224 Objectives 260 Shear box test 225 Shallow foundations 260 Effect of packing and particle nature 226 Definition 260 Constant volume condition 228 Geotechnical risk 260 Effect of density 228 Spread foundations 261 Effect of confining stress and crushing 229 Types of foundation 261

Shear strength of clay 229 Strip foundations 261 Effect of sampling 229 Pad foundations 262 Undrained strength, cu 230 Raft foundations 263 Strength index tests 230 Depth of foundations 263 Strength tests 231 Adequate bearing stratum 264 Unconfined compression test 231 Seasonal moisture variations 264 Field vane test 231 Effects of tree roots 265 Triaxial test 233 Frost action 266 Triaxial unconsolidated undrained River erosion 267

(UU) test 234 Water table 268 Multi-stage (UU) test 236 Super-elevation 268 Effect of clay content and mineralogy 236 Design requirements 268 Partially saturated clays 236 Traditional approach compared with Fissured clays 237 the Eurocodes 269 Variation with depth 238 Design approach 270 Frictional characteristics 239 Bearing resistance 270 Test procedures 239 MO<les of failure 270 Triaxial consülidated undrained Bearing capacity and bearing

(CU) test 240 resistance 270 Triaxial consolidated drained Shape factors 273

(CD) test 242 Depth factors 273 Critical state theory 243 Base inclination factors 274

Parameters 243 Bearing resistance - overturning 274 State boundary surface 243 Eccentric loading 274 Isotropie normal consolidation Inclined loading 275

line (ICL) 244 Summary 275 K0 normal consolidation line (K0 CL) 245 Different soil strength cases 275 Critical state line (CSL) 245 Effect of water table 276 Roscoe surface 246 Net ultimate bearing capacity 276 Tension cut-off 247 Effect of compressibility of soil 277 Hvorslev surface 248 Sliding 277

Contents IX

Bearing capacity - two layers Permissible settlements 317 a) two clay layers 278 Definitions of ground and foundati_on b) sand overlying clay 279 movement 317

Allowable bearing pressure of sand 280 Criteria for movements 318 Introduction 280 Routine settlement limits 318 Settlement limit 280 Summary 320 Allowable bearing pressure 280 Further Reading 320 Corrected SPT N values 281 Case Studies 321

Summary 283 · Worked Examples 326 Further Reading 283 Exercises 333 Case Studies 284 Worked Examples 287 10 Pile foundations 335 Exercises 292 Objectives 335

Pile foundations 335 9 Shallow foundations - Introduction 335

settlements 295 Types of pile 336 Objectives 295 Loading conditions 336 Introduction 295 Uncertainty of design calculation

Are settlement calculations required? 296 methods 337 Elastic methods - assumptions 296 Limit states 337 Elastic displacement method 296 Pile load tests 337

Immediate settlement 298 Compressive resistance from static Introduction 298 load tests 338 General method 298 Compressive resistance from ground Principle of superposition 299 test results 339 Principle of layering 299 Compressive resistance from other Rigidity correction 299 methods 340 Depth correction 302 Design values of compressive Effect of local yielding 303 resistance 341 Estimation of undrained modulus E

0 303 Weight of pile 341

Consolidation settlement 304 Bored piles in clay 341 General 304 Geometrie data 341 Compression index Ce method 305 End-bearing resistance qb 341 Oedometer modulus Eoed or m, method 306 Adhesion c. 342

Total settlement 307 Driven piles in clay 343 Skempton-Bjerrum method 307 End-bearing resistance qb 343 Elastic drained method 308 Adhesion c. - installation effects, Estimation of drained modulus E 310 soft clay 343 Proportion of immediate to total Adhesion c • - installation effects,

settlement 310 stiff clay 344 Secondary compression 311 Adhesion c - values • 344

Introduction 311 Effective stress approach for adhesion 345 General method 312 Driven piles in sand 346 Estimation of Ca or E values a 312 Effects of installation 346

Sands 312 End bearing resistance qb 346 Methods of estimating settlements 313 Critical depth 346 Schmertmann's method 314 Skin frictionf. 348 Burland and Burbridge's method 315 Bored piles in sand 350

X Contents

Mobilisation of base and shaft loads 350 Ultimate limit states 389 Downdrag (negative skin friction) 350 Loss of overall stability 389

Causes of downdrag 350 Overturning or rotational failure 390 Determination of downdrag 351 Bearing pressure under the toe 390

Pile groups 352 Bearing capacity 391 Stiffness of pile cap and structure 352 Sliding 391 Pile spacing 352 Failure of structural elements 391 Stressed zone 352 Design of embedded walls 391 Load variation 353 Cantilever embedded walls - general 392 Efficiency 353 Cantilever embedded walls - design 392 Ultimate capacity 354 Single anchor or propped embedded Settlement ratio 355 walls - general 393 Settlement of pile groups 357 Single anchor or propped embedded

Summary 358 walls - design 393 Further Reading 358 Design methods 394 Case Studies 358 Gross pressure method 394 Worked Examples 361 Net available passive resistance Exercises 367 method 394

Factor on strength method 395 11 Lateral earth pressures and BS 8002 method 395

retaining structures 369 Anchorages for embedded walls 395

Objectives 369 Strutted excavations and cofferdams 396

Lateral earth pressures 369 Introduction 396

Introduction 369 Strut loads 397

Effect of horizontal movement 370 Base stability of excavations 398

Effect of wall flexibility and propping 373 Reinforced soil 398

Effect of wall friction 373 Reinforced soil walls - construction 399

Coulomb theory - active force 374 Effects of reinforcement 399

Coulomb theory - passive force 375 Reinforced soil walls - design 400

Limitations of the Coulomb theory 376 Extemal stability 400

Earth pressure coefficients 376 Intemal stability - general 402

Effect of cohesion intercept c' 378 Tensile rupture 402

Minimum equivalent fluid pressure 379 Pull-out resistance or adherence 403

Effect of water table - gravity walls 380 Intemal stability - tie-back wedge

Effect of water table - embedded walls 380 method 403

Undrained conditions 381 Coherent gravity method 405

Earth pressures - undrained condition 382 Summary 406

Tension cracks 382 Further Reading 407

Uniform surcharge 383 Case Studies 407

Line loads and point loads 384 Worked Examples 411

Earth pressures due to compaction 384 Exercises 424

Retaining structures 385 Introduction 385 12 Slope stability 426

Basement walls 387 Objectives 426 Bridge abutments 387 Natural and artificial slopes 426 Gabions and cribwork 388 lntroduction 426

Design of gravity walls 389 Types of mass movement 426 Serviceability limit states 389 Natural slopes 426

Contents xi

Artificial slopes or earthworks 429 Construction expedients -Short-term and long-term conditions 430 improve the earth structure 471

Methods of analysis 433 Construction expedients -Plane translational slide - general 433 improve the underlying ground 473 Plane translational slide - limit state Earthmoving and fill materials 474

approach 434 Earthmoving plant 474 Plane translational slide - Purpose and types of materials 475

special cases 435 Suitability of fill material 477 Stability of vertical cuts 437 Suitability of granular soils 478 Circular arc analysis - general 438 Suitability of cohesive soils 478 Circular arc analysis - undrained Moisture condition test 479

condition or <P. = 0° analysis 438 Efficiency of earthmoving 481 Undrained analysis - the overdesign Material problems 482

factor 439 Softening 482 Tension crack 439 Bulking 483 Undrained analysis stability charts - Soil compaction 485

Taylor's method 439 Factors affecting compaction 485 Effective stress analysis 441 Field compaction 486 Effective stress analysis - method of Compaction plant 487

slices 441 Specification of compaction 488 Bishop simplified method 443 Control of compaction in the field 490 Bishop method - overdesign factor on · Laboratory compaction 492

strength 443 Light compaction test 492 Bishop method - overdesign factor on Heavy compaction 493

moments 444 Vibrating hammer 494 Favourable and unfavourable actions 445 Air voids lines 494 Pore pressure ratio r. 445 Correction for oversize particles 494 Stability coefficients - r. method 446 Califomia Bearing Ratio ( CBR) test 495 Stability charts - ultimate limit state 446 Addition of lime 496 Submerged slopes 449 Lime stabilisation 496 Rapid drawdown 449 Cement stabilisation 497 Non-circular slip surfaces 450 Summary 497 Wedge method - single plane 451 Further Reading 498 Wedge method - multi-plane 451 Case Study 499 Factors affecting stability and slope Worked Examples 501

design 452 Exercises 507 Summary 453 Further Reading 454 14 Site investigation 509 Case Studies 454 Objectives 509 Worked Examples 458 Site investigation 510 Exercises 468 lntroduction 510

Relationship with geotechnical design 510 13 Earthworks and soil Stages of investigation 511

compaction 471 Desk study 511 Objectives 471 Site inspection 512 Earth structures 471 Preliminary studies report 512

Introduction 471 Ground investigation 512 Typical examples 471 Extent of the ground investigation 512

xii Contents

Depth of exploration Choice of method of investigation Methods of ground investigation Undisturbed sampling- sampling

quality Types of samples Methods of in situ testing Groundwater observations Investigation of contaminated

land

513 Ground investigation report 531 513 Presentation of geotechnical information 531 514 Factual report 531

Geotechnical evaluation 532 518 Summary 536 518 Further Reading 536 522 527 Glossary 537

References 539 530 Index 551