Code of Practice 1989 – Lateral support in surface excavations

CONTENTS

Page

CHAPTER 1: INTRODUCTION 1

1.1 SCOPE 1

1.2 BASIC CONSIDERATIONS 2

1.3 PRELIMINARY ACTION 3

1.3.1 Appointment of Engineer 3

1.3.2 Legal requirements and precautions 3

1.3.3 Notifications 4

1.3.4 Insurances 4

1.3.5 Contractual aspects 5

1.4 PLANNING, DESIGN AND EXECUTION 5

1.5 MEASUREMENT 5

CHAPTER 2: SITE INVESTIGATION 7

2.1 INTRODUCTION 7

2.2 OBJECTIVES AND PLANNING 7

2.3 DESK STUDY AND PRELIMINARY FIELDWORK 8

2.4 DETAILED FIELD AND LABORATORY INVESTIGATION 9

2.4.1 Extent and intensity 9

2.4.2 Methods of investigation 11

2.4 3 Description of strata 11

2.4.4 Sampling 11

2.4.5 Laboratory testing 12

2.4.6 Field testing 12

2.4.7 Investigation during construction 12

2.4.8 Adjacent activities 12

2.4.9 Monitoring 13

2.5 EXISTING STRUCTURES AND SERVICES 13

2.6 REPORTS 13

2.6.1 Site investigation report 13

2.6.2 Completion report 14

CHAPTER 3: SELECTION OF A SYSTEM OF LATERAL SUPPORT 15

3.1 INTRODUCTION 15

3.2 SITE CONDITIONS 15

3.3 PROPOSED DEVELOPMENT 16

3.4 ADJACENT PROPERTY 17

3.5 CONSTRUCTION SYSTEMS 17

3.5.1 Construction within an open unsupported excavation 17

3.5.2 Temporary support strutted against a central dumpling 18

3.5.3 Temporary support by fully braced

trench 19

3.5.4 Fully braced temporary support 19

3.5.5 Permanent wall construction prior to excavation and braced from a central permanent construction 20

3.5.6 Ground anchors 20

3.5.7 Soil nailing 21

3.5.8 Rock bolts 22

3.5.9 Cantilever walls 23

3.5.10 Caissons 24

3.5.11 Concurrent upward and downward construction 25

3.5.12 Unconventional methods of support 26

3.6 DEWATERING AND LEACHING OF SOLIDS 28 CHAPTER 4: EARTH PRESSURES 31

4.1 INTRODUCTION 31

4.2 DESIGN PARAMETERS 31

4.2.1 Soil strength parameters 31

4.2.2 Wall friction and wall adhesion 31

4.2.3 Confidence limits 34

4.3 DEFINITION OF SYMBOLS 34

4.4 EFFECT OF MOVEMENT OF SUPPORT SYSTEM ON EARTH PRESSURES 35

4.4.1 General 35

4.4.2 Movements required to develop active

and passive earth pressures 36

4.5 AT-REST EARTH PRESSURES 38

4.6 ACTIVE AND PASSIVE PRESSURES 39

4.6.1 Active pressures – general guidance 42

4.6.2 Active pressures – minimum design pressures 42

4.6.3 Passive pressures – general guidance. 44

4.7 RESTRICTED MOVEMENTS 45

4.8 EMPIRICAL METHODS 45

4.9 SURCHARGE LOADING 47

4.10 EFFECTS OF WATER PRESSURE 47

4.11 PRESSURE INDUCED BY BACKFILLING 47

4.12 RESTRAINT OF EXCAVATIONS IN ROCK 48

4.12.1 General 48

4.12.2 Failure mechanisms and methods of analysis 49

CHAPTER 5: DESIGN OF LATERAL SUPPORT SYSTEMS 53

5.1 INTRODUCTION 53

5.1.1 General 53

5.1.2 Design objectives 53

5.1.3 Deformations 54

5.1.4 Factors of safety 54

5.2 DESIGN CONSIDERATIONS 56

5.2.1 General 56

5.2.2 Cantilever wall systems 58

5.2.3 Anchored wall systems 58

5.2.4 Soil nailing 60

5.2.5 Strutted wall systems 64

5.2.6 Underpinning 65

5.3 OVERALL STABILITY 67

5.3.1 Methods of analysis 67

5.3.2 Factor of safety 72

5.4 STABILITY OF EXCAVATED FLOOR 72

5.5 DESIGN OF STRUCTURAL ELEMENTS 73

5.5.1 General 73

5.5.2 Anchors 74

5.5.3 Struts and rakers 75

5.5.4 Soldiers 76

5.5.5 Waters 77

5.5.6 Lagging 77

5.5.7 Factors of safety 79

5.6 MOVEMENTS 81

5.6.1 General 81

5.6.2 Effect of movement on adjacent structures 81

5.6.3 Effect of movement on buried services 83

5.6.4 Limiting deformations 84

CHAPTER 6: DESIGN AND CONSTRUCTION OF GROUND

ANCHORS 85

6.1 INTRODUCTION 85

6.2 DUTIES OF THE VARIOUS PARTIES 86

6.3 ANCHOR TYPES 86

6.3.1 General 86

6.3.2 Untensioned anchors 86

6.3.3 Tensioned anchors 87

6.4 FIXED ANCHOR DESIGN 89

6.4.1 Fixed anchor design in rock 89

6.4.2 Fixed anchor design in cohesionless

soil 89

6.4.3 Fixed anchor design in cohesive soil 90

6.4.4 Internal design 90

6 5 ANCHOR ARRANGEMENT 91

6.5.1 Anchor layout 91

6.5.2 Transfer of anchor forces against rock faces 92

6.5.3 Transfer of anchor forces against soft ground faces 92

6.6 CONSTRUCTION 92

6.6.1 Materials 94

6.6.2 Drilling 96

6.6.3 Manufacture 98

6.6.4 Homing 98

6.6.5 Grouting 98

6.7 ANCHOR STRESSING AND TESTING 99 6.7.1 Stressing and testing 99

6.7.2 Proving test anchors 99

6.7.3 Site suitability test anchors 102

6.7.4 Working anchors 103

CHAPTER 7: GROUND WATER CONTROL 107

7.1 INTRODUCTION 107

7.2 GROUND WATER LOWERING 108

7.2.1 Pumping from open sumps 108

7.2.2 Pumping from well-points and deep

wells 108

7.2.3 Electro-osmosis 109

7.3 AQUIFERS AND SETTLEMENT 109

7.4 SPECIAL CONSIDERATIONS 112

CHAPTER 8: CONTROL OF THE WORKS, MONITORING AND

RECORDS 113

8.1 INTRODUCTION 113

8.2 CONTROL OF THE WORKS 113

8.2.1 Extent of control measures 113

8.2.2 General precautions 113

8.3 MONITORING AND RECORDS 114

8.3.1 Site conditions and stratigraphy 115

8.3.2 Water regimes 115

8.3.3 Movements and control measurements 116

8.3.4 Forces in struts and props 118

8.3.5 Forces in anchors 118

8.3.6 Forces in soil nails and bolts 119

8.4 SURCHARGE LOADING 119

APPENDICES

APPENDIX A. STEP-BY-STEP GUIDELINES FOR

INVESTIGATION AND ACTION FOR LATERAL SUPPORT OF A BUILDING BASEMENT 123

B. LABORATORY AND FIELD TESTS 127

C. FIELD DESCRIPTION OF SOIL AND ROCK 135

D. LEGAL ASPECTS OF LATERAL SUPPORT

FOR SURFACE EXCAVATIONS 143

E. REMOVAL OF LATERAL SUPPORT RISKS-CONTRACTUAL ASPECTS AND INSURANCE 153

F. CASE HISTORIES OF WALL MOVEMENTS 165

G. EXTRACTS FROM CERTAIN RELEVANT REGULATIONS AND ACTS 173

H DRAFT SPECIFICATION FOR BLASTING IN BUILT-UP AREAS 181

I. SUGGESTED METHODS OF MEASUREMENT FOR LATERAL SUPPORT, EXCAVATION AND ASSOCIATED WORKS 187

J. DEFINITIONS OF SOME ANCHOR TERMS 199

K. REFERENCES 205

TABLES

TABLE 1.1 Engineering an excavation 6

2.1 Investigation planning 8

2 2 Factors to be investigated during desk study and preliminary field work 9

4.1 Appropriate shear strength parameters for various soil types 32

4.2 Wall movement (rotation about base of wall) required to develop active and passive earth pressures 38

4.3 Coefficient of earth pressure at-rest (K) 39

4.4 Equivalent fluid “densities” to be used in the computation of minimum active thrust 43

4.5 Analysis methods for various failure modes 51

5.1 Thickness of lagging 79

5.2 Minimum safety factors recommended for design of individual anchors 80

5.3 Permitted relative vertical movement 82

6.1 Corrosion protection guide 93

6.2 Typical relaxation of stress in tendons 94

B.1 Laboratory tests on soils 128

B.2 Field tests 132

B.3 Laboratory tests on rocks 134

C.1 Consistency of granular soils 136

C.2 Consistency of cohesive soils 137

C.3 Weathering 138

C.4 Discontinuity surface and micro structure spacing. 139

C.5 Grain size classification 140

C.6 Hardness 141

F.1 Movements of anchored walls (Peck) 167

F.2 Movements of anchored walls – South African case histories 170

H.1 Maximum particle velocities (vibration) 184

H.2 Maximum charge levels 185

FIGURES

FIGURE 3.1 Construction within an open unsupported excavation 18

3.2 Temporary support strutted against central dumpling 18

3.3 Temporary support by fully braced trench 19

3.4 Fully braced temporary support 19

3.5 Permanent wall constructed prior to excavation (Diaphragm or contiguous augered pile) and braced for central permanent construction 20

3.6 Ground anchors 21

3.7 Soil nailing 22

3.8 Rock bolts 22

3.9 Cantilever walls 24

3.10 Caissons 25

3.11 Concurrent upward and downward construction 26

3.12 Unconventional methods of support 29

a) Unbraced peripheral walls constructed from the top downwards (and stabilised by grout jacking)

b) Chemical and cement grouts used to provide lateral support

c) Examples of ground freezing

d) Examples of mix-in-place grouting

4.1 Schematic relationship between wall movement and coefficient of earth pressure 36

4.2 Lateral pressure distributions associated with different wall movements 37

4.3 Active and passive earth pressures for vertical retaining walls with horizontal backfill assuming no wall friction 40

4.4 Active and passive thrusts for granular backfill, taking account of wall friction, wall inclination and sloping backfill 41

4.5 Terzaghi and Peck pressure diagrams 46

4.6 Typical rock cut failure mechanism related to stereo plots of discontinuities 50

5.1 Simplified earth pressure distributions 60

5.2 Typical soil nails 61

a) Temporary application

b) Permanent application

5.3 Stability analysis of nailed soil structure 63

a) Single wedge analysis

b) Multiple wedge analysis

5.4 Examples of overall stability considerations 68

a) Anchored support system

b) Strutted support system

5.5 Forces acting in wedge mechanism of failure 69

5.6 Recommended location of anchorage in soft ground 70

5.7 Sliding block method of analysis 71

5.8 Anchors at re-entrant corners 75

a) Corner using cross-over anchors

b) Corner without cross-over anchors

5.9 Recommended location of lagging boards for king-post/soldier pile walls 78

a) For king-post installed against excavated face

b) For driven soldiers or soldiers cast into auger holes

6.1 Ground anchor nomenclature 85

6.2 Four main types of tensioned anchors 88

6.3 Fixed anchor beyond critical failure plane 91

6.4 Typical detail for permanent bar anchor 97

6.5 Typical detail for restressable permanent strand anchors 97

6.6 Typical detail for non-restressable permanent strand anchors 97

6.7 Schematic stressing procedure for permanent and temporary anchors 100

6.8 Typical methods of measuring tendon displacement from a fixed datum 101

6.9 Test period versus loss of lock-off load 103

7.1 Aquifer beneath excavation level 110

7.2 Use of recharging to maintain the surface of groundwater close to the original level 111

F. 1 Typical wall movements outside an excavation 166