Structural Foundation Designers Manual

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Structural Foundation Designers Manual

Contents

1.11.3 Example 8: Reliability of the soils investigation 13
1.11.4 Example 9: Deterioration of ground exposed by excavation 13
1.11.5 Example 10: Effect of new foundation on existing structure 14
1.12 Design procedures 14
1.13 References 14

2 Soil Mechanics, Lab Testing and Geology

A: Soil mechanics 15
2.1 Introduction to soil mechanics 15
2.2 Pressure distribution through ground 15
2.3 Bearing capacity 17
2.3.1 Introduction to bearing capacity 17
2.3.2 Main variables affecting bearing capacity 19
2.3.3 Bearing capacity and bearing pressure 19
2.3.4 Determination of ultimate bearing capacity 20
2.3.5 Safe bearing capacity –cohesionless soils 21
2.3.6 Safe bearing capacity – cohesive soils 22
2.3.7 Safe bearing capacity – combined soils 22
2.4 Settlement 22
2.4.1 Introduction to settlement 22
2.4.2 Void ratio 23
2.4.3 Consolidation test 23
2.4.4 Coefficient of volume compressibility 24
2.4.5 Magnitude and rate of settlement 25
2.4.6 Settlement calculations 25
2.5 Allowable bearing pressure 26
2.6 Conclusions 26
B: Laboratory testing 26
2.7 Introduction to laboratory testing 26
2.8 Classification (disturbed sample tests) 26
2.8.1 Particle size and distribution 26
2.8.2 Density 27
2.8.3 Liquidity and plasticity 29
2.8.4 General 29
2.9 Undisturbed sample testing 29
2.9.1 Moisture content 29
2.9.2 Shear strength 29
2.9.3 Consolidation tests
(oedometer apparatus) 29

PART 1: APPROACH AND FIRST CONSIDERATIONS 1

1 Principles of Foundation Design 

1.1 Introduction 3
1.2 Foundation safety criteria 3
1.3 Bearing capacity 4
1.3.1 Introduction 4
1.3.2 Bearing capacity 4
1.3.3 Presumed bearing value 4
1.3.4 Allowable bearing pressure 5
1.3.5 Non-vertical loading 5
1.4 Settlement 6
1.5 Limit state philosophy 7
1.5.1 Working stress design 7
1.5.2 Limit state design 7
1.6 Interaction of superstructure and soil 8
1.6.1 Example 1: Three pinned arch 8
1.6.2 Example 2: Vierendeel superstructure 8
1.6.3 Example 3: Prestressed brick diaphragm wall 8
1.6.4 Example 4: Composite deep beams 9
1.6.5 Example 5: Buoyancy raft 9
1.7 Foundation types 9
1.7.1 Pad foundations 10
1.7.2 Strip footings 10
1.7.3 Raft foundations 10
1.7.4 Piled foundations 11
1.8 Ground treatment (geotechnical 
processes) 11
1.9 Changes of soil properties during excavation 12
1.10 Post-construction foundation failure 12
1.11 Practical considerations 13
1.11.1 Example 6: Excavation in waterlogged ground 13
1.11.2 Example 7: Variability of ground conditions 
2.9.4 Permeability tests 32
2.9.5 Chemical tests 32
2.10 Summary of tests 32
2.11 Analysis of results 37
2.12 Final observations on testing 37
C: Geology 37
2.13 Introduction to geology 37
2.14 Formation of rock types 38
2.15 Weathering of rocks 38
2.16 Agents of weathering 38
2.16.1 Temperature 38
2.16.2 Water 38
2.16.3 Wind 38
2.16.4 Glaciation 38
2.17 Earth movement 38
2.17.1 Folds, fractures and faults 38
2.17.2 Dip and strike 39
2.17.3 Jointing 39
2.17.4 Drift 39
2.18 Errors in borehole interpretation 40
2.19 Geophysical investigation 42
2.20 Expert knowledge and advice 42
2.21 References 42

3 Ground Investigation 

3.1 Introduction 43
3.2 The need for investigation 44
3.2.1 The designer’s need 44
3.2.2 The contractor’s need 45
3.2.3 The client’s need 45
3.2.4 Site investigation for failed,or failing, existing foundations 45
3.3 Procedure 45
3.3.1 Site survey plan 47
3.3.2 Study of existing information 47
3.3.3 Preliminary site reconnaissance and site walkabout 47
3.4 Soil investigation 48
3.4.1 Borehole layout 48
3.4.2 Trial pits layout 49
3.4.3 Hand augers 50
3.4.4 Boring 50
3.4.5 Backfilling of trial pits and boreholes 50
3.4.6 Soil sampling 50
3.4.7 Storage of samples 50
3.4.8 Frequency of sampling 50
3.4.9 Appointment of specialist soil investigator 51
3.5 Site examination of soils 52
3.6 Field (site) testing of soils 52
3.6.1 Standard Penetration Test (SPT) 52
3.6.2 Vane test 52
3.6.3 Plate bearing test 53
3.6.4 Pressuremeters 53
3.6.5 Groundwater (piezometers and standpipes) 53
3.6.6 Other field tests 55
3.7 Recording information – trial pit and borehole logs and soil profiles 55
3.8 Soil samples and soil profiles 56
3.9 Preliminary analysis of results 56
3.10 Site investigation report 61
3.10.1 Factors affecting quality of report 61
3.10.2 Sequence of report 62
3.10.3 Site description 62
3.10.4 The ground investigation 62
3.10.5 Results 62
3.10.6 Recommendations 62
3.11 Fills (made ground) 63
3.12 Legal issues 63
3.13 Time 64
3.14 Conclusions 64
3.15 Further information 65
3.16 References 65
PART 2: SPECIAL AND FURTHER CONSIDERATIONS 67

4 Topography and its Influence on Site Development 

4.1 Introduction 69
4.2 Implications from surface observations 69
4.2.1 Changes in level, ground slopes and movements 69
4.2.2 Mounds, depressions and disturbed ground 70
4.2.3 Past or current activities 71
4.2.4 Vegetation 72
4.2.5 Surface ponding or watercourses 72
4.3 Effects on development arising from topographical features 73
4.3.1 Sloping sites 73
4.3.2 Slope stability 75
4.3.3 Groundwater 77
4.3.4 Settlement 78
4.4 Summary 79
4.5 References 79

5 Contaminated and Derelict Sites 

5.1 Introduction 80
5.1.1 State of the art 80
5.1.2 Contamination implications 81
5.2 Redundant foundations and services 82
5.2.1 Identification 83
5.2.2 Sampling and testing 83
5.2.3 Site treatment 83
5.3 Chemical and toxic contamination 83
5.3.1 Part IIA risk-based approach 83
5.3.2 Soil Guideline Values 84
5.3.3 CLEA Model 84
5.3.4 Risk to humans and animals 85
5.3.5 Risks to plants and the wider ecosystem 89
5.3.6 Risk to the water environment 89
5.3.7 Risk to buildings and construction materials 89Contents vii
5.3.8 Toxic contamination – site identification 91
5.3.9 Contaminant investigation 91
5.3.10 Sampling and testing 92
5.3.11 Site treatment 92
5.4 Foundation protection 93
5.5 Examples of site investigations on potentially contaminated sites 94
5.6 References 94

6 Mining and Other Subsidence 

6.1 Introduction 95
6.2 Mechanics of mining subsidence 95
6.3 Methods of mining 97
6.3.1 Longwall workings 97
6.3.2 Pillar and stall workings (partial extraction methods) 97
6.3.3 ‘Bell-pits’ 99
6.4 Associated and other workings 100
6.4.1 Abandoned mine shafts and adits 100
6.4.2 Fireclay and other clays 100
6.4.3 Iron ores 100
6.4.4 Other metals 100
6.4.5 Limestone 100
6.4.6 Salt 100
6.4.7 Chalk 100
6.5 Faulting 100
6.6 Natural and other cavities 100
6.6.1 Dissolving rock 100
6.6.2 Dissolving soils 100
6.7 Treatment of abandoned shallow workings 100
6.7.1 Introduction 100
6.7.2 Excavate and backfill 101
6.7.3 Partial and full grouting 101
6.8 Treatment of abandoned shafts 101
6.8.1 Capping 101
6.9 Effect of mining method and method of treatment 101
6.9.1 Introduction 101
6.9.2 Bell workings 101
6.9.3 Pillar and stall 102
6.9.4 Longwall workings 103
6.9.5 Rafts founded over longwall workings 103
6.10 Design principles and precautions in longwall mining subsidence areas 103
6.10.1 Introduction 103
6.10.2 Rafts and strips for low-rise, lightly loading buildings 104
6.10.3 Rafts for multi-storey structures or heavy industrial buildings 105
6.10.4 Jacking points 105
6.10.5 Service ducts 105
6.10.6 Piling 105
6.10.7 Articulated foundation 105
6.11 Superstructures 106
6.11.1 Introduction 106
6.11.2 Rigid superstructures 106
6.11.3 Flexible superstructures 106
6.12 Monitoring 107
6.13 References 107

7 Fill 

7.1 Filled sites 108
7.1.1 Introduction 108
7.1.2 Movement and settlement 108
7.2 The container 108
7.2.1 The container surface 108
7.2.2 The container edges 108
7.2.3 The container base 110
7.2.4 The container sub-strata 110
7.3 Water 111
7.3.1 Effect of water on combustion 111
7.3.2 Effect of water on chemical solutions 111
7.3.3 Water lubrication 111
7.3.4 Water inundation 111
7.3.5 Organic decay 111
7.3.6 Information from water 111
7.4 The fill material 111
7.4.1 Introduction 111
7.5 Fill investigations 112
7.5.1 Special requirements 112
7.5.2 Suggested procedures 113
7.6 Settlement predictions 113
7.6.1 Settlement: fill only 113
7.6.2 Settlement: combined effects 115
7.7 The development and its services 116
7.7.1 Sensitivity 116
7.7.2 Treatment and solutions 117
7.7.3 New filling for development 118
7.8 Case examples 118
7.8.1 Introduction 118
7.8.2 Example 1: Movement of existing building on fill 118
7.8.3 Example 2: New development on existing colliery fill 119
7.8.4 Example 3: New development on new filling 120
7.8.5 Example 4: New developments on existing preloaded fill 120
7.8.6 Example 5: New development on existing backfilled quarry (purchase of coal rights) 121
7.8.7 Example 6: Development on new fill (prevention of flooding) 122
7.9 References 123
7.10 Further reading 123

8 Ground Improvement Methods 

8.1 Introduction 124
8.2 Surface rolling 124
8.2.1 Introduction 124
8.2.2 Method 124
8.2.3 Soil suitability and variation 125
8.2.4 Site monitoring 125
8.3 Vibro-stabilization 126
8.3.1 Introduction 126
8.3.2 Working surfaces 127viii Contents
8.3.3 Method 127
8.3.4 Vibro-compaction 128
8.3.5 Vibro-displacement 129
8.3.6 Vibro-replacement 129
8.3.7 Summary of vibro-stabilization 130
8.3.8 Design considerations – granular soils 130
8.3.9 Design considerations – cohesive soils 130
8.3.10 Testing 131
8.3.11 Vibro-concrete 131
8.4 Dynamic consolidation 133
8.4.1 Introduction 133
8.4.2 Method 133
8.4.3 Usage 133
8.4.4 Site checks 133
8.5 Preloading 133
8.5.1 Introduction 133
8.5.2 Method 134
8.5.3 Design of surcharge 134
8.5.4 Installation of drainage systems 134
8.6 Grout injections 135
8.6.1 Introduction 135
8.6.2 Loose soils 135
8.6.3 Swallow-holes 136
8.6.4 Shallow mining 136
8.6.5 Mine shafts, wells and bell-pits 136
8.7 Lime/cement stabilization 137
8.8 Reinforced soil 138
8.8.1 Introduction 138
8.8.2 Foundation applications 139
8.8.3 Patents 139
8.8.4 Research and development 139
8.9 Reference 139
PART 3: FOUNDATION TYPES: SELECTION AND DESIGN 141

9 Foundation Types 

9.1 Introduction 143
9.2 Foundation types 143
9.3 Group one – strip and pad foundations 143
9.3.1 Strip footings 143
9.3.2 Masonry strips 143
9.3.3 Concrete strips – plain and reinforced 144
9.3.4 Concrete trench fill 145
9.3.5 Stone trench fill 145
9.3.6 Rectangular beam strips 145
9.3.7 Inverted T beam strips 145
9.3.8 Pad bases 147
9.3.9 Shallow mass concrete pads 147
9.3.10 Shallow reinforced concrete pads 147
9.3.11 Deep reinforced concrete pads 147
9.3.12 Deep mass concrete pads 147
9.3.13 Balanced pad foundations 148
9.3.14 Rectangular balanced pad foundations 148
9.3.15 Trapezoidal balanced pad foundations 148
9.3.16 Holed balanced pad foundations 148
9.3.17 Cantilever balanced pad foundations 149
9.4 Group two – surface spread foundations 149
9.4.1 Nominal crust raft 149
9.4.2 Crust raft 150
9.4.3 Blanket raft 150
9.4.4 Slip-plane raft 151
9.4.5 Cellular raft 151
9.4.6 Lidded cellular raft 151
9.4.7 Beam strip raft 151
9.4.8 Buoyancy (or ‘floating’) raft 151
9.4.9 Jacking raft 152
9.5 Group three – pile foundations 152
9.5.1 Introduction 152
9.5.2 Stone/gravel piles 153
9.5.3 Concrete piles 153
9.5.4 Timber piles 155
9.5.5 Steel piles 156
9.5.6 Anchor piles 156
9.5.7 Anchor blocks 156
9.5.8 Pile caps and ground beams 157
9.6 Group four – miscellaneous elements 
and forms 157
9.6.1 Suspended ground floor slabs 158
9.6.2 Floating ground floor slabs 159
9.6.3 Pier and beam foundations 159
9.6.4 Retaining walls 161
9.6.5 Grillage foundations 162

10 Foundation Selection and Design Procedures 

A: Foundation selection 164
10.1 Introduction 164
10.2 Foundation selection 164
10.3 Information collection/assessment 164
10.4 General approach to choice of foundations 165
10.5 Questioning the information and proposals 169
10.6 Exploitation of foundation stiffness and resulting ground pressure 172
10.7 Conclusions 173
B: Foundation design calculation procedure 173
10.8 Introduction 173
10.9 Definition of bearing pressures 173
10.10 Calculation of applied bearing pressures 174
10.11 Structural design of foundation members 178
10.12 General design method 180
10.13 References 185

11 Design of Pads, Strips and Continuous Foundations

11.1 Unreinforced concrete pads and strips 186
11.1.1 Introduction 186
11.1.2 Trench fill 186
11.1.3 Trench fill design decisions 187
11.1.4 Sizing of the design 189
11.1.5 Design Example 1: Trench fill strip footing 190Contents ix
11.1.6 Design Example 2: Deep mass concrete pad base 192
11.1.7 Unreinforced concrete strips 193
11.2 Reinforced concrete pads and strips 194
11.2.1 Introduction 194
11.2.2 Design decisions 194
11.2.3 Sizing up of the design 194
11.2.4 Design Example 3: Reinforced strip foundation 195
11.2.5 Design Example 4: Reinforced pad base 198
11.3 Pad foundations with axial loads and bending moments 200
11.3.1 Design Example 5: Pad base – axial load plus bending moment (small eccentricity) 201
11.3.2 Design Example 6: Pad base – axial load plus bending moment (large eccentricity) 202
11.3.3 Design Example 7: Pad base –axial load plus bending momentsabout both axes 206
11.3.4 Design Example 8: Pad base – axial and horizontal loads 207
11.3.5 Design Example 9: Shear wall base – vertical loads and horizontal wind loads 209
11.4 Rectangular and Tee-beam continuous strips 212
11.4.1 Introduction 212
11.4.2 Design decisions 212
11.4.3 Sizing of the design 212
11.4.4 Design Example 10: Continuous Tee beam footing with uniform bearing pressure 213
11.4.5 Design Example 11: Continuous rectangular beam footing with trapezoidal bearing pressure 
11.5 Grillage foundations 221
11.5.1 Introduction 221
11.5.2 Design decisions 221
11.5.3 Sizing of the design 221
11.5.4 Design Example 12: Grillage foundation 221
11.6 Floating slabs (ground slabs) 224
11.6.1 Introduction 224
11.6.2 Design decisions 224
11.6.3 Sizing of the slab 225
11.6.4 Design Example 13: Floating slab 225
11.7 References 226

12 Tied and Balanced Foundations 

12.1 General introduction 228
12.2 Tied foundations 228
12.2.1 Introduction 228
12.2.2 Design decisions 228
12.2.3 Sizing the foundations 228
12.2.4 Design Example 1: Tied portal frame base 229
12.3 Balanced foundations (rectangular,cantilever, trapezoidal and holed) 230
12.3.1 Introduction 230
12.3.2 Design decisions 230
12.3.3 Sizing up the design 230
12.3.4 Design Example 2: Rectangular balanced foundation 232
12.3.5 Design Example 3: Cantilever balanced foundation 233
12.3.6 Design Example 4: Trapezoidal balanced foundation 235
12.3.7 Design Example 5: Holed balanced foundation 236

13 Raft Foundations 

13.1 Design procedures for semi-flexible rafts 238
13.1.1 Design principles 238
13.1.2 Design of raft layouts 238
13.1.3 Bearing pressure design 239
13.1.4 Design span for local depressions 240
13.1.5 Slab design 240
13.1.6 Beam design 243
13.2 Nominal crust raft – semi-flexible 245
13.2.1 Design decisions 245
13.2.2 Sizing the design 245
13.2.3 Design Example 1: Nominal crust raft 249
13.3 Crust raft 
13.3.1 Introduction 251
13.3.2 Design decisions 251
13.3.3 Design Example 2: Crust raft 252
13.4 Blanket raft 256
13.4.1 Introduction 256
13.4.2 Design decisions 257
13.4.3 Sizing the design 257
13.4.4 Design Example 3: Blanket raft 257
13.5 Slip sandwich raft 261
13.5.1 Introduction 261
13.5.2 Design decisions 262
13.5.3 Sizing the design 262
13.5.4 Design Example 4: Slip sandwich raft 263
13.6 Cellular raft 265
13.6.1 Introduction 265
13.6.2 Sizing the design 265
13.6.3 Design Example 5: Cellular raft 266
13.7 Lidded cellular raft 270
13.7.1 Introduction 270
13.7.2 Sizing the design 271
13.7.3 Design Example 6: Lidded cellular raft 271
13.8 Beam strip raft 271
13.8.1 Introduction 271
13.8.2 Sizing the design 271
13.8.3 Design Example 7: Beam strip raft 272
13.9 Buoyancy raft 272
13.9.1 Introduction 272
13.9.2 Sizing the design 274
13.9.3 Design Example 8: Buoyancy raft 274
13.10 Jacking raft 276
13.10.1 Introduction 276
13.10.2 Sizing the design 276
13.11 References 276x Contents

14 Piles 

14.1 Introduction 277
14.2 Applications 277
14.3 Types of piles 278
14.3.1 Load-bearing characteristics 278
14.3.2 Materials 278
14.4 Methods of piling 283
14.4.1 Driven piles 283
14.4.2 Driven cast-in-place piles 283
14.4.3 Bored cast-in-place piles 283
14.4.4 Screw piles 284
14.4.5 Jacked piles 284
14.4.6 Continuous flight auger piles 284
14.4.7 Mini or pin piles 284
14.5 Choice of pile 284
14.5.1 Ground conditions and structure 285
14.5.2 Durability 285
14.5.3 Cost 285
14.6 Design of piled foundations 285
14.6.1 Factor of safety 285
14.6.2 Determination of ultimate bearing capacity 286
14.6.3 Pile loading tests 288
14.6.4 Pile groups 288
14.6.5 Spacing of piles within a group 289
14.6.6 Ultimate bearing capacity  of group 289
14.6.7 Negative friction 289
14.7 Pile caps 289
14.7.1 Introduction 289
14.7.2 The need for pile caps – capping beams 290
14.7.3 Size and depth 290
14.8 Design of foundations at pile head 291
14.9 Design examples 293
14.9.1 Design Example 1: Calculation of pile safe working loads 293
14.9.2 Design Example 2: Pile cap design 295
14.9.3 Design Example 3: Piled ground beams with floating slab 296
14.9.4 Design Example 4: Piled ground beams with suspended slab 299
14.9.5 Design Example 5: Piled foundation with suspended flat slab 300
14.10 References 303

15 Retaining Walls, Basement Walls,Slip Circles and Underpinning

15.1 Introduction 304
15.2 Retaining walls and basements 304
15.3 Stability 305
15.4 Flotation 306
15.5 Buoyancy 306
15.6 Pressures 307
15.6.1 Liquid pressure 307
15.6.2 Earth pressure 307
15.6.3 Surcharge 307
15.7 Slip circle example 307
15.8 Continuous underpinning 308
15.9 Discontinuous underpinning 310
15.10 Spread underpinning 311
15.11 References 311
Appendices 313
Introduction to appendices 313
Appendix A: Properties and Presumed 
Bearing Pressures of Some Well Known 
Engineering Soils and Rocks 314
Appendix B: Map Showing Areas of 
Shrinkable Clays In Britain 317
Appendix C: Map Showing Areas of Coal 
and Some Other Mineral Extractions 318
Appendix D: Foundation Selection Tables 319
Appendix E: Guide to Use of Ground 
Improvement 322
Appendix F: Tables Relating to 
Contaminated Sites/Soils 325
Appendix G: Factors of Safety 341
Appendix H: Design Charts for Pad 
and Strip Foundations 343
Appendix J: Table of Ground Beam 
Trial Sizes 348
Appendix K: Design Graphs and Charts for 
Raft Foundations Spanning Local Depressions 349
Appendix L: Table of Material Frictional 
Resistances 357
Appendix M: Cost Indices for 
Foundation Types 358
Appendix N: Allowable Bearing 
Pressure for Foundations on 
Non-Cohesive Soil 359

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