the design and constructions ofr shett-piled cofferdams - special pubblication 95

Special Publication 95 1993

THE DESIGN ANO CONSTRUCTION OF SHEET-PILED COFFERDAMS

8 P Williams D Waite

..

l

CONSTRUCTION INDUSTRY RESEARCH ANO INFORMATION ASSOCIATIO)\j

THOMAS TELFORD PUBLICATIONS Thomas Telford House,

6 Storey's Gale Westminster

London SW1 P 3AU Tel 071-222 8891 Fax 071-222 1708

tbftot ce Centra ecolt Ingegneri

1 Heron Quay London E14 4JD

Tel : 071 -987 6999 Fax: 071-538 4101

POLITECNICO BIBL. FAC. INGEGNERIA

E B 0723

OON34839

2

Summary

This report brings together from many sources information which is likely to be needed for the successful design and construction of a cofferdam up to IO metres deep in steel sheet piling. It points out the need for the inclusion of the requirement for such a cofferdam in the initial project planning so that the sile investigation will give the information necessary for its design. Other sections include generai planning of the cofferdam, earth and water pressure calculations, various methods of analysis for the detailed design of the wall and suppor! system, and the construction, maintenance and removal of the cofferdam. Emphasis is given lo the considerable effect that water pressures have on the loading on the cofferdam wall, and various methods are given to establish these pressures, including the use of flow nets. There are nurnerous checklists, comprehensive references and a bibliography, together with a number of worked examples in an appendix. While computers are used extensively in design offices, an engineer must understand the basic principles of the design, and the report helps to achieve thls understanding.

B P Williams and D W ai te The design and construction of sheet-piled cofferdams Construction lndustry Research and lnformation Association Special Publication 95, 1993

Keywords

Cofferdams, Earth pressures, Retaining walls, Sheet piling, Soil properties, Temporary works, Water pressures

Reader lnterest

Ci vii engineers, consultants, contractors, geotechnical engineers, water authorities, river and coastal management authorities, centrai and local government engineers

Ali rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, including photocopying and 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.

CIRIA ISBN O 86017 361 5

Thomas Telford ISBN O 7277 1980 7

© CIRIA 1993

CLASSIFICATION

A V AILABILITY Unrestricted

CONTENT

STATUS

USER

Guide to design and construction

Committee guided

Engineers involved in design, construction or management of cofferdams

Published by CIRIA, 6 Storey's Gate, Westminster, London SWIP 3AU in conjunction with Thomas Telford Publications, Thomas Telford House, l Heron Quay, London E14 4JD

CIRIA Special Publication 95

Foreword

This report sets out the latest good practice on the planning, design, construction and maintenance of steel sheet pile cofferdams as used for the suppor! of temporary excavations. Users are expected to understand structural design and basic soil mechanics.

Whlle the report is primarily concemed with cofferdams for temporary works constructed with steel sheet piles, some of the conteni is relevant lo other forms of construction, such as secant piles, soldier piles and poling boards, and diaphragm walls.

The report has been written with reference lo UK construction legislation, with appropriate reference to European Standards. However, the principles embodied in the report can be applied to the design and construction of cofferdams in any part of the world.

The theory of retaining wall design is under review as a result of the preparation of the forthcoming BS 8002 and Eurocode 7. These new standards will propose new design approaches. However, as these approaches were not standard in the industry at the lime of writing, these new approaches have not been incorporated into the present document.

The research leading to this report was carried out under contraci lo CIRIA by W A Dawson Lirnited in conjunction with Barker & Hodgson.

Research Team

B P Williarns BSe CEng FICE MIWEM

D W ai te CEng FIStruetE MICE AWeldl

Steering Group

D J lrvine BSe CEng FICE (Chairman) D W Calkin BSe MSe DIC CEng FICE

T F J Cunnington BSe CEng MICE

N J K Davies BSe CEng FICE MIQA

FLane A D Masters BA CEng MICE

D A Randle BSc{Eng) AKC CEng MICE

K W ard BSe CEng M ICE

P E Wilson BSe CEng MICE

Research Managers

A R McAvoy BSe CEng MICE

J H Sakula MA CEng MIStrnetE MICE

Sponsors

W A Dawson Ltd Barker & Hodgson

Tarmac Construction Ltd Kier Group Pie British Steel, Generai Steels Cementation (Major Projects) Ltd Sir Robert McAJpine and Sons Ltd Posford Duvivier Wessex Water Engineering Services Sir Robert McAlpine and Sons Ltd Property Services Agency

(To July 1991) (July 1991 onwards)

CIRIA CIRIA

The project was financia!Jy supported by Anglian Water, North W est Water, Severn Treni Water, Soulhern Water, British Steel pie, the Property Services Agency and CIRIA's Core Programme.

CIRIA Special Publication 95 3

4

Acknowledgements

CIRIA acknowledges the help given by many people and organisations, in particular the following for reviewing parts of the drafts:

G J W King MSc(Eng) PhD

R J Mair MA PhD CEng FICE

B Simpson MA PhD CEng MICE

l F Symons BSe MSc CEng MICE MIHT

University of Liverpool Geotechnical Consulting Group Arup Geotechnics Transport and Road Research Laboratory

Following the fina! Steering Group meeting a Task Group was established to resolve outstanding technical and other issues. This Task Group comprised D W Calkin, D J lrvine (Chairman), D Waite, B P Williarns and J H Sakula. CIRIA wishes to express particular thanks to members of this group for their efforts during the fina! stages of the work.

Extracts from Briti~h Standards are reproduced with the permission of BSI Standards. Complete coptes can be obtamed by post from BSI Sales, Linford Wood, Milton Keynes, MK14 6LE.

The cover photograph is Sprotborough Lock, Sheffiled and South Yorkshire Navigation, British Waterways Board. Photograph by David Lee Photography Ltd., Barton upon Humber.

Figure 17 is reproduced from 'Foundation Engineering' by Peck Hanson & Thombum, 1974 with the permission of John Wiley & Sons !ne., New York, who hold the world copyright©


Contents

List of Figures List of Tables Notation

8 9

IO

l INTRODUCTION 1.1 Cofferdarns

13 13 14 14 16 16

1.2 Temporary works 1.3 Scope of the report 1.4 Supervision of design and construction 1.5 Causes of failure

2 PROJECT PLANNING AND SITE INVESTIGATION 2.1 Planning of the whole project

17 17 17 17 17 18 18 19 20

2.2 The cofferdarn in the context of the whole works 2.3 Layout, clearances and access 2.4 Movement of ground around the cofferdarn 2.5 Environmental considerations 2.6 Sile investigation 2.7 Check Iist of information required from site investigation 2.8 Planning for construction

3 CONCEPTUAL DESIGN OF SHEET PILE COFFERDAM 21 3.1 Appraisal of information 21 3.2 Lirnitations on design 22

3.2.1 Availability of plant and materials 22 3 .2.2 Extemal supports 23 3.2.3 Internai supports 23

3.3 Ground pretreatment 25 3.4 Choice of pararneters for design and driveability 26

4 DESIGN OF W ALL 27 4.1 Water and earth pressures on wall 27

4.1.1 Total and effective stress 27 4.1.2 Water pressures 28 4.1.3 Earth pressures 35 4.1.4 Earth pressure coefficients 38 4.1.5 Choice of ground pararneters 38 4.1.6 Layered ground 41 4.1.7 Sloping ground surface 41 4.1.8 Check Iist for calculating active and passive pressures 44 4.1.9 Variation of ground pressare due to wall flexibility 44 4.1.10 Earthquake Ioads 45

4.2 Other loads on cofferdarn during construction and use 45 4.2.1 Construction plant 45 4.2.2 Spoil heaps 45 4.2.3 Adjacent structures 46 4.2.4 Other loads 46 4.2.5 Latera! pressure on the wall due to Ioads other than uniform surcharge 46

4.3 Analysis of wall 47 4.3.1 Methods of analysis (cantilever or single prop walls) 47 4.3.2 Strength factored 48 4.3.3 Moments factored 48 4.3.4 Choice of method of analysis 51 4.3.5 Design for free or fixed earth suppor! conditions 51 4.3.6 Use of computers for design 54

Cl AIA Special Publication 95 5

4.3.7 Multi-prop walls 55 4.3.8 Soldier piles 58 4.3.9 Design stresses in steel sheet piles 59

4.4 Factors of safety 60 4.4.1 Cantilever walls and propped walls with free earth suppor! 60 4.4.2 Propped walls with fixed earth suppor! 61

4.5 Overall stability 61 4.5.1 Sloping sites 61 4.5.2 Circolar slip instability 62 4.5.3 Bottom stability 62 4.5.4 Pressure due lo river or tidal flow 64 4.5.5 W ave forces 64 4.5.6 Overtopping 65 4.5.7 Scour 65 4.5.8 Protection from vessel impact 65

4.6 Choice of pile section 65 4.6.1 Section profùe 65 4.6.2 Choice of section to suit driving conditions 66

4.7 Generai layout of sheet piling 67 4.8 Check lists 70

4.8.1 Design of cofferdam 70 4.8.2 Check list for analysis of cantilever wall (simplified method) 73 4.8.3 Check list for analysis of a propped wall with free earth suppor! 74 4.8.4 Check lisi for analysis of a propped wall with fixed earth suppor! (simplified

method) 75 4.8.5 Check list for analysis of a multi-prop wall (using the stage-by-stage method) 76

5 DESIGN OF SUPPOR T SYSTEM 78 5.1 Generai 78 5.2 Walings 78

5.2.1 Straight walings 78 5.2.2 Circolar walings 82

5.3 Anchorages for external suppor! 84 5.3.1 Walings 84 5.3.2 Anchors 84 5.3.3 Passive anchors 85 5.3 .4 Ground anchors 85

5.4 Struts for internai suppor! 86 5.5 Double-skin, earth-filled cofferdam 89

6 CONSTRUCTION, MAINTENANCE AND REMO V AL OF THE COFFERDAM 91 6.1 Contro! of work (including design) 91

6.1.1 Generai 91 6.1.2 Contractual requirements 91 6.1.3 Statutory requirements 91 6.1.4 Other legai liabilities 92 6.1.5 Organisation of temporary works contro! 92 6.1.6 Responsibilities of the temporary works coordinator 93

6.2 Installation of sheet piles 93 6.2.1 Types of pile driving equipment 93 6.2.2 On si(e storage of piles 94 6.2.3 Pitching and driving 96 6.2.4 Order of driving 100 6.2.5 Safety 101

6.3 Excavation 101 6.4 Sealing of leaks and separated interlocks (or isolated piles not driven to leve!) 103 6.5 Contro! of water 103 6.6 Access and safety 104 6.7 Maintenance of the cofferdam 104

6 CIRIA Special Publication 95

6.8 Monitoring of the cofferdam 6.9 Removal of the cofferdam

References

Bibliography 1.0 GENERAL COVERAGE AND REFERENCE 2.0 IDSTORICAL 3.0 DESIGN, EAR1H AND WATER PRESSURES 4.0 CONSTRUCTION

Appendix A Soil and water pressures on retaining wall Generai principles Earth pressures on the wall Granular soils Cohesive soils

Appendix B Worked design exarnples Exarnple No l: Design of sheet pile wall for a cofferdam Exarnple No 2: Design of internai frame for a cofferdam Exarnple No 3: Use of flow net diagrarn Exarnple No 4: Earth pressures for 1ayered ground with non-uniforrn slopes

Appendix C Safety regulations

Appendix D Dimensions and properties of steel sheet piles manufactured in the United Kingdom


105 105

107

111 111 111 112 115

116 116 118 118 119

121 121 158 170 185

188

196

7

8

list of Figures

Figure l Types of single sldn sheet pile cojferdams Figure 2 Double wa/1 earth-filled cofferdams Figure 3 Movement of ground around the cojferdam Figure 4 Externally suppor/ed cofferdams Figure 5 Typica/layouts of internally supported cojferdams with straight sides Figure 6 Types of interna/ support [or cojferdams with straight sides Figure 7 Types of circular cofferdams Figure 8 Water pressures - hydrostatic Figure 9 F/ow net diagram Figure lO Water pressures on cofferdams - typica/ cases Figure 11 Typica/ cofferdam f/ow nets Figure 12 Water pressures - simplijied methods Figure 13 Effect of width of a cofferdam on the flow net Figure 14 Pressure diagram [or mixed rota/ and ejfective stress design Figure 15 Coe[ficients of active earth pressure (horizontal component) [or horizontal

retained suiface ( qfter Caquot and Kerisef'!) Figure 16 Coe[ficients of passive earth pressure (horizontal component) [or horizontal

retained suiface ( qfter Caquot and Kerisef'!) Figure 17 Estimation of ~'[or sands and gravels (after Peck, Hanson and Thornburn!25!J Figure 18 Coejficients of active earth pressure (horizontal component) [or genera/ case

on inclined backf/1/ with wal/friction (qfter Caquot and Kerisef'!J Figure 19 Coe[ficients of passive earth pressure (horizontal component) [or genera/ case

of inc/ined backf/1/ with wa/1 friction ( after Caquot and Kerisef'!) Figure 20 S/oping ground-pressure diagram on active side of wa/1 Figure 21 S/oping ground-pressure diagram on passive side ofwa/1 Figure 22 Concentra/ed and line load surcharges Figure 23 Pressure diagram [or a /ine /oad Figure 24 Gross pressures method - pressure diagrams Figure 25 Nel pressure method - pressure diagrams Figure 26 Burland-Potts method - pressure diagrams Figure 27 Analysis of cantilever wa/1 Figure 28 Analysis of propped wa/1 with fv:ed earth suppor/ Figure 29 Analysis of propped wa/1 with free earth support Figure 30 Construction of multi-prop walls Figure 31 Pressure envelope method- Terzaghi and Peck Figure 32 Use of pressure enve/opes [or structura/ design Figure 33 Soldier piles - passive resistance Figure 34 Overa/1 stability with difference in ground leve/ Figure 35 Overa/1 stability, raking struts or tie rods Figure 36 Circular slip instability Figure 37 Bottom stability (after BS 8004(14!) Figure 38 Pressure due to river or tida/ [low Figure 39 Rotation of Z Section pro[/ l es Figure 40 Layout of stee/ sheet piling Figure 41 Use of timber packing to accommodate junction pile between guide wa/ings Figure 42 German wateifront method of driving Figure 43 Analysis o[ a cantilevered wa/1 Figure 44 Analysis o[ a propped wa/1 with free earth support Figure 45 Analysis of a propped wa/1 with fv:ed earth support Figure 46 Multi-prop wal/s Figure 47 Packings [or sheet piling Figure 48 Detail of raldng strut Figure 49 Details of stee/ framing Figure 50 Waling detai/s

15 15 18 22 23 24 24 29 30 32 33 34 34 38

39

39 40

41

42 43 44 46 47 49 49 50 52 53 53 56 57 58 59 61 62 62 63 64 66 68 69 70 73 74 75 77 79 80 81 82


Figure 51 Circular reinforced concrete walings Figure 52 Types of anchor Figure 53 Strut details Figure 54 Direction of accidental /oad on struts Figure 55 Effective length of struts Figure 56 Double-sldn, earth1illed cojferdams Figure 57 Pressure diagram [or a double-s/dn, earth-filled cofferdam Figure 58 Construction of a double-s/dn, earth-filled cofferdam Figure 59 F/ow chart of principal activities and responsibilities [or design and

construction of a temporary cofferdam Figure 60 On sile storage of piles Figure 61 Recommended storage procedure [or steel sheet piling Figure 62 Genera/ arrangement and detail of the buti we/ding /empiate Figure 63 The 'Pane/' driving technique - guide frame Figure 64 Detail of hammer leg guides Figure 65 The 'Pane/' technique - stages in driving Figure 66 Pitching piles Figure 67 C/osing inter/ocked sheet piling Figure 68 Excavation under water

List of Tables

Table l Cofferdams without sheet piling (after Packshaw">) Table 2 Cofferdams dependent on sheet piling (after Packshaw<"l) Table 3 Basis of calculation of soil pressures Table 4 Maximum values for wall friction and adhesion Table 5 Tabular Iayout for pressure calculations Table 6 Typical ground parameters Table 7 Estimation of <!>' for cohesive soils Table 8 Estimation of <j>' for weak rocks (from Draft BS 8002<12>) Table 9 Steel sheet piles - steel stresses in bending Table lO Guide for the selection of pile size to suit driving conditions in granular soils Table 11 Guide to selection of pile size to suit driving conditions in cohesive soils Table 12 Reinforced concrete walings for circular cofferdams Table 13 Types of pile driving equipment


83 84 87 87 88 89 89 90

92 95 96 97 97 98 99

100 101 102

13 14 28 35 37 39 40 40 60 66 67 84 94

9

10

Notation

Note: The following symbols have been chosen to be consistent with those given by the Intemational Conference on Soil Mechanics and Foundation Engineering(45).

A

B

c'

c,

D,d

E

e

F

F.,

F,

F,

F,

H, h

I

linear dimension

linear dimensions

effective cohesion intercept

factored effective cohesion intercept

apparent cohesion intercept (undrained shear strength)

factored apparent cohesion intercept

wall adhesion

factored wall adhesion

depth, diarneter

Y oung' s modulus

eccentricity

force

factor of safety for net pressure method

factor of safety for factoring passive pressure

factor of safety for Burland-Potts, method

factor of safety for factoring soil strength parameters

depth, head

second moment of area

ij depths

hydraulic gradient

K. active pressure coefficient

K., active pressure coefficient for cohesion

K. coefficient of horizontal earth pressure at rest

K, passive prèssure coefficient

~ passive pressure coefficient for cohesion

k coefficient of permeability

L length

L, effective length

N standard penetration test resistance

P A total active force

PN total force on wall due to line load (Krey's method)

P, total passive force

P. total nel water force

p, total pressure active

p,' effective pressure active


P o maximum pressure on wall from line load (Krey's method)

p, total pressure passive

p,' effective pressure passive

concentrafed load surcharge

line load surcharge

q flow of water, surcharge pressure

R resultant force

r radius

T prop force

t linear dimension

u pore water pressure

W totalload

x,y linear dimensions

z depth

y bulk unii weight of soil

y, bulk unii weight of saturated soil

Y. unit weight of water

o angle of wall friction

om factored angle of wall friction

OJJ; vertical stress (total, effective)

~ shear stress

<!>' effective angle of shearing resistance

<!>'m factored effective angle of shearing resistance



1 lntroduction

The purpose of the report is lo guide practical engineers and designers lowards the construction of safe, economie and effective cofferdams for tempornry wmks. The report is not intended in any way lo resbict innovation and the development of new techniques. The use of alternative methods of design and construction techniques, provided they can be substantiated, is not precluded by omission from this document.

The sections on design are intended to guide the user in the preparation of manual calculations. While many designers will have access to a computer, they should have a working lmowledge of the basic methods of design and analysis so that they appreciate the significance of the various pararneters entered inlo a computer run and so that they can check by manual calculation the results produced by computer.

The ability to make manual calculations will also greatly enhance the value of a computer, since the designer will be better able lo predici the effect of variations lo the structural fono. In cofferdams these will be the number and spacing of frarnes, the use of fixed or free earth support, the use of lotal or effective stress soil values, and the sensitivity of the design lo small changes in soil pararneters.

1.1 COFFERDAMS

The function of a cofferdam is to exclude soil and water from an excavation below the existing surface lo facilitate construction of pennanent works. Total exclusion of water is rarely necessary, but the effect of water ingress should be included in the design calculations. With good design and construction, single skin cofferdarns can be used in marine conditions, but for large excavations in marine works, double skin earth filled cofferdams may be preferable.

Tables l and 2 show the various types and categories of cofferdam.

Table 1 Cofferdams without sheet piling (after Packshawt"l)

Gravity Types Concrete

Earth or rock

Sheeted Types Timber

Concrete

Steel Combinations of materials

Special Methods

Other Types


In bags Blockworlc In bags Earth bank with impenneable surfadng or core Rock-filled timber crib with timber sheeting or puddled clay

Sheet piles Runners Poling boards Double-skin with puddled clay Precast sheet piles Precast panels Cast-in-piace panels Wall of cast-in-piace bored piles Trench sheeting Vertical wide-flange beams with poling boards or concrete slabs

W eli points, borehole pumps, chemical cònsolidation, freezing, electro-osmosis, bentonite suspensions,long-tenn deep borehole pumping in soft clays

Concrete arcb, caissons, precast concrete or cast iron lined shaft

13

14

~~~~~"~"~~~~,,~~~~----------------------------------

Table 2 Cofferdams dependent on sheet piling (after Packshaw"'l)

Slngle Skin: One or More Stages

Double Skin

Supported by struts and 'Yalings. Timber, steel, concrete or combinations of materials, One or more frames

Tied back or anchored

Supported by circular rings or arcs

Supported by soil

Supported by gravity wall

Double wall type Cellular type

The following requirements must be fulfilled:

Raking struts or raking piles

Horizontal struts (walings and struts separately or combined)

By tie rods (to anchorages or to reinforced concrete ring) To raking tension piles T o raking tension and compression piles

Circular cofferdam Figure eight cofferdam Arch Arcs with struts

Cantilever Rock-filled timber crib with skin of steel sheet piling Earth dam with steel sheet pile core Steel sheet piling supported by benns

Blockwork with skin of steel sheet piling

the cofferdam must withstand the loads upon it

the amount of water entering the cofferdam must be controllable by reasonable pumping

when excavated to the full depth, the formation leve! musi be stable and not subject to excessive heave or to boiling

deflection of the cofferdam walls and any internai framing must not interfere with construction of the permanent works, and must not be detrimental to existing adjacent structures or services

the cofferdam musi have overall stability against unbalanced earth pressure or ground movements such as circular slip.

1.2 TEMPORARY WORKS

These are works of short duration which are required to enable the permanent works to be constructed. A cofferdam must always be conceived to suit the construction which will be carried out within it Also, it musi be so constructed that as much as possible of the materials can be recovered when its purpose has been fulfilled.

Temporary cofferdams can often be incorporate<! into the permanent structure with significant savings in overall cost and construction lime.

1.3 SCOPE OF THE REPORT

This report is intended primarily for cofferdams not more than IO metres deep. The guidance given is applicable to cofferdams of greater depth, although expert advice may be appropriate when complex soils or loading occur, or where severe water seepage through the soils associated with the cofferdam is possible,

The report deals in detail with single skin sheet pile cofferdams where the formation leve! is situated below the natura! surface.


Such cofferdams may be designed so that the sheet piles cantilever above formation leve!, or to have additional support from internai props or extemal ties (see Figure 1).

(a) Cantilever wall

Waling

=:J~--~=/ ,

(c) Propped wall. External support

(b) Propped wall.lnternal support

Tiero{, /

Anchors

Figure 1 Types of sing/e skin sheet pile cofferdams

A brief section is also devoted to double wall earth-filled cofferdams which are often used in marine situations (see Figure 2).

Tie rod

l Waling

' Fili

Fili

Figure 2 Double wa/1 earth-fil/ed cofferdams

As with any structure, the stiffness of the materials employed has an effect on the way a cofferdam performs. This report concentrates on cofferdams built with steel sheet piling, which is much more flexible than reinforced concrete contiguous and secant pile walls or diaphragm walls.

The effect of corrosion on steel piles has not been considered since the timescale for temporary works is short. Advice on corrosion rates is, however, given in the British Steel Piling


10

Notation

Note: The following symbols have been chosen to be consistent with those given by the International Conference on Soil Mechanics and Foundation Engineering""·

A linear dimension

B linear dimensions

c' effective cohesion intercept

' cm factored effective cohesion intercept

c. apparent cohesion intercept (undrained shear strength)

c= factored apparent cohesion intercept

c. wall adhesion

c.m factored wall adhesion

D,d depth, diarneter

E Young's modulus

e eccentricity

F force

F., factor of safety for net pressure method

F, factor of safety for factoring passive pressure

F, factor of safety for Burland-Potts, method

F, factor of safety for factoring soil strength parameters

H, h depth, head

I second moment of area

ij depths

hydraulic gradient

K. active pressure coefficient

K,., active pressure coefficient for cohesion

K. coefficient of horizontal earth pressure at rest

I<. passive prèssure coefficient

~ passive pressure coefficient for cohesion

k coefficient of permeability

L length

L. effective length

N slandard penetration test resislance

PA total active force

PN total force on wall due to line load (Krey's method)

P p total passive force

P. total net water force

p, total pressure active

P.' effective pressure active


P,

p,'

O c

QL

maximum pressure on wall from line load (Krey's method)

total pressure passive

effective pressure passive

concentrafed load surcharge

line load surcharge

q flow of water, surcharge pressure

R resullant force

r radius

T prop force

u

w x,y

z

y

Y.

Yw


linear dimension

pore water pressure

totalload

linear dimensions

depth

bulk unit weight of soil

bulk unit weight of saturated soil

unit weight of water

angle of wall friction

factored angle of wall friction

vertical stress (total, effective)

shear stress

effective angle of shearing resistance

factored effective angle of shearing resislance

11


1 lntroduction

The pUIJlOse of the report is to guide practical engineers and designers towards the construction of safe, economie and effective cofferdams for temporary works. The report is not intended in any way to restrict innovation and the development of new techniques. The use of alternative methods of design and construction techniques, provided they can be substantiated, is not precluded by omission from this document

The sections on design are intended to guide the user in the preparation of manual calculations. While many designers will bave access to a computer, they should bave a worlcing knowledge of the basic methods of design and analysis so that they appreciate the significance of the various pararneters entered into a computer run and so that they can check by manual calculation the results produced by computer.

The ability to make manual calculations will also greatly enhance the value of a computer, since the designer will be better able to predici the effect of variations to the structural form. In cofferdams these will be the number and spacing of frarnes, the use of fixed or free earth support, the use of total or effective stress soil values, and the sensitivity of the design to small changes in soil parameters.

1.1 COFFERDAMS

The function of a cofferdam is to exclude soil and water from an excavation below the existing surface to facilitate construction of permanent works. Total exclusion of water is rarely necessary, but the effect of water ingress should be included in the design calculations. With good design and construction, single skin cofferdams can be used in marine conditions, but for large excavations in marine works, double skin earth filled cofferdams may be preferable.

Tables l and 2 show the various types and categories of cofferdam.

Table 1 Cofferdams without sheet piling (after Packshawt"l)

Gravlty Types Concrete

Earth or rock

Sheeted Types Timber

Concrete

Steel Combinations of materials

Special Methods

Other Types


In bags Blockwork In bags Earth bank with impenneable swfacing or core Rock~filled timber crib with timber sheeting or puddled clay

Sheet piles Runners Poling boards Double-skin with puddled clay Precast sheet piles Precast panels Cast-in-piace panels W ali of cast-in-piace bored piles Trench sheeting Vertical wide-flange beams with poling boards or concrete slabs

Well points, borehole pumps, chemical consolidation, freezing, electro-osmosis, bentonite suspensions, long-tenn deep borehole pumping in soft clays

Concrete arch, caissons, precast concrete or cast iron lined shaft

13

14

Table 2 Cofferdams dependent on sheet piling (after Packshaw1"')

Single Skin: One or More Stages

Double Skln

Supported by struts and 'Yalings, Timber, steel, concrete or combinations of materials. One or more frames

Tied back or andtored

Supported by circular rings or arcs

Supported by soil

Supported by gravity wall

Double wall type Ce11ular type

The following requirements must be fulfilled:

Raking struts or raking piles

Horizontal struts (walings and struts separately or combined)

By tie rods (to anchorages or to reinforced concrete ring) T o raking tension piles T o raking tension and compression piles

Circular cofferdam Figure eight cofferdam A.ch Arcs with struts

Cantilever Rock·filled timber crib with skìn of steel sheet piling Earth dam with steel sheet pile core Steel sheet piling supported by berms

Blockwork with skin of steel sheet piling

the cofferdam musi withstand the loads upon il

the amount of water entering the cofferdam must be controllable by reasonable pumping

when excavated to the full depth, the formation leve! must be stable and not subject to excessive heave or to boiling

deflection of the cofferdam walls and any internai framing musi not interfere with construction of the permanent works, and musi not be detrimental to existing adjacent structures or services

the cofferdam musi have overall stability against unbalanced earth pressure or ground movements such as circular slip.

1.2 TEMPORARY WORKS

These are works of short duration which are required to enable the permanent works to be constructed. A cofferdam musi always be conceived to suit the construction which will be carried out within it. Also, it musi be so constructed that as much as possible of the materials can be recovered when its purpose has been fulfilled.

Temporary cofferdams can often be incorporated into the permanent structure with significant savings in overall cost and construction time.

1.3 SCOPE OF THE REPORT

This report is intended primarily for cofferdams not more !han lO metres deep. The guidance given is applicable to cofferdams of greater depth, although expert advice may be appropriate when complex soils or loading occur, or where severe water seepage through the soils associated with the cofferdam is possible.

The report deals in detail with single skin sheet pile cofferdams where the formation leve! is situated below the natura! surface.

Cl AIA Special Publication 95

Such cofferdams may be designed so that the sheet piles cantilever above formation leve!, or to have additional suppor! from internai props or extemal ties (see Figure 1).

{a) Cantilever w ali

Waling

=J~-----,;<=""ll/ 1

(c) Propped wall. External support

(b) Propped wall. Internai support

L

Tiero~ /r Anchors

Figure 1 Types of single skin sheet pile cofferdams

A brief section is also devoted to double wall earth-filled cofferdams which are often used in marine situations (see Figure 2).

Tierod

l Waling

Fili Fili

~

Figure 2 Double wa/1 earth·filled cofferdams

As with any structure, the stiffness of the materials employed has an effect on the way a cofferdam performs. This report concentrates on cofferdams built with steel sheet piling, which is much more flexible than reinforced concrete contiguous and secant pile walls or diaphragm walls.

The effect of corrosion on steel piles has not been considered since the timescale for temporary works is short. Advice on corrosion rates is, however, given in the British Steel Piling


16

Handbook(1). The condition of second hand piles should be checked if they are to be used at the same stresses as ne w.

1.4 SUPERVISION OF DESIGN ANO CONSTRUCTION

The design and construction of cofferdarns should at ali times be supervised by a suitably qualified and experienced engineer. The actual calculations are frequently delegated to Iess experienced personnel, but the overall concep~ interpretation of soils infonoation and supervision of installation should always be under the contro! of the experienced engineer, who must also ensure that ali calculations are checked and approved.

The cost of a cofferdarn may fono only a small part of the whole project. However, the consequences of inadequate or inappropriate design can result in very significant additional costs and delays. These can be further increased by darnage to adjacent structures through settlement of inadequately supported soils.

1.5 CAUSES OF FAILURE

There are many possible causes of cofferdarn failure, but in most cases the cause is confined to a relatively small range of possible defects. These include:

oversight of small, but important, details in the design

failure to make sufficient allowance for variation of water Ievels due to seasonal effects, constriction of flow by the cofferdam itself, overtopping by high tide or flood conditions, etc.

failure to provide balancing and/or flooding valves where appropriate and to use them when occasion demands

variations between the ground conditions assumed for design and those revealed during excavation

failure to contro! the excavation Jevels to those indicated in the design at ali stages of construction

failure to provide adequate suppor! to frames and latera! restraint to compression flanges of waling beams

• use of frames as platfonos for the suppor! of ancillary equipment such as pumps and generators when no allowance for such loading has been made in the design

frame members darnaged by impact from skips, grabs, etc., but not repaired or replaced

unauthorised strut removal or substitution

uncontrolled water ingress through separated interlocks or short driven piles into the passive soil below fonoation leve!.

Frequently the failure is the result of a number of factors occurring simultaneously, any one of which by itself would not have caused the failure.


2 Project planning and site investigation

2.1 PLANNING OF THE WHOLE PROJECT

Planning of the project should include consideration of any temporary works which may be necessary to facilitate construction of the penoanent structure. Any construction below the existing ground surface will entail excavation and, most probably, ground water contro!. Excavations greater than 1.2 metres deep and not cut to a stable slope are required by UK law to be adequately supported to ensure the safety of personnel working in or around the excavation. The planner must, therefore, be sufficiently experienced to be able to envisage appropriate fonos of temporary suppor! for any construction below ground leve!. Good planning will ensure that the penoanent structure can be built economically with the minimum difficulty and danger to sile personnel.

2.2 THE COFFERDAM IN THE CONTEXT OF THE WHOLE WORKS

Temporary works are sometimes given insufficient thought at the planning stages of a project. The design and construction of a cofferdarn is usually the responsibility of the contractor, but many decisions which will indirectly affect its efficiency and effectiveness have to be made long before the contractor is appointed. Any cofferdarn is an important structure because it is the primary safeguard both for people called upon to work below the surface leve! and for the integrity of the soil underlying and adjacent to the penoanent structure. The fono it Jakes and the degree to which it can be made compatible with the penoanent construction may make a very Jarge difference to the overall cost and speed of construction.

2.3 LAYOUT, CLEARANCES ANO ACCESS

Congested sites or those where the penoanent structure is to be built very near to existing structures cali for special consideration at the planning stages. For example, unless sufficient working space is allowed for pile driving equipment, it may be impossible to achieve the desired building Jines. There may be noise and vibration Jimitations. On congested sites it may become difficult to gain access to the piling after the penoanent works rise to ground leve!. Consequently expensive piling materia! has to be abandoned in piace instead of being extracted for re-use. Such waste can often be avoided by planning for the penoanent structure to be designed to facilitate access in its partially built state, or for the temporary cofferdarn to fono part of the penoanent structure.

2.4 MOVEMENT OF GROUND AROUND THE COFFERDAM

Ground movement around the sides of an excavation is unavoidable. Suppor! to the sides of an excavation, even of the most stiff fono, cannot eliminate such movement which will arise from global movements of the ground in which the cofferdarn is situated as well as deflections in the walls and support system (see Figure 3). However the magnitude of these movements is very difficult to calculate because of the complex interaction between the ground and the structure retaining it and other factors. Details of observed ground movement at a number of sites are given by Tomlinson''l, Potts and Day""· Flemming et at.''1l and BS 8081"n.

W eli designed and constructed cofferdarns will nonoally result in acceptable ground movements. Computer programmes using finite element analysis are available to mode! the soil pile interaction and give predictions of ground movements. However at present their use in design is limited to major constructions, particularly where the presence of existing structures requires a prediction of ground movements.


18

Originai ground level

l """ ---..__ l

\inal ground level •---.._ l •-.,.._ ·---... l

Soil movement ·-"' l

·~ l

•-,.._ H l

·~ l

•-,__ ·~ ~

Deflected wall

Figure 3 Movement of ground around the cofferdam

Concrete diaphragm walls, and secant and contiguous bored pile walls, are stiffer !han steel sheet pile walls of equivalent strength, but bave the disadvantages of entailing the removal of soil during their installation, thus allowing some relaxation of the soil even before the main excavation commences, and of being non-recoverable after completion of the works. If deformations are important, steel walls can be designed for equivalent stiffness and some very stiff sections are available.

Steel sheet piling, on the other hand, while leaving the soil intact during installation, is generally more flexible and will deflect more under the action of soil and water pressure. If it is necessary to contro! the movement this can be done to some degree by preloading struts and anchors or by using more levels of internai supporting frarnes or tiebacks than are necessary for limiting the bending moments in the piles. Such measures to contro! movement of external ground can add significantly to the cosi of temporary works and, if additional internai frarnes are used, may complicate the construction of the permanent structure.

2.5 ENVIRONMENTAL CONSIDERATIONS

Where cofferdarns are to be constructed in urban and city areas attention must be given to undesirable effects in terms of both noise and vibration. The Contro! of Pollution Act 1974 gives power to local authorities to set limits on the noise emitted from construction sites where this will adversely affect the quality of !ife. BS 5228, Part 4"" gives guidance on piling methods and equipment for the contro! of noise and vibration. While noise is more Iikely to give rise to complaint, vibration is more dangerous as it can cause physical darnage to adjacent structures and their contents.

2.6 SITE INVESTIGATION

Site investigations should be carried out in accordance with the recommendations of BS 593()0 >.

The primary purpose of a site investigation is to discover the suitability of the ground for the intended construction. Modifications to the originai layout of the permanent works are sometirnes made in the light of information obtained from the site investigation. On occasions, structures may be resited to avoid difficult construction or foundation problems. Unfortunately, such changes are not always accompanied by an adequate investigation of the new Iocation. ldeally, every significant structure should be based on the fullest possible knowledge of the soils with which it is associated.

Permanent works designers tend to have the fina! structure in mind when planning a site investigation. Unfortunately the site investigation sometimes ignores completely the need for information which can be used in the design of the temporary works.

A typical exarnple is that of boreholes taken for the design of bridge foundations. Very detailed investigation will be made of the deeper soils to be used as the founding stratum, but the upper


soils will often be reported only by description since they contribute nothing to the foundation. In the design of a temporary cofferdarn for such a project, the properties of the upper soils are very important.

The reverse situation applies in the construction of substructures such as underground purnping stations, where the site investigation is taken only deep enough to obtain information for the permanent structure and the soils immediately below it. The information obtained in this case will enable the active pressures on a cofferdarn to be determined, but there may be no information on the lower soils, which is necessary for calculation of the passive resistance to the toes of the piles and of the stability of the bottom of the temporary excavation.

In both of these exarnples the information needed by the temporary works designer could be obtained in the course of the investigation for the permanent works at very little extra cost.

As an aid to an understanding of the ground condition by ali parties involved, ali relevant sile information, including interpretive reports, should be included in the tender docurnents.

Tender documents frequently require the contractor to be satisfied as to the relevance and accuracy of the information contained in the site investigation report. This is an impractical demand as there is usually insufficient time for the tenderer to make any further site investigation and it is unrealistic to expect every tenderer to carry out independent boreholes. When it is clear that the client's site investigation does not include the necessary details for design of the temporary works, it is advisable for the tenderer to allow for further investigation on their own behalf before commencement of work.

2.7 CHECK LIST OF INFORMATION REQUIRED FROM SITE INVESTIGATION

location of boreholes with respect to the cofferdarn

date on which each hole was bored

ground leve! at each borehole to the same datum reference as the construction drawings

diarneter of borehole, type of rig used, and whether water was added during boring

depth and thickness of ali soil strata to at least l \6-2 times the proposed excavation depth, and possibly more in weak soils

bulk unit weights for each soil type

Standard Penetration Test results and any other relevant test results such as eone penetrometer tests

grading curves

undrained, and where appropriate, drained shear strength and Plasticity Indices (Atterberg limits) for clays

in situ vane tests for ali soft cohesive soils together with remoulded values and sensitivity values

ground water strike, rate of ingress, standing levels and permeability test data

casing depth at ali stages of boring

position and details of ali piezometers and readings

tidal variations and lag

information on previous history of the site including the geology.

If it is available, an interpretive review of the information obtained from the investigation should be passed on to the temporary works designer, even though it may not appear relevant.


20

2.8 PLANNING FOR CONSTRUCTION

The construction of the cofferdam must be set within the overall pian and programme for the whole project. The type of cofferdam will depend on the permanent structure to be built, the ground conditions and constraints. A feasibility study will probably show that more than one solution is possible. Preliminary designs with costs, advantages and disadvantages will help narrow this down to a preferred solution so that a more detailed design and estimate for tendering purposes can be produced.

Tbe designer will need the following information:

ground information - see check list in Section 2. 7

• details of permanent structure and site

• any 'size' limitations for construction of permanent work - e.g. access for precast units or

steel work

• other procedures to be carried out inside the cofferdam - e.g. connection to a sewer or driving bearing piles

• any obstructions, services or other constraints affecting the cofferdam

availability of plant and materials

• any limitation on ground movement.

After the contrae! has been awarded a further review of the planning and design of the cofferdam should be made before the fina! design and drawings are prepared.

It should be appreciated that planning and design affect each other and plans may have to be altered as design proceeds.

CIRIA Spacial Publication 95

3 Conceptual design of sheet pile cofferdam

The design process is frequently divided into two distinct phases.

The first is the preparation of a preliminary design for tender purposes and will genemte the basic approach to tbe method of construction and sufficient calculations to indicate frame positions and pile section, so that the cost can be estimated.

If the tender is successful, the second phase will consist of a complete and detailed reappmisal of ali available information and a new set of calculations covering every aspect of each stage of excavation, frame or anchor installation and removal of the cofferdam. It should also consider restrutting during construction of the permanent structure.

3.1 APPRAISAL OF INFORMATION

The temporary works designer needs to review information from any source for omissions and contradictions. The reliability of information should not be taken for granted.

Ideally, full details of the permanent structure and its construction procedure should be available, but these are not always complete at the time the cofferdam is designed.

The contents of the site investigation report must be very carefully assessed in terms of their relevance to the life of the cofferdam. Total stress parameters for clays will usually be appropriate for the conditions applying during and immediately after the construction of the cofferdam.

Some clays soften much more rapidly than others when subject to change of stress, e.g. Kimmeridge clay, and in these conditions effective stress values should be considered if the cofferdam is to be left fully excavated for more than a few days without the exposed surface being blinded off or covered by the permanent construction.

Standard penetration test values are the usual means of assessing the in situ angle of internai friction for granular soils. The 'N' values should be reviewed for unexpectedly low or high values. Low values at depth may be an indication that hydraulic seepage into the bottom of tbe borehole casing has loosened the soil, resulting in false 'N' values. Unexpectedly high values may indicate the presence of cobbles, boulders or other natural or man-made obstructions which may cause difficulty in pile driving. The regolar occurrence of such high values could be the dominant influence on the choice of pile size and method of inslallation. Occasionai high values may be of less significance but the designer would need to bear in mind the consequences of a declutched pile.

It is important to understand that even the best site investigation will give only an indication of the depths and thickness of the various strata and that soil strengths, whether determined in the field or the laboratory, will not be truly representative of the undisturbed soil. Ground water levels should be studied carefully as pressure below a clay stratum can cause the bottom of a cofferdam to heave.

Water levels may be subject to seasonal variations andare likely to be affected by the construction work itself. Cofferdams constructed across water flow paths (including drainage paths within the sòil) will consbict the flow and can result in higher water levels than would exist naturally.

In cofferdams likely to be subject to flood conditions it may be uneconomical to design for the highest flood condition. In this situation overtopping must be allowed for in the design, so that lrapped water does not cause failure. Equally protection against, or allowances for, scour should be considered.

CIRIA Spacial Publication 95 21

22

When boreholes are at some distance from the location of the cofferdam, it will be necessary to construct a 'design borehole' (in effect two boreholes, one ascribing the weakest likely properties to the soil far earth pressure calculations, and the other ascribing the strongest likely properties to the soil far driving considerations), based upon the conservative intei]Jretations of the available information.

3.2 liMITATIONS ON DESIGN

3.2.1 Avallablllty of plant and materlals

Before the detailed design of the cofferdam begins it is worth reviewing available plani and materials. Many contractors keep stocks of secondhand sheet piles and structural steel which could be used without the need far buying new, or there might be piles or tubes available far purchase on the secondhand marlce~ or far hine. The design of the cofferdam c:ill-often be tailored to suit such materials by adjusting the number and levels of the supports.

The availability of cranes and pile driving equipment can also affect the design and cast. The crane musi be capable of lifting both the pile and pile driver at the largest radius required by the piling layout and available access. !t must also have a boom long enough to allow pitching

"" Concrete block or beam anchor

Ground anchor

Figure 4 Externally supported colferdams

' Sheet pile

anchor

Driven raking steel tension

pile

Multilevel ground anchors

fordeep cofferdam


of the piles one above the next. Far instance, if a cofferdam is at the side of a river and a large crane is available it may be more economica! to use it than to provide temporary access far a smaller crane to reach the outer wall.

3.2.2 External supports

An extemal support system (see Figure 4) will give unhindered access to the cofferdam but there are limitations. Tie rods and their anchors require a considerable space outside the cofferdam. This space will be excavated aver most of the area to allow construction and fixing of the various components. and will allow virtually no access in that area unti! backfilled. There will be similar problems if the anchorage system is to be removed after the permanent works have been completed, otherwise the cast of the materials will have to be written aff.

Raking tension piles and ground anchors can be placed without much ground disturbance, and the area required will probably be less than that needed far tie rods and their anchqrs, but the materials are not normally recoverable. They can be placed under an adjacent structure provided that they do not jeopardise its stability. If the land is under dilferent ownership, arrangements far the temporary construction and use of ground anchors musi be agreed. A frequent condition far the agreement is that the anchors are destressed at the end of the construction period.

Mixed systems using both extemal anchors and internai struts should be avoided since the comparatively large ground movement needed to mobilise the anchor force may induce increased loading ìn the struts and the possibility of failure.

3.2.3 Internai supports

Cofferdams with straight sides

The arrangement and location of latera! supports to the sheet piling is a most important factor when designing a cofferdarn. While structural integrity musi be the first priority, the suppor! layout must be compatible with the permanent works construction and cause the least obstruction to excavation plant and materials access.

Frame levels should be set to allow concrete lifts to be completed before removal of a frame is necessary. Pian arrangement of frame members should create as much clear space between the members as possible without recourse to excessively large member sizes (see Figures 5 and 6).

Figure 5 Typica//ayouts of internal/y supported colferdams with straight sides


24

Walings and struts

~---==-__,~ T emporary berms left

until struts in piace

Longstruts h==----4 t---...--1 supported by

kingpiles Multiframewith vertical bracing

t<;;so;===='=~:)6~==;~ if necessary or king pii es far deep cofferdam

Figura 6 Types of interna/ support !or cofferdams with straight sides

Il Il

\\ u

Where severa! levels of frames are required, the frame members should be detailed so that the length of the individuai pieces of the lower frames can be inserted and removed through the previously erected upper frames. Alternatively, complete frames can be assembled at the top and lowered as the work proceeds. This is particularly useful if the frames have to be installed under water.

Circular cojferdams

As an alternative to conventional internai cross strutting in some cases, a completely open structure can be achieved by having a circular cofferdam around the perrnanent structure, or an are or series of arcs to suit the site conditions (see Figure 7).

r------1

l

l l L ____ _

-------, l l l

l l l l l _ ____ .J

Figure 7 Types of circular cofferdams

c

Morethan onearc

Single are at entranceto dock or lock

Full circle cofferdam


The internai supporting frames consist of steel or cast-in-piace reinforced concrete ring 'bearns', which in fact are struts rather than beams. The ring beam is subject, in theory, to pure compression, although, in reality, it is designed for nominai bending in addition to the ruda! load, to allow for the non-uniforrn ground pressures around the circumference and irnperfect circularity of the cofferdam.

If the cofferdarn is subject to uneven loading, such as varying ground levels or soil strengths around the cofferdarn or not entirely surrounded by ground, e.g. in water, then particular care must be taken in the design of the walings due to the larger bending moments that may be induced.

A circular cofferdarn as above should not be confused with a ceUular cofferdarn. The forrner is a single skin structure whereas the latter is a particular type of double skin cofferdarn which is not dealt with in detail in this report (but see Section 5.5).

3-3 GROUND PRETREATMENT

One of the most important aspects of any design is the ease with which it can be built In the case of cofferdarns, this is affected not only by the arrangement of the members and their connection to one another, but whether the sheet piles can be driven to the designed penetration.

The section profile ('U' or 'Z'), the size within the profile ranges, and the quality of steel, have a direct bearing on the ability of any particular section to penetrate the ground, as do the driving technique and equipment.

The choice of appropriate section is dealt with in detail in Section 4.6, but the use of ground pretreatrnent is an option which should be considered early in the design process. Pretreatrnent may take various forrns, but the generai objective is to reduce the resistance the ground offers to the penetration of the pile. In dense granular soils and hard clays the ground may be loosened or softened by augering to disturb the soil without actually removing it. Alternatively the remava! of small quantities for the full depth of the intended penetration will produce a similar effect, but this is not recommended if the foundation of adjacent structures could be affected.

When it is required to achieve penetration into rock, shock blasting may be employed to disrupt the integrity of the rock, so that it has the properties of a coarse grave!. This is usuaUy achieved by means of a patented system, which is operated by a specialist contractor workilo3 in cooperation with the piling contractor.

Ground pretreatment is expensive. !t is norrnaUy resorted to only when the use of a heavier pile section than is necessary to fulfil the structural requirements of earth retention would be either impracticable or uneconomic. There are, however, some new developments in pile driving machinery which incorporate pre-augering tools, but currently these can only handle relatively light pile sections.

When piles are to be driven into very dense medium/fine grained granular soils, penetration may be assisted by jetting. This consists of fixing pipes to each pile so that a high pressure supply of water can be delivered to the pile toes to loosen the soil during driving. The pipes are attached in such a manner that they can be recovered as the driving of each pile is completed. Where jetting is carried out there may well be ground movement which could affect adjàcent and buried structures or services.

The driving of piles into stiff clays may be assisted by welding a lip near the base of the piles to reduce shaft friction, though this may make it more difficult to extract.


26

3.4 CHOICE OF PARAMETERS FOR DESIGN ANO DRIVEABIUTY

It is important to recognise that the soil parameters which are appropriate to the calculation of soil pressures will not necessarily be the sarne as those for assessing the driving resistance.

Granular soils will always have effective stress values. which do not vary with the passage of lune. Overconsolidated clay soils, however, can with time exhibit very large loss of undrained strength.

The ch~ice of appropriate soil pararneters for the calculation of active and passive pressures is dealt With m Section 4.1, but should always tend to reflect the weaker end of the possible range of values. When considering the resistance which the soil will present to pile penetration, the values should reflect the stronger end of the range. It is important to appreciate that, for clay smls, rrrespechve of whether total stress (undrained) or effective stress (drained) pararneters are used in pressure calculations, total stress values apply at the time of construction and immediately afterwards. In the case of overconsolidated clays, the undrained shear strength may be such that the soil will have a resistance to pile penetration similar to that of a soft rock, even though at some later time its strength might be very much Iess.

Where the range of values is great the design will give long piles (using the weaker end of the range) and heavy ~iles (at the stronger end). If during construction the driving becomes very difficult or Impossible, then a redesign may be necessary.


4 Design of wall

4.1 WATER ANO EARTH PRESSURES ON WALL

The guidance given in this document is intended for practising engineers to produce safe designs for cofferdams. They must, however, have an understanding of soil mechanics. Brief notes with an explanation of 'Total' and 'Effective' stresses are given in Appendix A, but anyone wishing to enquire more deeply into the theories of soil mechanics upon which these notes are based should refer to the standard text books.

4.1.1 Total ancl effectlve stress

Terzaghi's principle of effective stress is centrai to an understanding of soil strength. In ali soils, be they sands, gravels, silts or clays, shear strength is derived mainly from internai friction characterised by the effective angle of shearing resistance <)>'. This frictional principle means the greater the effective stress on a piane, the greater the available shear strength. Some engineers find it helpful to think of the effective stress as a way of expressing the sum of the forces exerted between the soil grains or as a measure of the load carried by the skeleton of the soil particles. No matter what the conceptual mode!, it is important to know the definition:

effective stress cr'

= total stress - pore water pressure = a-u

In sand and grave! it is always possible to make estimates of water pressures. Earth pressures for such strata are always calculated in terms of effective stresses.

Very fine-grained relatively impermeable soils adjust to changes in loads and differences in water levels only slowly. In the short term, a clay exhibits a shear strength which depends on the locked-in effective stress which cannot change without an increase or decrease in volume which results from a change in water content. This strength, in the short term, is independent of any recently applied loads or changes in hydraulic conditions, because of the relatively impermeable nature of clays. The immediate strength is termed the 'apparent cohesion' or 'undrained shear strength' (cj. Over a period of time, water is squeezed out of, or is drawn into, a clay and, as the ground adjusts to a new set of conditions, this 'undrained shear strength' changes. It is impossible to calculate water pressures inside the clay unti!, eventually, the water settles down to a new steady state regime (either static or steady state seepage).

Thus, in the short term, earth pressures in clay layers are calculated using only 'total stresses' and 'undrained shear strengths'. In the long term, when once again it is possible to estimate the water pressures, the pressures can be calculated using the 'effective stress' strength pararneters c' and <)>' appropriate to the clay. Soil pressures due to sands and gravels (granular or noncohesive soils) are calculated using effective stresses forali stages of construction and service. This is summarised in Table 3.

The problem facing the designer is to decide what analysis to use in the clay (or cohesive soil) strata. The question of what constitutes long- or short-term conditions depends very much on the mass permeability of the clay strata. The valid period for an undrained, total stress analysis can vary from a fe.w days to severa! months. Often clay strata are larninated with bands of sand or silt which can drarnatically shorten the drainage time which would apply if the whole soil mass were pure clay. In such circumstances a drained, effective stress analysis may well be more appropriate !han an undrained approach for that particular clay layer. For stiff clays, a long-term effective stress analysis will usually be more criticai than the short-term undrained case. For soft clays. the reverse often applies and the short-term total stress calculation can be more criticai !han that for the longer-term. In doubtful cases both drained and undrained analyses should be done and the most criticai used for design.


28

Table 3 Basis cf ca/cu/aticn cf scii pressures

Soil type

Time period Clays after loadlng (coheslve soils)

Short term - undrained - total stress

-c. - water pressures unknown

Long tenn -drained - effective stress -c' and cf~'

- warer pressures known

Granular solls (non·coheslve solls)

- drained - effective stress - f (c'= O) - water pressures known

-drained - effective stress - f (c'= O) - water pressures known

Note: For temporary cofferdams c' is nonnally taken as zero for clays as well as for sands and gravels.

4.1.2 Water pressures

The estimation of water pressures is a very important stage in the analysis of a cofferdam. It is essential for designers to have an understanding of the principles involved and of the strengths and shortcomings of the theoretical methods in relation to practical situations.

The coefficient of permeability of soil, k {m/s), varies over a very wide range of values from l x w-w m/s for practically impervious clays to l m/s for clean gravels. A range of values for various soils is published in BS 8004: Foundations: Figure 6°4

) and in Henry""· Most real sites have several strata of differing soils varying in thicknesses and properties, both with depth and with pian position over the area of the site. Individuai soil layers often have different properties at different points within their mass, i.e. are non-homogeneous and at any one point may no! have the same properties in ali directions, i.e. are anisolropic. For instance, it is common for the bulk permeability of soils to be greater in the horizontal direction !han the vertical.

Where the water table is present at a shallow depth below the soil surface, a large proportion of the load on the active side of a cofferdam wall is due to the water. In a true water cofferdam it will represent 100% of the active load and even in a typicalland-sited cofferdam in granular ground the water load may well be 65-75% of the active load. However, it is important to note that on the passive side of the wall the effect of water pressure forms a much smaller proportion of the overall passive resistance. Additionally, if the water is flowing through the ground, this will affect the value of pore water pressures and may significantly reduce the value of the passive resistance of the soil in front of the toe of the wall and increase the active soil load.

Notwithstanding the difficulties due to soil variability, a study of simple idealised situations which are amènable to calculation can be a valuable basis for judgements on the more complex situations encountered in real design problems.

The aims of an analysis of water pressures are:

to establish the magnitude of the actual water pressures acting on the cofferdam walls and on any undrained soil strata

to enable accurate calculation of vertical and horizontal effective stresses in the soils

to check that the soil formation inside the cofferdam will not fail by 'piping' or 'boiling'

to check that the plug of soil inside the piles below forrnation level will not fail by 'heaving'

to estimate the volume of water inflow so that the necessary pumping capacity can be assessed.


Where the sheet pile toes penetrate a virtually impermeable stratum, e.g. as in Figures 8 al and IO(a), then the pore water pressures on each side of the wall abov~ that_ slratum are eq~ to simple hydrostatic pressures related to the water level on the respechve sides of the wall.

i.e. u w h ere: 'Yw

x

= =

=

"fwX density of water (fresh water 9.81 kN/m')

(salt water 10.00 kN/m3)

depth below water table

Permeable ground

h

d

lmpermeablel ground UL

U ~ Yw(d- i) U ~ Yw(h +d-j)

(a) Gross water pressures

Figure a Water pressures - hydrostatic

The maximum net pore water pressure, u, = y.(h + i - j).

(b) Net water pressures

The situation where there is flow or seepage of water in the ground near th~~sh;:"t piles isf :~~ complicated. Practising engineers usually fmd the 'flow net' t<:<'hni_que an e ecd v; ;a~~orrn calculating the water pressures and of visualising the flow regime ·~ th~ groun · o u ground conditions and uniform permeability, this is done as shown m Figure 9.

1. Draw a true scale cross section (assuming uniform ground permeability).

2. Draw the datum line, either at an actual impermeable boundary or an arbitrary low horizon.

3. Identify the boùndaries of flow:

• extemal water at H, above datum

• internai water at H2 above datum

• axis of symmetry on centreline of cofferdam

• flow lines parallel to the faces of the sheet piles and to the impervious base or datum.

Conslruct a net of 'squares' formed from flow lines and 'equipotentiallines' which intersect 4

' h other at ri ht angles. This is hand sketched by trial and error. S~me pract_ICe IS needed ;: be able 10 dr;w good flow nets, but even simply drawn nets can g1ve suffic~ently ~c~ate results. An equipotential is a line joining ali points for which the water levels m stan -plpes

would rise to the same height H above the datum level.


30

Gravel Sand

H,

Hz

Gravel (very permeable}

Water table

Equipotential

~lines~

Sand

B/2

Datum (impermeable base stratum}

Figure 9 Ffow nel diagram

tt l B/2 Standpipe piezometer

water level a t poi n t A

T

' ' !

l l

A: ~-

~t

__ 1 H

z

'-y--J Head at pointA

To calculate the pore water pressure 'u' - e.g. at point A outside the cofferdam in Figure 9:

l. Calculate the potential head 'H' at point A

Note: The potential drop between successive equipotentials is always the same once the 'square' net is drawn. "

H = Hi - (H1-H2) • .!!.. N d

where n = number of drops to the point under consideration N. = total number of drops

Hence at point 'A' H = H1

- (H1

- H2).~ lO

2. At any point H = .!!.. + z 'Yw

where u = pore water pressure 'Y. = density of water z. = height of the point above datum

as H, 'Y. and z are known, then u can be calculated

u = (H- z).y,.

~ow nets can also be used to estimate the volume of flow of water around the toes of the piles mto the cofferdam. The flow volume 'Q' m3/s per metre run of wall is:


= coefficient of permeability of the ground (m/s) = t o tal head drop (m) = number of flow channels (in half width of cofferdam) (3 in Figure 9) = number of potential drops (lO in Figure 9)

Flow nets can be produced by methods other than hand sketching.

The theory underlying distribution of electrical potentials through conducting media provides an analogy to equipotential values in seepage problems. This is used in the field plotter technique in which the problem is modelled using electrically conducting paper and voltages are applied to represent the hydraulic potentials on the boundaries. Plotting equipotentials using the field plotter is very easy and the method is recommended as a simple and graphic aid to flow net production< 10

• 48J.

A spreadsheet computer program may be used to determine potential heads using the finite difference method, as shown by Williams et af'"·

There are also finite element computer programs which output plots of equipotentials and streamlines. The value of these is the ability to explore sensitivity to varying parameters which can be dane rapidly once the design mode! has been established. These programs also allow problems with stratified soil of diffcrent permeabilities to be analysed.

Having drawn a flow nel it is now possible to calculate the water pressure on the wall, and the effective stresses and earth pressures. The flow net is also used to check the factors of safety against piping and heave (see Section 4.5.3).

The flow of water in effect exerts a hydrodynamic pressure on the soil in addition to the hydrostatic pressure. As the water on the active side of the wall flows downwards towards the pile toe, the friction of the water on the soil will dissipate some of the flow energy, resulting in a downwards fon:e which will increase the effective overburden pressure. Conversely the reverse effect occurs on the passive side of the wall as the water flows upwards towards the free water surface at formation leve!, and the effective overburden pressure and earth pressures are reduced. The effect on the active pressures is, in most cases, small, but the resulting reduction of passive overburden will have a significant, possibly criticai, effect on the passive resistance and bottom stability of a cofferdam.

Figure 10 shows some typical cases for water pressures on cofferdams.

Where it proves impractical to drive piles to a cut-off leve! in clay, orto drive them deep enough to eliminate problems from loss of passive resistance, piping or heave, then pumping or pressure relief wells often have to be used to contro! water pressures. The design of such wells and the procedures to ensure the system cannot fai! due to minor purnp or power supply failures are covered in CIRIA Report 113(23).

When drawing flow nets it is necessary to be very clear on the boundary conditions of the problem, including the location of the source of recharge water. Figure 11 shows some typical situations with and without pumping wells and the generai form of the resulting flow nets.

Various short-cut methods are used for estimating water pressures and these may be adequate for some simple cases and for preliminary designs. When using such methods it is important to appreciate the way the cofferdam width affects the water pressure. Figure 13 shows three cofferdams of different widths but otherwise identica!. As a cofferdam becomes narrower the loss of head becomes more concentrated in the soil plug between the pile walls which causes the water pressure at the toe of the pile and the hydraulic gradients in the soil plug to increase.

CIRIA Report 104''' contains details of a simple approach to the problem based on the assumption that the differential head of water is dissipated uniformly along the length of the flow path adjacent to the wall as in Figure 12(a), assumption l. While this method is


32

s~tisfactory far retaining walls and wide cofferdams, where the seepage pressure is free to dissipate honzontally, through the passive soils, the method leads to optimistic results when the flow pa_th IS reslncted m the bottom of a narrower cofferdam. Figure 13 demonstrates this and c?mpanson of results of Exampl~ 3 in Appendix B shows the danger of using the unifonn dissipatiOn ~ethod. Except far wide cofferdams (width greater than four times the differential water head) Il IS recpmmended that the flow nel method is used.

Sand

Clay

Clay

Sand

(b)

(c)

(d)

)o)

m

- Sheet pii es driven t o lorm 'cut·off' in clay

- Hydrostatic water pressures in sand

- Minimum active pressure in clay should be taken not less than hydrostatic water

- Equal hydrostatic water pressures either side of piles penetrating sand

- Check lactor against uplift of clay 'plug' between piles

- Equa l hydrostatic water pressures either side of piles

- Check factor agairist uplift of concrete plug

- Steady state seepage {analyse by flow nel)

- Equal water pressures only al pile toe

- Check for 'piping' al 'A' - Check factor against

'heave' of plug due to water pressures al pile toe level

- Coarse materia! over fine - Effectively hydrostatic

water pressures in gravel - Analyse steady state

seepage in sand by flownet

- Check lactor offailure

- Fine over coarse materia l is potentially dangerous as head loss concentrates in the passive sand plug

- Jf permeability of coarse more !han 1 O times fine, approx lailure head is ok

- Check piping, heave & loss ol passive resistance

- Consider alternative designs - Rei. Marsland {30)

Figure 1 O Water pressures on cotferdams - typica/ cases


Recharge a t

horizontal su riace

Recharge remote fra m

cofferdam

Pumping from formation in cofferdam

Figure 11 Typica/ cofferdam 1/ow nets

<. l

<. l

i i l i i i

Pumping from external wells

,......,.......

~ ,...

.P\ 1 !r l' ~ (

Another rule of thumb, which gives results more closely matching a flow net analysis far a typical cofferdam, is shown in Figure 12(a), assumption 2. In this approximation the water pressure at the toe of the sheet piles is taken to be the average of the hydrostatic pressures calculated far the active and passive sides. Once again, final designs are always best checked using a flow net analysis.

If, as is often the case, the horizontal penneability kH is greater than the vertical penneability kv, then a corree! solulion far the water heads and pressures can be obtained by sketching a 'square' grid flow net in the usual way but on a cross section in which the horizontal dimensions are modified by a scale factor.

Far instance, if = horizontal dimensions are drawn at approximately 0.5 of the natura! k8 4

scale. As an example, in Figure 13 the result would be to change the pressures and heads far the IO m wide cofferdam (which is drawn far the kv = kH case) to those far the 5 m wide cofferdam. As real soils have significantly different kv and kH values, the importance of making conservative assumptions on water pressures can be appreciated.

Further infonnation and guidance is contained in Tomlinson1'>, Henry146', Harr'"'. McNamee'31}• Marsland130>, King13", Dickin and King'"i, Kaiser and Hewitt13'' and Cedergren110

'.


34

= Assumption (l}

Uniform dissipation of differential head along the flow path adjacent !o the wall

U o 2(d + h - j)(d -i) "(w

2d +h- i- j

= Assumption (2)

Average hydrostatic pressure at toe

_u,+u2 [ (h-i-J)] U - -2- = Yw d + --2 --

Netwater pressure

(a) Uniform ground: alternative simplifying assumptions far calculating water pressures far steady state seepage conditions

Gravel

Fine sand

(b) Layered graun d- high aver low permeability

Figure 12 Water pressures - simpfified methods

20m

Figure 13 Effect of width of a cofferdam on the ffow net

Steady state seepage

Netwater pressure


Tidal effects

Where cofferdams are located near to the shoreline or adjacent tidal reaches of rivers, and at least some of the ground is permeable, the porewater pressure will fluctuate under the influence of tidal variations. Water pressures will vary with every tide and the possible detrimental effects of the resulting load variations on the supporting frarnes and cofferdarn bottom stability should be carefully considered. At depth there may be a lime lag in the variation of pressure compared with the norma! tides. BS 6349°" gives useful advice.

4.1.3 Earth pressures

Earth pressure coefficients are not only dependent on cf the effective angle of shearing resistance, and c' the apparent cohesion, or c;. the undrained shear strength, but also on o the angle of wall friction, and c. the wall adhesion.

The values far cj>' and c' or c. can usually be chosen by reference to the borehole logs, standard penetration tests, and laboratory tests and descriptions.

The value of c' is normally calculated during the course of laboratory tests of clay sarnples, and its value is influenced by many factors including the stress leve! of the test, the rate of strain, the degree of wealhering and the amount of swelling experienced before the test. The values of c'are normally only of the arder of O-IO kN/m2 and there is generally some uncertainty about them. Therefore, unless the engineer is confident about the value of c' it is recommended that c' be taken as zero far the design of temporary cofferdarns, as it can have a significant effect on the design. See CIRIA Report No 104''' far further information.

The values of o and c,, however, musi usually be estimaled by the design engineer, and the values chosen will have a significant effect on the earth pressure coefficients, particularly far the passive case. For temporary steel sheet pile cofferdarns it is recommended that the maximum values of these parameters should not exceed those shown in Table 4.

Table 4 Maximum values for wa/1 friction and adhesion

Actlve

0.67f

Angle of wall friction O

Passive

0.5$'

Note: c' nonnally taken as zero

or but max

Wall adheslon c ..

Active Passive

0.5c' O .Se' O .Se,. 0.5cQ 50 kN/m2 25 kN/m2

Where the toe of the wall penetrates into hard rock the above values should be reduced by 50% far any overlying dense granular materia! or over-consolidated clay. Far overlying loose granular materia! the values should be taken as zero. Far anchor walls which have the freedom to move upwards on mobilisation of the passive pressure then zero values should be taken.

Horizontal soil pressure

In undisturbed ground the horizontal pressure at any depth is the 'al rest' pressure and is

Kocrv' where: Ko = The 'at rest' pressure coefficient

and cr,' = Effective vertical stress

CIRIA Specia/ Publication 95 35

36

For nonnally consolidate<! ground K. = l - sin <jl', but for overconsolidated clays K, may be significantly greater.

Short-term, undrained, tota/ stress analysis

The limiting active and passive pressures acting on the wall at any depth z are given by:

p, = K,a. - K,.,c, Total horizontal pressure active P, = K,a. + J(,.c, Total horizontal pressure passive

w h ere

a. = rz+q 'Y = q =

Total vertical stress Bulk density Any unifonn surcharge on ground surface

K, = K, =

Active pressure coefficient (taken as 1.0 in this case) Passive pressure coefficient (taken as 1.0 in this case)

K" = 2R K,.

c, c.

=

= =

2R Undrained shear strength W ali adhesion

If the active pressure calculated in this way is less than hydrostatic water pressure, then it is in the potential region of a tension cmck which can fili up with water. In Ibis region Ibere can be no wall adhesion so c. must be taken as zero, and the depth of the tension cmck will be (2c, -cj)/y. Within the crack depth the active water pressure will be Y •. z where z = depth from the surface (see Figure 14).

Where the ground surrounding the cofferdam is well protected from the accumulation of surface water so that tension cracks will not fili with water, then, lo take account of any softening of the clay, the total active pressure at any leve! should be assumed to be not less than 5z kN/m2•

This is known as the 'Minimum Equivalent Fluid Pressure' with the density of the equivalent fluid being 5 kN/m'.

For passive pressures it is recommended that a reduction factor is applied to the undrained shear strength, c., to allqw for any generai softening of the clay during the period of the tempomry work. In Londo"n Clay for example a reduction factor of 0.7-0.8 is usual. In addition c, should be taken as zero al excavation leve! and increased linearly to its reduced value at a depth of one metre (see Figure 14).

Long-term, drained, effective stress analysis

Increasing pore water pressure reduces the vertical effective stress in the soil and thus reduces the effective horizontal pressure on the wall.

The active and passive pressures acting on the wall at any depth z are given by:

p/ = Kacrv' - ~cc' Effective horizontal pressure active P,' = K,a; + J(,.c' Effective horizontal pressure passive


where

a; = yz - u + q Effective vertical stress y = B ulk density ( use saturated density y, if below water leve!) u = Pore water pressure q = Any unifonn surcharge at ground surface K, = Active pressure coefficient K, = Passive pressure coefficient

K,., = 2 [K• (l + ~ )]

K,. =

c. = c' =

2~ [K, (l + ~ )] W ali adhesion Effective shear strength

Note, however, that it is recommended that c' (and c.fc') be taken as zero for the design of temporary cofferdams unless the engineer is confident of its value.

The pore water pressure must be added to the effective horizontal pressure to give the total horizontal pressure acting on the wall. See Section 4.1.2 for the calculation of pore water pressures.

i.e. Pa = Pa' + U

Pressure calculations are best carried out in tabular fonn such as in Table 5.

Table 5 Tabular layout far pressure calculations

R.L. Ground Parameters

Depth Total Pore water Vertical Pressures

Effecllve Effective Total Vertical Horizontal Horlzootal

Mlnlmum Equlvalent

Stress Stress Pressure Presso re Fluld Pressure

Effective Stress All ground y .y, cjl~, c~

3. c. K., K,. K,. K,.

Total Stress clay only y c,.c. K •• K,. K,.K,.

z

See exarnples in Appendix B. l

"·

Mixed total and effective stress ana/ysis

u

u

Water Pressure in tension crack only y.,.z

cr,' p."( or p',) p,(or p,) Sz

Active (jv 'K. -K..,.c'

crv- u Pt,' + u Passive

cr,'K, +~.c'

Active larger of

N/A N/A cr,k, - K..,..c;, or 5z

Passive

cr,K, + K,..c,

Mixed total and effective stress design can be appropriate for temporary works design in stiff clays. Effective stresses with full water pressures are used for the active pressures al the back of the wall, al least over the zone of potential tension cracks, i.e. to a depth of (2c, - cj)/y.


38

Total stresses can be used below this zone provided that there is an active pressure of not less than the equivalent minimum fluid pressure, 5z kN/m2• Total stresses are used for the passive pressures in front of the wall using appropriate undrained shear strength values, c" making due allowance for generai softening of the clay. In addition a reduction of c, to zero over in the frrst metre of ground below excavation leve! should be made. CIRIA Report No 104"t discusses this and Figure 14 illustrates it.

Crack depth

W.L.

Earth pressure

Porewater pressure Softened

zone

\-+-- Minimum equivalent

Passive pressure calculated in terms of

total stress assuming eu profile and allowing for

softening in the upper layers of soil

fluid pressure

Active pressure calculated in terms of effective stress aver

crack depth and the total

stress below""'

Figure 14 Pressure diagram for mixed total and effective stress design

4.1.4 Earth pressure coefflclents

Active and passive pressure coefficients, K,. and K,, are given in the char1s in Figures 15 and 16. The coefficients will give horizontal pressures, though the actual pressures are inclined at the angle o to the horizontal.

Values forI<., and K,o may be calculated from the equations given in Section 4.1.3.

4.1.5 Cholce of ground parameters

Wherever possible the various parameters should be based on the results of a site investigation, borehole logs, in situ tests and laboratory tests. Nevertheless there will be occasions when the engineer will have to choose parameters without this infonnation and the following figures and tables will help in this choice. Due allowance should be made for soils likely to deteriorate on exposure where this may affect the strength of the soils in the passive zone.

Bulk Densities Table 6 <jt' for sands and gravels Table 6 and Figure 17 <jt' for clay soils arrd weak rocks Tables 7 and 8 c, for clay soils Table 6 K,. and JC. Figures 15 and 16 K,., and K,o Calcolate from equations (Section 4.1.3)


Angle of shearing resistance, cjl'(degrees)

Figure 15 Coefficients of active earth pressure (horizontal component) for horizontal retained surface (after Caquot an d Kerisef'!)

Table 6 Typical ground parameters

3 o

o /

5 //

o ~ ~

2

8 ,/ 6 l / 5

/h / / 4

l d-~ ........ -

~~ .......... i-l

1.

10 15 20 25 30 35 40 45

Angle of shearing resistance, ~'(degrees)

Figure 16 Coefficients of passive earth pressure (horizontal component) for horizontal retained surface (after Caquot and Kerisef''J

Saturated Type of materlal

Gravel W eli graded sand & gravel Coarse or medium sand W eli graded sand Fine or silty sand Rock fill & quany waste Brick hardcore S1ag fili Ash fill

Topsoil Rivermud Silt Peat

Very soft day Soft day Soft to finn day Finn day Finn to stiff day Stiff day Very stiff day or hard clay

"' See Table 7


Bulk Unlt welght

y kN/m'

Loose Dense

16.0 18.0 19.0 21.0 16.5 18.5 18.0 21.0 17.0 19.0 15.0 17.5 13.0 17.5 12.0 15.0 6.5 10.0

16.0 19.0 14.5 17.5

18.0 12.0

16.0 17.0 17.5 18.0 18.5 19.0 20.0·21.0

Bulk Unlt weight

y, kN/m3

Loose Dense

20.0 21.0 21.5 23.0 20.0 21.5 20.5 22.5 20.0 21.5 19.5 21.0 16.5 19.0 18.0 20.0 13.0 15.0

20.0 21.0 19.0 20.0

18.0 12.0

16.0 17.0 17.5 18.0 18.5 19.0 20.0-21.0

Effectlve stress

~-Degrees

Loose Dense

35 40 35 40 35 40 35 40 30 35 40 45 40 45 30 35 35 40

25 5-10 25 15

• • • • • • •

Total stress Coheslon

c,. kN/m'

<.W 20-40 40-50 50-75 75-100 100-150 > 150

39

sw 1.00

0.67

0.58 0.33

0.00

40

Very loose Loose

E E

\ / Medium dense

g o

~ 10

~ - 20 ~ 25- 30 c m

'" ·~ 40 c o 1 50

~ 60

~ "O

'""'~

.......... ..........

""' ""'

Dense Very dense r----"'

Ì'..

" 1'-.

"' ~

70 28 30 32 34 36 38 40 42

Angle of shearing resistance, <fl '(degrees)

44

Figure 17 Estimation of$' for sands and grave/s (alter Peck, Hanson and Thornburni25!)

Table 7 Estimation cf $' far Table 8 Estimation cf $' far weak rocks cohasive soils (!rom Draft BS 80021"1)

Plasticlty ~· Stratum r lndex

Chalk 35° 15 30° Oayey mari zgo 20 zgo Sandy marl 33° 25 27° Weak sandstone 42° 30 25° Weak siltstone 35° 40 22° Weak mudstone zgo 50 20° 80 15°

These values may he used for preliminary calculations. Fina! designs should be based on inforrnation from site investigations. Note lhat on the passive side it is unsafe to overestimate the unit weight of the soil.

Cohesive soi(s

The value of the angle of shearing resistance •l>' of a cohesive materia! is dependent on its mineralogy and is influenced by small percentages of silt or sand homogeneously mixed with the clay-sized particles.

Where test results for <j>' are not available, Table 7 may be used, and c' taken as zero, to obtain a conservative design.

Clays containing veins or seams of sand or silt will exhibit lower plasticity indices !han the clay itself if sarnples containing such seams are remoulded for the plasticity index tests. Care should he taken to carry out the tests on lhe clay alone. If there are doubts lhen lhe above table should he used wilh a higher plasticity index than recorded in the tests.


Weak rocks

Table 8 gives indicative values of the effective angle of friction related to rocks which can conservatively he treated as composed of granular fragments, i.e. they are closely and randomly jointed or olherwise fractured. (Rock Quality Designation near zero.)

The presence of a preferred orientation of joints, bedding or cleavage in a direction near that of a possible failure piane may require a reduction in the above values, especially if lhe discontinuities are filled wilh weaker materials.

4.1.6 Layered ground

At lhe leve! of lhe interface hetween different ground layers there will he a change in the calculated pressure acting on the wall, according to the pressure coefficients used for the two layers. Unless lhe ground stiffness at the interface changes greatly it is unlikely lhat there will actually he a sudden alteration in pressure, but for practical design purposes it is satisfactory to use a pressure diagrarn so calculated. See Appendix B, Exarnple 4, which gives calculations for pressures with multi-layered ground.

4.1.7 Sloplng ground surface

Where the ground surface slopes from lhe face of lhe piles the soil pressures will not he lhose which would apply if the surface was horizontal. There is no precise method available for evaluation of lhe pressures, particularly when dealing wilh layered soils, but lhe following methods may be used.

Method A:

For ground which slopes directly from the wall, lhe coefficients of active and passive pressure K,. and ](, should he modified.

Figures 18 and 19 show a series of charts which give values of K, and ](, for vertical walls and varying ground slopes.


(a) o~ O (b) o~~·

Figure 18 Coefficients of active earth pressure (horizontal component) for genera/ case on inclined backfi/1 with wa/1 friction (alter Caquot and Kerisef'1)


42

"'-"

" -~

"' ~ o u ~ t ro ~

~ > ·;n w ro a.

5 4

3

2

o l

o- '"1\j,., -"'~-- L _ -ìlr+veL _ V

1

o-f- df:-c :S ~-- -o . -- )- --5 t _j -T'v o ·-· . +-- t-71--7'--

- ____:_ - . ...... -~ -

-·-

;2-5 / /7 .... _ -

--

V/ -3 -~ v--- ........ _ f.-- ....

21b-" 1. 5 -- -

1 -10 15 20 25 30 35 40 45


(a) o~ O

PW 1.0

0.5

o

-0.2

0.4

-0.6

10 o 8 6

)<" 4

c' ~ 2 ·u

"' ~ o u ~ t ro ~

~ > w w ro a.

1.

o_~·~

~ o r ----o ~(+ve)

/ -T --+ 0 ___ o(+ve) i /

5- _-- -;, l / o .-' i/ v -

.L-8- - z

-""t;/ 6~ ~ - =· é.? c.

4f-- /

~ rs v v ---2.6 :--t-

5~ c--- - --

0.:

o

o

o

O.

1 O. 10 15 20 25 30 35 40 45

Angle of shearing resistance, <fl'(degrees)

(b) o~ ~'/3

15

10 8 6

o , Il W 1.0

p /q ,-,--,--,--,---,-----,---L 1.0

O te o 0 = f-.::::: ]l( ~ve)-~

"'-" 4 o-bi-c' ~

t ~

i g; ·~ a.

- . _'i+~=~:: 2 ~-- ,..V-o-8 ,c- -

f--4f-- -/.

:~::;-:: 1.

1

/

--

-r-- =F-

~t=--~ ~ -

0.5 )<"

c' ~ ·u

-/-/ /

o "' ~ o u

0.2 ~ t ro ~

0.4 ~

-~

P'- -= w w

-0.6 ro a. -

-0.8

10 15 20 25 30 35 40 45 25 30 35 40 45

Angle of shearing resistance, q, '(degrees) Angle of shearing resistance, <fl'(degrees)

(c) ò ~ ~'/2

o 30 20 o 15 o

o ~n ~~ 6 or---o

4 or-- -

2 1 o 5

10 8 6 4 3

f---

~ft

""' ~(+ve)

o(+ve)

IY

-

(d) 8~2~'/3

~ l -

~·-

= ce- f·c-- -c::: c---~ '/

/ / /

-

~ T -

/ /

........

~w 1.00

0.50

0.00

0.20

0.40

0.60

2 0.80 :~ 1.5 1 IO 15 20 25 30 35 40 45

Ang!e of shearing resistance, ~'(degrees)

(e) o~~·

Figure 19 Coefficients of passive earth pressure (horizontal componenlj !or genera/ case of inclined backli/1 with wa/1 friction (alter Caquot and Kerisef''!


o

It should be noted that these modified coefficients asswne that the sloping ground continues at that slope unti! it is beyond the potential slip planes taken from the toe of the pile, i.e. approximately at an angle of (45° + <j>'/2) from the horizontal for the active case and (45° -<j>'/2) for the passive case.

If multi-layered ground is being considered then modified coefficients should be used for every layer, sin ce the effect of the slope will be felt over the full length of the pile.

Method B:

The graphical method shown in Appendix B Example 4 may be used. This method is applicable to ground which has non-uniform slopes and is multi-layered, and to both cohesive and cohesionless soils using effective stress parameters-

Metbod C:

The latera! pressure on the wall due to a sloping ground surface or a berm can be calculated by the following simple method for cohesionless soils. This method can be used for spoil heaps.

On the active side of the wall, as shown in Figure 20, the latera! pressure on wall at A is zero, at B is y h,K. and at C is y h2K,.

45" + ~/2 -c -~------

Figure 20 S/oping ground-pressure diagram on active side of wa/1

On the passive side of the wall, as shown in Figure 21, the latera! pressure on the wall al A is zero, at B is y h1K, and at C is y h2K,.

A check should also be made for sliding on the horizontal piane at the base of the berm, especially if the berm has a higher angle of friction than the soil beneath. Ifa grave! berm overlies stiff clay it is probably not appropriate to use the undrained strength of the clay in checking sliding on the interface.


44

G.L.

--c

Figure 21 S/oping ground-pressure diagram on passive side of wa/1

4.1.8 Check llst for calculatlng actlve and passive pressures

l. Calculate the total overburden pressure a. for the active side of the wall, including any surcharge loading, at each soil stratum interface and at any other significant leve! where a change in slope of the pressure diagram will occur, e.g. excavation or ground water levels. Similarly for the passive side of the wall, but surcharge from live loading should never be taken on this side. Use bulk or saturated unit weights as appropriate.

2. Establish the water pressures at the various levels for both active and passive sides, from steady state seepage or static conditions as appropriate.

3. Calculate the effective vertical pressures, a.'. by deducting the pore water pressure, u, from the total overburden pressure, a •.

4. Using the appropriate coefficients convert the overburden pressure to a horizontal pressure for each ground stratum at each interface and al the other significant levels. Adjust if necessary to allow for the effect of any berrns or other loading on the wall not accounted for in the surcharge loading in l above (see Section 4.2).

5. Plot diagrams of gross active and passive earth pressures, and net water pressures.

6. Depending on the particular method of analysis to be used (see Section 4.3), construct the appropriate modified pressure diagrams.

4.1.9 Varlation of ground pressure due to wall flexlblllty

In steel sheet pile temporary cofferdarns it is commonly assumed for design that sufficient movement occurs due to the construction sequence and wall flexibility to allow the full shear strength of the ground to be mobilised and hence the ground pressures to be reduced to active values. Similarly, full passive resistance is usually assumed to be mobilised.


In certain circumstances these assumptions may not be realised. Earth pressures may be higher than the limiting active pressure if the cofferdarn is constructed in such a way that the piles are reslrained from deflecting to the norma! degree. This might occur when the piles are supported by prestressed anchors, or exceptionally stiff walings and struts preloaded by jacking or with stiff walls in overconsolidated clay (i.e. K, is high). If the toes of the piles are embedded in a particularly stiff stratum, such as rock, the length of pile irnmediately above the stiff stratum will not be able to deflect freely and the passive resistance of the overlying soil will not be fully mobilised.

Deflection of the pile may cause arching of the ground and thus bring about a redistribution of active pressure, transferring some of the pressure at mid-pile span to the waling leve! and the pile toe. This action is directly related to the stiffness of the pile section - the more flexible the pile section, the greater the reduction of pressure and bending moment at mid span. Rowe's moment reduction method'., is explained by Clayton and Milititsky''>, although in practice the effect is usually ignored. It is important to note that water pressures are not subject to redistribution, only the ground pressures.

4.1.1 o Earthquake loads

In some regions of the world earthquake shocks can have severe effects on cofferdarn structures, not only in terrns of increased active pressures and decreased passive resistance due to direct effect of the associate<! horizontal accelerations (vertical accelerations are usually ignored), but also by the indirect effect on the actual strength of some soils. Fine grained submerged soils can momentarily lose ali their shear strength due to the sudden increase in porewater pressure generated by the passage of the shock wave. This is referred to as liquefaction. The Kawasaki Steel Sheet Piling Design Manual gives details for design for earthquake conditions in Japan"'·

4.2 OTHER LOADS ON COFFERDAM DURING CONSTRUCTION ANO USE

The most likely additional loads are those from construction plani, spoil heaps and adjacent structures. These can usually be represented by a uniforrn surcharge or by concentrated or linear surcharges in various combinations. No surcharge should be allowed on the passive side of the wall for a temporary cofferdarn.

4.2.1 Constructlon plant

A minimum uniforrn surcharge of IO kNfm2 should be used to allow for plant up to 30 tonne loaded weight. This can be increased to 20 kN/m2 for plant up to 60 tonne loaded weight.

Larger items of plant, however, can be considered individually by representing the loads from tracks and wheels as concentrate<! surcharges. This approach allows for the dissipation of loading within the ground as the depth increases, which does not occur with a uniforrn surcharge.

Working cranes and excavators can give heavy loads on one side of the machine, since a driver may load the crane to the point of overturning (although this is strictly against regulations). For tracked cranes it is suggested that the total weight of the crane + lifting tackle + heaviest load to be lifted be taken on one track with a uniforrn loading on the bearing length of the track. For lorry-mounted or wheeled cranes and trucks the manufacturer's figures for the loadingsshould be used.

4.2.2 Spoil heaps

A spoil heap, if it is not of great size, can be represented by a concentrated surcharge. Otherwise a uniforrn surcharge equivalent to the height of the spoil heap times its unit weight should be used. If the spoil heap is set back from the wall the method shown in Figure 20 should be used.


46

4.2.3 Adjacent structures

These can he represented by surcharge loads at the appropriate depths. Loads from wall foolings can he represented by a linear surcharge if they are parallel to the cofferdarn wall, or by a concentrated surcharge if norma! to it. Column footings will be represented by concentrated surcharges.

If various combinations of additional loads are possible, then care should he taken to check the cofferdarn design for the worst combination of load at each stage.

4.2.4 Other loads

Cofferdarns may he constructed adjacent to railways or machines which give rise to vibrations. In these cases the wall friction may he destroyed and the values of o and c. should he taken as zero, even then earth pressures may exceed active values. Arching of the ground (see Section 4.1.9) should not he allowed.

Where ground movement could fracture water mains behind the wall consideration musi he given to the water pressures that might occur.

4.2.5 Lateral pressure on the wall due to loads other than unlform surcharge

For granular soils Krey's simple method (British Steel Piling Handbook, page D8(7)) has heen successfully used for surcharge loads of limited area:

Let: Qc = Concentrated load in kN or QL = Line load in kN/m run of wall.

Fora concentrated load the equivalent line load Ot = Qd(2A +L) kN/m run of wall (see Figure 22) which is effective over the length of wall (2A + L) metres.

Line load QL

Concentrated load Oc {kN) run of wall)

Figure 22 Concentrated and fine load surcharges

Fora line load (or equivalent line load as above) the totallateral force on the wall due to 0c is:

P. = QL . .[K. /clf/m run of wall


The pressure diagram (see Figure 23) for this force is triangular over the depth D where:

2P p = -• lcN/m2

R D

It should he added to the earth pressure diagram.

The resulting pressure diagrarn is, of course, simplistic and the values approximate. As the width B increases the pressures become more conservative and a check should he made to see if an equivalent uniform surcharge would he more economica!.

For situations where the pressure from limited area surcharges need to he calculated more accurately there are various other methods of calculating the latera! pressure distribution on the cofferdarn wall due to concentrated or linear surcharges as outlined in the CIRIA Report 104''' (Section 5.7.2). Terzaghi modified Boussinesq's elastic theory to match field and mode! results to give relatively simple formulae for the latera! pressures, and details are given in the CIRIA Report.

Clayton and Milititsky'" (Chapter 5) give an interesting solution using Boussinesq's theory to give the increase in vertical or overburden pressure at any leve!, which can then he converted to latera! pressure using the relevant earth pressure coefficients for differing ground strata. However, it is pointed out in Soil-Structure Interaction'"' that this approach may in some cases be unsafe.

The latera! stress is very sensitive to the variation, with depth, of the soil stiffness. Pappin et al.',., give a method to suit these circumstances.

D

45" + ,ì/2

Figure 23 Pressure diagram for a fine /oad

For most situations, particularly where water pressures are present, it is not criticai which method is used for estimating the effect of surcharges as long as a reasonable provision is m ade.

4.3 ANALYSIS OF WALL

4.3.1 Methods of analysls {cantilever or single prop walls)

This section considers four of the methods of analysis in current use for determining the required depth of penetration. Each can be used with cantilever walls, or with walls propped at or near the top and having either free or fixed earth conditions at the toe (see Section 4.3.5).


48

When the penetration of the pile is such that the Factor of Safety on overall rotational stability is 1.0 then the condition is known as the 'Limiting Equilibrium Condition' (LEC). Ali methods give the same penetration at this condition. The prop load and maximum bending moment must also be calculated at LEC and the values will also be the same for ali methods.

To calculate the design penetration an appropriate Factor of Safety is applied to the method being used.

Note that no factor is applied to the water pressures. If the water levels vary, sue h as in ti dal, river or seasonal conditions, then conservative levels should be chosen to give the largest net active water pressure.

It should be bome in mind that the soil parameters chosen for use in any analysis will never be precise. The analysis should therefore take account of the sensitivity of the resulting solutions to likely variations both in assumed and actual soil parameters (see Section 4.3.6). Also note that however carefully now net or other solutions are made the resulting water pressures are only an approximation to the rea! conditions in the ground. Effects of varying ground permeabilities can give rise to different water pressure distributions. Furthermore calculated factors of safety can suffer big changes for apparently minor variations in the water pressure diagrams. In sensitive cases it can be prudent to alter the basic cofferdam design in arder to eliminate such risks.

4.3.2 Strength factored

In this method the Factor of Safety is applied to the strength parameters of the ground.

Strength factored method (Factor of Safety = F,)

The factored parameters are:

Effective stress

!an <P' m = tancp'/F, c'm = c'/Fs

an d lì.Jcpm' = lì/cp' c' wuJc' m = c.jc'

Total stress

The factored strength parameters are then used to select factored earth pressure coefficients for the production of factored active and passive pressure diagrams. The effect is to increase the active and reduce the passive earth pressures as the Factor of Safety (F,) is increased. The analysis is then carried out using the earth pressure diagrams based on the factored parameters to find the penetration, which is at the leve! where:

Moment of factored passive earth pressure

Moment of = factored active

earth pressure

Moment of + net active

water pressure

Bending movements and prop loads derived from the factored pressure diagrams can be used for the design of the wall if they are treated as ultimate limit state values.

4.3.3 Moments factored

In these methods the Factor of Safety is applied to the moments of pressure.

Gross pressures method (Factor of Safety = F,)

This is the method described in CP21'>.


,l

The active and passive pressure diagrams in Figure 24 are not modified, so the analysis is carried aut using the gross pressures to find the design penetration which is at the leve! where:

Moment of Moment of Moment of

passive earth + F,= active earth + net active

pressure pressure water pressure

Earth Net water

Passive Active Active

F,

Figure 24 Gross pressures method- pressure diagrams

Far total stress analysis this method can give rise to the value of F, rising to a peak and then decreasing as the penetration increases. This is clearly not logica! and if the method is used it may be uneconomic.

N et pressure method (Factor of Safety = F.,J

This method is used in the British Steel Piling Handbook(7>.

The active and passive pressure diagrams are modified to produce net pressures, i.e. the passive pressures are subtracted from the active pressures - the remaining positive values become the net active pressures and the negative values become the nel passive pressures, as shown in Figure 25.

Passive Active

Figure 25 Net pressure method - pressure diagrams


50

The analysis is then carried out using these net pressures to find the design penelration which is at the leve! where:

Moment of Moment of nel passive (earth + water pressures) + F"" = nel active (earth + water pressures)

This method has bèen used successfully for more than 50 years or so and is stili used widely for the design of cofferdams. It has been suggested that its success over the years has been due to the use of conservative values for the strength pararneters of the ground. Provided strength pararneters are chosen so as to provide a satisfactory margin of safety, the method can be used for predominantly granular soils. It is not recommended for cohesive soils (see CIRIA Report 104''').

Burland-Potts metlwd (Factor of Safety = F,)

This method was developed by Burland, Potts and Walsh'"' who were looking for a method which gave consislenl results while avoiding some of lhe illogicalities of lhe other methods.

In lhis melhod lhe moment of lhe net available passive resistance is factored.

When using effective stresses with c'= O and where there is no surcharge on the passive side of the wall (which applies to temporary cofferdams) the active earth pressure diagrarn in Figure 26 is modified by altering the pressure at any leve! below excavation to be equa! to the pressure at excavation leve!, i.e. the pressure diagmm becomes a vertical line below the excavation leve! (when the soil is consistent).

Earth

Passive Active

Figure 26 Bur/and-Potts method- pressure diagrams

Net water

Active

The passive earth pressure diagmm is modified by deducting the difference between lhe gross and modified active pressures from lhe gross passive pressure al any leve!.

The diagram shows these modificalions for a uniform ground where an area equa! to the shaded area of lhe active diagram is deducted from lhe passive diagrarn to give the modified aclive and passive diagrarns (unshaded).

Where a value for c' is used and for lotal stress conditions reference should be made lo the paper by Burland, Polts and Walsh""· With total stress conditions any waler above excavalion leve! musi be trealed as a surcharge.


The analysis is carried out using the modified pressures to find the design penetration which is at the leve! where:

Moment of modified passive earth pressure

Moment of + F,. modified active

earth pressures

Moment of + nel active

water pressures

This method is relatively new and there is not a lot of experience in its use. It should nevertheless give consistent and satisfactory results. As experience is gained it is expected that lhis method will become widely used.

4.3.4 Cholce of method of analysls

The choice of ground pararneters will have as great an effect on the depth of penetration as the choice of method of analysis. Because of the different mathematical approach of each method it follows that different values for the factors of safety will be required for each method to achieve a consistent penetration depth.

Water in the ground affects the pressures considerably. In average granular materia! below water leve! only some 25-30% of the increase in total active pressure is due to the ground, and the passive earth pressure is reduced by some 40% when it is below water leve!. If there is any water seepage around the toe of the wall then this can also have a significant effect on the design of the wall. The effect of water pressures is common to ali methods of analysis.

The engineer shouÌd choose whichever method of analysis is used regularly by colleagues/organisation etc, subject to the comments given above for each method since this aids the checking process. It is preferable to check the design by using an alternative method of analysis.

4.3.5 Design for free or flxed earth support condltlons

Where the penetration is large enough to give stability without being sufficient to prevent rotation at the toe of the pile, the situation is described as 'free earth' suppor! condition. Where the penetration is enough to prevent rotation at the toe then the condition is described as 'fixed earth'. These two conditions are illustrated in Figures 28 and 29.

A propped wall depends for stability on the prop at or near lhe top and the passive resistance of lhe ground in front of the pile where it penetrates below excavation leve!. It can be designed using either free earth or fixed earth conditions.

A cantilever wall, however, depends entirely on the suppor! of the penetrated ground for its stability and could noi be in equilibrium withoul lhe toe being prevented from rotating. It musi lherefore be designed with 'fixed earth' suppor! condilions. Figure 27 shows lhe idealised pressure distribution al limiting conditions.

For lhe design of bolh propped and canlilever sheel pile walls il is assumed lhat there will be enough movement'of the ground lo allow full aclive and passive pressures to be generated al limiling conditions.

For propped walls the fixed earlh suppor! requires a grealer penetralion lhan lhe free earth case lo give the fixily, bui has a lower maximum bending momenl and prop force when compared with lhe free earth suppor!. If hard driving is expecled then lhe free earth suppor! may well provide the more economica! solution as it will need a stronger pile which will be of shorter lenglh.

Calculations lo find lhe necessary penetration for fixed earth suppor! follow from the worlc of Blum'"· 33

· "'· They are complicated and tedious, and have become known as the 'full method'. Details can be found in standard text books''· "'· However, il is usual to make simplifying assumptions to reduce the complexity of the calculations.


52

A

d

(a) (b)

Cross-section ldealised pressure diagram

Figure 27 Analysis of cantilever wa/1

Cantilever walls

(c) (d)

Shear Bending moment

(e)

Deflection

A

D

ID Simplified pressure

diagram

Figure 27(f) shows a simplified pressure diagram where the pressures at the toe of the pile have been replaced by a resultant force F3 at C some distance above the toe. The forces F1 and F2 acl through the centres of gravity of their respective areas.

The depth BC is found by assuming a leve! for C and calculating the moments for the forces F1

and F2 about leve! C. This is repeated unti! the moments are in balance.

To correct the error caused by the use of the 'simplified method' the depth BC should be increased by 20% to give the design penetration BD.

The maximum bending moment occurs at the point of zero shear at leve! X-X.

Propped wal/s with fixed earth suppor/

As with cantilever walls the calculations follow from the work of Blum132•

33•

341 and the

'full method' is complicated and tedious. However, Blum proposed a further simplification called the 'equivalent beam method'.

This method uses the same assumption as in the cantilever case, that the forces at the toe of the pile are replaced by a resultant force F3 some distance above the toe, additionally it is assumed that the point of zero bending moment (leve! Y-Y) occurs at the leve! where the active and passive pressures balance i.e. the net pressure is zero.

Figure 28(b) 'shows the idealised pressure diagrarn and the forces on the wall, T the prop Ioad and F1, F2 & F3 acting through the centres of gravity of their respective areas.

The simplified pressure diagram is shown in Figure 28(f) where the force F3 acts at the leve! of C. The prop load T can then be found by taking moments about and above leve! Y-Y (assumed point of zero bending moment). Finally the depth of C can be found by assuming its leve! and calculating the moments about and above this leve!. This is repeated unti! the moments balance.

As with the cantilever wall, to correct the error caused by the use of the simplified method the depth BC is increased by 20% to give the design penetration BD.

The maximum bending moment in the pile occurs at leve! X-X, the point of zero shear.


(a) Cross-section

A

(b)

ldealised pressure diagram

(c)

Shear

(d) (e)

Bending Deflection moment

Figure 28 Analysis of propped wa/1 with fixed earth support

Free earth suppor/

A

D

ID Simplified pressure

diagram

Figure 29(b) shows the forces on the wall, T the prop load, F1 and F2 which act through the centres of gravity of their respective areas.

A

[ F,

D

(a) (b)

Cross-section Pressure diagram

(c)

Shear

Figure 29 Ana/ysis of propped wa/1 with free earth support

(d)

Bending moment

(e)

Deflection

To calculate the penetration BD at the limiting equilibrium condition the depth of D is assumed and moments of the forces F1 and F2 are calculated about the leve! of the prop T. This is repeated until the moments balance.

The prop load T can then be found by balancing the forces, i.e.:

T= F1 - F2

and the maximum bending moment in the pile will be at the leve! of zero shear X-X.

Genera/

Where there is water present in the ground then the active pressures shown in Figures 27, 28 and 29 must include the net water pressures.


54

To calculate the design penetration for cantilever and free earth suppor! the pressure diagrams should be modified as required by the chosen method of analysis (see Sections 4.3.2 and 4.3.3) and factor of safety (see Section 4.4).

Note that with total stress ground c?nditions the net pressure method of wall analysis (F "") is not recommended for use (see Sectlon 4.4).

For fixed earth suppor! the design penetration should be calculated at limiting equilibrium conditions. The penetration will be greater than that for free earth suppor! in the sarne situation and will give an adequate factor of safety against failure by rotation about the prop.

For cantilever and both free and fixed earth suppor! the calculations to find the working load in the prop and the maximum bending moment in the pile for use with working stresses should always be made at limiting equilibrium conditions (i.e. with a factor of safety of 1.0).

The simplified form of analysis for cantilever or propped walls with fixed earth suppor! conditions assumes that the pile penetrates into ground of uniform strength. Where there is a reduction in the ground strength near the toe of the wall the extra 20% of penetration allowed in the analysis to provide for the toe force F3 may not be adequate and the analysis should be carried out by the 'full method' or, if the wall is propped, the free earth analysis could be used instead.

Caution should be observed when designing cantilever walls in soft clays as unacceptable deflections can occur. Whatever the ground, cantilever walls are likely to move to a greater extent than propped walls, and if there are vulnerable services or buildings nearby then careful monitoring will be necessary. In some circumstances it would be prudent to have available on site materials that could be used to provide some temporary props.

4.3.6 Use of computers far design

There are a number of programmes on sheet pile design available for purchase from software suppliers, and many contractors and consulting engineers have developed their own in-house programmes. Some carry out equilibrium and factor of safety calculations in accordance with one or more of the recommended methods described above, while others divide the wall into a number of elements and attempt to mode! the interaction of soil and wall for each of these elements to determine the structural loading and magnitude and distribution of ground movements.

This latter category requires the input of additional soil pararneters (particularly the coefficient of horizontal subgrade reaction and the coefficient of earth pressure at rest) which are not easily determined and usually are estimated.

The use of a computer programme is encouraged subject to the following:

the programme should be subject to detailed validation before generai use

the output must include ali the input data necessary to carry out an independent check

use of the prograrnme must be under the direct supervision of an experienced engineer.

The great benefit in the use of a computer prograrnme is in the time saved. With a typical programme it will take a few minutes to type in the data, and only a few seconds to calculate the results, whereas the hand calculations would take some hours depending on the experience of the engineer. This enables a number of analyses to be made in a relatively short tirne, and thus the sensitivity of the structure to changes in ground conditions. water levels and prop levels can be assessed. It is advisable to carry out an independent manual check on the final computer analysis for ali but the most routine designs. It is recommended that true scale pressure diagrarns showing ground strata, water, excavation and strut levels are drawn by hand if they are not part of the output from the programme.

CIRIA Special Publlcation 95

!t must be emphasised that the use of a computer programme does not !essen the need for the design process to be under the direct supervision of an experienced engineer. The computer is only a tool which, in the right hands, gives understanding of the way in which the ground and the wall interact.

4.3.7 Multl-prop walls

Analysis of walls with more than one level of props is complicated by the complex soil/structure interaction arising from the construction sequence. The ground is usually excavated in stages and the props are introduced at each leve!. This modifies the behaviour of the surrounding ground so that the classical active state cannot develop. Consequently it is necessary to use engineering judgement and make simplifying assumptions.

The major concern is to assess the strut loads at each leve! of props. Experience has shown that most failures occur due to overload in the props, often accompanied by local web failures of the walings if they are not adequately stiffened. Failures by bending of the walls or walings are rare, and are, in any case, more easily monitored during regolar inspections.

The positions of struts are usually selected to prevent excessive deflections during construction of the cofferdam and to suit the construction sequence for the works within it. Therefore, it is sensible to check the overall stability of the cofferdam and its base, before proceeding with the more detailed analysis of the walls.

There are three methods commonly used for analysis.

Stage-by-stage method

The draft BS 8002'12' suggests that the most suitable method of analysing for bending moments

and prop loads is by successive analysis of each stage of construction. This is done using a pressure diagrarn, based on a classica! pressure distribution. Consideration should be given to increasing the calculated prop loads to allow for arching.

The British Steel Piling Handbook''' gives an exarnple of this method. Hinges in the wall are assumed at ali prop levels except the first, the spans between the frames are designed as sirnply supported bearns loaded with the active earth and water pressures and the span between the lower frame and excavation leve! is designed as a single propped wall with its appropriate active and passive earth and water pressures. Prop loads include the respective load from adjacent spans. The wall must be analysed al each stage in the cofferdam construction sequence, for example:

Stage l - excavate to depth allowing erection of 1st leve! props or other suppor!, i.e. cantilever wall.

Stage 2 - continue excavating to allow erection of 2nd leve! props or other suppor!, i.e. single-prop wall.

Stage 3 - further excavation to allow erection of 3rd leve! of props or other suppor!, i.e. double-prop wall.

And so on, including backfilling and removal of, or alterations to, the leve! of any props as the construction of the permanent work proceeds (see Figure 30).

The prop loads and bending moments during the earlier stages may be greater than those at later stages and should be used for the design of the various members. Also the effects of repropping during dismantling should be checked as in ali methods.

A check list is given in Section 4.8.5.


56

C!RIA Report 104 Method

Strictly, this method was proposed for use in stiff clays but is likely to yield safe results with other soils if the assumptions are met. The major assumption is that the wall will move sufficiently to allow active conditions to develop behind the wall. (Note: this is more likely to occur with raking than with horizontal struts).

The resultant of two or more propping forces is taken as acting at their centre of action and a calculation performed as if there were only one leve! of props. The load is then shared between the actual props taking account of the increased load which may act temporarily on tbe upper props prior to installation of the lower ones. The calculated prop loads should be increased by 25% (upper leve!) and 15% (lower leve!) to allow for the unquantified effects of pressure redistribution because of yield of the wall. In some circumstances, the lower prop may take a much larger share of the load, e.g. when base failure is imminent in clays.

Stage 1 Stage 2 Stage 3 Stage 4

ìiHR1J Drives piles lnstall frame (l) lnstall frame (2) lnstall frame (3) Excavateto Excavateto Excavate to Excavateto below frame (1) below frame (2) below frame (3) finallevel le v el leve! leve!

Cantilever Single prop Multiprop Multiprop

~ ~ --Cantilever Singleprop Doubleprop Tripleprop (fixed earth (fixed earth (fixed or free (freeearth

support) support probably) earth support) support)

Figure 30 Construction of multi·prop walls

Pressure envelope methods

Successive stages in analysis of multiprop cofferdam.

(Excavation stages only shown. Also consider building

and strut remava!)

L1= Jss bea m

LJ= ps bea m

~]Single prop free earth

Typical multiple support

prop analysis affinai stage

These are empirica! methods which use the loads actually measured in struts to construct an envelope for the horizontal earth pressure at the back of the wall. This envelope, which is usually trapezoidal, allows for the variation in strut loads that occur in practice. Measurements show that the maximum strut load can occur at different levels of the excavation varying with time, and it can exceed the average strut load by 33% or more. The totalload represented by the area of the trapezoidal diagrarn is larger than the area of the norma! Rankine pressure diagrarn by a factor which varies from about 1.3 to 1.75 depending on the type of ground.


The most frequently used envelopes are those developed by Terzaghi and Peck which were subsequently modified by Peck0 ". For granular and mixed soils any water pressures and/or pressures due to surcharges should be added nel to give the design envelope which can be used to calculate the prop loads (see Figures 31 and 32).

It should be noted that no allowance should be made for continuity of the wall when calculating the strut loads. Using the ground pressure envelope each load is simply the summation of the appropriate tributary area of the diagrarn. This load is the maximum likely to occur from ground pressure and no additions are required to allow for arching. However, engineering judgement will be required when assessing the additional loads arising from water and live load effects.

Draft BS 8002°2' does not recommend the use of the pressure envelope for the calculation of

bending moments in the sheet piles and suggests that these should be found by the successive analysis of stages of construction as above.

Clayton and Milititsky'", however, suggest that the wall units can be designed using two-thirds of the Terzaghi and Peck earth pressure envelope. In this case engineering judgement should be used to decide if tlÌe sheet piles are continuous over the supports (WL/10) or simply supported (WL/8). The span below the lowest prop can be designed as a cantilever supported by the lowest frame, or as a propped wall designed as the last excavation stage, as in the successive

~1'\l ~~ -[=1 l

l \ 0.65K"y~ l \ 1-----~

KayH

l l ( • l 0.4yH

Equivalent Rankine active

PRESSURE DISTRIBUTION

(a) Sands K0~ lan'(45- ~'/2)

~ (1 - s1n ~)/(1 + sin~) Add groundwater pressures where groundwater is above the base of the excavation

(b) Soft to medium clays• (N> 5-6) K, ~ 1 - m(4c/yH) ~ 1 - (4/N) m = 1.0 except where cut is underlain by deep soft normally consolidated clay, when m= 0.4

(c) Stiff clays' Far N< 4(for 4 < N< 6 use the larger of diagrams (b) and (c))

TOTALFORCE

P1 = trapezoid = 0.65 Ka yH2

P a :;;; Rankine = 0.50 Ka yH2

PtfPa = 1.30

m=1.0 P,~ 0.875yH'(1 - (4/N)) P,~ 0.50yH'(1 - (4/N)) PtfPa = 1.75

P1 = 0.15yH2 so 0.30yH2

PaiN= 4,Pa =O N<4,Pa<O Note: equivalent Rankine active = O.

• Far clays, base the selection on N= yH!cu.

Figure 31 Pressure enve/ope method- Terzaghi and Peck


l il ,, l l

58

analysis given above. The passive resistance to provide the ground prop force can be calculated by conventional methods. Walings in bending may be designed using the two-thirds envelope, but the full envelope must be used for the design of the walings in bearing and shear and for the design of struts. No increase in working stresses are allowed for the design of sheet piles and walings when using this method.

4.3.8 Soldler plles

Soldier or king piles are installed at suitable intervals around the perimeter of the cofferdam and bave a system of ground suppor! between them, which is commonly horizontal tirnber laggings. The soldier piles are installed first and the laggings are placed between the soldiers as excavation proceeds. The soldier piles are supported intemally or externally at various Ievels in the sarne manner as sheet piling. The system is not suitable for conditions where water needs to be excluded as there may be a considerable inflow of water between and below the laggings.

If the laggings are of timber or other flexible materia) there will be enough deflection to allow arching of the soil and warrant a reduction in the bending moment in the lagging. In this case half the Terzaghi and Peck earth pressure envelope to calculate the bending moment in the laggings should be used. To qualify for this reduction the ground must be in contact with the rear flange of the soldier pile and the lagging must span between the front flanges of the soldier.

Surchar~e WkN/m

Groundwater

+

Terzhagi & Peck (f & P) envelope appropriate

to soli conditions

+

(f & P) 1~1

l

i l l l l l l l l l / 1/

Surcharge (f & P)

~l Water Composite

strutload envelope H

=KaW

Ground model

~~(f&P)

Envelope far design of walings, soldiers and sheet piles bearing

~l

Envelope far design of:(a) Axial strut load (b) Walings, soldiers or sheet piles

in shear and bearing

H j(f &P)

l l l N.B. Water pressure may l be relieved by drainage 1 through lagging but l 1 this cannot be relied l l -+·-r· upon

l l l l l l l l

Envelope far design of horizontallagging

Figure 32 Use of pressure envefopes far structuraf design


According to Clayton and Milititsky''l the bending moments in the soldier piles can be designed using two-thirds of the Terzaghi and Peck envelope as above. The span below the lowest prop can be designed as a cantilever supported by the prop alone, or as a propped wall supported by the prop and the passive resistance of the ground into which it penetrates (see Figure 32). The full Terzaghi and Peck envelope should be used for the design of soldiers in bearing and shear. Use norma! working stresses for the design of the soldier - no increase in stress is allowed with this method.

The passive resistance for the toe of the soldier piles can be calculated by the following methods suggested by Clayton and Milititsky (see Figure 33).

r 1.5b r 3Kp y l

1 Kp/

Pile width = b 1v 9 eu. b 3a~b. Kp

~l

Undrained strength Effective stress parameters (cohesive) (cohesiontess or cohesive)

Figure 33 Sofdier pifes -passive resistance

4.3.9 Design stresses In steel sheet piles

British Steel manufactures steel sheet piles in two grades of steel given in Table 9.

There are two approaches in common use for the structural design of steelwork.

Ultimate /imit state approach to BS 5950 - Structural use of steelwork in buildin!l1'!

This approach applies partial factors to the various loads on the structure from which factored bending moments are calculated. The moment capacity of the pile section is calculated using the characteristic strength for the grade of steel in use, factored by an appropriate partial factor for materials, and this ·moment capacity must be greater than the factored bending moment


60

Table 9 Steal sheet piles - steel stresses in bending

Grade of steel Characteristic strength BS 595otttl

BS 436l11 '~ BS EN 10 025 N/mm2

43A FE 430A 265 50A FE 510A 345

Working stress BS 449<11l

N/mm1

180 230

Note: The 'A' subgrades (which are not lmpact tested) appear only in Annex D of the UK edition of the European Standard. Other editions use the 'B' sufftxes.

When applied to the loads on a sheet pile wall this method alters the geometry of the bending moment diagram, and the analysis becomes complicated.

Therefore when designing to BS 5950 it is recommended that the factored bending moment is calculated as follows:

a) A factor of 1.4 is applied to the bending moment found when the wall is analysed by any of the methods given in Section 4.3.3 at limiting equilibrium conditions (i.e. with the F of S = 1.0).

b) Alternatively, moments derived directly by the Strength Factored method (see Section 4.3.2) should be used, with pressure diagrams factored by using an appropriate F of S.

Working /oad approach to BS 449 - The use of structura/ stee/ in bui/dingr181

In this approach the loads are not factored and are used to establish the working bending moment. The moment capacity of the pile section using the working stress for the grade of steel in use must be greater than the working bending moment. The bending moment found by any of the methods of analysis in Sections 4.3.2 and 4.3.3 at limiting equilibrium conditions may therefore be used as the working moment.

4.4 FACTORS OF SAFETY

4.4.1 Cantllever walls and propped walls wlth free earth support

The various Piling Handbooks and other design guides(7· '· '· "' give guidance on safety factors to use with the different methods and conditions.

When used with moderately conservative ground strength pararneters the following safety factors can be used for temporary cofferdams:

The term 'moderately conservative' is described in CIRIA Report No 104''' as 'a conservative bes t estimate'.

l. Effective stress analysis:

a) Factor on Strength

•V ~ 3oo cj>' > 30°

F, = 1.2 F, = 1.2 - 1.1


b) Factor on Moments

Gross pressures (CP2)

cj>' ~ 20° F, = 1.2 cj>' = 20--30° F, = 1.2- 1.5

= 1.5 cj>' <: 30° F,

Net pressures (British Steel)

cj>' ali values

Burland-Potts

cj>' ali values

2. Total stress analysis:

Factor on Strength Gross Pressures Net Pressures Burland-Potts

(F.,) = 2.0

(F,) = 1.5

(F.) = 1.5 (F,) = 2.0 Not recommended (F,) = 2.0

4.4.2 Propped :walls wlth flxed earth support

The penetration will always be greater than that required for free earth suppor!, and will give an adequate factor of safety against rotation about the prop.

If the 'simplified method' is used for the calculation of penetration, the 20% e.xtra penetrati~n is not a factor of safety but the additional depth required to corree! for the Simplification used m the method.

4.5 OVERALL STABILITY

4.5.1 Sloping sites

Cofferdarns are often Iocated in river banks where the ground level may vary considerably across the width of the cofferdarn. The resulting unbalanced horizontal loading must be adequately catered for, otherwise there is risk of serious instability (see Figure 34).

Passive-----_ resistance

pressure line

····· .... ........... .....

Hl====== H

Figure 34 Overa/1 stability with difference in ground leve/

Active / pressure

When the difference in ground level is small, it may be sufficient to ensure that the top frame is set sufficiently deep so that the excess active pressure from the h1gh s1de can be res1sted by developing passive resistance from the soil at the sarne level on the low s1de.


62

In olher cases the excess active pressure may be lransferred to a lower level on the opposite side of the cofferdam by means of raking slruts. In 1his case 1he resulting induced vertical forces musi also be catered for. Altematively, the unbalanced active pressure can be resisted by ground anchors installed from the top frame level into the soil on the high side. Again, if the anchors are inclined at an angle below the horizontal, 1he resulting induced vertical forces must be catered for (see Figure 35).

' / •

_-' / , /

.# Tie rods and anchorages if":~==fl"- Raking

struts

Figure 35 Overa/1 stabi/ity, raking struts or tie rods

4.5.2 Clrcular sllp lnstablllty

On sloping sites where 1he founding stratum is cohesive, the possibility of overall failure by circolar slip must be checked. The toes of 1he sheet piles musi intercept 1he criticai slip circle, which means 1hat the part of the circle in front of the line of the piles becomes ineffective and the shear strenglh il would have contributed must be replaced by passive resistance from the piles. When 1he pile loe level has been established, a check on the slip circle passing under 1he loes of the piles should be carried out lo ensure an adequate factor of safety (see Figure 36).

H H

~ Hl=== H

----------------/' / /

Figure 36 Circu/ar s/ip instability

4.5.3 Bottom stablllty

Stability of lhe base of 1he cofferdam may be disrupted by water pressure causing 'piping' or 'heave' failures.

Piping occurs whep the drag on 1he soil grains near forrnation level due to the upward flow of walls is so great that 1he effective slress in the soil approaches zero. In 1his state the soil has no shear strength and will not support any vertical load, even 1hat of a footfall. Hence 1he situation can be very dangerous to personnel and to 1he stability of the cofferdam. Prediction of lhe possibility of piping is carried out by constructing a flow net as described in Section 4.1.2.


The flow net allows the calculation of 1he 'exit hydraulic gradient' just below 1he forrnation level inside 1he cofferdam. Hydraulic gradient 'i' is defined as loss of head per unii length in the direction of flow, i.e. melres divided by melres, which is a dimensionless number. In Figure 9 if B/2 is half lhe widlh of 1he cofferdam, then the width of each exit flow nel square is B/2Nf where Nf is the number of flow channels in the half wid1h of cofferdams and the exit hydraulic gradienl ~ is given by:

For ground wilh a salurated bulk weight of approximately 20 kN/m3 the criticai hydraulic gradient at which 1he effective soil slress reduces to zero and piping occurs will be i, = 1.0. The factor of safety against piping is defined as

l 10 _; , whtch approxlmates to -·-~ ~

A flow net such as 1hat in Figure 9 is strictly a slice from a very long cofferdam. For square or circolar cofferdams, the 3-dimensional nature of the flow has the effect of further concentrating lhe head loss within the soil plug between the sheet pile walls. The following correction factors should be applied to 1he head loss per field on the inside face of the cofferdam:

Circolar cofferdams parallel wall values x 1.3 In the comers of a square cofferdam : parallel wall values x 1.7

For clean sands 1he fuclor of safety against piping l.O should be between 1.5 and 2.0. t.

The faclor of safety can be improved by increasing lhe penelration of the piles. Figure 37 shows the minimum deplhs required in medium uniforrn sands which extend some distance below the toe level of 1he piles. In fine sands a greater penelration may be required and in coarse sands a redùced penelration may be sufficient. Note that the deplhs in Figure 37 are given to avoid piping, but they do not necessarily provide enough penetration to ensure adequate passive resistance for stability.

= f777011"-'\

GWL //AWN ~

w H

'lAVI"' _Dc Figure 37 Bottom stability (after BS 8004141

}

Wìdth of cofferdams Depth of cut-off W 'D'

2Hormore H

0.5H

0.4H 0.5H 0.7H

The flow nel also allows calculation of 1he factor of safety against base heave. Base heave can occur if the force due to water pressure under a block of soil becomes greater !han 1he bulk weight of that block of soil. This can be assessed using Figure 9 by calculating 1he average water pressure and hence the force on the piane BC (defined by the line between 1he pile toes) and comparing 1his with 1he total weight of the plug of soil between 1he sheet piles. In an exlreme situation, where ali 1he head loss is concentrated in the soil plug, the pressure on the


64

piane BC would be hydrostatic from the upper water leve!. In that case a factor of safety against uplift comfortably greater than 1.0 would be required. Such a situation can arise if inward movement of the sheet piles loosens the soil on the active side to such an extent that it becomes more permeable and feeds high pressure water directly to the toe leve! of the sheet pile s.

Figure lO(b) shows the situation where sand containing water at pressure underlies a clay plug in the base of a cofferdam. Again it is essential to ensure an adequate factor of safety against heave of the clay with the worst possible combination of pressures and levels. A similar situation can occur if fine sands overlay grave! layers.

It should be noted that the normally adopted defmitions of factor of safety against piping and against heave are fundamentally different and if applied to the sarne block of soil will only give the sarne result if the factor of safety is 1.0.

4.5.4 Pressure due to rlver or ti dal flow

Cofferdams situated in river and marine locations may be subject to unbalanced loading from the pressure of flowing water (Figure 38). In particular, cofferdams which conslrict the natura! flow of a river may cause back-up of the upstream water leve!, resulting in a substantial differential head on apposite sides of the structure. In such circumstances vertical bracing may be required to provide stability and the penetration of the piles into the river bed should be checked to ensure an adequate factor of safety against the induced vertical forces. Allowances should be considered for the effect of floating debris accumulation and debris impact where appropriate.

: i l r: l

l ---- l -l

l l l

""'"' l l l l l l

l l0m Wl Wl 77' ""' 777, m; "l l l l l l l l l l l

Figure 38 Pressure due to river or tida/ f/ow

4.5.5 W ave forces

Cofferdams exposed to wave conditions may be subject to extremely high overturning forces, which are not necessarily at their maximum under high tide conditions, when breaking waves would tend to overtop the cofferdam rather than expend energy against them. The evaluation of wave pressures is dealt with in BS 6349 : Part I1"l. It is a complex subject and it is advisable to seek the advice of an expert if significant wave loading could occur.


4.5.6 Overtopplng

Where cofferdams may be overtopped and internally flooded, the pile support system musi be designed for this situation i.e. flooded internally with the external water leve! at the bottom ~f the wave trough. This may require internai ties to resist outward movement of the walls. Slmce gates or flap valves should be provided to allow the cofferdam to drain as the external water leve! drops, which can happen quickly.

4.5.7 Scour

Where cofferdams are exposed to fast flowing river or tidal conditions, or to wave action, care should be taken to either protect the bed adjacent to the piles from the removal of materia! by scour, or to design the cofferdam to be stable after scouring has occurred. The upstream and downstream ends of a cofferdam may be shaped to provide cutwaters.

4.5.8 Protectlon from vessel lmpact

Cofferdams located in navigable waterways should be clearly marked as a hazard to navigation but should also be suitably protected from accidental impact. Protection may take the form of independent dolphins or strong points and fendering designed into the cofferdam.

4.6 CHOICE OF PILE SECTION

4.6.1 Sectlon proflle

Steel sheet piling to resist bending is manufactured in two profiles, U and Z. In the United Kingdom these are referred to as Larssen and Frodingham respectively. The interlocks between piles are at the face of the piles with the Z profile and on the ~entre line of the line of piles . with the U profile. Other profiles including H section and stra1ght web are produced for special purposes. Manufacturers issue brochures giving details of the prolùes available.

There is no preference between U or Z profile for most cofferdams. However, there are differences in the characteristics of the two profiles which can inlluence the choice in certain conditions.

z profiles have advantages in marine situations because the interlocks are mor~ closely fi~ing and are more watertight than U profiles. They are also to be preferred where p!les are des1gned as cantilever walls, because dellections tend to be less than those of the equivalent U section.

U profiles depend upon transfer of longitudinal shear between adjacent piles, by friction in the interlocks, to develop the full modulus of the combined section. Experience over many years shows that this generally happens, the shear being generated by surface irregularities, rusting, lack of initial straightness and soil particle migration into the interlocks during driving. Concentrated transfer of horizontal soil pressure from front to rear piles at points of support, such as walings, where only alternate piles are directly supported also generates resistance ~ shear. However. there are particular conditions where shear transfer may not be fully effecllve:

lengths of pile shaft not driven below soil surface leve!

lengths of shaft retaining water only

piles forming cantilever walls, or cantilevering above or below walings

piles driven into and supporting silts and very soft clays.

In such circumstances it may be advisable to weld together the tops of alternate piles after driving. This may lead to difficulties when extracting the piles.

Failure to develop the full modulus of the combined section is indicated by excessive deflection of the completed wall. There will always be a considerable amount of friction in the interlocks.


66

The piles are never considered to acl as individuai units and may be taken 1o develop al Ieast 40% of the combined modulus in the above situation.

Z profiles will also lose modulus if they are allowed lo rotate during driving. As a rough guide, so of rotation will result in a 15% reduction of modulus. Rotation of the occasionai pile can be ignored, but consistent rotation, as shown in Figure 39, is not acceptable.

Figure 39 Rotation of Z Section profiles

U profiles have greater single section moment of inertia than the equivalent z section and, consequently, are ress prone lo deviate from the theoreticalline when penetrating dense or difficult soils. For the same reason they tend to give a greater number of re-uses than z profiles, which is of parlicular benefit in temporary works. They are also used on occasions as light walings and struts and for various other secondary purposes. A further advantage is the greater. arnount of rotation which can be obtained in each interlock during pitching, approx•mately 9o compared with 3° for Z profiles, which is useful when constructing walls to small radii.

4.6.2 Cholce of sectlon to sult drlvlng conditlons

While the primary pmpose of the sheet piles is lo resist bending moments induced by soil and wat_er pressures, the section musi also be chosen so that it will be capable of being driven to the des•gned penetrallon WI!hout undue darnage or deviation. The successful fulfilment of the Iatter is very much a matter of judgement based upon experience.

Table IO, based upon a relationship between Standard Penetration Test results and wall modulus, has been evolved to provide guidance to the less experienced designer. It must be noted that the guide is based upon experience with piles of British manufacture and of approximately 500mm width, driven by the pane! driving method. Sections of greater width, or those driven by other methods, may require somewhat heavier sections than those indicated in Table IO.

Table 10 Guide far the selection al pile size to suit driving conditions in granular soils

Domlnant SPT (N value)

O- IO Il- 20 21-25 26- 30 31-35 36-40 41-45 46-50 51-60 61 -70 71-80 81 - 140

Mlnlmum wall modulus cm3/m ofwall

BSEN lO 025 BSEN lO 025 Grade 430A, Grade SIOA, BS 4360 BS 4360 Grade 43A Grade SOA

450

450 850

850 1300

1300 2300

2300 3000

3000 4200

4200

Remarks

Grade FE510A for lengths greater than 10 m

Lengths greater than 15 m not advisahle Penetration of such a stratum greater than 5 m not advisable • Penetration of such a stratum greater than 8 m not advisable •

Some declutching may occur

Some declutching may occur with pile lengths greater than 15 m lncreased risk of declutching. Some piles may refuse

* If the stratum is of greater thickness use a larger section of pile


The table requires the designer to assess the relevant dominance of each soil stratmn which the pile is required to penetrate. Strata of greater thickness and density will be more dominant than strata of similar thickness but !esser density, or thinner strata of similar density.

The table is derived from the fact thal in granular soils the major pari of the resistance to penetration results from point resistance at the toe of the pile. Shaft friction with the surrounding soil contributes relatively little lo the overall resistance to pile penetration. The required section is, therefore, related to the density of the soils being penetrated by !be pile toe at ali stages of the drive, the length of embedded shaft having only a small influence. The criteria for adequacy of the pile section are !hai the pile head shall not be darnaged by the harnmer impact, and the pile toe shall not be darnaged by the soil resistance.

For cohesive soils, the resistance to pile penetration results primarily from shaft adhesion with the soil, virtually no point resistance being offered to the toe of the pile. The overall resistance is, therefore, a function of the undrained shear strength of the soil, the perimeter dimension of the pile section, and the length of pile shaft embedded in the ground. A pile ~ver with . . sufficient power lo overcome this resistance will be necessary to advance the pile. The cntenon for adequacy of pile section is that the pile head and shaft shall sustain this force without buckling. Darnage to the pile toe is far less likely !han when penetrating granular soils.

Table Il is intended as a guide when no other information or experience is available.

Table 11 Guide to selection al pile size to suit driving conditions in cohesive soils

Clay descriprlon

Soft to firrn Firm Firm to stiff Stiff Very stiff Hard (c,. >200)

Minimum wall modulus cm3/m

BSEN lO 025 BSEN lO 025 Grade 430A, Grade SIOA,

BS 4360 Grade 43A BS 4360 Grade SOA

450 400 600-700 450-600 700-1500 600-1300 1600-2500 13000-2000 2500-3000 2000-2500

Not recommended 4200-5000

Maxlmum length

m

6 9 14 16 18 20

The above table has been derived from a similar table published in 'Specification for Steel Sheet Piling', published by the Federation of Piling Specialists'"'·

Note: The ability of piles to penetrate any type of ground is also a function of attention to good pile driving practice and these tables assume that this will be the case.

4.7 GENERAL LAYOUT OF SHEET PILING

When considering the layout of a sheet pile cofferdarn it is important lo consider the problems encountered in pitching and driving the piles, and in the construction of the permanent work.

The most economica! layout for driving sheet piles is a straight line which can be pitched in long panels. Comers, junctions and particularly special piles cost mo~e and are ?enerally m~ch more expensive to pitch and drive as they will upset the regular routine. If poss1ble these plles should be accommodated within the width of the sheet wall as they can be pitched between the norma! guide walings as pari of a pane! of piles (see Figure 40). Altematively packings can be attached to the guide walings to allow for the increased width (see Figure 41).

Engineers should remember that there are lolerances in the rolling margins of the p~es which can ali be either plus or minus. Driven dimensions may vary from those when the plles were


68

pitcbed. Piles may not drive vertically even wben pitcbed correctly. Tbe tolerances wbicb sbould be acbievable under norma! conditions are about:

Position of top of pile Verticality after pitcbing Verticality after driving

± 50mm l in 200 l in 75 transverse to line of piling

The rolling tolerances for the nominai width of a pair of interlocked piles is normally 3%. This will probably reduce lo about 0.5% over a 30 m lengtb, but should be checked after eacb pane!

between guide walings

Gap between guide walings

corner can plug up toe of pile causing spreading the clutches

,~F--..,---------------------------

-------~--..l

Larssen section 9W

Welded junction pile

... ------------, l l l l l l l l

Double bent corner pile

Welded junction pile

Frodingham section 2N

Figure 40 Layout of stee/ sheet piling


bas been pitched. Manufacturers of sbeet piles publisb production tolerances and these should be bome in mind wben planning the cofferdam, as on occasions the tolerance can be ali plus or ali minus.

In addition to tbe above, working space may be needed for access, sbuttering and safety. Space may also be needed for sumps for dewatering pumps, wbicb can often be allowed for wbere the outline of the permanent work varies from the straight. Space musi be allowed for any walings and pile deflection during excavation and/or dewatering.

It is preferable to use open corners or specials. In hard driving conditions closed corners (especially Larssen type) can plug up inside in the manner of an open-ended box pile. This will make the driving of tbe corner more difficult, and if it causes the corner pile to open up al the lower levels tben the next pane! to be pitcbed will bave to bend as it is driven down causing further difficulties in driving. It may even declutcb and make dewatering more onerous and, in some ground conditions, cause loss of soil in tbe gap between the declutcbed piles. If this leads to the bearing capacity of tbe formation being affected by the movement of the ground and/or water then there could be a very difficult situation.

It may be possible to use junction piles instead of corner piles. If any junction pile can be designed so tbat the offset is witbin tbe norma! widtb of the sbeet walls, tben tbe junction pile can be pitcbed within the guide walings and will not bave to be pitcbed separately. It should be ensured that tbe piles leading from the junction will bave enougb clearance for tbe harnmer legs.

L---~~--~~~--~~---Guide waling

Sheet piles ,_,.......(,.~=~,..,.~~-,,.,.,-~+4=.,.,~,.........., Timber packing

L-------\-------------' Guide waling

Junction pile

Figure 41 Use of timber packing to accommodate junction pile between guide walings

Tbere are occasions wben a circular layout can prove economica!. The ground levels around the cofferdam sbould be relatively leve! so tbat an even loading is applied to the waling. Tbe waling is largely in direct compression and bending stresses are low. A complete circle will not need any internai strutting, giving unobstructed access to tbe cofferdam. If the outline of the cofferdam is made up from a series of arcs tben it is important tbat the geometry leads to no out of balance forces al tbe junction of the circular walings and any struts.

If site conditions require tbe use of a pile driver wbicb uses hydraulic rarns to force tbe piles into tbe ground and relies on otber piles to provide tbe reaction lo this force then tbe layout musi reflect any limitations necessary for this type of driver. Tbere are two types of drivers in use. Tbe first drives a pane! of piles normally consisting of eigbt single piles in a straigbt line, tbougb six or seven may be acceptable depending on ground conditions. Tbe second type drives a single pile w bile depending on the reaction from previously driven adjacent piles to. provide tbe driving force. It is recommended that tbe plant manufacturer or supplier is consulted before finalising any layout.

It is important that cofferdams, as temporary works, are capable of being constructed quickly, economically and safely, and are unlikely to give problems in service. Steel sheet piles whicb can be extracted v.iithout damage to tbem will often bave a recovery value close to tbat of new piles. It can therefore be a false economy to use piles with tbe least section modulus and length compatible with the design requirements. If ground conditions vary and driving becomes


70

difficult the piles can be damaged both at the head and toe, and may even declutch at the toe giving rise to infiltration of water and/or ground into the cofferdam. Extraction of the piles will be more difficult and the recovery value will be reduced.

Ground strata are often variable in nature and as cofferdams are frequently designed with limited ground information, it is wise to allow for the possible variability, i.e. both better and worse ground. An extra metre or so of length in the pile and the use of heavier sections and/or higher grades of steel will allow far some variations. If the ground is poorer then the extra driving will be at no cast since driving conditions will be easier. If the ground is better, then the piles can be left with the tops above ground forming a strong safety fence. Remember that while it is usually possible to revise or strengthen the suppor! system if poorer ground is found during construction, it is not possible to increase the pile section after driving. It is very costly and time consuming to lengthen piles by welding, though it may be possible to drive every other pair of piles down a short distance. As suggested in the German Waterfront Code'm, the gaps at the top can be filled in with short lengths of piles if necessary (see Figure 42).

---- ...,

1-

Gaps fil led in with light section pile if necessary

...,_A -f- -1-+_ji--l--l-o---~

LL

Originai penetration

_ Effective L L -t increase in

penetration

Figure 42 German waterfront method of driving

4.8 CHECK LISTS

Reference should be made to Section 4.3 far details of the various methods far analysis of the wall.

4.8.1 Design of cofferdam

l. Collect ali information available an:

• structure to be built within the cofferdam

• drawings and other documents

• ground and soil

• water levels in the ground

• if cofferdarn is in water then:

- variations of tide levels and currents

- variations of river levels and flows

- wave climate

- any water traffic


• restraints an construction, e.g.:

- access

- working area and obstructions

- any adjacent structures, buildings and services

- any height restrictions

- any noise or vibration restrictions

• availability of plani and materials, e.g.:

- cranes and excavators

- piles

- steel far walings and struts etc. (assume secondhand steel to be Grade 43A unless documentary evidence is available to certify the steel quality).

2. Sketch aut preliminary scheme having regard to:

• above information

• sequence and method of construction of permanent work

• excavation procedures

• suppor! system where necessary (walings, struts etc.)

• remava! of supports as construction proceeds

• remava! of cofferdam.

Discuss with construction team and arnend if necessary.

3. Study ground and water leve! information including any artesian conditions.

4. Decide an ground profile far design including water levels and pressures (static or steady state seepage).

5. Decide an use of effective or total stress parameters far each ground stratum.

6. Decide an values for ground parameters.

7. Decide an values far any surcharge or additional loads an the wall.

8. Decide an levels far any suppor! to the wall and estimate suitable excavation levels to allow supports to be fixed at each stage of cofferdam construction. Make allowance far overdig especially under water when up to 0.5m should be allowed.

9. Far each stage calcolate and draw to true scale gross pressure diagrarns with ground strata, water and suppor! levels far:

• por e water pressure, active an d passi ve

• net water pressure for design

• horizontal earth pressure, active and passive.

IO. Far each stage carry aut an analysis of the wall (see Section 4.3):


if not a cantilever wall decide whether to use free or fixed earth toe suppor! conditions

decide an method of analysis and factor of safety far rotational stability

71

72

analyse at limiting equilibrium conditions (F of S = 1.0) for:

- penetration

- prop load

- leve! and value of maximum bending moment

modify or factor the pressure diagrams as required by the method of analysis

analyse for design penetration at the chosen faclor of safety

• for multi-prop walls calculate the prop loads at each leve!.

11. Consider length of pile required and ground conditions relative to difficulties in driving. Make decision on section of pile and grade of steel.

12. Consider ali results and decide whether another analysis with different support levels may be more economica!. If so, repeat from Step 8 until satisfied with the result

13. Consider fresh analysis using a different ground profile and parameters etc. lo test the sensitivity of the results to changes in ground strengths. If necessary, repeat from Step 6.

14. In some circumstances it will be prudent lo consider the worst credible conditions. CIRIA Report 1041'l describes the values of the parameters for these conditions as the worst credible values that the designer could realistically believe might occur:

decide on the worst credible values for the ground parameters, water levels, and seepage conditions. If strata thicknesses are variable decide on worst combination

carry out analysis at limiting equilibrium conditions (i.e. F of S = 1.0)

the length of pile, section and penetration chosen for construction should not be less than that resulting from this analysis, and the bending moments and forces in the sheet piling (and support system) should be satisfactory when considered as ultimate loads when designing to BS 5950°'l.

15. Make fina! decision on penetration, pile length, section and steel grade.

16. Design support system with regard to:

restraints and availability of plant and materials (see Step l)

excavation procedure and construction of permanent work

access and safety requirements.

17. Discuss with construction team and amend if necessary.

!8. Produce working drawings including details of soil strata and groundwater etc. with instruct)ons for:

construction of cofferdam

procedures for excavation and erection of supports

details of groundwater contro! or dewatering if required together with any fai! safe procedures or standby plant

procedures for construction of permanent work including any alterations to supports

procedures for backfilling and remava! of supports

any checks- required during !ife of cofferdam

remava! of cofferdarn


any special instructions e.g.:

- accommodation of services

- pipes or accesses at lower levels.

Note: It is important that working drawings should show the various soil strata used in the design, so that a check can be made during excavation to confrrm the validity of the information used by the designer.

4.8.2 Check llst for analysls of cantllever wall (slmpllfled method)

l. Using gross pressure diagrarns analyse wall at limiting equilibrium conditions i.e. Factor of Safety = 1.0 (see Figure 43).

2. Assume a leve! A-A at a depth 'd' below excavation leve! and calculate the areas above this leve!:

a) Passive earth pressure P.

b) Active earth pressure P A

c) Nel water pressure P w

3. Calculate the moments of the above areas about leve! A-A. If moment of Pp does not equal moments of PA +P w then alter depth 'd' unti! they balance.

Passive Active

Earth pressures

Figure 43 Analysis of a cantilevered wa/1

Net water pressure

4. Find the leve! where there is zero shear in the wall. Assume a leve! B-B and calculate the areas of the pressure diagrams above this leve! as in Step 3 above. If the area Pp does not equa! the area of p A + P w then alter the leve! B-B unti! they balance. This is the leve! of zero shear.


l l l 'l '

:l

74

5. Calculate moments of these areas about leve! B-B for P,, P A and P w as above. The maximum bending moment is equa! to:

Moment of P A + moment of P w - moment of P,

6. Decide on method of analysis for penetration and the value of the Factor of Safety to be used i.e. F., F,, F"" or F,.

7. If using the strength factored method then apply the Factor of Safety (F,) to the ground parameters and draw the factored pressure diagrams. Repeat the Steps 2 and 3 above unti! the moments of area balance. Add 20% to the depth 'd' to give the design depth of penetration.

8. If using one of the moments factored methods (F,, F"" or F,), modify the pressure diagrams as necessary for the method chosen. Assume a depth 'd' below excavation leve! and calculate the moments of area about and above this leve! as in Steps 2 and 3 above. The moment of P, divided by the Factor of Safety should equa! the moment of P A plus the moment of P w. If not then adjust the depth 'd' unti! they balance. Then add 20% to 'd' to give the design penetration.

Note: If the soils vary in strength in the lower part of the wall a further check should be made that the moments of ali the forces about the toe of the wall balance, and that the horizontal forces are in equilibrium.

4.8.3 Check llst for analysls of a propped wall with free earth support

l. Using gross pressure diagrams analyse wall at limiting equilibrium conditions i.e. Factor of Safety = 1.0 (see Figure 44).

GL

Prop T -->·l\ WL

d

---A

Passive Active

Earth pressures Net water pressure

Figure 44 Analysis of a proppad wa/1 with free aarth support

2. Assume a leve! A-A at a depth 'd' below excavation leve! and calculate the areas above this leve!:

a) Passive earth pressure P,

b) Active earth pressure PA

c) Net water pressure


3. Calculate the moments of the above areas about prop leve!. If moment of P, does not equa! moments of PA +P w then alter deplh 'd' unti! they balance. When this occurs then 'd' is the depth of penetration at limiting equilibrium conditions.

4. Calculate the value of the prop force 'T' which equals PA +P w- P,.

5. Find the leve! where there is zero shear in the wall. Assume a leve! B-B and calculate the areas of the pressure diagrams above this leve! for P A and P w as in Step 2 above. If the sum of the areas P A and P w do noi equa! the prop force 'T' then alter the leve! B-B unti! they balance. This is the leve! of zero shear.

6. Calculate moments of these areas and the prop force 'T' about leve! B-B. The net moment is the maximum bending moment in the wall.

7. Decide on the method of analysis for penetration and value of the Factor of Safety to be used i.e. F3 , FP, F np or Fr.

8. If using the strength factored method then apply the Factor of Safety (F,) to the ground parameters and draw the modified pressure diagrams. Repeat the Steps in 2 and 3 above unti! the moments of area balance. The depth 'd' is the design depth of penetration.

9. If using one of the moments factored methods (F,, F., or F,), then modify the pressure diagrams as required for the method chosen. Assume a depth 'd' below excavation leve! and calculate the moments of area above and below the prop leve! as in Steps 2 and 3 above. The moment of P, divided by the Factor of Safety should equa! the moments of PA +P w.· If not then adjust the depth 'd' unti! they balance. The depth 'd' is the design depth of penetrati an.

4.8.4 Check llst for analysls of a propped wall wlth flxed earth support (slmpllfled method)

l. Using gross prossure diagrams analyse the wall at limiting equilibrium conditions i.e. Factor of Safety = 1.0 (see Figure 45).

GL

T-- WL

WL

P w B

P, c

d

A R A

Passive Active

Earth pressures N et water pressure

Figure 45 Analysis of a proppad wa/1 with fixad aarth suppor!

2. Find the leve! C-C where the net pressure is zero, i.e. where the passive pressure equals the active earth pressure plus the nel water pressure. Let 't' be the distance between the leve! of the prop and the leve! of C-C.


76

3. Calculate the areas above leve! C-C:

a) Passive earth pressure P,

b) Active earth pressure P •

c) N et water pressure

4. Calculate the moments of the above areas about leve! C-C.

5. Calculate the value of the prop force 'T' which equals moments of (P • + P w - P,) divided by distance 't'.

6. Assume a leve! A-A at a depth 'd' below excavation leve!. Calculate the areas a), b) and c) above this leve! as in Step 3 above.

7. Calculate the moments of these areas and the prop force 'T' about leve! A-A. If moments of P,+ T do not equa! moments of P.+ P w then alter depth 'd' unti! they balance.

8. Find the leve! B-B where there is zero shear. Calculate the areas of the pressure diagrams above this leve! for b) and c) as Step 3 above. If the sum of the areas P. and P w do not equal the prop force 'T' then alter the leve! B-B unti! they balance. This is the leve! of zero shear.

9. Calculate moments of these areas and the prop force 'T' about leve! B-B. The net moment is the maximum bending moment in the wall.

IO. Add 20% to depth 'd' to give the design penetration which allows for the simplified form of analysis.

Note: If the soils vary in strength in the lower part of the wall a further check should be made that the moments of ali the forces about the toe of the wall balance, and that the horizontal forces are in equilibrium.

4.8.5 Check llst for analysls of a multl-prop wall {uslng the stage-by-stage method)

By successive analysis of each stage of construction with hinges assumed at each support leve! below the first.

For the final stage with the layout shown in Figure 46(a):

l. Analyse top span AB as a simply supported bearn as in Figure 46(b ).

2. Analyse intermediate span BC as a simply supported beam as in Figure 46(c).

3. Analyse bottom span CD as a sheet pile retaining wall with single prop at the top, for free or fixed earth as in Figure 46(d).

• at limiting equilibrium conditions to give:

- maximum bending moment and leve!

- load on suppor! No.3 from span CD = C0

• with selected method and Factor of Safety to give penetration.


4. Totalloads on supports are:

Support No.I A = A Suppor! No.2 B = s. + Be Support No.3 C = c. + C0

5. Check any alternative frame arrangements required far re-propping during permanent works construction and final removal of the cofferdarn.

6. The maximum bending moments occur at leve! X-X far each of the spans, i.e. at position of zero shear. Far design take the largest load o n each support and the largest bending moment at any stage of construction.

Support no.

Exc. L

Passive ---pressure

Figure 46 Multi-prop wa/ls


T o tal active pressure BA _L.,._4-8:'-'-'"' including net (b) water pressure

(a)

(c)

77

78

5 Design of support system

5.1 GENERAL

Ali walls except cantilevers require a suppor! system which, if exposed, is more vulnerable lo abuse than the wall itself since excavators and cranes have lo manoeuvre between the various members of the suppor! structure. In genera! there are more failures due to inadequacies of the suppor! system than failure of the wall. (See also Section 5.2.1 for cantilevers.)

The ground can vary considerably around the perimeter of the cofferdam and the pressures locally may lead lo overstressing of the wall. This will only lead to a !oca! failure of the wall, but if the suppor! system fails in turn then it could lead to progressive failure and total collapse of the whole cofferdam. !t is therefore important to design the suppor! system lo be robust enough that the risk of progressive failure is minimised.

The suppor! system normally consists of walings at each suppor! leve!. These are usually exposed at the front of the wall, though the top waling can be at the back of the wall if required. The walings are commonly supported by internai struts to form a horizontal frame inside the cofferdam. Altematively the walings can be supported by tie rods and anchorages or by ground anchors at the back of the wall. Two or more frames can be braced together vertically lo give a greater ability for the whole structure to resist unbalanced horizontal loads.

Note that mixed suppor! systems (i.e. extemal anchors together with internai struts) should not be used (see Section 3.2.2).

5.2 WALINGS

5.2.1 Stralght wallngs

Cantilever piles in theory do not need walings, but, if the contro! of wall movement is important, then a light waling will help to even out any differential movement of the wall due to variations in the ground pressures. Similarly if each pair of piles is supported by a strut or tension member i t is not necessary, in theory, to ha ve a waling. !t is advisable, however, to provide a waling so that the load from one pile can be transferred to severa! adjacent piles in the event of the failure of one suppor!, and thus limi! the onset of progressive failure.

!t must be appreciated that, although the waling will be designed for a uniform loading, the actualloading will vary considerably, depending on the variation of the ground and its movement, any arching effect, the construction methods, quality of packings between the pile and waling, etc. !t is norma! to use a simplified approach which takes these faclors inlo account.

The design load for the waling should be greater than the prop load given by the wall analysis, to allow for the possibility of arching of the ground and stress redistribution behind the wall. CP2<'l gives a 10% increase in the prop load for cohesive soils and 15% for cohesionless ground. CIRIA Report No 104<'l recommends for stiff clays an increase of 25% for a wall with a single leve! of suppor!, or the upper leve! of a multi-frame suppor!. Lower levels should be increased by 15%. Note that it is not strictly necessary to factor any loads arising solely from water pressures.

Draft BS 8002°2) gives an increase of 25% in the prop load when a reduction of bending

moment in the pile has been allowed (see Section 4.1.9).

The walings should be continuous over more than two supports where possible and designed for a maximum bending moment of WL/10. If continuity is not possible then it should be designed for WL/8.


The faces of sheet piles when driven are never truly vertical or in one piane, and it is necessary to provide packings between the piles and the waling to transfer the load. Packings may be tirnber or steel plates, pairs of folding wedges, concrete or dry mortar or a combination of these. Under water a generous gap between the waling and piling should be allowed and concrete-filled bags supported by wire mesh cages used as packings (see Figure 47).

Waling

\ l

Sheet pile

Waling l Larger gaps filled with (1) Wedge to bring its face

parallel to waling -----(2) Pairs of folding wedges

to close gap

Never use a single wedge or packing. lt will distati waling an d induce eccentric loading into struts

--------Concrete or mortar in bags can be placed by divers underwater

Figure 47 Packings for sheet pifing

Care musi be taken in the fixing of packings as there will usually be some uneven loading of the waling tending lo twist or distort it. !t is therefore advisable lo choose large square section members for walings such as Uni versa! Column (UC), Universal Bearing Pile (UBP), twin Universal Beams (UB) or twin Rolled Steel Joists (RSJ), which are more stable under these conditions than a single Universal Beam (UB).

Welding of walings is best carried out under workshop conditions, i.e. 'shop weld- si te boli'. Site welds should be generously sized.


80

Where diagonal struls forrn part of the frarnework or a walìng acts as a strul for another walìng, then the waling musi be designed to resisl both lhe axial and bending loads, together with suitable shear plates at lhe conneclions (see Figures 48 and 49).

Sheet pile w ali

Figure 48 Detail ot raking strut

Brackets

Raking strut

/Folding wedges

Design of the waling should be in accordance with BS 5950''", with a load factor of 1.4 on the design load (see above) or with BS 449°".

Any waling joinl should be located at or near the point of minimum bending moment in the waling. For twin channel walings they should be staggered on each side of lhe suppor!.

Where tie rods or ground anchors are used as supports, the walìng is norrnally made from twin steel channels placed back lo back with spacers between to allo w lhem to be installed easily, taking inlo account any inclination. Spacers will be required al intervals. If the inclination is significanl then spacers should be provided at each side of the lie rods or ground anchors logelher wilh a suitably angled washer or olher seating arrangement. Altematively lhe waling itself can be inclined with special sealing arrangements between the piles and waling. If the waling is at the back of the wall lhen lhe piles must be tied back to the waling with suitable bolts and bearing plates. Ali lhese details must allow for the transfer of any induced vertical load to the piles lhrough the waling.

Where the walings are to be supported by internai struts they should be of generous size to allow for the conneclions to the struts. UC or UBP seclions are commonly used for the less heavily loaded walings (see Figure 50).

Supports lo the waling should be located on pans of the piles which are in contaci wilh the waling and a check. must be made for web buck:Jing. Where loads are heavy, il is good practice to provide web stiffeners even where noi theoretically necessary. This will reduce the possibility of the walings twisting due to uneven loading or poor packing belween the pile and waling.

Brackets can be fixed to the wall below the walings to facilitate their erection. If the tension member is inclined downwards then the brackets must be designed to transfer the verticalload to the wall, and to stop any twisting of the waling. Similarly if raking struts are used inside the cofferdarn then brackets will be required above the walìng (see Figure 48). Where verticalloads are involved a check should be made to ensure that the wall will be able to suppor! the Joad. It may be necessary lo weld the clutches of adjacenl piles together to spread the Ioad along the wall.


Detail A Oetail B

DETAILA

Shear p late weldedto waling

DETAILC

Figure 49 Details of stee/ framing

Detail C

Use nominai size bolts to hold strut Web and filler plate in piace

stiffener

When removing frame:

1. Sling strut 2. Remove ali bolts 3. Remove filler plate

by blowfrom hammer

Diagonal End Filler Shear Waling

pl'ìte plates stlut pl'ìte

~ + l + .,.

~ + + + ~

DETAILB

DETAILD

If lhe walings and struts are prefabricated as a complete frame, il is necessary to provide a gap between lhe piles and waling to allow for variations in the line of piles as driven and for any movemenl during excavation. This is norrnally not less lhan 100 mm and can be up lo 300 mm at lower levels for the larger frarnes where a greater tolerance is required to allow for the possibilily of piles being deflected from the vertical.


82

Strut

Figure 50 Waling details

- Universal Column frequently used

Don't use a single Universal Beam as a waling as it tends to be unstable

Better to use:

Twin Universal Beams

or

.........._ Universal Column

Sheet piles

More steel required but better design

Waling supported on a bracket welded to sheet piles at intervals suitable to ensure latera! stability of flanges an d always a dj acent t o struts. Bracket can be made from an offcut of Universal Beam or Column. Check weld for ability t o withstand any verticalload applied to waling. Bracket w ili be required above waling if raking struts apply verticalload upwards. Assume verticalload is applied to bracket at si de of waling away from the sheet piles.

Rolled steel sections used for walings and struts may have their webs horizontal and, with the various stiffeners added, become a trap for water to collect. They are often used as access ways, so holes should be cut at suitable places to allow this water to drain away. The design should also be checked to allow for the possibility of water and/or debris collecting in these areas.

Where there are more than two frarnes, consideration should be given to the procedures for erecting and removing the lower frarnes as construction proceeds to make sure that they can be installed and taken out without undue difficulty.

5.2.2 Circular wallngs

Circolar walings are designed as a ring bearn. In practice they will probably vary from a true circle and will therefore be subject to some eccentric loading. Draft BS 8002°2

) gives the following equation for calculating the size of waling.

W = 1.5 x El kNfm R3 x 10'


where W E I

= Safe radiai waling load in kN/m run = Young's Modulus of waling materia! in N/mm2

= Moment of inertia about x-x axis in cm4

R = Radius on centre line of cofferdarn piles in metres

The above basic formula is based on Tirnoshenko's work'"'l wherein the formula is given as

W = k El kN/m • R3 x 10'

where W, is the ultimate radiai waling load and k is a factor, the value of which is dependent on the stiffness of the retained medium. 3 is the value for water, e.g. in a marine cofferdarn built to facilitate construction on the sea bed. Progressively higher values are, in theory, applicable for weak/medium/strong soils. However, it is common practice to use the value of 3, to which a factor of safety of 2 is applied. Hence the value of 1.5 in the basic formula

The ring bearn can tolerate very little distortion from a true circle before the onset of catastrophic instability. Hence the empirica! rule

d;, D/35

where d is the depth of the ring beam, i.e. the difference between the outer and inner radii of the bearn, and D is the diameter of the cofferdam (i.e. the diarneter of the inner face of the piles).

Because stiffness is a prime requirement of the ring bearn, it is usually constructed in reinforced concrete but steel can be used provided the depth rule is observed and particular attention is paid to ensuring latera! stability of the inner flange of the bearn (see Figure 51).

Figure 51 and Table 12, from the British Steel Piling Handbook"\ give some details of bearn sizes for various loadings and cofferdam diameters. If the sheet piles deflect to any great extent then the load on the walings will be concentrated at the top or bottom of the waling and will impart torsion into the bearn. This should be checked in the design.

''"""/' .... Circularreinf:Ced con/te walings

~Steel sheet p1l1ng

.• '1/i:-,Y//

"'"""

Figure 51 Circular reinforced concrete walings


Steel sheet piling

Waling

83

84

Table 12 Reintorced concrete walings !or circular cofferdams

Safe wallng Joad kN/m

Dlameter of cofferdam (m)

Slze of wallng 'd' x 'b' In m m and number of relnforcing bars (In mild steel to BS 4449(•:1J. Characterlstlc Strength 250 N/mm~

450 x 300 600 x 400 750 x 500 900 x 600 1050 x 700 6 No ZO mm 10 NolO mm 10 No 25 mm 14 No 25 mm 12No32mm bars bars bars bars bars

5.0 280 500 10.0 140 250 390 15.0 90 165 260 375 20.0 125 195 280 380 25.0 !55 225 305 30.0 185 255 35.0 215

The tabulated safe loads are based on:

l. The pennissible compressive stress in the concrete not exceeding 5.2 N/mm1•

2. W = 1.5 BUI WRJ Where W = waling load in kN/m E Young's Modulus far concrete= 13800 N/mm2

I Moment of inertia about 'xx' axis in cm4

R Radius of cofferdam in metres

3. Depth of beam 'd' to be not less than D/35.

5.3 ANCHORAGES FOR EXTERNAL SUPPORT

5.3.1 Wallngs

When the upper part of a cofferdam is supporied by exiernal ties rather !han by internai props, the anchorage system is essentially the same as that used for the support of sheet pile retaining walls. Walings consist of two channel sections placed back to back with sufficient space between them to accommodate a tie rod or tendon.

Bending moments in the walings are usually calculated ignoring the benefits of continuity, steel stresses being taken as for structural steelwork.

5.3.2 Anchors

The anchor resistance may be derived from passive deadmen, from driven tension piles or from grouted ground anchors. The latter form may be preferable when the soil outside the cofferdam

09~;-?tt .. w / . ....._ n

l / ......... i ./ ......... i /·j ~ Anchor behind

j/ these lines

///

-w;rn""-1 / t::;_s' + ~12 /

w n

l i

i i

A Passive anchors

Figure 52 Types of anchor

/ /

/

A Ground anchors


carries services, shallow foundaiions, roads or railways, where ihe ground water table is above the tie rod leve!, or where the ground surface outside the cofferdam rises steeply. In most other cases a passive anchorage is usually more economica!, provided there is sufficient open ground available around the cofferdam.

The anchors must derive their resistance from soil which lies entirely outside the potentially unstable wedge supported by the cofferdam, as shown in Figure 52. The starting leve! for the unstable wedge is point A on the diagram, for the stage of construction under considera1ion.

5.3.3 Passive anchors

Passive anchors may be constructed of sheet piling or of reinforced concrete. The latter is less common in temporary works because of the problem of disposing of the concrete at the end of the contract In either case the actual anchor is usually designed as a balanced system, i.e. the tie rod is arranged to attach to the anchor at the centre of pressure of the passive resistance. Thus, in granular soils, the depth to the tie rod at its connection wiih the anchor wall will be two-thirds of ihe depth to the toe of the anchor.

Where a balanced anchor is difficult to achieve, a cantilever anchor may be used. The deflection of this type of anchor is difficult to predici, and will always be more !han the equivalent balanced anchor. This could lead to larger loading on the lower frames than that of the nominai design.

Anchor walls may be continuous or in isolated short lengths. Occasionally, when the loads are very small, the anchor may even consist of only a single Larssen pile. The available passive resistance should be calculated on the basis of net available resistance, i.e. passive pressure less active pressure. When anchors are composed of sheet piles, no account should be taken of wall friction as there is insufficient weight in ihe anchor wall to provide the necessary resistance to uplift. Under no circumstances should advantage be taken of any surcharge loading on the ground surface in front of the anchor, but surcharge loading musi be allowed for on the surface immediately behind the anchor e.g. for construction equipment and materia!. Care must be taken to see that the ground in front of the anchor is not disturbed.

Tie rods up to 75 mm diameter are designed using ihe following working stresses with a factor of safety of 2.0.

Mild Steel (BS 436ci'" Grade 43A) High Yield Steel (BS 4360 Grades 50B or 50C)

111 N/mm2

140 N/mm2

Tie rods are also obtainable in higher grades of steel, and design data for these should be obtained from the supplier. It should be noted that increased working stresses will result in increased strain at working load.

5.3.4 Ground anchors

Ground anchors require no disruption of the surface area surrounding the cofferdam and can be installed at more than one leve!. However, they do occupy space outside the limits of the cofferdam and hence may not be permissible if they project beyond the site boundary. Also, they must be abandoned in piace at the end of the contrae! and may thus constitute a hindrance to other adjacent construction.

Ground anchors are usually installed at an inclination below the horizontal. The sheet piles must in such cases be designed to resisi the induced vertical component of the inclined anchor force. This may require deeper penetration since the force can only be transferred to the soil by friction on the passive face below formation leve! and by end resistance. No friction can be taken on the active face.


86

No advantage of wall friclion musi be taken in lhe calculalion of active pressures when using inclined ground anchors due lo the tendency of the wall lo move downwards under lhe influence of lhe induced vertical load (see Section 4.1.3).

Ground anchors are usually pretensioned during installalion. This prelensioning may prevent the piles moving sufficiently to allow full realisation of active pressures. BS 8081"0 provides guidance on the design of ground anchors.

5.4 STRUTS FOR INTERNAL SUPPORT

Of ali the various components of a cofferdarn, the internai struts will be at the greatest risk of darnage due to falling objects, swinging buckets or loads from excavators and cranes. The consequence of these members failing is also likely to be more serious than the failure of a waling or a pile.

If a strut fails it is unlikely that there will be any warning such as graduai movement, or any lime to take remedial measures. The failure will be sudden and unless such a failure has been allowed for in the overall design then it may lead to progressive failure and to the complete collapse of the cofferdarn. It is therefore unwise to economise in the design of struts, particularly for long ones.

Modero construction procedures tend to use large cranes and excavators. In order to reduce the risk of accidental damage il is best to adopt the largest practicable spans for the walings to keep the struts further apart. This will require larger walings and struts which are better able to withstand an accidental load.

The best strut of ali is a tube since there are no projections to snag a swinging load, but box piles, Universal Colurnns and twin Universal Beams (suitably battened together) are commonly used. A single Universal Bearn has little value as a column or strut because of the relatively small radius of gyration in one piane. Diagonal corner struts can help by creating a larger open area at the end of a cofferdam, though at the cost of more difficult access to the corner area (see Figure 68).

The detailing of the end fixing of a strut is very important because the load from the waling should be transferred to the strut as close as possible to its axis. There are proprietary end fixings available which include a heavy screw with square threads which can take up a limited tolerance in length and assist in removal of the strut by the easier release of the load. It also ensures a nearly axial load on the strut. Some tolerance in length can be accommodated by the use of packing pieces between the strut end and the waling (see Figure 53).

Consideration must also be given to the installation and removal of the struts particularly al the lower levels if thete is more than one frame.

The struts should be designed in accordance with BS 449°'' or BS 5950°". In addition to the strut load, allowance should be made for bending due to self weight and any anticipated imposed loads such as pumps, access ways, interrnediate struts or staging for materials, and any debris or water which might collect on the strut. A common occurrence is the temporary stacking of bundles of steel reinforcement on the frarning and struts as the perrnanent work is being built.

The various steel stresses for columns make allowance for various imperfections including variation from the ideai straigh1ness of the colurnn. If secondhand steel is to be used for struts then it is very important that the straighmess is checked and any variation should be wilhin the tolerance allowed for new steel.

Unless careful detailing of the connection between the strut and the waling ensures axial loading, e.g. spherical bearings, then some eccentricity of the axial load should be allowed.


For walings made from a single section UC or UB, the eccentricity should be approx IO% of the overall dimension of the strut in the vertical piane. Where the walings are constructed from twin bearns then the eccentricity in the vertical piane should be half the distance between the webs of the two bearns (see Figure 53).

Strut Waling

D

necessary

\

Sheet piles

4"C~c Eccentric load onstrut

Note: Packings must be secured to stop them falling aut before load is taken up or if load is reduced (e. g. during tidal movement}.

Figure 53 Strut details

y !

x-$-x

l y

r-y l

Circular struts

___ Accidentalload Worst direction vertical

x---I- -x \ Sr~.~~: ~:rversal Beam as strut

\ fx > ry

l Acc1dentalload Y Worst direction horizontal

y

·-IIf· l y

Twin Universal Beam as strut Calculate stress for both vertical and horizontal accidentalload Use worst result for design

Figure 54 Direction of accidenta//oad on struts


88

Provision for an accidental load will depend on both the degree of risk and on the consequences of failure. The engineer will have to use judgement in this respect and it is suggested that a load of 10 to 50 kN be applied norma! to the strut at any point in any direction (see Figure 54).

It may be prudent to consider temperature effects if there are likely to be large differences in temperature during the period of construction. Struts can be painted white to reduce thermal gain.

Struts which are fixed to walings should have the ends regarded as fixed in position but free to rotaie. If they are not supported at any intermediate point the effective length, J..., will equa! the total length, L. Longer struts can be supported at intervals to reduce the effective Iength. If the suppor! is at mid-point in the horizontal piane only, then the effective length will be Lfl in this piane but will remain equa! to L in the vertical piane. This layout can be useful where the struts are column or beam sections with the web vertical. If support can be given to the vertical piane as well then the effective length will be reduced accordingly (see Figure 55).

Waling Waling

/ L

l Ar

(

[ Strut A y [ l

r

x-I--S:ction A-A

y

L/2 l L/2

n Strut il lJ

l

i

Strut ~~Kingpile i

Figure 55 Effective /ength of struts

b;-lxx= L lyy :o= L

lxx= l lyy = L/2

lxx= L/2 lyy = L/2

Support in the vertical piane can he provided by either king piles or by bracing in the vertical piane. It is very important that if the strut design relies on king piles for support then they must he very frrmly fixed in the ground and must on no account be removed or disturhed until the struts are ready to he taken out. Details of the joint between the king pile and strut must allow for tolerance in position and level so that the strut can be held truly straight.

Vertical bracing can he used where there are two or more levels of struts. The upper strut will he designed without any vertical support as it will have to be erected and take its load hefore the next frame is ready. The bracing takes the form of a lattice beam in the vertical piane similar to that shown in Figure 6. It should be designed to take all the vertical loads on the struts with a generous margin of safety.


5.5 DOUBLE-SKIN, EARTH-FILLED COFFERDAM

When large cofferdarns are required for construction in marine situations, e.g. cut-off walls across harbour entrances, large bridge foundations in rivers etc, an alternative to an internally strutted cofferdarn is two parallel walls of sheet piles tied together, with earth fili between them.

Stability is derived from the internai friction of the fùling materia!, hence the fili must be granular soil. The fili must also be free draining in order to minirnise pore water pressures.

There are several methods of analysing the stability of such structures in respect of the overturning effects of unbalanced soil and water pressures"'), but for practical purposes the structure is considered to be safe if the width of the cofferdarn B is at least 0.8 x the retained depth of water H (see Figure 56).

""'

Figure 56 Doub/e-skin, earth-filled cofferdams

The inner wall (nearest to the excavation) is the most heavily loaded and is designed as a tied retaining wall. The outer wall is less heavily loaded and is usually detailed to be similar to the inner wall. However, it is permissible to design the outer wall on its own merits, but it must be remembered that it is in effect an anchor wall to the inner wall and will be subject to pressure at the top of the fill which is greater than the natura! active values, because of the tie rod force (see Figure 57).

Outerwall

Additional pressure to resist tie force

Active pressure

lnnerwall

Figure 57 Pressure diagram far a doub/e-skin, earth-filled cofferdam


90

Weep holes are provided at the foot of the inner wall in order to reduce hydrostatic pressures, and suitable drainage at formation leve! will be required.

On very long walls of this type, it is good practice to incorporate cross walls at regular intervals so that the whole structure is broken down into a series of rectangular cells. This will facilitate construction, enabling fili to be placed as the walls progress, and will also contain any failure in lhe completed structure to one celi.

Construction may be from a lemporary working platform erected alongside the cofferdam, or by using the partly constructed and filled cofferdam as an access way and working platform (see Figure 58).

Figure 58 Construction of a doubfe·skin, earth·filfed cofferdam

For further reading on lhe subjecl refer to 'Proceedings of the Conference on Design and Installation of Pile.Foundations and Cellular Structures'(22).


6 Construction, maintenance and removal of the cofferdam

6.1 CONTROL OF WORK {INCLUDING DESIGN)

6.1.1 Generai

The successful design and construclion of cofferdams will depend on good communication and management by ali concemed i.e. the Employer, the Permanenl Worl<s Designer, lhe Temporary Works Designer, and the Constructor.

The legai responsibilities between lhese parties will depend on the nature of the contracts belween them and on lheir respeclive statulory and common law obligations. Every effort should be made lo dea! wilh these matters explicitly in the contraci documents. Not ali standard conditions of contraci refer lo temporary works and il may not be clear where lhe divisions of responsibilily fall, for instance when a cofferdam is incorporated inlo the permanenl works.

Usually ali responsibililies for cofferdams as temporary works are left lo lhe construclor and it is importanl lhat a policy is established to contro! ali aspects of cofferdam construction fully from design lo removal. This should ensure the safely of lhe works and ensure that the design and execution will be to a high standard.

It is recommended that the duty of ensuring that ali lhe relevant procedures and checks have been carried oul should be done by one individuai in the construction organisation known as the Temporary Works Coordinalor.

A brief outline of lhe various responsibilities in accordance with norma! UK practice is given below.

6.1.2 Contractual requlrements

Under the Inslitution of Civil Engineers (ICE) Generai Conditions of Contraci, the arrangements are:

• Permanenl Works Designer: The Engineer (Employer's staff or consulting engineer)

• Permanent Works Construclor: The Contractor

• Temporary Worl<s Designer: The Contraclor, excepl for those ilems designed by lhe Engineer

• Temporary Works Construclor: The Contractor

The Engineer will have an overall responsibility lo the Employer and should be satisfied lhal lhe lemporary works are designed and executed properly.

6.1.3 Statutory requirements

The principallegislalion in lhe UK is lhe Health and Safely Acl 1974 (HSW Acl) which gives generai requirements for heallh and welfare. 1t musi be followed bolh by designers and construclors since the Employer (in lhe ICE lerminology) and the Permanent Works Designer will have obligations under the Acl as well as lhe Contraclor.


92

In addition lo the HSW Act there are severa! Regulations which give generai and particular guidance conceming the construction and contro] of cofferdams. The need for regular thorough examinations of cofferdams and for the results to be recorded in a register is emphasised (see Section 6.6).

6.1.4 Other legai llabllltles

Ali the parties listed in Section 6.1.2 have numerous duties of care under common Iaw. It should be noted, however, that anyone with a supervisory function on site who has the ability lo make appropriate judgements may bear some responsibility. Consequently, an engineer who is aware of some deficiency in the temporary works before hann befalls may be liable together with the Contractor for any damages awarded by a court of law arising from such an incidenl Further infonnation on construction law is given by Uff'44>.

6.1.5 Organlsatlon of temporary works contro!

Figure 59 shows an organisation flow chart fora typical temporary works cofferdam. It should be noted that a preliminary design will have been prepared at tender stage.

Site management

TWC distributes documents as necessary

Temporary works designer 1 Permanent works designer

Monitoring an d and advice

Permanent work drawings, key features,

liaison

Comment and information

Copies for engineers si te stati

~ Collaborative and liaison activity

~ lndicates responsibility for action

Figure 59 Flow chart of principa/ activities and responsibilities for design and construction of a temporary cofferdam


6.1.6 Responslbllltles of the Temporary Works Coordlnator

The Temporary Works Coordinator will usually be a member of the Constructor's line management, and must have sufficient experience to carry out the duties and be given the necessary authority. The responsibilities of the Temporary Works Coordinator include:

• coordinate ali temporary works

• ensure that the various responsibilities have been allocated and accepted

• ensure that a design brief has been established with full consultation, that it is adequate, and that it is in accord with the actual situation on site

• ensure that a satisfactory cofferdam design is carried out, including calculations, drawings and method statement

• ensure that the design is independently checked for:

- concept

- structural adequacy

- compliance with the brief

• ensure that safety is properly considered in the design. Drawings and method statements must include requirements for protection and access/egress as appropriate for the construction of the pennanent works

where appropriate, ensure that the design is made available lo other interested parties, e.g. the permanent works designer

• register or record the drawings, calculations and other relevant documents relating to the fina! design

• ensure that those responsible for on-site supervision receive full details of the design, including any limitations associated with it which should also be included on the drawings

• ensure that checks are made at appropriate stages during construction covering the more criticai faclors

• ensure that the pennanent works are capable of taking any transferred loads when removing or adjusting the temporary works

• ensure that the actual ground conditions and any proposed changes in materials or construction are checked against the originai design and appropriate action taken if necessary

• ensure that any agreed changes, or correction of faults, are correctly carried out on site

• take appropriate action if any changes are needed during the use of the cofferdam or if the environmental conditions change

• ensure that during use ali appropriate inspections and maintenance are carried out and registers are kept up lo date

• ensure that the cofferdam is dismantled in accordance with an agreed safe procedure.

BS 5975<40> gives useful guidance.

6.2 INSTALLATION OF SHEET PILES

6.2.1 Types of pile drlvlng equipment

Sheet piles can be installed by a variety of methods and equipment. Each has particular advantages and disadvantages and the fina] choice is, in most cases, a compromise between speed, accuracy and economy of installation. A further deciding element is the increasing concem for noise and vibration contro!.


94

Table 13 indicates the various options and relative merits.

Tabte 13 Types of pile driving equipment

Type or driver

Double acting aìr hanuner

Single acting diesel

Double acting diesel

Cable operated drop

Hydraulic drop

Vibrator

High speed vibrator

Hydraulic thrust

Sol t compatiblllty

Ali soils except stìff clay

All soils

All soils

Ali soils

A1l soils

Granular and soft clays

Granular and soft clays

Granular with jetting and Oays

Nolse Qui et output verslons

High N

High N

Medium/ N low

Medium/ y low

Mediwn/ y low

Medium N/A

Medìum/ N/A low

Low N/A

Key: N/A= Not applicable : Y = Yes :N= No

Vibration output

Low

Mediwn/ high

Mediwn/ low

Medium/ lo w

Medium/ low

Low

Low

N il

Extract

y

N

N

N

N

y

y

y

Rate of penetratlon

Mediumf lo w

Medium

Mediwn/ high

Medium/ low

Medium

High

High

Palrslslngle drlvlng

Both

Pairs

Pairs

Pairs

Both

Both

Both

Slow/Medium Single/Panel

Impact drivers (air, diesel, hydraulic, cable) are the most suitable for generai work in mixed soils. However, if noise contro! is important, careful selection is necessary and some reduction in speed of driving should be expected.

Whe~e the soils are esse?tiaily granular and of low to medium density then a vibrating driver will mstall piles very qmckly. However, where there are adjacent structures, care must be taken if the piles can encounter clay strata since ground resonance may then occur, with the possibility of structurai darnage due to vibration.

Similarly, where the soils are essentially cohesive a hydraulic thrust driver will prove vaiuable in urban and city centre locations.

On congested sites it is sometimes difficult lo obtain dose access to the pile posilion by the crane/ma~hine which will handle the pile driving equipment. In these circumstances the weight of the drivmg eqmpment becomes a further important consideration.

6.2.2 On slte storage of piles

Piles will usuaily be delivered to site in nested stacks of pairs or singles. H is preferable to off·load and store lhe piles in these stacks unti! they are required (see Figure 60).

Piles for temporary works will normaily be unpainted. Therefore the use of chain slings for handling the piles is not detrimental, provided that the interlocks are not darnaged.

Cl RIA Special Publication 95

Figure 60 On site storage of pifes

The storage area should be reasonably leve! and finn. H is essentiai to provide adequate suppor! between the ground, the piles and between successive stacks of piles in order to minimise sagging under self weighl leading to possible loss of straightness. The centres of timber dunnage should be no greater than that used for the transport of the piles to site, and, where the ground is relatively soft, lhe ground leve! dunnage should be at suitably closer centres. A typicai storage specification is shown in Figure 61.

When piles are to be lifted directly from the storage area into the upright driving position, il is useful to store them with the lifting hole end nearest to the crane position.

When handling new piles, it is wise to inspect for and remove any rough edges on the sawn ends of the piles, 'Hot saw rag' is very sharp and can cause serious injury to operatives' hands.

Similarly, the removai of such 'rags' from the inside faces of interlock ends will greally facilitate entry of one pile into its neighbour during the pitching process.

Buti welding multiple lengths

When multiple lengths are to be welded together to form longer piles, the resulting piles must be straight and the interlocks musi be accuralely aiigned al each butt weld Ends to be joined must be properly prepared for welding and the welds must be of good quaiity. Details of weld preparations and welding procedures are available from pile manufacturers.

Lengths to be welded should be selected so thal the weld positions in adjacenl piles are staggered by at least 0.5 m in the driven wail. Where possible, welds should be located at a point in the pile length well removed from the position of maximum bending moment

Manufacturing tolerances mean that piles of the same section size can have severa! millimetres difference in interlock centres. Interlocks must be aligned at the welds in order to ensure ease of pitching and driving. Alignment can be achieved by constructing a template into whlch the ends to be welded can be entered prior lo welding. The laying out bed on which the template is set will aiso be provided with pile supports to ensure overail straightness. Figure 62 shows the generai arrangement and the detail of the butt welding template.

The welds are completed across the full width of the pile except for the interlocks. The pile is then removed from the template so that the welds can be completed. H is important to avoid weld meta! being deposited on the inner faces of the interlocks and thls should be carefully checked before lhe piles are used.


96

l~ 'B' 'B'

Section Frodingham SW1, SW1 A

Frodingham 1N, 1BXN, 2N, 3N, 3NA

Frodingham 4N, No.5

Larssen (ali sections)

(1) Pii es are t o be stare d an reasonably flat ground. Base supports to be levelled to a taut string line.

(2) Ali packings are to be of equal thickness and strong enough to carry the weight of the sheet piles.

(3) Where piles are to be loaded or stored in tiers, the packings should be in verticallines.

(4) The spacing of packings is to be in accordance with the sketch and table shown above. (Applicable to site storage and transport.)

(5) Piles are not to be lifted using a single chain, multiple chains or lifting beams MUST be used.

f' 'B' 'B' 'A'

'A' 'B' 1 metre 3 metre max.

1 metre 3 metre max.

1.5 metre 4 metre max.

1.5 metre 4 metre max.

(6) Stocks should be limited to 4 slings high. (7) Stock in pyramids, with decreasing number of

bundles on each successive layer. (8) Bundle size to be 1 O tonne maximum

FOR GUIDANCE PURPOSES ONL Y

Figure 61 Recommended storage procedure for stee/ sheet piling

6.2.3 Pitchlng and drlvlng

The prime requirement of sheet pile installation is contro! of the verticality of the piles both parallel and transverse to the piane of the piles, coupled with consistent accuracy of Iocation on pian.


Buttweld

T empiate piles fixedtobed

Lengths to be welded

Figure 62 Genera/ arrangement and detail of the buti welding template

Far most projects the best way to achieve this is to use the 'Pane!' driving technique. This consists of setting up a guidance system so that a number of individua! piles can be pitched, (interlocked in the vertical position) before driving commences. A typical guide frame is shown in Figure 63. With such a system the hammer is operated in rape suspended leaders. The hammer is equipped with leg guides which project below the body of the hammer to maintain its corree! Jocation on the pile head (see Figure 64).

Top an d bottom gates must be rigidly

mounted and stiff

~ Transverseand ,.. )? '\ , longitudinal

Kelly blocks L-must be heavy

and on a firm base

Figure 63 The 'Pane/' driving technique - guide frame

racking must be minimised

The advantage of the system is that, since each pile is restrained in position, due to being interlocked with its neighbour, a good contro! of the alignment of the piles is possible as the piles are driven to 'depth in the ground, provided that no pile is driven too far ahead of its neighbour, Excessive penetration of any pile below its immediate neighbour can result in the driven pile deviating from the theoretical line, since the stiffness of the projecting length of pile shaft will diminish proportionately to the cube of the uninterlocked length. Figure 65 shows the progression of events in detail.


98

Rope suspended leaders

Hammer

Figure 64 Detai/ of hammer /eg guides

Disadvantages of the system are:

Leg guides

lnserts

the need for a crane with a long boom to lifl piles lo lwice lhe pile Iength when pilching (Figure 66)

• the very high interlock friclion which can develop if pile verticality is not maintained

• the top gale must be removed after roughly half the pile length has been driven, to allow passage of the pile driver. Thus, the pile head is only restrained in the transverse direction for the upper part of the drive.

Three factors are criticai to the success of the operation:

• the guide frame ìnust be sufftciently stiff and rigid lo hold the piles in position and Iine

• the frame musi stand on sufficiently finn ground lo maintain its position under the effects of vibration from the piling hammer and the latera! forces which the piles will exert if they tend to deviate from the theoretical line

the piles must be well secured in the frame by blocks and wedges.

When long piles are being driven lhrough soft soils, lhere is a lendency for the tops of lhe piles to creep in the longitudinal direction of the wall. This is due to the eccentricity between lhe centre of the driving force and the centre of lhe combined soil resistance and interlock friclion with lhe previous pile. Any lean must be corrected as soon as it is detecled, either by extracting and redriving the offending piles or, if things have gone too far, by lhe insertion of a taper pile inlo the run. The latter is only to be recommended as a Iast resort. lf the alignmenl of each pile with its neighbour is not maintained, lensile and compressive forces along the line of the wall will be generated, resulting in very high interlock friclion belween adjacent piles. In extreme cases this friction can be so great as to prevenl the piles being driven to full penetration.

While piles should have been selected to suil the anlicipated driving resistance, as well as the struclural requirements, there are occasions when unexpectedly hard driving is encountered. In lhis condition il is wise lo inslilute reduced pile loe leads i.e. the progress of any pile toe beyond thal of the immediately adjacenl pile. This will assist in maintaining pile alignmenl,


• l

although overall p(Ogress will be slower because of the more frequent movement of the driver from pile lo pile.

m Il

""

rr m Il Ili l

'""

rrr

'"

\r

l Pitch, align, plumb and part drive first pair

Il Pitch remaining piles,plumb, and part drive last pair

l! l Part drive remainder of pane!

IV Continue with successive panels, Last pair in each pane! becomes first pair of next panel

Figure 65 The 'Pane/' technique - stages in driving

On occasions the pile heads will lend to buckle under continuous hard driving. As soon as head damage is observed driving must be halted and the pile head trimmed by burning to restare an undamaged profile under the driving plate of the hammer. !t this is not done, little further penetration will be obtained and the pile head will become so deformed that it will become impossible to remove the hammer from the pile without dismantling the inserts on the hammer

leg guides.

A careful record of driving should be kept so that any difficulties encountered can be accurately Iocated when excavation commences, and particular care taken in areas where the continuity of the wall (separated interlocks, darnaged piles, misaligned piles, etc) is suspect.

It is most irnportant to keep a detailed record when pile heads have been trimmed during driving and the designer should be informed when any piles have not been driven to leve!. Where difficult driving is expected it is good practice to mark the piles at l m intervals. As the excavation approaches formation leve!, areas adjacent to those piles which were not driven to full depth can be controlled with extra care to avoid the danger of bottom instability. lf this occurs only occasionally around the perimeter of the cofferdarn, the difficulty can probably be


100

dealt with by the engineer on site. If the piles are grouped together the effect is more serious and the designer should be consulted about remedial measures.

Figure 66 Pitching piles

An alternative method of installation is referred to as 'pitch and drive', which means pitching smgle piles, or pairs of piles and driving them before proceeding lo the nexl pile. The syslem uses a pile driver mounted on a mast or hanging leader which is also used lo restrain the piles, thus dispensing with the need for the guide frame used when pane! driving.

Recently developed high-speed vibraling pile drivers used in conjunction wilh this syslem are proving to be very effective, producing high rales of progress and good accuracy of pile alignmenl. Vibrators of lhis type are limiled lo medium pile size and length.

The pitch and drive syslem is sometimes preferred when very long piles are to be driven, as mlerlock friclion due lo misalignmenl of adjacent piles is avoided, although the likelihood of piles declutching during driving is grealer.

Boli holes are oflen burned in piles for the temporary support of guide wales and various incidental attachments. When working in water bearing granular soils, it is important to seal such holes prior to driving below ground leve!, olherwise water entry during excavation may be a problem. This is particularly so when the hole will be in the length of pile below forrnalion leve!.

6.2.4 Order of drlving

In order lo make pitching and driving of the closing piles as easy as possible, installalion should commence at piles adjacent lo one of the corner piles and progress around the cofferdam working away from· the corner pile. The cofferdam should then be closed by pitching one plain pile and the last corner pile as an interlocked pair. The gap to be closed is unlikely to be the theoretical distance and piles at each side of the gap are urùikely to be truly parallel with each other.

Closing with an interlocked pair consisting of a plain pile plus a corner pile provides an articulated closure where the interlock centres can be varied at the lime of pitching and can continue lo vary as the closing piles are driven (see Figure 67).

Section 4.7 describes the advantages and disadvantages of the alternative types of corner pile. When hard driving is anticipa led it may be preferable to use junction piles at corner localions. Such sections are stiffer and less likely to be deflected from the theoreticalline. In particular, if


the pile layout can be adapted to use closed junctions, these will fit into the guide frame very conveniently.

Figure 67 C/osing inter/ocked sheet piling

6.2.5 Safety

The process of pile pitching can be very dangerous. Access lo lhe head of lhe piles in order lo guide the bottom of the lifted pile inlo the top of the previous pile should always be by safety cage, unless lhe operation can be perforrned with safely, directly from top gate leve!.

The top gale is used as an access platforrn and a suitable walkway width and handrail is a legai requirement in the UK. Ladders providing access lo the top gale should be securely fixed.

Some of the danger can be obviated by lhe use of patented pile threading devices which enable piles to be interlocked with each other without the need for access above top gale leve!.

The crane cable sh(mld be attached to the pile heads by means of a remote release lifting shackle. Such shackles musi be inspected frequently lo ensure safe operation.

6.3 EXCAVATION

The method of excavation will vary according to circumstances and lhe ingenuity of the contractor. Constraints on access and spoil disposal will be important.

If spoil is to be temporarily stored on sile before being used as backfill it must noi be placed in an area which would affect the stability of the cofferdam, unless specifically allowed in the cofferdam design. Careful contro! musi be maintained lo ensure that excavation levels do not exceed those allowed for each stage of construction, particularly when excavating under water when it is ali too easy to overdig.

In an open cofferdam or before any frarnes have been fixed, excavation can be by any suitable means, frequently by dragline, back hoe, tracavator or loading shovel directly into trucks, with a rarnp down to excavation leve! if necessary.

Once any supporting framework is in piace the excavation is usually limited to using a small tracavalor belween the frames lo feed spoil lo a suitable point for hoisting by grab or back hoe. Below water leve! excavation may be lirnited lo grabbing, with an airlift pump lo clear the


102

fonnation leve! of debris and silt. When excavating by grab under water the tendency is to dig a series of holes between the struts leaving ridges under the struts, and the grab will stop further excavation at some point. The corner of a cofferdam under a diagonal strut is also a difficult area. In these circumstances the most open arrangement of framework is preferable. In some circumstances an airlift pump can be used for the main excavation. Big airlifts of 450 mm diameter have been used with an offset toothed end to operate under struts and walings. Divers with high pressure water jetting equipment may be needed sometimes (see Figure 68).

Walings

o

o

Figure 68 Excavation under water

WL

Grabs tend to dig holes between struts and then stop

Difficult zone to excavate

Air lift pump with offset end

It is important to seal a clay or chalk formation with a blinding layer of concrete as quick:ly as possible to reduce any softening.

It is essential to check that the ground strata and water levels do not vary from those taken for the design. If this occurs, the design team must be notified immediately so that a check on any variations can be made.


l

6.4 SEALING OF LEAKS ANO SEPARATED INTERLOCKS (OR ISOLATED PILES NOT DRIVEN TO LEVEL)

In cofferdams with substantial differential head between internai and external water levels, and where the pile cut-off is through fine grained granular soils, the existence of separated interlocks is a potential source of fonnation leve! instability.

When driving records indicate that an interlock has declutched this often results in an unexpected local seepage at the excavated surface, and precautions to stem the flow of water into the passive soils should be taken. Precautions may take the fonn of external grouting in the vicinity of the suspect interlock, or by driving a pair of covering piles down the outside of the cofferdam. As an additional precaution the soil between the main wall and the covering piles can be removed by air lift and the void filled with mortar grout.

Where there is heavy leakage through interlocks above fonnation leve!, these should be sealed if the cofferdam is to be left open for severa! weeks. Graduai wash-out of fines from the external soil may occur, resulting in local collapse of the adjacent surface. This is especially dangerous when the retained soil supports structures or services.

In marine or river cofferdams the ingress of water through the interlocks may require special measures to contro! the flow. These usually consist of tipping power station bottom ash into the water iromediately outside the piling. This will be drawn into the interlocks as it sinks and fili the gaps. In tidal cofferdams the treahnent may need to be repeated after each tide cycle. Proprietary sealing compounds are available.

6.5 CONTROL OF WATER

Contro! of groundwater in excavations is dealt with in CIRIA Report 113"3)·

Bottom instability of cofferdams founded in saturated penneable strata is a particular hazard. Water ingress from soil below the pile toes should be carefully controlled. Fonnation leve! sumps, from which excess water is pumped, should be located as far from the cofferdam walls as possible. This will keep drainage paths out of the passive soil iromediately inside the piles. Such sumps are kept accessible for pumping during and after casting of the bottom slab of the pennanent works by inserting a stand pipe into the formation soil and projecting it upwards to a greater height than the top of the slab. The stand pipe can be sealed with concrete when the permanent construction achieves sufficient weight to ensure an adequate counter to the uplift force.

Wells around the cofferdam should draw water from below pile toe leve!.

The cofferdam design should include an investigation of such potential hazards, but site personnel must maintain contro! systems very carefully and keep a vigilant watch for signs of instability. This may be very localised but on occasions can involve the whole formation and can occur with little warning. In these circumstances the time for evacuation of personnel may be critically short.

Standpipes or piezometers can also be used to monitor the water leve! in the soils at formation leve!, so that potentially dangerous levels can be detected and dealt with before they become criticai.

As indicated in Section 6.4, loss of fines from the soil mass may occur and should be controlled by the use of suitable filters.


104

6.6 ACCESS AND SAFETY

Ali construction work should have regard to the safety of employees and the public at large. In the UK the Health and Safety Act 1974 (HSW Act) applies to ali persons. The publication 'Construction Safety'""" is a very good generai guide to the main provisions of the Act as far as they affect the construction industry. In generai the cofferdam should be inspected once a day and thoroughly examined once a week by a competent person. The examination must be recorded on Form 91 (Part l, section B) under the Factories Act. See Appendix C fora copy of this form and the accompanying notes.

The structural safety of the cofferdam an d any space needed for access is covered in Section 4. It is essential that there is suitable and substantial fencing around the cofferdam, and that access is safe and properly constructed whether by ladders or steps. There should be at least two access routes to allow for escape in an emergency, except for the smallest cofferdam. In certain ground conditions such as old tip areas there may be a problem with noxious gases and suitable precautions must be taken.

If cofferdams are adjacent to water or may be flooded there is the added risk of a person falling into the water with the risk of drowning or being carried away by the current. Special measures must be taken lo prevent the fall in the first piace and to provide rescue as quickly as possible.

Platforms, guardrails, ladders, etc. should be robust enough to resist being carried away in swell or by waves. Safety nets and harness must be provided if protected platforms or gangways are impracticable. Non-slip footwear should be wom. Knee and thigh length rubber boots are almost impossible to remove when in the water and increase the risk of drowning. Their use should be discouraged unless absolutely necessary. For any night work good illumination is necessary and should include the nearby water area.

Full lifejackets are too bulky for generai use on construction sites but buoyancy aids should be provided, They should be strong enough to stand up to continuai wear. Lifebuoys and lifelines should be placed at suitable points along the work and also upstream and/or downstream as necessary depending on the tidal and river flows. A rescue boat should be on standby, powered if in fast-flowing or tidal waters. It should be sufficiently stable to allow a man to be pulled inboard from the water; inflatables are good in this respect where conditions permit their use. Boats should carry oars (even if powered) together with 1ifebuoys, lifelines, torches, etc.

6.7 MAINTENANCE OF THE COFFERDAM

Once a cofferdam has been completed it is ali too easy for the sile staff to forge! it under the pressures of other work. This must be resisted. Regular inspections are not only good practice but are required under the Health and Safety Act. They have to be made weekly and the details entered up in the official register (see Appendix C).

Suitably qualified sile staff should be appointed to carry out these inspections. Points to watch out forare:

• integrity of access routes and fencing

stability of framework and packings

accidental damage

movement of the piling

excess spoil at the back of the piling

ground movement, particularly cracks adjacent to or even some distance away from the back of the piling

abnormal variations in level or inflows of water


signs of soil being carried through the ground by the inflow of water, or boiling of the

formation level

pumping equipment (including any standby) should be working and maintained properly.

Any problems which cannot be dealt with by the sile must be reported immediately to the

design team.

It may be necessary to alter the designed Jayout of a cofferdam after it has been construc~. The design team must be notified of any change needed and their approvai g1ven before 1t IS

carried out. Ali changes must be recorded and entered in the register.

6.8 MONITORING OF THE COFFERDAM

The design of cofferdams does not normally include any consideration of the likely deflection of the various components. Sheet pile movements tend to be. n:uch larg~r ~an those ~ormally associated with other structures, due mainly to the compressibility of s01ls m the passiVe mode

and flexibility of the piles.

Experienced cofferdam personnel expect and accept visible inward deflection of the piles at formation level, provided it occurs almost as quickly as the cofferdam 1s excavated and does not

continue afterwards.

All excavations should be regularly monitored for signs of un.expected d~formation. The most frequent cause of failure is buckling of struts or latera! instab1lity of wal.mg flanges. Both constitute a very dangerous form of localised failure which can lead ra~ndly to progressive failure of an entire frame, followed by successive overloading of assoc1ated frames and

eventually to complete collapse of a cofferdam.

Monitoring by experienced personnel is essential. Visual inspection from the top of th~ cofferdam at the start of each working day, prior to commence~ent of any work w1thm the cofferdam, is a good rule. This should include checking the ad]acent ground for cracks. Formai inspection and signature of a certificate is a legai weekly requrrement.

Al the first sign of unexpected deformations, boiling or piping at formation level, or accidental damage to the supporting frames (e.g. struts impacted by grab, skips, elc.) personnel should be

evacuated until the situation is properly assessed.

6.9 REMOVAL OF THE COFFERDAM

This generally follows the pattem of:

• backfilling

removal of framework

extraction of piles.

The first two of these may be repeated in successive stages. Any special pr~edures necessary will bave been given by the design team, including a method statement g!Vmg a safe method of

work.

It is important that backfilling is carried out properly and that good compactio.n is ~hieved. Poor compaction will not only lead to excessive ground movement but the p11ing wi11 deflect and will become more difficult to extract. Where the gap between the piling and the perm~ent work is narrow, compaction may be difficult and extra attention must be given to the work m

these areas.


106

If the pennanent work is not capable of taking the loads imposed on it by the backfill, some temporary strutting will be required. Procedures for this will have been given previously. Before removing any pennanent struts or walings with ruda! loading, the packings should be removed to release the load. If these members are stili under load then extra care must be taken, slings musi be securely fastened and steadying ropes arranged to limit the sudden movement that may occur when the load is released. If members have to be cut to release them, the operative doing the work must be Ìn a position which is unaffected by the movement. Careful burning of the steel so that a neck is left to yield will release the stress slowly and reduce the likelihood of sudden movement.

Space is often at a premium when the piling is to be extracted because of the continuation of the pennanent work. Careful planning and prograrnming may be needed to allow access for the extraction plani, storage of the piles and their transport off sile.

Extraction of piles is often difficult, particularly the first one. The cast of more than adequate plant will often be repaid by the increased speed of extraction. The force extended to the pile is a function of the extractor itself and the pull exerted by the crane. Choosing the smallest crane required for the extractor will not be economie. The crane should be placed as close to the pile as possible. If difficulty is experienced, e.g. no apparent movement after ten minutes, then the next pile should be tried. Piles should be tried successively around the cofferdarn unti! one pile moves. Once one pile is extracted then adjacent piles should move more easily as the friction of the pile interlocks is reduced.


References

l. BRITISH ST ANDARDS INSTITUT!ON Code of practice for site investigations BS 5930 : 1981

2. TOMLINSON, M.J. Foundation design and construction Pitman, London, 5th Edition, 1986

3. CAQUOT, A., KERISEL, J. and ABSI, E. Tables de butée et de poussée Gauthier-Villars, Paris, 2' édition, 1973 Revised as: KERISEL, J. and ABSI, E. Active and passive earth pressure tables A.A. Balkema, Rotterdarn, 3rd edition, 1990

4. ROWE, P. W. A theorelical and experimental analysis of sheet pile walls P roe. lnstn. Civ. Engrs. January 1955, Vol. 4, Paper No. 5990, p 32

5. CLAYTON, C.R.I. and MILITITSKY, J. Earth pressures and earth retaining structures Surrey University Press, 1986

6. KA W ASAKI STEEL CORPORA TION Steel sheet piling design manual Kawasaki Steel Corporation, Japan, 1982

7. BRITISH STEEL GENERAL STEELS Piling handbook British Steel pie, 6th Edition, 1988

8. PADFIELD, CJ. and MAIR, R.J. Design of retaining walls embedded in stiff clay CIRIA Report 104, 1984

9. CIVIL ENGINEERING CODES OF PRACTICE JOINT COMMITTEE Earth retaining structures Code of Practice No.2, The Institution of Structural Engineers, 1951

10. CEDERGREN, H.R. Seepage drainage and flow nets. 2nd Edition John Wiley & Sons, New York, 1977

11. BURLAND, J.B., POTTS, D.M. and WALSH, N.M. The overall stability of free and propped cantilever retaining walls Ground Engineering July 1981, 28 to 38

12. BRITISH STANDARDS INSTITUTION Code of practice far earth retaining structures Draft BS 8002 : 1987

13. TERZAGHI, K. and PECK, R.B.


Soil mechanics in engineering practice Wiley, New York, 2nd Edition, 1967

i !

107

108

14. BRITISH STANDARDS INSTITUTION Code of practice far foundations BS 8004 : 1986

15. BRITISH STANDARDS INSTITUTION Code of practice far maritime structures BS 6349: Part l : 1984

16. BRITISH STANDARDS INSTITUTION Specification far weldable structural steels BS 4360 : 1990

17. COMMITIEE FOR WATERFRONT STRUCTURES Recommendations of the Committee far Waterfront Structures EAU 1975 Willhelm Emst and Sohn, Berlin, 1978

18. BRITISH ST ANDARDS INSTITUTION Specification far the use of structural steel in building BS 449: Part 2 : 1969 amended 1989

19. BRITISH STANDARDS INSTITUTION Structural use of steelwork in building BS 5950: Part l : 1990

20. TIMOSHENKO, S.P. and GOODIER, J.N. Theory of Elastic Stability New York, McGraw-Hill 1970, 3rd Edition

21. BRITISH STANDARDS !NSTITUTION Code of practice far ground anchorages BS 8081 : 1989

22. FANG, H-Y. and DISMUKE, T.D. Design and installation of pile foundations and cellular structures Proc. Conf., Lehigh University, USA, Envo Publishing Co. !ne., 1970

23. SOMERVILLE, S.H. Contro! of groundwater far temporary works CIRIA Report 113, 1986

24. THE BUILDING EMPLOYERS CONFEDERATION Construction Safety The Building Advisory Service, updated annually.

25. PECK, R.B., HANSON, W.E. and THORNBURN, T.H. Foundation Engineering John Wiley, New York, 1974

26. PAPPIN, J.W., SIMPSON, B., FELTON, P.J. and RAISON, C. Numerica! Analysis of Flexible Retaining Wa!ls Proceedings of Symposium on Computer Applications in Geotechnical Engineering Midland Geotechnical Society UK, 1986

27. PACKSHAW, S. Cofferdarns Proc. Instn. Civ. Engrs. February 1962, Vol. 21, Paper No 6588, pp 367-398


28. POTTS, D.M. and DAY, R.A. Use of sheet pile retaining walls far deep excavations in stiff clay Proc. Instn. Civ. Engrs. Voi 88 (Pt l) December 1990, Paper No 9585, pp 899-927

29. BRITISH STANDARDS INSTIT!JTION Noise Contro! on construction and open sites BS 5228: Part 4: 1986

30. MARSLAND, A. Mode! experiments to study the influence of seepage on the stability of a sheeted excavation

in sand. Geotechnique. Voi 3 1953, pp 223-241

31. McNAMEE, J. Seepage into a sheeted excavation Geotechnique. Voi l 1949, pp 229-241

32. BLUM, H. Einspannungsverhtiltnisse bei Bohlwerken. Wilh. Ernst und Soh, Berlin, 1931

33. BLUM, H. Beitrag zur Berechnung von Bohlwerken. Die Bautechnik 27 (2), 45, 1950

34. BLUM, H. Beitrag zur Berechnung Von Bohlwerken unter Beriicksichtigung der Wandverfonning, insbesondere bei mi t der Tiefe linear zunehmender Widerstandsziffer. Wilh.

Ernst und Soh, Berlin, 1951

35. UNITED STATES STEEL Steel sheet piling design manual United States Steel, Pittsburg, USA, 1974

36. KING, G.J.W. Design charts far long cofferdarns. Geotechnique, Voi 4 1990, pp 647-650

37. DICKIN, E.A. and KING, G.J.W. Computer methods for Ci vii Engineers. (Eds. Cape, Sawko and Tickell) McGraw-Hill (U.K.) Ltd., 1982 Chapter 5: Applications in Soil Mechanics, pp 273-279

38. KAISER, P.K. and HEWITT, K.J. The effect of ground water flow on the stability and design of retained excavations Canadian Geotechnical Journal Voi 19, 1982, pp 139-153

39. Specification far Steel Sheet Piling Federation of Piling Specialists, London, 1991

40. BRITISH STANDARDS INSTITUTION Code of Practice far Fa!sework BS 5975 : 1982

41. FLEMMING, W.G.K., WELTMAN, A.J., RANDOLPH, M.F. and ELSON, W.K.


Piling Engineering (2nd Edition) Blackie & San London, 1991

109

110

42. BRITISH STANDARDS INSTITUTION Carbon Steel bars far the reinforcement of concrete BS 4449 : 1988

43. INSTITUTION OF STRUCTURAL ENGINEERS Soii-Structural Interaction Pre-arnble and Part II, fundarnentals and retaining walls, March 1989

44. UFF, J.F. Construction Law Sweet & Maxwell, London, 3rd Ed., 1981

45. Proceedings of the Ninth International Conference an Sai! Mechanics and Foundation Engineering, Tokyo, 1977, Volume 3, pp 156-170

46. HENRY, F.D.C. The Design and Construction of Engineering Foundations pp 216-220 Chapman & Hall Ltd., London, 1985

47. HARR, M.E. Groundwater and Seepage New York; McGraw Hill, 1962 Section 5-9

48. PEA TT!E, K.R.

A conducting paper technique far the construction of flow nets Civil Engineering & Public Works Review Voi 51 No 595 January 1956, p 62

49. W!LLIAMS, B.P., SMYRELL, A.G. and LEWIS, P. J. Flownet diagrams - the use of finite differences and a spreadsheet to determine potential heads. Ground Engineering, Val 26 No 5 June 1993, pp 32-38


Bibliography

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l. LEE, D.H.

2.

Deep foundations and sheet piling Concrete Publications Ltd, London, 1961

AMERICAN SOCIETY OF CIV!L ENGINEERS Latera! stresses in the ground and design of earth retaining structures State of art papers, Speciality Conference at Cornell University Sai! Mechanics and Foundation Engineering, ASCE, 1970

3. CANADIAN GEOLOG!CAL SOCIETY

4.

Canadian foundation engineering manual, Part 4, excavation and retaining walls CGS, Montreal, 1978

SACILOR Guide practique pour l'utilisation des palplanches metallique Paris, 1980

5. ENGINEERING DEVELOPMENT OFFICE Guide to retaining wall design, Geoguide No.I Engineering Dcvelopment Office, Hong Kong, 1982

6. NIPPON STEEL CORPORATION Steel sheet piling manual Nippon Steel Corporation, Japan, 1984

7. LACKNER, E. The W est German approach to the code of practice far waterfront structures P roe. fnstn. Civ. Engrs. August 1984, Vol. 76 (Part l}, 671

8. DANISH GEOTECHNICAL INST!TUTE Code of practice far foundation engineering DG! Bulletin No.36, Copenhagen, 1985

9. ARBED Practical design of sheet pile bulkheads Luxembourg

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2.0 HISTORICAL

l. BELL, A. L. The latera! pressure and resistance of clay and the supporting power of clay foundations Proc. fnstn. Civ. Engrs .. February 1915, Vol. 199 Also published in 'A Century of Sai! Mechanics', ICE, 1969, pp 233-272

2. JENKIN, C. F. The pressure of retaining walls Proc. fnsrn. Civ. Engrs. February 1933, Vol. 234, pp 103-154


112

3. PACKSHAW, S. Earth pressure and earth resistance Proc. lnstn. Civ. Engrs. February 1946, Vol. 4, Paper No. S497

4. GOLDER, H. Q. Coulomb and earth pressure Géotechnique June 1948, Vol. l (No. l), pp 66-71

S. COOK, G. Rankine and the theory of earth pressures Géotechnique December 19Sl, Vol. 2 (No.4), pp 271-279

6. SUTHERLAND, H. B. Rankine his !ife and times Institute of Ci vii Engineers, London, 1973

3.0 DESIGN, EARTH ANO WATER PRESSURES

l. ROWE, P.W. Anchored sheet pile walls P roe. lnstn. Civ. Engrs. January 19S2, Part l Vol. l, Paper No. S788, pp 27-70

2. SKEMPTON, A. W. and WARD, W.H. lnvestigations concerning a deep cofferdam in the Thames estuary clay at Shellhaven Géotechnique September 19S2, Vol. 3 (No.3), 119

3. ROWE, P.W. A theoretical and experimental analysis of sheet pile walls Proc. lnstn. Civ. Engrs. January 19SS, Vol. 4, Paper No. S990, p 32

4. BJERRUM, L. and EIDE, O. Stability of strutted excavations in clay Géotechnique March 19S6, Vol. 6 (No.I), 32

S. ROWE, P.W. Sheet pile walls subject to line resistance above the anchorage Proc. lnstn. Civ. Engrs. August 19S7, Vol. 7, Paper No. 6233, pp 879-896

6. ROWE, P.W. Sheet pile walls in clay Proc. lnstn. Civ. Engrs. February 19S7, Vol. 7 Paper No 6201, pp 629-6S4

7. PACKSHAW, S. The application of steel sheet piling to engineering construction Civil Engineering 1933

8. GRAY, H. and NAIR, K. A note on the stability of soil subject to seepage forces adjacent to a sheet pile Géotechnique June 1967, Vol. 17 (No.2), 136

9. CORNFIELD, G.M. Direct reading nomograms for design of anchored sheet pile retaining walls Civil Engineering and Pub/ic Works Review August 1969, pp 7S3-7S6


10. LAMBE, T.W. Braced excavations Latera! stresses in the ground and design of earth retaining structures Speciality Conference at Cornell University Soil Mechanics and Foundation Engineering, ASCE, 1970

11. GOULD, J.P. Latera! pressures on rigid structures Latera! stresses in the ground and design of earth retaining structures Speciality Conference at Cornell University Soil Mechanics and Foundation Engineering, ASCE, 1970

12. SCHNABEL, J.R. Sloped sheeting Civil Engineering ASCE, February 1971, pp 48-SO

13. LITTLEJOHN, G.S., JACK, B.J. and SLIWINSKI, Z.J. Anchored diaphragm walls in sand - some design and construction considerations J. lnstn. Highway Engrs. Aprii 1971, lS

14. BROMS, B. and STILLE, H. Failure of anchored sheet pile walls Proc. Am. Soc. Civ. Engrs. -l. Geotech Engng. Div. March. 1974, Vol. 100 (No.GT3), 23S

!S. HOEG, K. and MURAKA, R.P. Probabilistic analysis and design of a retaining wall P roe. Am. Soc. Civ. Engrs. - J. Geotech Engng. Div. March. 1974, Vol. 100 (No.GT3), 349

16. IRWIN CHILDS, R., SHERRY, J.H. and BARDEN, L. A comparison of quay wall design methods CIRIA Technical Note 61, 1974

17. ZANKER, A. Nomograph - how to calculate the earth pressure on a retaining wall Civil Engineering November 1974, 86

18. HANNA, T.H. Design and construction of ground anchors CIRIA Report 65, 1977

19. JONES, C.J.F.D. Current practice in designing earth retaining structures Ground Engineering September 1979, pp 40-45

20. SYMONS, !.F. Assessing the stability of a propped, insitu retaining wall in overconsolidated clay Proc. Jnstn. Civ. Engrs. December 1983, Vol. 7S (Part2), 617

21. POTTS, D.M. and BURLAND, J.B. A parametric study of the stability of embedded earth retaining structures Transport and Road Research Laboratory, Supplementary Report 813, 1983

22. MILLEGAN, G.W.E.


Soil defonnation near sheet pile walls Géotechnique 1983, Vol. 33 (No.I), 41

113

114

23. NEALE, R.H. et al The use of computers in temporary works Proc. lnstn. Civ. Engrs. August 1984, Vol. 76 (Part 1), 761

24. POTTS, D.M. and FOURIE, A.B. The behaviour of a propped retaining wall: results of a numerica! experiment Géotechnique 1984, Vol. 34 (No.3), 383

25. HUBBARD, H. W. et al Design of the retaining walls for the M25 cut and cover tunnel at Beli Common Géotechnique 1984, Vol. 34 (No.4), 495

26. TEDD, P. et al Behaviour of a propped embedded retaining wall in stiff clay at Beli Common Géotechnique 1984, Vol. 34 (No.4), 513

27. HEAD, J.M. and WYNNE, C.P. Designing retaining walls embedded in stiff clay Ground Engineering Aprii 1985, 30

28. SMITH, G.N. The use of probability thcory lo assess the safely of propped embedded cantilever retaining walls Géotechnique 1985, Vol. 35 (No.4), 451

29. POTTS, D.M. and FOURIE, A.B. The effect of wall stiffness on lhe behaviour of a propped retaining wall Géotechnique Seplember 1985, Vol. 35, 347

30. SYMONS, I.F. and KOTERA, H. A pararnetric study of the stabilily of embedded canlilever retaining walls Transport an d Road Research Laboratory, Research Report 116, 1987

31. ILLINGWORTH, J.R. Temporary works Thomas Telford, 1987

32. SYMONS, I.F. et al Behaviour of a temporary anchored sheet pile wall on Al(M) at Hatfield Transport and Road Research Laboratory, Research Report 99, 1987

33. MaCLEOD, LA. Guidelines for checking computer analysis of building structures CIRIA Technical Note 133, 1988

34. SYMONS, I.F. and MURRAY, R.T. Conventional retaining walls: pilot and full scale studies Proc. lnstn. Civ. Engrs. June 1988, Vol. 84 (Part 1), 519

35. CARDER, D.R. and SYMONS, I.F. Long term performances of an embedded cantilever retaining wall in stiff clay Géotechnique March 1989, Vol. 39 (No.!), 55

36. FOURE, A. B. and POTTS, D.M. Comparison of finite element and limiting equilibrium analyses for an embedded cantilever retaining wall Géotechnique June 1989, Vol. 39 (No.2), 175


r 37. BICA, A.V.D. and CLAYTON, C.R.I.

Limit equilib1ium design methods for free embedded cantilever walls in granular materials P roe. lnstn. Civ. Engrs. October 1989, Vol. 86 (Part !), 879

38. DAY, R.A., and POTTS, D.H. A comparison of Design Methods for Propped Sheet Pile Walls The Steel Construction Institute 1989 (Publication No 077)

39. INSTITUTION OF CIVIL ENGINEERS International Conference on Retaining Structures, July 1992

4.0 CONSTRUCTION

l. TOWNSEND, G.H. and GREEVES, !.S.S. The design and construction of Gallions surface water purnping station Proc. lnstn. Civ. Engrs. November 1979, Vol. 66 (Part l), 605

2. FLEMMING, J.H., McMILLAN, P.H. and WILLIAMS, B.P. The river Hull tidal surge barrier Proc. lnstn. Civ. Engrs. August 1980, Vol. 68 (Part 1), 417

3. SARSBY, R.W. Noise from sheet piling operations - M67 Denton relief road Proc. lnstn. Civ. Engrs. February 1982, Vol. 72 (Part 1), 15

4. GERRARD, R.T., LONG, J.J., and SHAH, H.R. Barking creek tidal barrier Proc. lnstn. Civ. Engrs. November 1982, Vol. 72 (Part 1), 533

5. GRICE, J.R. and HEPPLEWHITE,E.A. Design and construction of the Thames Barrier cofferdams Proc. lnstn. Civ. Engrs. May 1983, Vol. 74 (Part 1), 191

6. McGIBBON, J.I. and BOOTH, G.W. Kessock Bridge : construction Proc. lnstn. Civ. Engrs. February 1984, Vol. 76 (Part l), 51

7. QU!NN, W.L. Foyle Bridge : construction of foundations and viaduct Proc. lnstn. Civ. Engrs. May 1984, Vol. 76 (Part 1), 387

8. BANKS, D.J., BROOKS, l. and JOHNSON, W.M. Construclion of Riding Mill Weir Proc. lnstn. Civ. Engrs. February 1985, Vol. 77 (Part 1), 195

9. HUTCHINSON, D.G. and SMITH, R.A. Brecon flood alleviation scheme Proc. lnstn. Civ. Engrs. February 1986, Vol. 80 (Part !), 121

10. MANN, T. and DUNN, M. Lock construction and realignment on the Sheffield and South Yorkshire navigation Proc. lnstn. Civ. Engrs. October 1986, Vol. 80 (Part 1), 1183

Il. CALKIN, D. W. and MUNDY, J.K.


Temporary works for the pumping stations at Piover Cove reservoir, Hong Kong Proc. lnstn. Civ. Engrs. December 1987, Vol. 82 (Part 1), 1121

115

116

Appendix A Soil and water pressures on retaining wall

Generai prlnclples

The designer needs to estimate soil and water pressures acting on the walls in order to be able to analyse the streÌigth and stability of a cofferdam. In practice, the soil-structure interaction and the relative stiffnesses and displacements of the wall and the ground will have a major influence on the pressures. It is often possible to carry out satisfactory calculations without regard for this fact, but in ali cases an understanding of basic soil mechanics is needed. If the brief outline given below is unfamiliar the designer should seek experienced advice.

The term 'soil strength' usually means shear strength, and by that is meant the ability to withstand differences in principal stresses. Shear strength is what enables the soil to suppor! the pressure of ground above without requiring an equa! pressure to act horizontally.

The Mohr circle is a geometrica] analogy for the equations describing the states of stress acting in different directions at a point. Shear stress ~ is plotted against direct stress cr. Given the stresses on any two planes at righi angles the stresses on any other planes can be calculated.

On the Mohr diagram 'ali round' equal hydrostatic pressure plots as a single point, and pure shear plots as a circle centred on the origin.

"'

"

"

"'

o, ? "' c,

~~ /~"'

Hydrostatic pressure

Pure shear


The classica] equation of Coulomb derives from experiments sliding blocks of material with different norma! loads.

When combined with the Mohr circles representing individuai soil tests, parameters c and <1> can be used to describe a failure line. This allows simple mathematics to predici one principal stress at failure given the other principal stress.

Soil consists of a skeleton of solid particles and voids filled with fluid or gas. At depth we are usually concemed with ground fully saturated with water.

A change in total vertical load will be carried by a change in load in the soil skeleton and a change in load carried by pore water pressure.


L c

t

c

t f l '::oc+cr.tan~

? ----

Total change

in vertical

lo ad

Change inload onsoil

skeleton

+ Change inload carri ed by pare water

pressure

cr

Mohr Couloml diagram

117

118

The fonnulation by Terzaghi of the principle of effective stress was a breakthrough in 20"' Century soil mechanics. He realised that soil shear strength depended an the effective stress, which he defined as total stress minus pare water pressure.

The Mohr-Coulomb diagram can be drawn with effective stress cr', rather than total stress, as the horizontal axis. The Coulomb equation then describes the behaviour in tenns of effective stress parameters c' and cf.

Earth pressures o n the wall

The problem of assessing the earth pressures on a cofferdam wall can now be solved, where the effective stress parameters, the total vertical stresses, the water pressures, and hence the effective vertical stresses, are known.

Granular solls

Far a typical granular soil far which c' = O and strength is defined by cf, simple mathematics from the Mohr diagram gives 'active' and 'passive' earth pressure coefficients K. and ~ according to whether the horizontal stress is less than or greater than the vertical stress.

This is the simple 'Rankine' theory which far rea! walls is rather conservative, as it makes no allowance far how wall friction reduces active pressure and increases passive resistance.

c;';= Ci- u

Effective Total Porefluid stress stress pressure

r= c'+ cr'tan<jl'

aH rrV '-y----}'-----~-__)

aH= Kacr~ aH= KpaV Active Passive

1-sinljl' Where Ka = -;--==c

1 +sin tV'

1 +sincp' Kp =

1 - sin <P'

cr'

Effective normal stress


The earth pressure coefficients used in practice are calculated by wedge theory (in fact using slightly curved wedge faces). Figures 15 and 16 in the main text show the relationship between the earth pressure coefficients, <Il' and the angle of wall friction.

Note that only a small movement of a wall away from a soil face is required to reduce the 'at rest' earth pressure coefficient K, to the active pressure K,. A very much larger movement towards the soil face is needed to mobilise the full passive resistance coefficient ~- (This applies to nonnally consolidated clays and to sands and gravels but not to stiff overconsolidated clays whlch have high K. values.)

The above theory would be sufficient if the water pressures in the pores in the soil could always be detennined. This is usually the case with sands and gravels which are relatively penneable and thus 'drain' quickly. Far these the water pressures can be estimated from extemal conditions such as an existing ground water table or a predictable regime of flow through the ground.

Cohesive solls

A problem occurs in the case of clays or other soils of very low penneability. These take changes of load initially on the pare water, which is incompressible, and which is effectively locked into the minute pores of the soil skeleton. The result is no immediate change in effective stress.


Active wedge

Passive wedge

Earth pressr coeffic1ent

Active

Movement awayfrom soil face

Passive

Movement towards soilface

L'l..u "" 6.a

fl..a' "" L'l..a - 6.u "" O

119

120

As the shear strength is detennined by the effeclive stress, the available strength is also inilially unaffected by changes in the total load.

With lime, pore water seeps into or out of the soil, causing a change in volume, a change in the locked in effective stress and consequently a change in available shear strength. The 'undrained strength' c, is for any clay dependent on the water conteni. High water conteni gives low undrained strength and low water conteni gives high strength. If identica! samples of a clay are removed from the ground and tested without allowing any change in water conteni then no matter what confming pressure is applied they will ali fai! at the same shear stress. As we can have no knowledge of the actual pore pressures, such tests are plotted on a Mohr diagram in tenns of total stress. The failure line is horizontal and gives the undrained strength c, for this particular moisture conteni (note that <jl, = 0).

c, is sometimes called the 'apparent cohesion' to distinguish it from the effeclive pararneter c'.

Knowledge of the undrained strength allows earth pressures on a cofferdam wall due to clays to be assessed in the short tenn, before moisture contents can change and when actual pare water pressures are not known.

Once again this simple theory is conservative as it takes no account of the effects of wall adhesion, which reduces aclive pressure and increases passive pressure. The pressure coefficients K., and K"" used in praclice have been assessed by wedge theory to allow for adhesion. Note that in this case K. = I<., = l.

With lime, the pore water pressures in clays change and in the long tenn come to a steady state controlled by extemal conditions. These long tenn water pressures can be estimated just as for sands and gravels, and long tenn effective stress pressure calculations can and should be made for clays using the relevant effective stress pararneters c' and <jl'.

When setting out to calculate pressures on cofferdam walls the designer must be clear about what type of analysis is to be applied to each layer and type of soil.

crH=crv-2Cu = Kacrv- KacCU

Active

Long term

" Draìned

Effe~tive stress

c'~·

crH = crv + 2Cu = Kpcrv + KpcCu

Passive

Short term " Undrained

r~'tal stress

eu <Pu

(Note: usually c' = O and always <Pu = O)

cr

cr

Total normal stress


Appendix B Worked design examples

Example No 1: Design of sheet pile wall for a cofferdam

A sub-surface tank is to be constructed in reinforced concrete. The tank is rectangular and 8 metres deep, with the roof at ground leve!. The overall dimensions of the tank are 16.5 m x 11.5 m. Two boreholes have been sunk. Borehole A is within the confmes of the proposed tank and borehole B is some 50 metres distant. Borehole A extends to a depth of 10.5 metres and tenninates in clay. Borehole B extends a further 3.5 metres and tenninates in dense sand containing ground water under a sub-artesian head of 7.5 metres.

Ideally a further borehole should be put down at the site of the cofferdam to confrnn ":hether. the sand stratum with sub-artesian head is present in that location. Due to shortage of ttrne thJS is not possible, so an assurnplion must be made that it will exist, and a 'design borehole' constructed from the available infonnalion.

In arder to obtain the best compatibility between the cofferdam and the pennanent worlcs, it will be helpful to set the frame levels just above the lift intervals of the reinforced concrete. In the case of the top frame, it would be particularly helpful to locate the frame above the tank roof so that the tank roof •an be constructed to provide suppor! to the walls before the frame ts

removed.

Construclion of the pennanent work in the cofferdam will take approximately 6 weeks and will start as soon as the excavation is completed.

Prepare calculations for the design of sheet piling.


SUBJECT

E. XCI mele No.1

BDRLHOL-E- A

+12 5 ~ Top soil +120 '----

tiO·O _ Sand and grave l

+B 5 -

Firm eia~

+bo -

S;lf~ sand

+30 - Sotr/çirnn eia~

tZ·O '-- End ot borehole

Sl-onding wa~er leve! +10·0

122

P age l Date Made by:

BDRE-HDLLB

tiO·Sr- ·i tiO·O f-- Top SOl

+9 o __j Sand <>nd eravel

Firm da~

t60 -

Si l f-~ Sand~ sravel

+3·5 f-Sofr eia~

+1·5 -

Dense medium/course sand

-1·5 -End of borehole

Wa~er s~ruck à tl-4 rose ro +9·0 in \5minu~e

CIRIA Special Publication 95 CIRIA Special Publication 95

SUBJECT

Aie heod

+12·5 t "

O ve~ i

E.xcavahon levels:

fina l.

Sro5e no. l Sro5e no 2 Shl13e no. 3

R L + 7·5 R L + 5·0 RL + 4·0

P\cm dimensions ins.ide coFFerdom ore 1}011'1, \2·0m

Page z_ Date Made by:

.Scale 1 :1oo

123

124

SUBJECT Page 3

E...xomp\e ~io. \ Date Made by:

As lhe coH'erdarn w il/ be open Por less fhan 3 rnonft,s and open t-o f'ull excavahon deplh Por no more !han sa!-\ 2 weeks' Use f-o~al s.hess p<>rarne~ers Por fhe clatl la::Jers.

Allow generai surcha'"5e o( IOkN/m2 f'or s1l-e equipmen~ el-c.

tl2·5 .-t 12·0 f.- Top so;! ( G\SSume as j'or SG\nol and '3""vei)

t-10·0 G-.W.L.

+8·.5 -

t 6·0 f-

+3·0-

ti·O 1---

Sand and grave/ S · zo·; 15"

Firm da':! cw ·3ojzs

Si[l-':l sand 8·15°

Sof'l- / Firm o la~ cw· zojzo

Medium coarse sand. (Sub·arf-esian he:ad

f-o +9·0) 8· 20/15°

Take wafer densi~.'::l ~w= IO·O kNjm 3

( Nof-e' ~5 = Safura~ed densi~'j)

~ = 18 kN/m 3

~s = 20·3

~ ~ 35• ka' 0·25 kp=58 c 'o

~ = 21 kN/m 3

rjJ ~ o• kq = kp = 1

Cu = 60kN/m2

k.c = 2·4 kpc = 2·4

1!'5 • 20·3 kN/m3

~ = 3o• kQ =0·3 kp =4·5

c =o

lS =zo kN/rn3 id = o• ka = kp = l Cu =40 kNjmZ

kac = 2·4- kpc = 2·4

l\5 = 20·3 kNjm3

" = 35° c =o

ka=0·23 kp=5·8


SUBJECT Page 4 Date

E.xomp\.e t--b. 1 Made by:


Colculahon oF Pressures

Using f-he de.sitjn borehole1 a 9uick e)(arninq~ion of' the wafer pressures under me lowercla':1 la~er shows l-hai- fhe L~pliPf- al- lhis leve l (+1·0) 15 Bò kN7m

2

This cequire5 af leasf +·O m oF -amund obove ·,f f-':' b<elance 1-he upliPf; i.e. \-o I<L + 5·0 which is lhe excavahon leve! a 1- S 1-a:~e Z.

As fhe level oF lhe unders1de af fhe eia~ l.atler is an as:-s~mphon, bCI<>ed on fhe lhickness a~ fhe la~er m Borehole B, me wa~ec presoure musf be relieved fòr bofh Sfages Z ond 3.

This w,ll be done b~ ins.~oll'n'j a rel16ble pumpÌn<j S':jsl-ern ou~s·,de fhe sheel-piJ,·n'3, pumpin3 froma leve\ befow the roe of fhe p1\es In the sand fa::~er. The s~sl-ern musi- be capable of keepìn3 fhe wafer pressure ,n fhe sand la.je• so fha~ ~he pressure heod ,,., no hì5her ~hon I<L +O·O 1.e. I·Onn

below fhe cla~/sand in~erface. The coFFerdarn Wl Il however, be des15ned vvith fhe pressure head Prom RL + 1·0 l-o allow some varìah'on in pressure.

The deoign oF fhe pump.'"9 syo.l-em is covered 6'j C l R lA Reporf no. 113 'Confrol af Groundwafer ForTennporor'j Works' K'eç (23) The rql-e oF lncrease in w~\-er press.ure

iFa pump1n'3 Pallure occurs musi- be f-esi-ed af- an earl~ sf-a'3e ro assess fhe de5 ree oF 6ack- up planf-, moni~orin:J and supervision recyuired.

125

!ll

Q JJ i>

t "C §-

~ g. ~

~

Q JJ i> 1!1 ~-!!!. "C

" Q:

ii g· ~

"' "'

"' ....

Al\ 1.m1b in kN Qnd m C.ALCULA TION OF PRE.SSURES A C. TI VE.

i? L G-rour.d ~ramef-e..-s Deplh To~q l Pon=.vvct 1-.::r fFFechve HFechve Tofal Mmimum Verhcal Verhcol honzonf-ql ' horizonf-qj eq..uivolenf-2 pre.ssure

sf-ress (?5w "'IO·o) si-ress pressure pressurc: fl,rd 12-:::. Surcho.r,ge \0·0 <Yv " <Yv ' Po' Fi\ pres.sure

5 z -DO 10·0 o IO-O 2 3 2 3

rfi'=35° 15,. 18-o l

1t~o-o Sand s "- 20"

Ka= 0·23 W. L. 25 65·0 o 55-0 12 7 12 7 .

l55 ,_zo-3 +B-s 40 B5·5 15-0 70·5 16·2 31-2

" o o }!=ZI-O

(25 o) (-37-5) 20·0 20·0

~ Cu "' bo Cw~3o 5o tob-5 25·00 2.5-0

era~ kQ = 1·0 kqc,Z-4 l

+6-o kç:~c x Cu - 144- O 65 135-o b) 40·0 ' 32-5 40-0 95-0 2"1A- b9-4

~ f' o 30° 75 ISB-3 .50·0 106·3 32-5 82-5

±±.Q_ 3and 8 = 15° ~s "zo-5 8·5 178 6 60-0 115 6 35 b a5 6 kQ= 0·3

+3-o 9 5 195·9 70 o IZ5-9 35 7 105·7 "oD lS"- :z.o 102·9 47-5

Cl a~ C~.t=40 Cw = zo Ko= 1-o kac=. :2-4

+1-0 kGicxCu."' q6-0 Il 5 238-4 142·9 57-5 80·0 IS8 CJ 36 6 lib-b 51--a':)<!'! 1 rzl'" 35° ~5 = 20-3 o 238-9 54 a 54-9 S~Q':)~ 243

8 " 20°

/1oo-o ~ Sand kct: 0·23 13·:::. 27CJ·5 179·5 +l 3 1+1·3 S~aGie 1 Sub. Ar~e.sian Head l l 20 l i'S9 S 51-aje 243 S9·6 79·6

f-o +9·0

Pumprng \--o reduce r pre.,-,.sures l~ 51-a'jeS 2 ~ 3

A\1 unr~s tn kN and rn CALCLJLATION OF PRE.SSURES PASSIVE SI-age no l E;-:c. L t WL@ t 7 5

R. L Grou,-.,d Fbr.::~rne~ers D ept-h To~C! l Porevvo ~er i EFfechve E(Fechve Tofal

z verhcGll pressure Verhcal hor1.zonh::::. l honz.onlQ(

s~ress (6~ o IO o) s\-ress pres:sure pressure

<Yv u. 6v' Pc' Fp f\2·5

p

rzl'o35° ~ = 18· o 8 = 15"

~ Sand kp= .5·8

+8-5 2S5 =Zo-3

~ rzl o o 21" = 21· o

(o o l 1440 Cu = 60 Cv..~-= 25 00 o o C la~ kp= IO

kpc=Z-4 +6·0 Kx:.::Cu=l44 15 31'5 175-5

15·0 lb·S 743 89-3

~ ~J = 30"

~ Sand s ::0 15° ìS:s = 20·3

kp= 4-.5

+3·0 45 92-4 450 47 4 213·3 2.55·3 fl!oo ('} = 20·0 188·4

Cio~ Cu=40 Cw-= ZO·O kp= 1·0 Kpco z 4

+l-O Kpc Cu c% O b·5 132-4 225-4

80·0 52 A· 303 -Cj 353 .-q si' c 350 65 = zo-3 (Arl-esicm HeQd 8 = \5° tr~.':"'n;:,ao)

~- sQ..,d Kp"' s-e. '5 1'3-o 73·0 423 ·4- 523-4

Sub. Dired-ion Head ~o +q. o

- ·---

m " Sl

3 li ~ z:.. p -

(/) c rn '-m o """

;;;:o-u

'" '" '" Q. ~(Q (1) (1) (1)

O" ':" "'

r 3 ili ~ \) __,

(/) c rn '-m o """

;;;:o-u

'" '" '" n.~"' (1) (1) (1)

O" ':" ~

ilii

Q :Il 'i>

_g> ~ ![ .., ~

g Al ['j· ~

(O

"'

Q :Il 'i> _g>

~-!!!. .., ~ .,.

I ~ (O

"'

\ll

Ali Uni~s in kN Qnd m. CALCULATION OF PRES.SURE5 PASSIVE Sl"oge no.2 Exc. L t WL @ + 5·0

R'L G,-ound Forome~ers Depf-h T o~ q l PorevV"'l 1-er EFfechve l E ff_ed,ve Tol"q l

z Verhco l pressur-e Verhça( 1

honzonf-al horiz.on/<:1 J

srress (2(w ~IO-o) srre.ss l pressure pressure

"v u. <Sv' . Pp' o-n Fp 12·5

<;i' o 35" <5= 18·0

s = 15°

t1o-o Sand kp= 5"8

~s = zo-3 +8-5

" o o 2S'- 21· o

~ Cu = 60 Cvv=2S·O

C la~ kp= 1-0 Kpcoo z-4

+6-o kpc x:Cu = 144-0

+5o <j' o 30" o·o 00 00 o o 00 0·0

f±±_Q_ Sand 8 ::. 15° 2S'5 = zo-.3 l<po +s

+3·0 z·o 40-6 zo-o 2.0·6 azy 112·7

~o o l> = 2 o 136·6

Cl a~ Cu:::40 Cw o;:.20

kp=-f-0 kpc= 2--4 ti-O k~xCu=q6·0 4.0 Bo 6 176 6

00 80·6 46J5 467·5

~·" 35" ~5 = 2.0·3 1 s = 15° ':±Q_ Sand kp= 5-8 60 121-2 20·0 IOI·Z 587·0 bOlO

Sub Ar~e..sion Kec.d ro +9-o Pùrnp1-n<;j f-o

reduce pressute

All uni~s in kN ond rn. eALCULATION Of PRESSUR'ES PASSIVE. Srage no.3 (f<nQ[) be L+ W L @ +4 O

l?. L. G-round Paro rne~erS Depl-h lof-qj ft>revva'rer ETFechve EF fec ~.ve l, Tofcd

z verhce<l pre..;.sure verhca l hooizon~al ! horizorJal

sf-ress (owoiOO) S~reSS pressure pressure

"v u. ' Pp' ' P p <>v

+12·5

<j'::: 3.50 ;>}"' 15·0

s :; 15"

±JQ:.Q_ Sand Kp= 5·8

21: 5 ,zo-3 l

+>< 5

" o o (5" = 21 o

~ eu= 6o Cw= 25·0

Cl a~ kp" 1·0 kpc= 2-4 l

tb·O Kpc>< Cu- 144-0

~ il' o 30"

1+4·0 SQnd s .:: 15" ;>;5 = 20·3 00 DO 00 o o o o DO

Kp=- 4-5

+3·0 IO Z0-3 IO-O 10·.3 4b 4 56 4

,? o o o 20·0 lib-3

C la~ Cu""40 Cw = 20

kp= 1·0 Kpc= 2-4

+l·O K=· Cu- 96 o 3·0 b0-3 156 3

00 f,0-3 349 7 349 7

f/!'" 35" "(5"5-= 20·3 t 200 & ::; 15°

~ 50 IOO·q 50·9 469-2 459·2

Sand Kp· 5·8 Sub. Arh~!.sion Head

l

1-o t-C! ·O Pu rnpin'J t"-o reduce p~re

'" "' p

~ fl>

c::-p -

(f) c ro '-m o -4

;s::o-u "' "' "' Cl.~(O <P <P <P

O" " ""

m ~ :l -o t'il z:.. !J ~

(/)

c ro '-m o -4

;s::o-u

"' "' "' Cl.~(O <P <P <P

O" <::JJ

""

130

bamp/e No.1 Prop No. l@ +13·0

SUBJECT Page '1 Date Made by:

S~age no.I Exc. leve l @ t 7·5

Pressure dìco5rams - Nef- pres.sure melhod- Fnp

CL o

o': N E ..__,_

7 _x

o 0'-

~ -

o o --

o ON

o o_ r() L{) --' !li !::!

o o l

o

f

Q_

cE

<Il ~~,

l ' l ', l '

l l ' l l ' l l '.:-\ l <Xl l l N Il N

~

00 o N

,... "' N

l l

'l" l 1\ oo: o

ti 6


r l


SUBJECT Fl-op. Page IO

St-age no.\

No.I@ +13·0

E~c. leve l @ + 7·5

An<ll'jsis using nd pres.sure.s - Fnp

Date Made by:

B':l inspechon ; f- is cleor thaf rh ere is enoush passive resis~ance below + ]-5 level t-o sat'e:l!:l assume Pi><ed eG\r\h Sl.lpporr and fhal- the evenf-l.!al pile len5f-h will be ade<J,ouaf-e ~or lhis sbge.

Assume lhaf- 1he poi n~ oF conJ-r.,rle~ure oç the pile a~ H,;s s~e is "f RL + 75.

An<>l\jse p·,\e using Blum's eq,uivalenf- bean mefhod. t 13 ·O- R-op. no. l -+ 12·0- K A-essure dia<;~ram

For design

t IO

+8-5 -

+7·5 -+7·35-

~~~ / Slope 12/m

2:\31

[7\ -~--- 2.5

Take momenl-s ot achve pressure abouf- RL +1·5

Area x Lever arm = Momenf-

(2/3x25)+265 (Y3xZ·5)+l65 (%x l 5) t 1·15 ( V3 x 1 ·s l + 1 15

('13xi·OJ +0·15 (Y3 x 1·0 1-0·15 ~.><0·/5

10·8 56 b 21· o 38·4 8·2. 60 O·Z

Yz x 2 x 2 5 = 2·5 Yzx 13x 2·5 = 16·25 \Zx 13 x 1·5 = 9·15 ~x 31 x 1·5 = 23·2.5 Y2x zox l·o = 10-o Yzx 25X /·O = 12·5 lZx25x0·5 = 1·9

Tel-a\ momenl- 141· ZkN.m

141·2 Load o n Prop. no.I = S·bS = 25 ·O kN per m. run

Leve\ oç zero shear @ K'L +9·59

Check are"' oç diasram above \t,is leve\.

Area = Yz >< 2 " 2·5 ~ x 13" 2·5

13 x 0·41 lZx 12." oA-1 2

To~<>l

= 2·50 =lb·25 = 5· 2.3 = 1·01 25 ·09 kf.J. OK

131

132

SUBJECT Page 11

bamp\.e. fJo. 1 Date Made by:

SCa~e no.I E><c. leve\ R'L +7·5

Anal':lsis using nel- pr.,ssures - FN P (Con~inued)

Maximum Bendi n~ Momen~ in pile

TC!ke momen~s C!boul- "nd above l< L + 9·59

Pr-o p + 13·0 -25·0kN.,~ +12.·5- No.I 2.

Pres.5ure. dlt:::tc:jr~m for desi5n

+10·0-+ 9·59-

~Siope: 12/m __ __!.-->~-

Aree> x Lever ~rm o Momen~ --2.5·0>< 3·41 +85·3

-Yzx2xZ·5 (% x z s) +0·41 -5·2

-Yz.xl3xZ5 (Y3x2.5) +0·41 -20·2

- 13x0·41 %x 041 -1·5

-Y2xl2x0·41 v3 x o·+l -O· l

Ma><. BM o 58·3 kN·m

per m r~n oç wall


r SUBJECT P age 12

E.~o.mple ~c.. l Date Made by:

51-a~e no.2 E><c. leve\ +5·0

Pressure dlagrarns - Gross pressure mefhod. Fp

'- o o(\o o ...l! l() + <() "' ~ "' !{) a. E

o~ ~'o-.

~ cnS! ("(l l)J :z o ...

~/ -t " -t l(\

> N :..c Q

CV]

<.J o ' ("() Il

o ~ Q_ o

:E a. <Il \...

_!!

\ o a (f)

"-'

\ .::!' l([._.

o o l([

"1 + -J1 .n o N (!"

~ N " <'l

<( ["(\ N

l'i i~ "' t'l -

l l l <( l o o .n b <() o L()

Q N ò " L()

~' N ò

~

<i- - N -.; uJ c o > ~ 1\i 0:. c "' ~ J o o t-- li.. u

& \Il rt .n o x <Il o': u.J !:" uJ

&: (1)

" Q o r

~ ·;n r--

>Il "' cr -Q_ o

::E .n-L E cr r--

w r::: ----\()

or o >.(]

0- f- -~ " N Q. o

<X) cn "' ....

o .n-- S< N

"'4- r--<X) >.(]


134

SUBJECT

Srage no.2

Prop. no. l RL t 13·0

Prop. no.2 RL + B·5

E.>< c. leve l RL + 5·0

Page /3 Date Made by:

Anal~sis o~ spqn bef-ween prop. nos. l and 2

Assume " hin5e ar RL + 8-5. Frqme no.2

- +13·0 Prop.no. l

- + 12.·5 z Achve pressure diasram f'or desi~n

- +10·0

~/ ~~~w" rer pressure

--~'--fT---' 31 Prop no.2 16

- +8·5

lake momenrs abouf- RL + 8·5

Area x Lever arm Momenf-

Yz x2x 2·5 ~ 2.·5

Y2xl3x2·5 = 16. 2.5

~xl3x 1·5 = 'l· 75

Yzxl3xi·5 = 23 ·25

Tor"l area= 51 75 k t.J

Load on prop. no. l =

(7'3 x 2·5) + l 5

(!3xz5)+15

~x l 5

V3 x l·s

7·92

37·"l2

9·75

11·63

Tol-al momenf- = 67·22

67·22 4·5 = 14·"l4 kN

Readion onprop. no.Z ~ 51·75-14·94 = 36·8 kN

bof-h per m. run

8~ inspechon Ma><. B.M. will be le.ss fhqn In sh,~e no.1


SUBJECT Page 14

lxamp\e t-b. \ Prop no.1@ RL + 13·0

Prop.noZ@RL+ 8·5

Exc. leve l K'L + 5 ·o

Date Made by:

Earrh Achve

Nef Wafer

Ear 1-h Passive

Cl AIA Special Publicauon 95

Anal~sis using Gross R-essure Mefhod- Fp

Calculohons Por pde wall below Prop. no.2

Assume a h•nge af- prop. level t B 5 and anol~se J'or 'free earrh' supporf-

CalculcJe roe level !'or limirine e9uilibruim cond·,hons

l. e. F oF 5 = l· O

Take mornenf-s oF pressures obouf- and below Prop. no.2 arRL+85

Try roe leve l ar RL + 0·92 (See P<>5e 13 fOr pressures ).

Area '/..

Yzxzoxl·O = 10·0 J2 x 25 x l o = 12·5 G.x25xl·5 = IB 8 Yzx+oxl5 = 30 o Jf x 29 x 3 ·O = 43· 5 lzx39x30 = 58 ·5 Y~xi03x2·o = 103·0 G.x143x 20 = 143. o

55 x 0·08 = 4 4

Yzx 40 x 1·0 = 20·0 Yz.x 50 x l· O = 25·0

50 x 2·0 = 100·0

JZ x 93 x 2 ·O : 93 ·O '2 x 137 x 2·0 = 13/' o Yzx 177 x z.o = 177·0

470 x 0·08 = 37· 6

Lever O.!'"'m

~x l o 33 x l o

(i3x15)+ IO (>'3x15)+ 1·0

l·5 (!3x3oJ+ (53 d O) + 2 5

(!3x20) +55 (

213x2 O) + 5·5 ( iZ xOoB)+ 7 5

l~xlo)+zs (>'3xlo)+Z·5 (Yzxzo)t35

(~xzo)+3·5 (i3xzo)+5·5 ('Y3X2o)+55 ( ~ x 0·08) + 7 5

2787· 3 = ~:-:-c:-+--;c;;c;o-,-2161·0 + 585·9

Toe level qf RL + O·"l2 t'or

= Momen~

3 3 8·3

ZB·Z 600

152·3 263 3 635·2 977-2

33·2 MA = 2161·0

56-7 79. z

4so·o

M w = 585·9

449·5 844 ·8

1209·5 283 ·5

Mp = 2 78 7. 3

Ok

135

136

SUBJECT

E ... ample ~o.\ Propno.l@ RL + 13·0

Prop no.2 @ R L + B · 5

Exc.level RL + 5·0

P age Date Made by:

15

Achve Earl·h

N d Warer

Pqssive Eacrh

See poge 13

t or %5

Sroge no 2

Analysis using Gross Pressure mdhod- F0 (conhnued)

Colculal-e Reach.on at Prop no.2 from spon below

Toke monnents obocJ ond below R'L t 0·~2

Areo. x --

y 2 x zo x l ·O " IO· o

Yz x 25 x l ·o " 12 5 Yz x 25 x l 5 " 18 8 ~ x 40 y, 1·5 " 30· o !i x 29 x 3 o " 43 5 !2 x 39 x 3·0 ; 58 5 Yz x 103 x Z· O "103 o Yz x 143 x z·o "143· o

55 x O·OB " 4-4

Y,_ x 40 x l o = 20 o Yz x 50 x 1 o " zs ·o

50 x 2 ·O , l 00·0

Yz x 93 x z o ; 93 o Yzxl37 x Z o " 137 o izxl7ìx2·0 " 177 o

470 x o 08 " 37 6

Lever arm

(73 x l o)+ b·58 (!3 x l· o)+ b 58 ('>'3 x l 5) t 5 08 (!3 x 1 ·s) + 5 08 (73 x 3 o) + Z ·og (>3 x 3·0) + 2 08 (%xzo)+ o 08 (Y3 x2o) t 0·08

y2. ){ o. 08

(Ì§xi0)+408 ( Y3 x 1 o ) t 4 08 (Yz x 2 o) t 2 08

(V3xZ o) t z 08 ( '13 x z o) t o 08 ( y3 x z o) + o 08 Yzx008

=

MA =

Momenr

72· 5 8b 4

114·3 167·4 177 5 1602 145 ·6 106 ·13

0·2 1050·9

94 9 IlO 3 308 o 513·2

255·4 193 ·6 132·2

15 582· 7

io l-o l moment "MA+ Mw- Mp = 1050 9 + 513 2- 582· 7 = 981 ·4

Reachon or Prop no.2 " 9_8_1_· _4 " 129·5 kN 7· 58

Tora\ lood on A-op no.2 = 129 · 5 t 36 ·8

= 166·3 kN/m run


SUBJECT P age 16 E><o.mple l-1:.. \ Prop no.1 @ RL t 13·0 Date

Prop no 2@ RL t 8·5 Made by:

A ch've

E ari-h

Pro p

Eacrh Achve


Srage no.2 E><c. \evel RL t 5·0

Anal~sis USin':j G-ross Pr-essure Merhod - Fp ( conhnued)

Calculafe Maxirnum BM in sp"'n pelow Pr-op no.2

Find leve\ oF zero shear i.e. where orea oç pressure dialjro.ms

q ce equa l ro the reqchon oF the pro p.

Tr:J leve l a t t 5 ·2.1

Area

Yz. " zo x 1·0 = IO·O

Yz x 2.5 x l o = IZ. · 5

Yzx 25 x [·5 = 18· 6 Vz x 40 x 1·5 = 30·0

2.9 x o 79 = zz·9 Yz. x 3-3 X0·79z = l ·O

95 2

40 X0·7"\; 31 6 Yz x 10 x 0·7"\2= _3_·1_

34·7

To!-ol aree> = 95·2 -t 34·7 = 129·9

Readion ar pcop. = 12.9·5

near enou:3h Ok:

So level oF z.ero sheor = t5·21

Take nnomen~s abouf- and above fhis leve\

Area Lever qrm Momenl---12.9 5 3·29 + 4Z6·1

,... (%xiO)t-Z29 - 29·6 \0·0

IZ · 5 (Y3 x l o)+ 2 29 - 32·8

1/3 8 (%x 15) t079 - 33·7

AreQS 30·0 (Y3 x 1 5) +O 79 - 38 7 22· 9 Yz x 0·79 - q o os

a bave l· o J3 x o 79 - 0·3

31 b Vz x 0·79 - 12·5

3· l V3x0·79 - 0·8 '- Maximl.lm BM " 2b8 7 kN·m

137

Achve E~rf-h

138

SUBJECT Prop no. I @ RL + 13 ·0

Prop no.2@ RL + 8 · 5

Exc. leve l RL + 5 ·O

P age Date 17 Made by:

Anc::dljsis using Gross Pressure Mel-hod -Fp (conhnued)

Clllcul a~e pile ~oe leve l Por F oF 5 ( Fp ) = l· 5

Take momenrs obour and be/ow RL +8·5

Tr'j 1-oe leve l a~ R L t O -44

Area ><.

~ 2. x 2 · 0 x l · o = IO · O Yz x 2·5 x... l · o = 12 ·5 Yz x 2 · 5 x l· 5 y2. x 40 >< l· 5 Yz x 2.9 )( 3 · 0 JZ x 39 )( 3·0

JZ x 103 x Z·O .lZ.x 14-3 x Z · O

55 x 0·5J ~x l ·5 x 0 ·5 12

Yz.x 40 x I ·O Y:z_ x 60 x 1 · o

5o x. 2. ·O

= 15 ·8

= 30·0

" 43·5 :: 58·5

= 103 · 5 = 143·o = Z5·J

" 0·3

= ZO·O

= 2.5 ·O

"100·0

Y:z.x'l3 x 2 ·0 = 93 · 0 Yzx l37x 2·0 = 13]-0

Yzx l77x 2·0 = 177 ·0 468 .x 0 ·51 = 238· 7

Jf x 5'l5 x 0·517L = 7· 7

Lever arm

~x I·O :Y3x1·o

(Y3 :x 1·5) + l·o

(~x 1· 5 ) +l· O

(!3x3 ·0)+2 ·5 (73 x 3·0) +2.:5 (J.§ x z ·o) +5·5 (~ )( 2 ·0} +5·5 (Yz. ><0·51)+7·5 (~ xO·SI) + 7·5

MA

(~x l·o) t 2·5 (~3 x l· o) +- 2 ·5 (Yzxz·o) +3·5

M . 'N

(~x 2·0) + 3 ·5 (Y3 ><2·0) + 5·5 ($"3 )( 2 ·0) + 5·5 ( Yz x 0-51) + 7. 5 (~x o·sl) + 7·5

=

=

Mp =

44-15· 3

2.345·1 + 585· 9

: 1·50

= Momen~

3 · 3

8 ·3

28· 2

60·0 152· 3 263·3 b3S'Z 977·2 2. 17· 4

2·4 2345· 1

56·7 79.2

4-50· o 585· 4

444·5 844·8

12.0"! ·5

1851· l

60·4-44 15·3

OK

Toe feve / or pile q~ !<?L + 0-44


SUBJECT Srac:3e no.3

(Fina/) E><c. level +4·0

Page 18 Date Made by:

Pressure dia:~n::~ms. Ne~ pressure mel-hod Fnp.

........, L

~

~ ìJ c C!

:E 1.. o

\.1..1 '--'

~ >

..L u

<(

O-rl{)

o o -

o -i- o l{'J - '<t

~

u.l ~

o -l()

J o ~ o - r-ILI OL IL \l > J iii a: Il) o --1- tf ~ s


o Q - '-N

o 1()+-N

o o ('() -_j \()

l\{ ~

o o l

ci c 0:. o

0:

l{'J ,.:.. l

ci c CL o tt

o 1(1 l[l N l l

o ... oo

'll ....... L ... ':::1 ....... Il) ...

~ v d:: l()

l(l

L[[ (j N

l l ~. O'

l{) l ro '

'<';y c-

~

*' o ~ l l l

139

140

tor 3f,·8 se e

P"'~~ l~

SUBJECT

SI-age No.3 Fino l.

Prop No. l RL + 13·0 No.2 RL t 8·5 No.3 RL. + 6·0

Exc. leve l RL + 4·0

Anal~sis using ne~ pressures - Fnp.

Assume hinses a~ RL + 8·5 and 6·0

Anal!:lse spon be~w~en Prop. nos. 2 and 3

+ 8 ·5 Prop.no.2

Page 1 q Date Made by:

~ +7·5 Ach've pressure dia~ram for desisn

+b·O Pr-op.no.3

Area

~ x 20 x 1-o = lo ·o

Yz x 2.5 x 1 · o = 12. · 5

Yz x25 x 1·5 " 18·75

Yz x4ox 1·5 = 30·o

-"

Reachon on prop. no. 2 =

40

Lever arm Momen~

(%xl-o) t 1·5 21 ·7

(V3 x l·o) +1·5 22·9

y3 x 1·5 18·75

y3 x 1·5 15·0

lo l-al momen~ =- 78·35

78 ·35 2·5

= 31·3 kN.

Reachon on prop. no.3 :. 71·3- 31·3 =40·0kN

L oad on prop. no. 2 = 31·3 t 3b·8 = b8·1 kN

ali per rn. run oç wal\

B!:l inspechon fhe max. BM will be less H-lan in sra'j~ no. z


Pro p No. l RL. + 13 ·0 Page 2.0

E.xamp\e ~o. 1

SUBJECT SI-age No. 3 No.2

No.3 RL. + 8 · 5 Date

RL. + 6·0 Made by:


Fina l. E,)(c . level RL. + 4·0

Ano l!:lsis using ne~ pressures - Fnp. (Conhnued)

Ancdcose s~n below prop. no.3 wi~h hin<je assumedoi-I?L+6·0

P"'55ive Achve

~+6 · 0 Prop. no.3 ~+-----/______,._,VIope 13-5/m

f-. +4-·0

-+3·0

-+1·0

'?.' 1.....· ~-

--1 ·0 409

Exc. leve! / \ 96 \\Y/A\Y/À\'Y/)).\Y/1 - -·~

Slope 57/ m

13 ........... -----:::::;_ ___ _.. ~

1.3

53

Pressure d1a5ram Por desi-an.

Take momen~.s or achve pressure aboc.J~ ond below +6·0

Area x Lever arm = Moment --Yz. x 6'1 >' 2. ·O = 6'l ·o J3 x 2 ·O 46·0

~ )( 96 x 2 ·0 :. '16 ·0 % x2·0 IZ.8·0

Yz. x 'lb )( 1·0 :. 48 ·o ( V3 ,)( 1·0) + Z·O 112. ·O

v2. x 53 x 1·0 = Z6 ·5 (~,c l·o) t 2·0 70 · 7 --To ra ! area = 239 ·5 To~a l rnoment =- 356 ·7 k N.m. -- --

Calcula~e pi le ~oe leve! tor li ,..,.... .• hn'ò e~uil'1brium condihons

1.e. F oF S " 1·0

So momen~ and pas.5iVe pressure obour + 6 ·0

~o be > = 356·7 x 1·0 = 35b ·7 kN.m .

141

142

SUBJECT Frop. No. 1 R"L +-13·0 P age 21 E."'"' rn pie l0o. \ fi'op. No z_ RL + 8·5 Date

Sro5e No.3 (Fino l) frop No.3 RL + 6 o Made by:

-

-

E<c. leve\ R'L t 4 O

Tr':l p·,!, roe level @ RL +0·83

Passive mornen r:-

Area

13 x 2·0 = 26·0 295 x 0·17 =502

Yz x 57 x. 0·\7'2. = 0·8

Lever arm

4·0 (Yz x o 17) t 5 o (%x0·17)+50

Momenf-

T o ~a l Passive Mome.n f- =

104·0 Z55-3

_+_·_l 363·4kN·m

fqke. Momen~s oF AchVe and fhssÌve Pressure.s oboul- cmd obove RL +- 0·83

Areo. Lever arm --Yz x b'1 li. 20 = 6'1·0 ('73xz.o)+317 Vz x 9b x 2 o =960 ( v3 X20)t3·1) Y:z x %x 1·0 = 48· o (% x l o) t 2 17 Yz x 53 x l·o = 26·5 (Y3 x l·o) +2.·17

13 x 2.·0 = -z6·o (.!Z x2o) to·l7

OK

Momen~

310·7 368·3 136. z 66 ·3

- 30·4 -2'15 ><0·17 =-5o·2 Yz x o·17 4·3 -X z !3 )< 0·17 2 x 57 x o . '7 = -o ·8

Total Momenf

K'e<>chon "f Frop Mo. 3 = 846

'8

= lb3 ·8 kN 5·17

Lood on frop No.3= lb3 lì t 40·0 = 203 8 kf-J

per m run oFwc>l\

Find level oF Ze.ro Sheor

Trj leve\ qf RL t 4· O l

-846·8 kN ·m

Are<> o P Active Pressure befween + 6·0 ond +4·01

69 x 1·99 ~ 137·3 Vz x 13·5 x 1·94 2 = ~

lof-al <>reo 164·0 kN


SUBJECT frop No l RL + 13·0 P age 22 E.><o.m p\e toJc:.. l frop No Z RL + 8 5 Date

Made by: Frop No.3


RL + 6 o Sro'je f.Jo. 3 (Final) Exc. leve l RL +- 4·0

Anai':Jsis Usin'j Nd Pre.ssures - Fnp ( Conhnued)

AreC\

163·8 69 x l 99 = 137·3

y2 x 13·5 < 1·99 2 = 26·7

Lever ar-m

l· 99 Yz x l· 99 y3" 1·99

Mo~imum B.M.

Moment

3 2 6·0 - 136·6 - _Il:]_

/71·7 kN·m

Co\cu\ote pile 1-oe leve\ Por FoF S = 2 O (Fnp)

Moment oF passive pre5sure obour RL + b·O robe > = 356·7 < Z·O = 713·4kN·nn

Tr:\ ptle roe \evel <>l RL +- O·bl

Po.s.sive Momen~ :-

AreCI 13 K 2·0 = 26·0

295 x 0·3'l = 115·1 Yz x 57 x 0·39 2 = 4·3

Lever ar-m

4·0 (Yz x 0·39lt 5O

Mornenl-

104·0

597 9 22·6 ( ?'3 x 0·39) t 5·0

Tof-al Momen~ = 725·5 kN·m

OK

File foe leve\ Por F oF 5 ( Fnp) = 2·0

is RL t 0·61

143

144

SUBJECT

E.><.o.rr.p\e Jo. l Pro p. No. l RL. + 13·0

No.2 RL. + 8·5 Sroge No.3 No.3 l< L. + b·O

Ex c. leve l + 4·0

Pressure diagr<>ms. - Gross pressure rne~hod - Fp.

o "'

o "'

P age Z3 Date Made by:

ot-----~~------------------~o ______________ J

o L() r--

o or-

o O f.N _j l[) 1\i N

o ò l

"' --

l{) lt)

'ì ';'

"' oJ1

o oJ1

"'

E ~ N

<u Q. o

\(}

l -9,

E ........_ L() .,.. L()

" Q. o

<f)

1!1'. èf"V cr-

"" ~ "'"


Ac.hve

Passive


SUBJECT Ftop. No. l @ RL + 13·5

Prop. 1-Jo.Z@ RL t 8·5 Prop.I-Jo.3@ RL + 6·0 leve/ RL + 4-·0

Analljsis using Gross Pressure - Fp

Anol'jsis o[ Spqn below Frqme No.3

\Vili; hin<je ossumed of RL + b·O

Page z4-Date Made by:

Ano l~sis Far Free Eodh Supporf wi fh F oF S ( Fp) = l 5

fake Momenfs obo~f and below RL + 6·0

Try pile roe leve/ af + 0·35

YL " 29 x 3. o o 4 3. 5 YL x 39 x 3·0 o 58·5 Yzxi03x z.·o =103·0 Yz. x 143 x z·o = 143·0

55 X065 = 35·8 Yzx2·5x0·652

o 0·5

Yzx40xZ·O ~4-0·0 Y,_x60xZ·O = 60·0

60 x l o = 60·0

Yz. x 46 x 1 . o = 23 o Yzx116xZ·O =116·0 Yzx156xZ·O =156·0

350x 0·65 = 227·5 Yz.x-59·5>'0·65

2::: 12.·6

Yz x 3·0 'Y'3x3·o

(Y3x20)+30 ('§X Z·O) +3 O

( .13 x o 65) + 5 o (3§x065)+5·0

V3 x z.o 3-3 x 2 o

Tofo l MA =

(V.,_ x 1 o)+Z.O Tof-ol Mw =

(~xi·O)+zo 03xz·o) + 3 o (?3xz·o) + 3·0 ( !Z x0·65)+s o (%x0·6s)+5 o

Torql Mp =

43.5 117. o 377. 7 619. 7 190· b

2·7 13SO·Z k.N·rn

26·7 80·0

150·0

256·7

b 1·3 425·3

6 76 D IZII 4

68·5 2442·5

Z442·5 = l· 5Z 1350·2 + 256·7

OK

File Toe leve/ Por F of S (Fp) = 1·5 i-=. RL + 0·35

145

---------------------------.... --~~~~~----

VI ~ ~

_l !; o -

~ "'"' VI ~

-'- L ~ (l z

o

~- -

~ ,.-..._ :.,;: l.. <J w.. <( g_ '-._.) --

o ~ -

§ f-.o

...J !Il N

146

SUBJECT No.2

S ~<>ge No. 3 No. 3 E.)< c. leve l

RL. t 8·5 RL. + b·O RL.+4-·0

Page Z5 Date Made by:

Pressure di<>gr<>ms - Burl<>nd-Po~~s Fc

o

9 o l

N o c o_ o

o':

'il

l

.,. N-o

M d c 0:. o

d:

o f

o -.!)

o

.o 7

o "' -.{) .oL-------


Wal-er

Pas~ive


SUBJECT

Sro5e. No.3

Prop No.2 @

Prop 1-Jo. 3@

Exc. level @

-f- 8·5

-f- b·O

t- 4--0

Anal:~sis usin5 Burlond- Pof+s - Fr (Conhnued)

Colculofe Pde Toe leve/ for FofS (Fr)= 1·5

Te'::\ roe leve l or +O· 60

Page 7..6 Date Made by:

Take rnornenl-s oç pressures abou~ and below RL t6·0

Area x

Yz x 29 ){ 2.·0 = 2CJ·O

)2. x 36 x 2.·0 = 36·0

36 x 3 ·4 " l z 2 4

Ji x4o x Z·o =- 4-o·o ~ x bo x z o = 60 o

bo x 1 o ~ bo· o

Yz x 43 x l· o " 21 · 5 -49x20 ~ 98·0

331 x 0·4 ~ 132·4 Yz X 57 X O· 4 2

" 4 · 6

Lever arm =

Y3 x z·o o/.:;)<20

{Yz x 3·4) +2·0

MA Toh"l

)3 x z-o 7:§ x Z·O

(Yzxlo)+z-o M w T o f-a l

l 7:3 x l· o) t z o (!Zxzo)+30 (.izxo4)+50 (%x04)t50

M p Tofq l

1162·0 szo·z t 256·7 = l· 50

Ok:

Pile Toe leve l Por F of S ( Fr) = 1·5 ,,. K'L + 0·60

Momen~

19 ·3 48·0

452·9 52.0·2

26. 7 80 o

150·0 256·7

57 3 392 ·O b88 5

24· 2 1162-00

147

g;

Q Il > cn "O

~. !!>_ ., c cr @ ~-0

" <O

"'

Q Il > .g> 1l. !!>_ ., c cr @ ~-0

" <O

"'

~

l l

i w c.<: rr !t

c n n n ~

n li' ~ c c O? '& <Y>(Yl --g._ Se .D' O' )> 3- )> )> )> ~ -" " - o ~ :0 l!> ~ " " " " " ,,

0 ,

" Q_ o :F D o o N -l' N (J- "' -N d r ~ -u cr 1}1 l]\ w ti" u: '-" o o o o o o 1}1 o 1}1 , ~- T V> ~ "' o o o "' 3 ~ o (ft :0 0\ " u; U)

(.]) 6 él o T ::>::r; X;<; ?\ x ~ O" ' o c o ll :0 g o o o c ...,..., "'

...,..., D o "" rr O? d lJ) c..o ' " il n n n n " 3 " 3 :0 o l"' "' Cf> u: lJ\ " " l " " 3~ n :0 ~ CL

~ (Jo -u 1l 3

c c o o i>' ~ c N- 3 N- " :f: " 'Q -o o o ~ò +6 3

W "'/N 3 o ~ c 3 :0 :0 o N 'G:/f u: ~ T ()) o !!: _j) il -· b' "' u- "' lJ " " " " " " v 0.... "' "' " "' ~ + 3 , , o "' N _,.. ()' " x T c ~ o ,... ""- o --'?._ ii o ~ -,.,...,., ,...

~ ~ ~ ;::,- ;p d - ""C] T o o 3 + ~ 1}1 ""CJ "" "' " ' (fl 7IJ ()' - "w :p L{) o o N w U1 gffi r :0 lJ tJl 71"?\ ?C ;.>; " •"' "' a /\?\ x ~lJ lJ1 lJ o IJl o " D -g-u x ' Cl

'"l w o ìT rrrr + 1J N =i' " CL N " N " l!' o c.r: -n (LO o o c " " " (JJ -· " "' z 3 o -l' ~ n N- N- lJl w .,., l.f'\ T ,...., ()

"' +o 1}1 w x ,, o o w "' +o 'Q o ' "' :l ife 3 - ~

"' :0 z "" N (J\ (Jo (Jo

3 (]) o o " " " " o (J\ ,... "- " "

,, ~ N N - + " " "' -. + -J w (Jo è?

.._, o N N w .... -J o 11\

" 1}1 o .,., w Q_ .Jl w w c c o ' --.l ' o (Jo 11\ ..,.., fiO ' "' "' (])

~ J;li];l c il " " CJ

1}1 N o Ul

il 3 U\

Ali Uni~s in kN ond m CALCULATION OF PK'ESSURES- ACTIVE. Mod1 çied Pressures usin5

Srcen3fh Facfoced Merhod. FofS (Fs) o 1·2/1·5

RL &round Pararne~ers Dep\-h Tol-a! Porevvo l-er Effechve E F Fechve T o!-al Minimum

z Verhcal pressur-e verhcCII horìzonfal honzon~\ e9.uivelent-

s~ress (~~=IO o) sf-ress prE.'SSLlre pressure fiu;d

Surcharge 10kN/m2 <Yv CL <Yv' P a' Po pre=.sore

+12·5 S·Z

IS=-16-o 0·0 IO-O 00 IO-O zq z.q

~= 30·3° Sm= 17"3°

t-IO-O Sand Ka= O· 29 WL. 25 SS-0 00 55 o 16-0 16·0

t-8·5 ~5 = 20·3

40 855 15·0 70·5 20·4 35-4

yi = o "lS = 21·0 (-10·5) ZO ·O 20·0

~ Curn"' 40 (w, o 17 50 ( 1065) (zs o) (!0-5) 2.5·0 25·0 C.lalj ka = 1·0 Assume wah~.r

Kac=Z-4- prl"!.s.sore in

t-b-o Kac x Cum=96·0 6-5 c""~7 dept-h 138-0 4Z-o -fo.Q' 32·5 40·0 '18-0 37-2 77·2

\+5-0 ~= zs-r 7·5 158-3 50·0 10~-3 41·2 91·2 Sand q,=. 12-9° 65 = zo-3

1+4 o ka" 0-38 85 175 b 60·0 115·6 4-5-l 105·[

+3-0 9·5 19P,·9 70-0 128·9 49-0 119-1 <i=O l'>=ZO·O 134-1 4]·5

Cl a~ Cum =27 Cwm:/4 KGl=l-o l-<ac= z-4

-t 1·0 kac )(Cu = 64-8 11·5 Z38·9 174-1 57·5 80·0 158-4 4-l:. . J 126·1 S l-q'3e l

Sand lit= 30·3' "lS = Z0-3 l o o 238-9 69-3 69·3

sra~e 2t"t3 Ò,= 17·3°

_::lcQ_ Ka = 0-29 13·5 279-5 100-0 17l1· 5 52-l [52·[ Sh=1qe \ Sub. Arl--e!>l an Head L zo-o 2.59-5 75·3 95·3 st-.,_~e2a3

t"o + 'l·O PuiTipin<J 1-o reduc.e Press.ures in(l-ages 2tt3

m x p

rt Il>

)> z:._ " " D

'-" -1!'. (1\

c (f) (JJ ~- <T c ~ m "' (5>

"' '-m (F> z o ~ -4

' o

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w T :r

ii' ~ ~ :0 n n O " Cl o d - -o

"' ' Zz: "' ~ D-. o o - N -3::

"' 3" 7IJ 71J}\} r r r o D-. + + + ' "' ur ()l w

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ModiçLed P.--e~~re& u!O,:'n'5 Sl-re.n<:Jh FClc~ord Me\hod FoFS (Fs)= r-z/r-s

Ali Unif-s in kN and m. CALCLILAIION OF PRE.sst..lRE:S- PASS\V[ Sh:l3e 3 Ex· c. L. + W.L. @-14. o

R. L Ground Pararnel-er-s .Depth Toh:~l Fbrevvo. \-er Effec.hve E Ffed;ve Tol-<ll z Verf-ic~J pressure verhcc:~l horLZOflf-a) horizonl-al

sh-ess (()w= 10-0) àress pressure pressure

·17 5 6v u 6v• PF P p

.• ---~---

)t)~" 30·3" i'5-= 18·0

8 " l Y.3 _±_lQ_:Q

Sand kp= 4-3

lls =20·3

+8·5.

~ .)6,.,.,=:.0 2{=:. ZI-O

Cu ::40 Cwm= 17 C la~ kp::. J·O

Kpc ::Z-4-

+b-O kpc x Cu ='16-0

~±-'= ~~=:. 25-7° So n d Sm= !2 ·9° 65 = zo-3

1H--o kp= 3·5 o o 00 00 00 00 o o

t-3·0 IO 20-3 IO-O 10-3 36·1 4b l sirn = O 85-1

Cio~ Cu,., = Z7 Cw,.,.,=l4-kp:ol-0 kpc= 2-4-

+l-O kpc ,.,c ... =64-B 3 o b0-3 125·1 00 b0-3 25"'1-3 25Cl-3

~,;,"" 30-3" (') :: 2.0·3 ! 20 o 8,.,.,= !3°

..:::.LQ_ Sand Kp" 4-3 5o I00-9 B0-9 347-9 367-9

5'-lb. Ar~esian Head ' ro + 9·0

Pumping l-o reduce pressure

MODIFIE.D PRE.S.:OURE. .DIAG-12AMS

kN/rn 2 Ner wo~er presstlres

Pqssive - Ear-~ pr~:ssur-es. - Acl-i ve R. L. 200 1!?0 IOO s,o PrO f.'· :';0 100 150 o 50

no.]~O

12·5' l l 3 l l l l

Modi ~i ed aàive and P'lssive lb o i-IO· O earfh pressures usin5 sl-renerh

pow<>meter:; modi F;,,d b~ FofS (F,;)=I·Ljl-5

,,_ 20 Prop. no.2

f-7·5 o

Prop. no.3 o 37 40

r-5·0

Exc.level o bO "·'m/\/ 4-9 134-85 bO 36

~ o

-2·5

2.59 b"' !'.. 125

Sloedo/~ 174-

t...·

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m x $)

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Po5:=:.ive

SUBJECT Fcop No.1 RL + 13·0 Page 31 Date M ade by:

Srage No 3 Fcop No.2 RL + 85

A-op No.3 RL + bo be. leve! RL + 4·0

Calculare Pile Toe leve l where:

Momef1f oF Passive Pr-essures = Momenf of Achve Pre=oure plus Momen 1- of W e r Wa rer Pr-essure

Toke momenfs abourand below K'L t 6·0 ( u.sing modi Fied pres..5ure d'togrqms)

Tr~ roe leve/ of RL +O· Zb

Areq x

~x 37 x 3·0 =55. 5 ~ x40x 3·0 = 73·5 Yz x 134 x Z·O = 134· O Yzxl74x2·0 =174· o

69 x o 74 =51. l

~x 3 x0·742 = O· e.

Yz x+o x 2·0 = 40·0

YzxbOxZ·O =60·0 bOxi·O=bO·O

lz x36 x 1·0 = 1/)·0 Yz x85x 2·0= 55·0 Yz Xl2.5 X 2·0 = 125 ·O

25'jx0·74 = 191·7 )f X 44-5 x O· 74

2= IZ · Z

Leverarm

Yz x 3 o ~x3o

(!3xzo)+3o (7'3xZ·O) +30 ( Vz x 0·74) +5o (~x0]4) +50

T o ho l

.\3 x 2 o 33 x Z·O

(Yzxto)+Z·O Tora l

13i.xlo)+zo (!3xzo) +30

~%xzo) +3·o Yzx0)4) -t-5o

(% )<074) + 5 o Tora l

MA+ Mw = 1726 · 6 + Z5b·]

=

MA

M w

M p

Momen~

55·5 14 7. o 491' 3 754· o

27+ + 4 +

= 17 26 6

26 7 80·0

150·0 = zsb·]

480 311· 7 541• 7

IOZ9·4 67·0

= 1'197·11

= 19B3·3

neor ~nou-;3h

Ok

A'le Toe leve\ çor Fs= 1·2/1·5 's R. L + 0·26

CIRIA Special Publication 95 Cl RIA Special Publication 95

SUBJECT Page 3Z Date C.h~ck For clep\-h oF e::<.CGI'·IQhon before

l d prop is in piace ie. C.anhlever p ilo: Made by:

Tr~ vv·,fh exwvahon leve! or -t-7·5, fhe some as Sra5e no.1

Use Nef Fl-essure Diagram as Por Sf-a9e no.1 on F?,ge IO wifhour prop. no.1

-+12·5 2

\ -+IO· O 12/m 1&Siope

- +8·5 20

31

-H5+j·~35~~§~"~J~07x~·,~~i ~f~/~\ 25 -+6·5

130

-+60 f.. 136 ~Siope 12/m 20

Slope43/m

-HvD-:1<:-49::------, 85

Find leve\ of Zero Sheqr

Acl-i ve Aree~ S.

~ x 2 x 2. ·s = 2·5 ~xl3x2·5 = 16·25 4 x 13 x 1·5 = 9·75 Yz x 31 x 1·5 = 23·25

Yz.xzox 1·0 = 10·0 Yz x 25 x 1·0 = \2.·5

~X25x 0·15 = l q

Tora l 76·15

Tr:J level + 6·34

Passive Areas

Y2 !' 130 x O·BS = 55·25 130 x O· 16 = zo · B

Yz x 12 x 0·162 = o. 15

To f-ol 76· 2.0

DK

153

154

SUBJECT

Check Co.nhlever E..xco.vahon

(Conhnued)

Maximum B.M.


Take Momenf-s obouf- a above R'L + 6·34-

AreCI --r 2·5

16·25 9·75

Achve 23·25 10·0 12'5 l 9

{55 25 Passive 20·8

0·2

Lever arrn Momenf-

(%,x 2·5) t 3 (,(, 13 ·3 (Y3 x z s) + 3 i, i, 73 o (~x 1 5) t 2. l i, 30 8 ( V3 x 1 sJ t Z· 16 bi ·8

(~x lo) t l' 16 18 ·3

( .!3 x l o) t l. 16 18 7 ('3 x o /5) + l ·o1 2· l

Tof-.,1 ---2J8·DkN·m

(.13 x 085) t O· 16 24·5 Jfxo·/6 l• 7 !3 x o 16 ----

Tof-q[ Z6·ZkN·m

Maximum B.M. = 218-Zb·Z o l'li 8kN·rn per m. run of "'"Il

Cc.lculaf-e Penef-rahon required

Af- limihn3 equ;!ibruim condihons me nnornenf-s oF ochve and pqssive pre.ssure P.::::..lo.nce.

Tr~ RL t4o

A rea Leverarm Mornen~ --

2 ·s (53 x z 5) +i, o 19 ·2 16· 2.5 (!3 x 2·5) + 6 o IlO· O 9 75 (~x 1·5) +4·5 53·6

Achve 23·25 (.13 x l 5) +4 5 l 16 ·3 10·0 ( 73 x 1 o) +3·5 41 7 12·5 ( J3 x 1·0)+35 47·9

l' 9 (73 x 0·15) + 3·35 6,·b T oh> l A c ~i ve Momenf- 395 · 3 k N· m


SUBJECT

C'heck Canhlever ExcavCll-ion

( C.onhnued)

Areo. Leverarm

Page 34-Date Made by:

M ome n~

(73 x o 85) +25 153·8 (i3x o·s) +-2·0 75 8

{

Yz x 13o x 0·85 o s5·Z5 )±x 130 x 0·5 o 32·5

Pc.ss,ve Yzxl36 x0·5 o34 o (!§x 0·5) +z·o 73 · 7


Yzxz-o 4o·o !i3x2·o 5?·3

20 x z·o o40·0 .!z x 4 3 x 2 0 2

o 86 . o T o f-a! Pc.ssìve Mornenf4oo · 6 kN ·m

Pene~rahon " ( 7·5-4·0) +zo% OK.

" 4·Z m

Toe leve\ af- + 3·3

From inspechon ot' R-essure Diogr-om fhi~ level 15. 5hl\ wifhin fhe Sand la:Jer, rhou3h QS fhe cla:J "' on[::J 0·3 nn below Hlis, a- would be unwise ro excavate Qny deeper beFore plocin3 trames.

Even of fhis depfh there w,!\ be some def'lechon oF fhe canf;!ever piles qnd ;F movemenf oF me 5mcmd rs Cl

limihn:~ FcKfor f-hen it would be wise ro limif fhe excavahon ro a depHl oF 3-4 rnerres onl:J-

The max,mum BM. is less mon fh"~ af- sra3e 2.

OK

155

SUBJECT P age 35

&.ame\e No. 1 Date Summar:l oF Resulrs Made by:

Sra5e Lood on orop no. Max. Pile Toe Level I?.L. rr. no. l 2 3 BM Fnp Fp Fr F.s kN kN kN kN.m l 25 o 58·3

2 14. '1 166·3 265·7 0·49

3 14· 9 68·1 203·8 171. 7 +0·61 +0·35 +0·60 +0·26

ali per m. run or wall

Seledion o[ Aie Sechon

Ma x 6M af SI-age Z = 268·7 i<N· m

Permissable S~ress in sl-eel Far femporor~ works

175 N/mm z Gracle 43A 225 N/mm 2 &rade 50A

Re'!uired Sechan Modulus

z o 268· 7 x 1000::; \535cm 3 /'or43A 175

= 2.68 ·7 x looo o ll'l4cm3 for 50A 2.25

Avoiloble Sechons

Larssen lbW - Modulus 1601 cm 3

Frodinghan 3 N - Modulus 1685 cm 3

Grade 43 A wdl sul'fice 6ur consider ~rode 505 osa s~ronger pde vvh,ch wil/ be less ld<el~ ro sur er dama'3e be Fare be,·n:J exfraàed ond will have a berfer second hond volue.

156 CIRIA Special Publicatian 95

l


SUBJECT P age Date Made by:

36

Frorn resulf-s o[ 4 mdhods oF An"/'jsis

Frorn fhe sumrnC\r~ of resulrs fhe lowesf- pile roe leve! "fs\-o5e 3 15 R L +o 26 for \-h e Sfren5 fh FocJored mdhod (Fs)

The pressure cliq5rarn on Fb5e 19 shows fh"r lhere is l;f-He p<::~s.s1've re.slsJances un hl RL + l·O where the \owest- sond la;jer ls reached (e. a\- the unders1de oP fhe sof~ clqj la:1er.

Boreho/e A which was ,·ns,cle fhe cof'Ferdam qre" d,·c1 nolpendral-e deep enoush l-o esl-ablish fhe leve/ of fhe soPr cl<>'j/ lowesf s"nd inl-erPace. Thus the leve/ of +1·0 Wqs "ssumed b::J rqkin5 2Om for fhe fh,ckness a\- fhe sotr e/a:\ \""'er in Eorehole E and appi::J'n~ f-his f-o fhe lovver end qf- Bore ho/e A.

l f. however; fhe underside cf fhe sotf- cl":l /o::ler in fhe cofferd<>rn q reo IS ."c f-u a l lp lower rho n assumed fhqn fhe F oF S for the des15n wil be reduced.

To allow for fhe po,;sik>,[if~ incnease l-he p·,[e len5fh b'j I·Om l-o ~i ve the p·,[e f-oe cd- S<>:J K'L -O· 75

ond fhe P,/e len5fh wi[[ be 14·25 m

157

158

Example No 2: Design of Internai frame fora cofferdam

Using the cofferdam layout and results from Example No.I design the internai frame at Prop No.2 leve!, using BS 5950 as the design code.

The frame will be pre-fabricated under workshop conditions.


Reçerence in

BS 5"150

Tcoble Z


L

SUBJECT P age 1 Date

ExamEie No.2 Made by:

Usin'j desien code BS5950 Pqrl- l. 1'1'10

Nonni n<> l Frame b\:louf-

4·25 4-ZS 4 25 4·25

v ~ 4 25

IZ.Om VCen~re s~rur 3·5

Ro~lòcol v 4·25

~'v\lco l in5s l ll·Orn

w,lings

Assume rnembers conhnuous q long each side.

Frop lood trom sheerpile IM:Oil des'·':ln , l&bkN/m. (see c"lcu\ahon sheel- no.35)

For mi><ed granul"'r and cohesive Soils odd zo"/. l-o :~i ve

L oe>d o n i'r<>me ; zoo kNjm.

Use load ~Ciàor : 1·4 Por lood trorn so!'! (i e. de"d la<>d)

F.,. c l-ore d locod = ZOO >< l ·4 = 280 kt-J/m

A>< la\ load on walin5 ( ne><f- f-o ceni-r., sf-ru f-)

(a) . . 4·Z.5 Prorn ady,cenl- WCiilng 1n corner --y- >' Z.8o = 595

( ") fronn dia'3onco l àrul- 4-25 x ZBO = 1190 F = 1755 kN --

A\omenl- in wolin:;J ; WL/IO = 280 )( 4·25 2/10

L= 4·25rn

Mx, Sob kN. nn

Sh<:<>r torce Fv = 4·25/Z " 280 ; 5"15kl\l

159

160

SUBJECT E_xq mp\e No.2


\Ve> lings ( Conhnucd)

Propose.d

Sec~ion

352. ~T~ble 7

4 2 3

4. 2· b

Tr':J 356 x 368 x 177 kg U C. in grade 43 ,;\-ce(

D = 368 3m m r = 14·5 8 = 372·1 mm T~238 A9 ~ 2Zb cmZ

d= 2'10·2. b/T ~ 7 82 r ~ 15·2 d/r o zo-o

r, = 5 7200 cm4

z, = 3100cm3 Sx = 3460cm 3

r;x = 15·9 crn 3

Z'j = I100cm3

s~ = lb70cm 3

':l = 9 ·52 cm.

Desi:ln St-ren9\-h Pj = Z65t-.J/mm"- Por T~ 23·8 m.-.,

Effecfive Lengfh LE = l O x 4 25 = 4 25

for normcd !oadin:l cond1'hons

<>(z~s r2 = UI~t = 1 02

05 6/r z 8 5 E q nd d/T z 7'ì E r-he sec.h.on is p/qshc Class l

Shear 6ucklin5 Since d/T .(. 63

Shear Copacir~

Pv ~ 0·6 P~· Av (andAv=D·r)

buf

=o 6" 265 x 14s" 3bB·3 IOOQ

= 849 kN. > Fv = 595 OK

Fvj, = 595 = o·]O /' 0·6 Pv 849

so desi5n for rnornen\- copgcil-~ wifl- h'"jh sheor lood.

For plqshc sechon

Momenr CafX!cir~ Mcx = ~ (5x -5vp1)

h L aren Av ( t4· 02

) W ere Sv = plq sr i c mod<..l <.<5 oF Shear ~

: 1-45 X 36·82 = 4'ìl CY\')3

4 ond p1 = 2·5 Fv _1·5 = 2·5 x 5"15_ 1·5

Pv 844 - O· 2.5>


SUBJECT Page 3 Date

E_')(ample No. 2. Made by:

W al i n'35 ( Conhnued)

4.3.7. 7. Toble 19(o)

Table Z7c

4./3.3.2.

43 7.3.

4.8 3.3.1


Mcx = 265 (34b0- 4"11 x 0·2.5) /rooo

= 884 kN·m < I·Z. E:l Zx ( 9Bb)

Ok:.

X = DjT = 3b8· 3 = 15·5 Z3·8

Compressive s\-ren5f-h Pc = ZZZ. d.-H-o.

Locc,\ capaci ~Y check

_F_t Mx t~~ l A5 ·P;! Mcx Mc~

(Mlj =0)

1785 t 2.Zb x Zb5J\ 0

506

ll84 = O· B7 < J

Bucklin'J R'esisf-ance. Mb = Sx · Eb

O.k'.

= 34oo x 2.61/rooo

= 9o3 kN·rn.

Overal[ bucklin'J check (m= l, M~= o)

F Ag Pc

+ m-Mx + m·M'j ~ 1 Mb P':l ·Z~

178.5 + 506 = O ·92. z l O. k. 2.26 x 2z:;y; 0 903

161

162

SUBJECT Page 4-Date Made by:

Wa\in5s (conhl"'lLJed)

4 .5

4-·5 ·3

4·5 ·2 · l

Web bearin5 <:~nd 6ucklin3

Consider cel"'ll-ra l

Sl-rul-~ 3~1.3 20mm endplare 1:!

f::=FS~~ \Valing -

1368·3

!

! l

-- ------

St-ru~ IOCid 1190 k N

T= 23 ·8

$=f r =15·2

Local c~pac 1' tu = (b t ) '- R -.~ 1 n2 , . 'j vv

= 2.·5 x z x (zo t Z3·8 + 15·2) z '15 l"l"'li'"T'I

b 1 = lb ·Brn,.,

So l oca l Capaci!·~ = ( 295 t 16·8) x \4 ·5 x 265 j tooo

( :. 11"'18kN '? 1140 Ok

~Cip<'~cif-~ will be incre<::~sed if shPfeners are tixed are nxed in li'"'e ""' \-h 1-he web)

Bucklin~ re.sis~'"'ce

Pv., = ( b1 t n 1 ) t. P. c

b1

= l b · 8 m rtì

n 1 = Z x (368·.3/2 + 20) = 40B ·3 m m

For web A :. 2·5 d/t = 2·5 x z0 "'so T.,.ble 27c So Pc = Zo6 ~/mmZ

and Pw =- (1b·8+4o8 ·3)x 14·5 x 2.06/tooo 12.70kN? 11q0 OK

SI-FiF~ene~s required 1-opreven~ ro~Cih.on and lt:~tere~ l rnovemen\-~ <: rlan~~- use ISrnm fhid~shFFeners af-each junchon wi th sl-rut.s and adja~nl- walì~.

The proposed .5ec-l-'•on is O.k.

CJRIA Special Publication 95

Toble 2.

SUBJECT

E..xCI rY'l el e. ~c. L

Load on Fr<:~me Load on Sl-ru~

::zoo kN/ m = 200 x4·25 = 850 kN

:::: J.4-

Faàored load on Srru ~ = 850 x I-4-F -= 1/<10 k N

L = 12 ·0 rn

P age Date Made by:

Allow For load F robe eccen~n'c b';j Q dis~once ot IO'Yo

oP Depl'h oF Srrul- j.., verh'c"'l direc.hon.

use load Fac~or = l ·+

5

Allow 20kN at- qn':1 posihon and an~le ro ~~ru~- Fac~or-ed load = 20x \ .4 = Z.8kN. Wor.s~cond ihon al- cef'\h-e oF s~ruf-al"'ld hor.IZOI"'lf--o l

Propos.ed Tr':l 35 6 x 3b8 x 202 kg U C. in 'jr-ade 43 sh~el. Sechon

iable 24

Toble b

rc:~ble. 7


D = 374 · 7 mrn t- :::: 16 ·8 d ~ 290·2 b/T " b ·93 B = 374-4 T= 2]' o r = 15 ·2 d/ t ::: 17·3 Ag = Z.SB cm:z. Ix= b6300cm+ Zx :: 3540 cm~ z':1 :: 1260 cm 3

5x = 3980cm 3 s~ = 1920 cm 3

r x = lb ·O crn '':1 = '1 ·5J c m

EfPech've lentjf-h LE = 1·0 x IZ. ~ 12 ·0 rn

Design 51-r.engf-h P~ ::. 265 1-.1/mm z for T = 27 ·0 mrn

E. = l ·02. ( qs beFore P"''3e Z) b/r <. 5 ·Se and d/r < 79 e The Sechon i:S plqs.hc..

Hor'1zon~~~ Mornen~ due. ~o Accic:len ~al load on Sl-rucJ.

:::: WL ::: 1.8>< 12. 4- 4-

= Bt kN · m

163

4-· 2· 5

164

SUBJECT P age Date Made by: E.xample No.2

Verhca.l Mome-n!-5 on s!-rut-

Sei P ''""9h~ o P s~ruà = 202 x lz/Jooo

= Z·+x l·+= 3·+kW

DePiechon wifh un FacCored load = 5/354. IVL.3

EI

~ 5/384. _ _:2o._·4~. _:12.:::0=0_3 __

zos >< 100 x 66300

=o. o4 cm

Verhcal ESM due ~o seiF weigh~ anol eccencrici~y

(a) SeiP we,ght- =~L= 3 +; 12 = 5·1 kNm

{b) Eccencricl~'j IO% X 374 · 7 def'lech.on

Tof"' l

= 375 = 0·4 ~ 37· q Say 38m m

= Il q o x 38 = 45 · Z 1ooo My.= 5o· 3 kWno

soy 50 kN·rn,

Sheo.r forces are low so use.

an d

Mc = P!:j·S

M ex= 265 x 3980/looo Mcy = 2.65 x 1 9 zojlooo

~ 1055 khl·nn = 509 kN. m

1190 5 t o t- B+ l 55 x 26~o I0 55 soq

= 0·39 <l

O.K.

CIRIA Spacial Publication 95

SUBJECT

E..xo.rnp\e Mo.2

P age Date Made by:

7

Ce n t-re St-ru ~ (conhnued)

4·3·7· 3

+·7· 3

T ab le 2. 7c


Overall Bucklin"j Check

13ucklin:J Pesisf-ance Mb = Sx. Pb

AL. T = 0·5 (L/r':J) = o·5 ( 12.00/'l· 57)

= 62·5

Pb = 201 N/mm2

and Mb = 3980 x zoi/JOoO

= 1'>00 kN·m

Compressive Scren\lfh Pc

,\ = LE/r'j = 1200jq. 57 = 12.5

Pc = 90 N/mm2

_F_ t m· Mx + nn·M!J ,;;. 1 ~g. pc Mb Py · z~

= 119 o + so + 84 2.5B x a o;; o eoo 2.65 x IZbOj,ooo

= o. 83 <(,l

o 1<.

Proposed Sechon is O K

165

Tqble 2

Toble Z+

Toble {,

SUBJECT Page 5 Date Made by:

Load on Fro!"Y""e " zoo lqJ/m Load on Sfru f " 2 00 x 4. z.s x 12

c 1202 i< N Load FQc~or = 1·4

f,K~ored \oad on Sfruf F" 1202 x l·+" 1683 kN

Len~fh of Sfruf

EPFecf,ve len~ m

Allow eccenrn"cir~ ond OCcldenrol load QS Por cenrr<e sfruf (pc,je 5) Wi·fh X-X 'WIS horizonb[

Tr~ 356 x 368 x IZ'l k:J UC In •yode 43 sfeel.

D c 355·6 mrn r = 10·7 d= 290·2 bJr " lo·5 6 = 368·3 mm T" 17·5 r = 15·2 djT "27·1 A~ = lb5cm 3

Ix = +ozoo cm4 Ij = 14boo cm4

lx = 2260 cm 3 Zy = 790 cm3

Sx c 2450 cm 3 s~ = 1200 cm3

rx = 15·6 cm rLJ = q-39 cm

DesiBn Sfren5fh P:J = Zb5 N/mm 2

tar T= 17·5 mm

é " 1-oz ( os beFore po~e 2) 6/1 = 10·5 <( 15é and d/r "" 79f.

The SechOn Ìn Semi- Compoc~

Honzonh,[ Monnenf- due l-o acciden~al load on sfruf-

M~ = WL " 28 x b 4 4

=42 kNnn

1000

say BkN

Foc~ored load = 8x 1-4 =Il kN


4·Z.·S

4-8·3·2

4 3·7·3

+ 7. 7

Toble Il

4-·7·3

Tob\e 27c

CIRIA Spscial Publication 95

SUBJECT

E.>'C\mple t--\o. L

Verhca/ 13M

Page 'l Date Made by:

SeiF wei3hf WL = Il x b = 8 3 kN·m 8 8

E.ccenfrìcìf':l IO% D" 35·6

= 59 ·9 BM = lb83x 35·6 1000 Mx = b8·2 sa:~ 68kN-m

Loc" l Cqpqc,lj Check

Sheqr rorcee:. are low so

Me = P:J ·5

an d Mc" = 265 x 2.48ojJooo ~ b57 HJ ·m Mcy = 265 x 12.ooj1ooo ~ 318 kN·m

_F_t M,. + MLJ = lb83 + A:J P:\ Mc" ~ lb5 x

267i'o

= O· bZ. OK

Overa Il Bucklin5 Check

6uckling Res·,srance Mb= Sx · Pb

~ t 42 b57 31B

ALT =0·5 (L/r'j) = 0·5 ( 60o/9-3"'!)

=32

Pb = 265 N/nrm 2

ond Mb = Z4BO" Z.b5 = 657 kN ·m fOOO

A = L'/r:~ = 60o/9 · 39 = 6+

Compressive Srren<3fh Pc = IBB N/mm 2

167

SUBJECT P age IO

E:><" mele Nco.2 Date Made by:

DiC\goncol s~ruf- (con~int..~ed)

4.B.3. 3.1 F + mMx +m M~ ~l. (m • 1) Ag· Pc Mb p~.z~

- 1683 + 68 + 42 -165>< 188/10 657 2.65 y. 790/looo

= o 85 <l

Proposed sechon is OK.

Junchon beh.veen dragona\ sf-rurand walin;~

The axial load in ihe diagonol srrur resolves in\-o loads norma! ro li1e waling and parallel 1-o il-.

The load parallel ro ir has f-o ~ resisred 6~ the kicker plaf-es welded 1-o fhe f'"ce cf' fhe vvcding.

The load is 4·25 x zoo = 850 kN

wdh fhe load Faàor = 1·4-

The bearing load an kicker pia \-es

= 850 x 1·4 = 1190kN

D;agona J srru f-

~

+ + 1 t.----Walin5

Bolrs ~ ~i~ / ~8mm + + ~

li Fillèweld ·*'

l • 500mm ' l 7 Two kicker plal-es 125 x 300" zo mm l'hick: grade 43 sf-eel

End area = Z x 125 x 20 = 5ooo mm2

Bearing capqci~ • A.p~ = 5000 " 265/ 1000 = 1325 kN OK.

168 CIRIA Special Publication 95 Cl RIA Spacial Publication 95

SUBJECT

LenefhoPweld = (4x3oo)+(2xl25)

= !450 n->m

Use B rnrn p; llef weld

Weld cqpocir.':l = l '2 kfJ/mm

Tora l weld c"pacif~ = 1·2 x l4SO = 1740 kf.J

OK

Page Il Date Made by:

= 1700mm OK

Use B rnrn Frllef welds arot..~nd sfruf as beFore

169

170

Example No 3: Use of flow nel diagram

A cofferdam 10 m wide by 25 m long and 10 m deep is to be constructed in the following ground conditions:

3.0 m of grave! : 22.0 m of sand :

y = 18.0 y = 17.8

y, = 20.5 y, = 20.3

c,= O c'= O

with an impermeable layer below the sand. Standing water leve! is 2.0 m below ground leve!.

l. Construct a flow net diagram and calculate: a) Porewater pressures b) Factor of Safety against piping c) Factor of S"afety against heave d) Rate of flow of water into the cofferdam e) Factor of Safety for the stability of the wall using the gross pressure method - F,.

2. As a comparison calculate the Factors of Safety for wall stability using the same method (F ,) but with porewater pressures derived from:

a) Average hydrostatic pressures at the toe of the pile b) Uniform rate of dissipation of excess pressure along the faces of the pile c) Hydrostatic pressures

CIRIA Special Publication 95 CIRIA Special Publication 95

SUBJECT E><Cimple no.3

Use ot l'low nel- diagr<>m

~ ~ E -w--L._._E o ~ J

:c

~ N N Ji { ~

::r:

-" ·l t· ~

o -j

l l o 00 o o o '() ri"IN ,... JJ ,:.. N NN \J

~-~ -o ..<:. > ,., ~ c

~ ..r o J\ "' <f) D--;:: u 1

o...Jl V O ..J,.: ~~-c.>- E E " ~L. .:L

ull_

N


~ :J !;: q

~

Ji ~ E E " ~ li_"cr EJ:

-<Il

171

SUBJECT P age Date 2

Ex<>rnple no.3 Made by:

Pc.l'"e'-Vol-er fresst.Jre From Fio w Ne 1- Diagram

Consldec fhe çlow ne~ dlagram

Af on!::l poln~ olon'3 o Plow line fhe po~en ho l head

H= HdH,-H,_)Jcr

The nu m ber o P Porenha l Drops Nd =IO

Thus fhe dcop bàween equ<pofenhal line il h

L1h = (H 1-H2 )jNd = (z3-15)/lo

= 0·8m.

Take ow =IO kNjrn3

\Ila ~er Pressures are:-

leve l fielghl- Numbec Po~enhol Porewo f-e.r above oF Head Pressu1e Da~" m Dr-oe.s H Lt

z d H- 1:1 h. d IH-z).~w m m m kNjm 2

23·0 23·0 o 230 o

2.2·0 22·0 o 230 IO

15 o 15·0 l 2.2· z 72

10·0 IO·Q z Zl·+ 114-

75 75 3 ZO·b 131

7.0 7·0 4 l'l B 12-8

75 7·5 5 l'l o 115

8~ 89 6 lll .z_ 93

10·3 10·3 7 17·4 71

Il · 1\ ll·ll 8 lb 6 4-B

13 + 134 9 158 2.4

15·0 15·0 IO 15·0 o


Po5e l


SUBJECT

E. xc:! m P-le no. 3 F of S asalnsl- pipln~ Clnd hec:~ve Rare oF fio"' inro coFfecdann

From fhe Flow nd dlagrom'-

Nurnber"" of c.hannels l·n ho lf' w id rh oF coPferda,.,

Widrh oF coPPud"rn Nucnber of pofenhàl dcops

P age Date Made by:

Nr = 3

B = IO

N d = IO

3

Ocn

Toro l f.iead drop H 1 - Hz. = 8 ·O rn

= 8·0x Zx3 = oAB IO 10·0

= !,:.'2_ = l·o = Z·DB <e o·4B

Por-ei.A.I'a~er- pr-essure o~ ~oe o P pile =- 12.5 kNjm 2

The coeFFic<énf of permab,l,f-~

'he ra~e of Flow

= /{,2. x 1·2.7 12.8 --

k = IO-+ rn/sec

Q= k(H,-~ 2 )~~ = lo-+x B·ox ~o x bo•

=-0·<36 rno/hou;fm. run

op wall

Af fhe_ corners o~ fi-e coPFercbm fhis F'low will nof be sf-ridi:J correcr "'"f- por pradlcCll pLtrpose ~~ will be suPFicienr ro eshm"fe rhe ~o~al flow 'n~o !he coFPerd"rn as,

Q x perirnefec ~ 0·86 x (<o+zs)xz rof-ql Flow = 60·2 m:Yhour

173

174

SUBJECT Example no.3 Page + Date Made by:

From flow nefdiagram

76 '13

115

128 80

48

P~..ssive

Level 25·0

23·0 22·0

10·0 89

7-5 7·0

H~drosl-"1-ìc pressures

Ac.hve

114-121

For uni Porm rqf-e o P dissiP"hon o P di H'erenh<~l he"d "long a Plow P" l-h (~) adjacenf- 1-o lhe wall

Level25·0

23·0

Uni Porm rare or dio;siP-ahon an d a verage h!:jd rosi-o hc

UniPorm ra~e oF dio;sipahon

Average h~drosl-" h c aF 1-oe

80 7.0

Pa.ssive Ac.hve

8·0 x IO

23·0 = 3·48kN/m2

' '

per m. run

Average h~drosra~ic al- 1-oe

bi'-. so ' ' ' ' '

' ' ' ', ' ' ' '

l bo

CIRIA Special Publication 95 Cl AIA Special Publication 95

SUBJECT Exornple no. 3

Achve Earth Pre.ss.ures usin:3 pore.vve~,~e~"" pre!Ssures ff"orn fhe Flow Nel- p,·agram

C.alculafion or Pres.oure"' (ali unì f-s in kN an d m.)

N

o o

o

o

o N

o() !f\ lll

o

o N\

. o N . ~~oSa· aoo

''' n N±± Il l\ il 11 11 Il -i:l.:U >o~ "u oo1 >c

o 1(1 N

o N\ N

o N N

N N\

o 1(1

N N1

N r--

" o N

o «<

o o 9 <il

Nlo o IV\

~ ~ o~ 6 li Il Il 11 11

>o~ù.:c{l_

o o L{\ o

o r/1 N

-JI N\

o a-

<il N

r-Nl

o <{)

o

Page S Date Made by:

175

al

Q JJ

> .g>

l ., c g; g :s· ::>

"' '"

Q JJ > (/)

" ~ i[ ., c 2: ii ~. o ::>

"' '"

::::1

Reduced leve[

15.0

11·8

8·9

_lS

7·0

G-round Para-

mel-ers

~=20·3

•l= 350 ' C =O

s =20° Sand

kp= S·7

211

Deprh Tof-al Pore i'_ verhca! wa~er

st-ress pressure

6v u (~w= 10·0)

o o o

3·2 65 4-1'>

6·1 124 93

7·5 152 115

B·o lbZ 12.B

Pqssive Earf-h

Fn::~ rne n o. l

Frame no.2 __ _.._

97

E Ffechve EFFechve lof-<el Mihìmum Desi5n Verhcc.l horiz.onh:<4 l horizonf-al equivalen~ value sf-ress pr-essllre pre.s.sure flui d ov' P.' P p pressure p

s. ii': o o o N/A o

17 97 14-5 97

31 177 270 177

37 ZII 32.6 2.11

34 1~4- 322 194

Adive Eqrth Nef- Waf-er

2

13

53 56

K'.L.

25·0

23·0

22·0

17·0

15·0 +

IO

54

"lÉ" 10·0 3B 8·9 28

7·5 lb TO

'.72

~ r n c r }-4 6 z. o ~

'J ?>

"' "' "' c i'J ~ ~

:t c :0

" ~·

>c z o ~ <L

3 ~

c (j) '!'. c ~ OJ

LO '-m 11 o ~ ;,o -l

~ "' !!'. or {\ ' o -u rr1 f'1

-·m ~ 3 nln ,.3 ll:lc: ::rli , ~ D" ]) -3 , ro =p ~

a "' z. 3 "' o :r ; (jJ

" ~ , "' o

"' z ~

'

:;;:o-o oo a~ g-coro rr

""

~ l) ~ , ~ ~ c (}' ~ c IJ\ ~ ~~ O D

3a l :::?3 o fJI

~ c ;:1 ~. " :J .,-(.D

9:--u D O

dl , :l ~

~ g .ir ,

O'

(j) c OJ 'm o -l

[01 x D 3

-u

" :J 9 w

:;;:o-o

"' "' "' Q.~ co (!) (!) (!)

rr

"" '----l

178

Nef Wo~er

SUBJECT

Exarnple. no.3 Page l3 Date

Srobilir~ op Pile Woll usm9 porcwo~cr pr-es.sures t'rom P/ow nef- d7ogr~m

Made by:

Assume a ht'n5e af level oF Frame no.2 K'L + 17·0

Toke monnenf.s oF pressure obou~a ~lovv tt,,·s level

Arect x

Yzx Z7x 2·0 "270 Yz. x 32 x Z ·O o 3Z· O

Yz x 32 x 5 o " Bo·o !ix 45 x 50" IIZ·S Y:z.x 45 x 2·5 ~ 56·3 ~x 53 x 2·5 " 66·3 YL x 53 x o 5 o 13· 3 Jlz x 5b >< o·s =- 14-o

Yzx54xzo Yz >< 72 x Z·O

~x 72-x 3·2 !fx51x3Z ''lxslx 1·8 Yz x 3tlx IB Yzx38xll YzxZBx 1·1 v, x '-8 x l ·4

= 54-·0 ~ 72·0 = 115· 2 = 81· 6 ~ 45· q = 34· 2 o 20·Cj

o 1'5 4 o 19· 6

~ x 16 x l· 4- :: l\. z Y:èx16xos =+·D

Yz x 97x 32 = 155·2 V, x 97x 2·9 o 14-0·7 Y,_ x 177x 29 o 256·7 Y,_ x '77 x 1·4 = 123·'! Jix211 x1·4" 147·7 Yzx ZII xOS= 52·5

!ix 194x05" 48 5

F o p 5 - Fp =

LeverArm

y3 x 2·0

33 x 2 o (73 x so)+Zo W3x50)+ 2 o (~xZ5)t70 (%x 2·5) + 7·0

( !3 x 05) + 9 5 (3§ x os) + 9 ·5

.1:3xzo

~x Z·o {J3x32) +ZO

(~x32)+zo (13 x l B) t 5 2 (l>§ x 18) +5 Z (!3 x 1·1) +7·0 W3x 1·1) + 7·o (Y3xl4)+81 (7§ x 14) +B l (!3 x os) + 9 5

("3x32)+zo (!3xz'l)+5Z (~x2 9) +52 (i3x14)+81 (73 x l +l t 8 l (]3 x O 5) t9·5 (73xos)t95

b 723·2

2Z.35·9 t 1888 'b

= Morner,~

18·0

42·7 293·3 600·0 4-41· o 574 6 128.6 137 7

2235·9

36 o 96·0

353·3 337·3 ZU·z 218 ·9 154 ·O 119. l 16 7 ·9 IDI· 2

36 7 o 1888.6

b41·S

8677 1831. l IObl-4 1334. 2 510·4

476 9 MF=b723·2

: l· b3


~.~...E.

N

o

N

N

SUBJECT

E)<ample no, 3 Ec:trrh Pr·essures Ust'n'5 Avef"a.ge H~drosfd-;c pre5sure af- f-oe of" pile

o o .n o o-

o

i z

rfO

o..Q_ o ~ c u o • N

c:...-·L. 4-0 wL ~~~--~--~-----t----t-~---+---~


o r()

± ~ J

"' ~ " 1----+---+--+----.j--- o':

o

o cf) N

o N

" > 2

0

o !il N

o ri]

o N N

(!'

o N

o o

o ciJ

m o o M

~ ~ o~ 6 1\ 11 11 il Il

)o)s_ÙdO~

o t[)

" c cr

<f)

o r--

\)

>

o

o

o

o

N ...

o N

N ..!)

o r()

1\ Il Il li Il

Xl\s.'ì.Jro~

o

-cl c CJ

Cf)

o


179

180

Pressure diQcarams

Acl-i ve eadh

Ne~ wal-er

Passive eadh

SUBJECT Example no.3

Sl-abili\-'j oF wal l using 9ross. pre-ssure-s and porewal-er pressures rrom avera~e h drosh:~~ic pressores qf-lhe f-oe o~ ~he pile.


Achve earH1

Frame no. l -----'-1

2.39

Assume<=~ hin5e ol- Prame no.2 a l- I<L t 17·0 T<=~ke momen~ aboul- cmd belovv fhis leve l

Area )(

Jf x 28 >< 2 ·0 = ZB·O

.!z x 34- >< 2 ·o = 34 ·o Vz. x 34 x B·o = J36·o .l2 x 58 x B·o =Z32·o

\2 X 46 x Z·O ~ 46·0 ~x6 1 x Z·O ~ b i· O l2 x 61 )( 8·0 : 244·0

~x 23'lx8·0 = 'l56·o

Lever arm

_)3 .x Z ·O

33 x 2 ·O (~ .>< B·o)+Z·O (% x 8 ·o) t 2 ·o

J3 x 2 ·0 %x2·0

(Y3 x B·o) t 2·0

:: 7010)

25·0

61

= Momen~

18·7 45·3

634 · 7 170 l• 3

MQ = z+oo ·3

Mw =

Mp =

30·7 8 1·5

1138·7 12.50· 7

70 10 · 7

F oF 5 - Fp : MP MA+Mw Z400 · 0 + \250·7


SUBJECT Eadh pre.ss~..u-e s t.4.S i nc:3 porewe~l-er

pre.ssures derivecl fro rn 1..1n i forrn n:=~ h~ oF d iss 1 pe~ \-i o,-, "l long H,e p ile.

Ca\cl..l l o. h on of Pre.ssures ( A l\ uni f-s in kN and m .)

N

o o

N

N

o l[}

o C)

o

Q..(L o

~o

Page I l Date Made by:

r-----~--+----~--~~-+--+---- ~~~-----4-------~ \)_ · ~ Cl \1) 111 ~

_l... <.l <11- ~ o -.!) ('(l 11'1 :l ~.:.C~2 ~ - + r--. L() -..D 111

'-'- !.... _j_. u J lO - ~ l .fl

o ~~<Il li) ' " "'

li) ~ ~----~---9~-n~+-r-------1------1----+----+-----~ r+-------+------------~

L :::1 0 LL

~.E~:S" \l o o o 'Ù rJJ Il.)

o.:>~~ 5 > - l[) 2 ~ o

cO o

CL ~ .:_c lfl l--------~-- u,+-+-~~~----~----~---+---4----- lfl~-------4------------~

~ ~ ~

-o c ~ o '-

0


o os: -l:!

o

::l Cf)

o l[l N

o N

o {"()

N

o ('(l

o N N

tr" o N

o o o C()

Il Il Il 11 li

>O ~'uGO ~

----cl c a

<fl

o o l[) r--

o

o

o l[}

o cO

---o c a

<.f)

o r-

181

182

Pressure d iagrams

Passive ear~h

SUBJECT E.xClmple no.3 Page 1 z St-qbil ir'j of wall u.sing eross pressures

Qnd porewo~er pre&sòres der-ived Prom uni rorm ra~e d issipahon alon'ò fhe p ile

Date Made by:

PCissive ear~h Ach've earth

z

308

Assume a hinee a~ frame no.2 RL + 17·0 Tqke momenl-s Qboui- Cind 6 e low H-lis levei

60

25·0

23 ·0 22·0

15·0 56

A reGI >( Leverarm = Momenl-s

y2 x 30 x 2 ·0 = 30·0 ~ )( 2·0 20·0 Yz >< 35 x 2·0 = 35·0 ~ )( 2 ·0 46·7 Yz. x 35 x B·o = 140·0 f Y3 x 8·0) + 2 ·O 653·3 Yz >< 60 x 8 ·O = 240·0 %x 8·0) +Z ·O 1760·0

MA = 2480·0

)Z x 43 x 2·0 = 43·0 J3 x 2 ·0 lB·7 Yz. x 56 x 2 ·O = 56· o %x Z·O 74 · 7 ~ )( 5b )( 8·0 = 22.4·0 (Y3 x 8·o) + Z·O 1045·3

M = w 11 48·7

Jt x 308 >< 8·0 = 1232·0 (~x B·o)+ 2·0 Mp = '1034·7

Mp _ '!034) F oF S - Fp = MA t Mw- 2480·0 + 1148 ·7 ::. 2·49

Fp =- 2-49


SUBJECT ExCimple no. 3

Eor ~h Pressur es us i n:l

t-1ljdros h::! h c porewa~er p ressur'es

Calc.u loh on oP P ressures (A l i un'1ts in kN qnd m.)

o o

o () M '<t'

o o 1.() o-

o

N ('() N o o

o N

Q_Q. o

~_E ~ c

u Q al N

<...l.... ·c. LI... Q W L

o ('() a_O... o

Vl ~- ~ .:;: cr V1 Vl L

-t . ~ 1(1 - > ~ o -..J) l;ì ~ = ~ ~~~b ~ ~ - N ~

o N cO


~ ~ (/) ., 'U

1--- - =J- ~ 1------l--- -1----+--+- + --ct 1--1-- ---+-- ------j ~ .-,...0 111

3 9 0: u \11\Jl o o o >

::1 " o o cO ._!) · - o o co ~ ~ ~ - ~

a.. .!'3 4-- 1------Jf-------+-----+---t---t- -- \Il 1-1------+---------j 1------t- u ~ cf

1/) l 1.. Ci eu L J..

cf q)

E

-o c :J o L

0


-< e' o-2 - ~

o

o 1.() N

:J

~

o N

o (l')

oo N

o o

o (Xl

1'1'1 IY1 O~ b N N m 0 N 0 H Il Il Il Il

)O~Ùc..Q~

o o o N Ul r-N

o

o

o li)

-u c o

<1)

o r-

183

184

Pre55ure dia~rams

Ac~ive earfh

t-le!-wal-er

Pe~s.sive earlh

SUBJECT E)<ample no.3

Sh:abilir~ ofw~sing g ro.ss pr~ssure.s and porewal-er pressures ~'lual l-o

hljdros~ahc head.

Page 14-Date Made by:

Passive earlh Achve ear\h

Frame no.I

Frame no. 2 --~>-~...,.25

30

Assume a hinge a~ level oç' Prame no. 2 Take momenf5 aboul- and below fhis level

Area x Lever arm --.!t )( 25 x 2·0 ; Z5·0 l3 x 2·0 Yz x 30 x Z·O = 30·0 ~3 x 2·0 .Vz x 30 )( 8·0 ; 120·0 [)3 x B·Oj +2·0 ~x 49 x 5·0 = l'lb·O 'Y3x8 ·o +2 ·0

Yz x bo x 2 ·O :: 60·0 .!3 x 2·0 ~xBOxZ·O = Bo·o 53 .x 2 ·0

Box 8·0 = b40·0 ( J2 x 8 ·O) t Z · O

Yz x 467 x 8·0 = 1868· O (:Y3x 8·0)+2·0

M p 13,698·7

RL 25·0

23·0 22·0 IO

t T O-t--...... bo

15·0- 80

1·0

So RL + 17 ·0

=c Momen~

lb·7 40·0

5bO·O 1437· 3

MA= 2054·0

40·0 10b·1

3840·0

Mw = 3986 · 7

M p = 13,698 ·7

F or s- : = 2·Z7 Fp =MA +Mw 2054 ·0 + 398b·l

Fp = 2·27


Example No 4: Earth pressures for layered ground wlth non-unlform slopes

A cofferdam is to be built at the foot of an irregular natura! slope.

The ground is layered sand, grave! and clay.

Calculate the active earth pressures on the wall nearest the sloping surface.


186

Example no.4 SUBJECT

Appendix B

ExAMPLE.. No.4

P age Date Made by:

1

EARTH fRE.5SURE.S FOR LAY.E.R.ED G-Rc:JL . .lND w tTH Nor-..1 l.\N t FORM

5LoPE..5

/ / / /

l / . l

A6z·s~/ / //

6· 4 ..,..,...,,....,.,...,..,.,.,..-f /

i B+ . -'1 ·0-

/

&2·5° l

A4

Sand and 5ravel 9J' '= 3Zo 1S = 17 k N/m 3

Sand and grave! )?5

1= 35° ~= 18kN/m3

To calculahe verh cal s l-ress a~ the back oP 1-he wall :

Sei- oul- cri l-ica l Fa ilure p lanes( 45° + ~-) relevanl- l-o sl-ra~"' in l-erFaces, surPace proçile and e>CcavQ h tn level.

line Al - BI drawn Prorn Fool- oP slope downwards Line A2- 82 drawn Frorn scmd cmd graveljcla!:l ìnl-erf.'ace opwards Line A3- 83 drawn From excovahon leve l upwards Li ne A4- B4- drawn rrorn ~op oF s lope downwards

Levels BI and 84- are derived From t·n~ersechon oP F'adu re planes wilh back oF wa ll.

CJRIA Special Publication 95 CIRIA Special Publication 95

SUBJECT Page 2. Date Made by:

VE.RTI CAL STRESS 0V

= OkN/,rn2

Al +O·O = o :: 8·5kN/m 2

AT -o s = 17x.0 ·5 (20 ><0·7) = 22·5 kN/mz. AT -l . 2 = [17.x 05)+

7'l ·ZkN/m 2 17 x l . b)+ (zo >< 2 ·b\ ::

AT - 3 ·3 ::

( 18 x 3 · l ) = l S 7 · l k N/ m 2

Al - 6 ·4 = (17x 2 · 'l)1- (20 x. 2 ·6) + ( 1 B x 5 · 7 ) =- z l 'l · Z k N/ m 2

AT -Cj · O ::: (l 7 x 3· 8) + (20 )( 2 ·6) +

1 r 1 • novv procee:d~ e~s Ca lcuiClrion o~ acrtve pre~~ur-es b l . l ~~·, rhe a ove va ues. snown t n E:XCl nlple no. U 'j

L · b d 00 ,- p<=~ ssi ve A s i m i lar consl-r-uc rt on ca n e u &c:: P p res.sures usi n'3 an an'O ie or

187

188

Appendix C Safety regulations

l. Copy of selected pages from:

Records of weekly thorough examination of excavations, cofferdams etc.

Fonn 91 (Part l, Section B), Factories Act 1961 with notes and regulations.

Reproduced by kind pennission of the Controller of Her Majesty's Stationery Office.

2. Copy of Part IV - Excavations, Shafts and Tunnels and Part V - Cofferdams and Caissons

Reproduced by kind pennission of the Building Employer's Federation from its publication 'Construction Safety'.

CIRIA Special Pub/ication 95 CIRIA Special Publication 95

"' u -'" '" o :>< o< w z

<:: 2 c "' ~ < <:: "' c "' '-l "'

:>< !-

"' w ~

< 2' o< w :r:

189

:g

Q JJ i> m

"O

~ ei -o c E: ~· ~. o " ~

o 55 i>

.g> "' Q. e. -o c E: ~· ~. o

" <O

"'

"'

Notes The pages of this register are set out in the form prescribed by the Secretary ofState for Employment for recording the results of weekly examinations of excavations, shafts, earthworks, twmels, cofferdams and caissons (regulations 9 and 18 of the Construction (Generai Provisions) Regulations 1961, the text of which is given below).

2. The results of the examinations must be kept on the si te to which they relate or when so provided for by the Regulations, a t an office of the employer for whom the examinatìon was carried out, and must be kept available for inspection by HM lnspectors of Factories for at least two years (or other prescribed period) after the date of the Iast entry therein (section 141 of the Factories Act 1961).

3 The name and address of the employer or contractor for whom the examinations are carried out must be shown on each page.

4 This register contains only Section B of F91, PartI. Other registers for Section A (Scaffolds) and Sections C to F (Weekly lnspections, Examinations and Special Tests 0f Lifting Appliances) can be purchased from HMSO.

For reports on other periodic examinations of lifting appliances (including hoists), chains, ropes and lifting gear and on periodic beat treatment of chains and lifting gear-See F91 (Part Il) obtainable from HMSO.

SECTION B Const:ruction (Generai Provisioos) R.egulations 1961 Excavations, shafts, earthworks ami tunnels (Regulation 9) 9 (l) Subject to the provisions of paragraph (4) of this Regulation, .every part of any excavation, shaft, earthwork or tunnel where persons are employed shall be inspected by a competent person a t Jeast once on every day during which persons are employed therein; and the face of every tunnel and the working end of every trench more tban six feet six inches deep and the base or crown of every shaft shall be inspected by a competent persona t the commencement of every shift.

(2) Subject to the provisions of paragrapb (4) of this Regulation no person sball be employed in any excavation, shaft, earthwork or tunnel unless a thorough examination has been carried out by a competent person-

(a) of those parts thereof, and in particular any timbering or other support, in the region of the blast since explosives bave been used in or near tbe excavation, shaft, eartbwork or twmel in a manner Iikely to bave affected the strength or stability of the timbering or other support or any part thereof; and

(b) of those parts thereof in the region of any timbering or other support or any part thereof that has been substantially damaged and in the region of any unexpected fali of rock or earth or other material; an d

(c) of every part thereof within the immedìately preceding seven days;

Provided that sub-paragraph (c) shall not appJy to timbering or other support which has not been erected or installed for more than seven days.

Name or title of employer or contractor

(ii)

(3) A report of the results of every thorough examination required by paragraph (2) of this Regulation, signed by the person carrying out the examination, shall be made on the day of the examination in the prescribed form and containing the prescribed particulars:

Provided that in the case of a site where the employer for whom a thorough examination as aforesaid was carried out has reasonable grounds for believing that the operations or works will be completed in a period of less than six weeks, the provisions of paragraph (3) of this Regulation shall be deemed to bave been satisfied if the person in charge of the operations or works carried on by that employer at such a site has hirnself carried out the examination and is a competent person and if within one week of the date of *he examination he reports to his employer in writing the results of such examination, and the date of such examination and the results thereof together with the name of the person making the examination are entered by the employer in the prescribed form together with the prescribed particu!ars.

(4) This Regulation shall not apply-

(a) to any excavation, shaft or earthwork where, having regard to the nature and slope of the sides of the excavation, shaft or earthwork and other circumstances, no fall or dislodgement of earth or other materia! so as to bury or trap a person employed or so as to strike a person employed from a height of more than four feet is Jiable to occur; or

(b) in relation t o persons carrying out inspections or exarninations required by this Regulation or actually engaged in timbering or other work for the purpose of making a piace safe, if appropriate precautions are taken to ensure their safety as far as circumstances permit.

Cofferdams or caissons (Regulation 18)

18 (l) Subject to paragraph (2) of this Regulation, no person shall be employed in a cofferdam or caisson unless it has been inspected by a competent person at least once on the same or preceding day and unless it has been thoroughly examined by a competent person-

(a) since explosives bave been used in m near the cofferdam or caisson in a manner likely to ha ve affected the streogth or stability of the cofferdam or caisson or of any part thereof; and

(b) since the cofferdam or caisson has been substantially damaged; and

(c) in any case within the immediately preceding seven days:

Provided that sub-paragraph (c) shall not apply until seven days bave elapsed since the cofferdam or caisson was erected or placed in its position on the site.

A report of the results of every such examination, signed by the person carrying out the examination, shall be made on the day of the examination in the prescribed form and containing the prescribed particulars.

(2) This Regulation shall oot apply in relation to persons actually engaged in the construction, placing, repairing or alteratioo of the cotferdam or caisson or canying out inspections or examinations required by this Regulation if appropriate precautions are taken to ensure their safety as far as circumstances pennit.

Factories Act 1%1

Construction (Generai Provisions) Regulations 1961

SECTION B

Address of si te

Work commenced-Date ...

EXCAVATIONS, SHAFTS, EARTHWORKS, TUNNELS, COFFERDAMS AND CAISSONS

Reports of results of every thorough examination made in pursuance of Regulation 9(2) of an excavation, shaft, earthwork or tunnel or in pursuance of Rc:gulation fferdam or caisson.

18(1) of a

---~

l

l Date of Result of thorough examination Signature (or, in ca~e where Description or location

examination State whether in good order signature is not Jegally required, name) of person who made

the inspection

(l) (2) (3) (4)

-·

' --

·-- -~------- --------- ----- -· ·-··~-·-~~--···----~ -- --- -~ ~---·--·- -- ----· --- ·~---

l i~~ l

l --------· l

l ---~-----

l

l -

l

l --- ~·----·-·---------~

l - ·---·- ----------·-&e Nott'S and Rt'f(u.lations 9 and 18 on pagt' (ii) of cova

PART IV

EXCAVATIONS, SHAFTS ANO TUNNHS

8. Supply and use of timber.

Official T ext (l) An adequate supply of ti m ber of suitable quality or

other suitable suppon wll where n~ry be providod and used to prevenl, so far~ is rea.sonably practtc.abk a.nd a.s e.arly a..s is practicable in the course or thc: work, ~ IO any person <mploycd from A fall Or dislodgement or eanh, rock or other material forming a side or the roof ol or adja.oent to any excavatton, shafl, earthwork. or lunnd:

l'rovided that this Regulation shall not apply-(Q) to any excavation. sh.aft or earthwork ~· havin~ reg;.m.l to the nature O:tnd slop<: or the skles ~ the ex(:;;jvation. shaft or earthwork and other orcumstances. no f;rtll or dislodgement o( unh or other materia! so as to bury or trap a persoo employed or so as hl strike a pers<l:' e.mployed from ò1 hdght o( more than 1.20 metres IS hahle W tlCCUI'"";.

or {b) in relation lo a ~ a~uatly engag~ in lim

bering or orher work. whK.h 1S bemg carned out roe the purpcse of compliance with this Regulat~ if appropriate pro::autions are rakcn to cnsure bis saJcty a.s far as circumst.ances pennit.

(2) In thc case of tunnelling operations on work.s of engineering construction, no perwn shall be hc:!d not IO ha ve complled with a requirement of thc forc:gomg pan.g.rnph of this Rcgulation by reas.on of any mat~c:r proved to ha~ been due to phystcal conditions over wh1ch he had no contro1 and aga.inst whlch it .....-a.s not rc.asonably PfX· tk:able for him to makc provision..

Simple lnterpretation (11 In any excavation. Shaft or tunnel, ti m ber.

trench sheeting. or other shoring equipment must be used where necessary to prevent danger to persons from falling materials.

This Regulation does not apply:

(al where there is no risk ol materia! collapse. or persons being strucl by materia! falling from a height of more than 1.20 m fe.g. due lo a safe slope to the sides of an excavationl. or

(b) in relation to those engaged in timbering or shoring operatìons, provided reasonably practicable precautions are taken to ensure their safety.

(21 This Regulation does not apply lo tunnelling operations in circumstances which an employer c.annot be expected to guard against or contro!.

9. lnspections and examinations of excavations. etc.

Offici81 T ext (l) Suhjat to the provìsions or r;Jragraph (.t) or th-5

Re~uli:uion. tvcry part of otny cxc-dvauon. Mklft. e~n1hv.-nrt or tunnel v.-hcre persons are employed ~h.;.lll ht.- ms~cted by "' competent pcrson al lca..'l once on ev~ry da~ durin&. which Jlef~lOS ;sre employed thercin; and tht.• f:1cr or ('V('fV

tunnel 01nd thc= working end or evcry trench more than ~ mctr~ dcl!'p otnd thc ho1se or crown or ewry sh;•ft ~hall No inspeC1ed hy a compete n! (len.tln 011 the cnmmenct.'m.:nt nf evcry shifl.

(2) Subjecl to thc provisforu or paragr2ph (41 or thK Regulation no pcrs.on shall be cmployed in any c:ll.cavatkln, shaft, earthwork. or tunnel unlcn a lhorou(!h cxaminatton has been carrted out by a compc:tent person-

(n) or lhos.c parts lhc:reof. and in partic:ular any limbering or othcr support. in thc rcgion or thc: btast sinoe tllplosives havc been used in or nca.r the excavation. shaft. e.arthwork or tunnel in a manner likc:Jy lo ha ve affccaed 1he strength or slability ol that timbering or other support or any part thereof; and

(b) or lhose parts thereofin lhe region or any timberings or other support or any part lhcrcof that has been oubstantiany damased and in the region or any unexpected fati or rock or ean h or other material; and

(<) or cvery" part lhereof within lhe immediately pre· c:eding seven days:

l'rovjded ihal sub-parapaph (<) &hall not apply 10 li!Jo.. berins or Olher oupport whicll has not been erected « insl.alled ror more than sevm days.

(3) A repor1 or the results or cvery thoroush examinatiooa ""luired by pe.ragnoph (2) or ibis Regulation, sisned by lhe penon ~ out lhe examination, shan be mode oa

192

Simple lnterpretation (1) lnspection must be m ade by a competent per-

so n:

(a} every day that menare working in any type of excavation, shaft, tunnel. etc..

(bi at the beginning of every shift. of: (i) tunnel working faces.

Iii l working ends of trenches more than 2m deep. and

(iii! the base or crown of a shaft. No record need be kept of these inspeaions. 12) Thorough examination of every f\'pe of excava

tion must be carried ou1:

(al after explosive charges have been fired,

(bi alter any damage lo limbering. trench sheel· ing. etc .• or after any fall of earth or collapse of materia!. an<l

Ici in any case. every seven days.

(3) A record of Illese thorough el<l!minalions must be made in the official register: Form !li PM 1 Section B.


the day or the examination i_n lhe presaibed form and containW& the presaibed panoculan:

Provided thal ia lhe """ or a site where the employer for wt.ooo o thoroush examination as afo.--id wu canied ouc has reasonable srounds for believins lhat the operatiom .,. works will be completed in • period of less t han sia .-~<s. the provisions or p&rallrllph. (J) of llus R.<aul.ation shall be deemed IO ha..: been satosfied tf lhe person ia eharge of the opentions or worlu carried on by that.empleyer al such • sito has himself carried out the ex.amination and is a competent person and 1f Wlthm o~ wuk o1 the date of the examin.ation he reports to h1s e.mployer in writing the results of such exa.mination, a.nd the dal& of such examination aod the_ results t~f together with 1he name of lhe person malong the examma· tion ...., entered by the employer io the prescribed form logether with the prescribed paniculan.

(4) This R.egulatiOn shall not apply-(,g) to any ex.cavatlon. shaft Of earthwork. ~here.

h.aving regard t o thc nature and slok of the s1des ~f tbc excavation. shaft or earthwor and other C1fo cumstance5~ no fall or dislodgemenl of earth or oCha materiò!l ro as to hury or l rdp a per'!'>on emo&oved or so as to strike a person employed from a ~~ha of more than 1.20 metres is liah\.e to occur:

"" (b) io rdalion lo pcrsons carrying. out inspect.ions or eu.minations required by thiS Regulattan or actua11y eng:agcd in timberin.g or other work . (or 111e pÙrpose of making a plroe safe,_ if appropnate Pfl!Ciulions are taken to ensure the1r s.arety a.s far .a:s circumstances pcrmil.

Where operatòons are expecte<l lo be completed within si• weelol. an<l records are 11()( kept on site. the examination must be earried out by the site agent, or loreman in charge. provided he is competent, and the record completed within one """""-

(41 This Regulation does noi apply:

(al where lhere is no risi< of earth collapse, or persons being Struck by materia! falling from a heigb'l of more than 1.20 m, or

(bi in relation lo those engaged in timbering or shoring inspections or operations. provided reasonably practicable precautions are taken for their safety.

10. Supervision and execution of timbering and other work

Official Text (l) No timbering or other suppon ror any pan of an

c:xcava~tion. shaJt, ea.rthwork or tunnel s~l1 be crected or be substantially added to, altered or d1smantkd except undcr the direction of a competent perwn and so far a.s possible by competenl workmeo _possessing adequate expc:.rienoe or such work. Ali matenal ror any such w?rk shall be inspected by a competenl peno~ on e.ach occ.a.s•_on bcJon: being taken into usc and matenal found defectave in any rcspect shall noc be used.

(2) Timbering or othet support ror any pan or an excav:ation, shart, earthwork or tunnel shall be of good corutructton, sound materi.JI, rrec from pater.tt d~f~t and of adequ.alc Slrc:ngth ror lhe pu~ for whteh Il IS used .and sh.JII be properly maint.ained.

(3) AH struts and braces in any exc:avation, shaft, ~rth~ "'ork or tunnel shall be properly .a.nd adeq ua1ely secured so as to prevent thcir accidental displ:ace.ment or fall.

Simple lnterpreraùon (1) Ali materia! used lor timbering or sheeting must

be inspected before use; materia! lound defeetive must not be used.

Timbering and sheeting must only be er~t.ed, altered or dismantled under competent superv1510n and, wherever practicabfe. by experienced operatives.

(21 Ali limbering and sheeting must be properly constructed and maintained in good order.

{3) Struts and braces musl be secured against accidental displac.ement.

11. Means of egress in case of flooding

Official Text (Il In any excavation, shafl oc tunnel where there .i>

rea.soo 10 apprehend danger to persons employcd 1~11 from risin& watu or (rom an irruptioo of...,.....ter « mate.n.al there shaJi be provided, so (ar as pr&ct!Qble, means IO enable sudi persons to reach positions of safety.

(l) , .. the """' or tunndli!ll opcntions "" woritll o( engi~ construaion, no penoos ahaJI be hel<l - to ..... eomplied with & requirement or lhe foreg<>inl pani• llf1lPI> or this Regulation by n:ason or any maa•or l>fO"e<< IO ha"" been due IO physi<:al eonditions 0- -..bici> be ha4 no COO>Irnl and opinst w1tic1> it ....,. 11()( rasooably p<acticable ror him to malte provision.


Simple /nterpreta!i<Jn

{1) Il there. is risi< ai llooding in ony e•cavation. shalt. or tunnel. ladden or othe< ,.,.,., .... ol eseape ITIU>ll be providecl.

(2) This Regulation does noi apply 10 tunnelling operatioos in cirwmstances whi<:h .., employer ca.,. no1 be expected to gUMI against e< contro!.

193

194

12. Excavations, etc. likely to reduce security of a structure

Oflicial T ext (l) No excavation, shafl, earthwork or tunnel which is

likely to reduce, so as lo endanger any person employed, the securily or stability or any pari or a ny structure, whelhcr temporary or permanenl, shall be comrnenced or conrinued unless adequale sleps are taken before and during lhe progress of thc work IO prevenl danger IO any person employed from collapse of lhe slruclure or thc fall or any part thereof.

(2) In lhe case of tunnelling operalions on works of e ngineering conslruclìon, no pc:rson shall be held not lo ha ve complied with a requirement or the foregoing parasntPh of lhis Regulation by reason of any matter pro\'ed 10 ha ve been due lo physical condilions over which he had no contro! and aga insl whicfl il was noi reasonably prac· licable for him IO make provision.

Simple lnterpretation (1) lf any existing building or structure is likely t o be

affected by excavation work in the vicinity, shoring or other support must be provided to prevent collapse of the building or structure.

(21 This Regulation does not apply to tunnelling operations in circumstances which an employer cannot be expected to gua rd against or contro!.

13. Fencing of excavations, etc.

Official T ext Simpfe lnterpretation Every accessihle pari of an exc<~valion, shafl . pii or

opening in lhe ground near lo which employed per.;ons are working and into or down a side of which a person is liable lo full a dislance of more !han 2 metres shall be provided wilh a suilable harrier placed as dose as is reasonably practiCilhle lo lhe edge or shall be securely covered:

Provided thal thc forcgoing requiremcnt shall noi apply lo any part of an excavation, shaft, pit or opening while (and lo lhe exlenl lo which) lhe abscnce of such barrier and covering is necessary for lhe access or persons or (or the movemenl of plant or equiprnenl or malerials or while (and lo the exlent to which) it has noi yel been practicable lo erect such barrier 'or covering since the forrnation of that Dari of the excavation, shaft, pii or opening.

Excavations, shahs or pits more than 2 m deep, near which persons work or pass, must be protected at the edges by guardrails or barriers, or must be securely covered.

Guardrails, barriers or covers may be temporarily moved for access or for movement of plant or materials but must be replaced as quickly as possible.

14. Safeguarding edges of excavations, etc.

Official Text (l) Materia! shall noi be placed or stacked near lhe edge

of any excavation, shaft, pit or opening in the ground so as to e ndanger persons employed below.

(2) No load or plant or equiprnenl shall be placed or moved near the edge of any excavation, shaft, pit or opc:ning in the ground where it is likely to cause a collapse of the si de or the excavation, shaft, pit or opening and thereby endanger any pc:rson.

Simple lnterpretation (1) and (2) Materials, plant, machinery, etc., must be

kept away from the edge of a li excavations, shahs, pits, etc., to avoid collapse of the s ides and the risk of persons falling in, or of materials fall ing on to persons below.

PART V

COFFERDAMS ANO CAISSONS

15. Construction and maintenance

Official T ext Every cotferdam or caisson and every pari lhereof shall

be of aood conslruction, or suitable and sound materia!, free from patenl defcct and or adequate Slrength and shall be properly mainlained.

Simple lnterpretation Cofferdams and caissons must be properfy con

structed of sound materials and, after c:ompletion, must be maintained in good order.


16. Means of egress in case of flooding

Official T ext (l) In any cofferdam or caisson lhere l'hall, so far as is

reasonably practicable, be adequate means for persons lo reach places O( safety in the event or an inrush or water.

(2) No pc:rson shall be held not lo ha ve complied with a requirement of lhe foregoing paragraph or this Regulalion by reason of any rnatter proved IO ha ve boen due to phys!a1 conditions over which he had no control and aga•nst which it was not reasonably practicable for him to rnake provision.

Simple lnterpretation

(1) Every cofferdam or caisson must be provided with ladders or other means of escape in case of flooding.

(2) This Regulation does not apply in circumstances which an employer cannot be expected to guard against o r contro!.

17. Supervision of work and inspection of material

Official Text (l) No cotferdam or caisson or part thereof shall be

conslructed or be placed in position or be subslanlially added to or altered or be dismanlled except under lhe immediate supervision of a competent pecson and so far as possible by compc:lent workmen possessing adequale experience or such wort.

(2) Ali material for the constructioo or fixing of a cofferdam or caisson shall be inspecled by a competenl person on each oocasion before being taken into use for such a purpose and materia! which is unsuitabJe or defective in any respect shall not be so used.

Simpfe lnterpretation

(1) Cofferdams and caissons must be constructed. altered or dismantled under competent supervision and, where practicable, by experienced operatives.

(2) Ali materials for construction and fixing must be inspected and checked for defect before use. Unsuit· able or defective material must not be used.

18. lnspections and examinations

Official Text Simple lnterpretation

(l) Subject to paragraph (2) of lhls Regulalion, no person shall be employed in a cofferdam or caisson unless it has been inspecled by a compelent person at least once on lhe sarne or preceding day and unkss it has been thoroughly examined by a compelent persoo-

(a) since explosives have been used in or near the cofferdam or caisson in a rnanner Jikely to have affected the slrength or stability of the cofferdam or caisson or of any part 1hcreof; and

(b) since the cofferdam or caisson has been subslantially damaged; and

(c) in any case within lhe immediately preceding seven days:

Provided lhat sub-paragraph (c) shall not apply unti! seven days ha ve elapsed since lhe cofferdam or caisson was erected or placed in its position on the sile.

A report or thc results of every such examination, signed by the person carrying out the examinalion, shall be made on the day of lhe examinat ion in the prescribed form and containing thc prescribed particulars.

(2) This Regulal ion shall not apply in relation lo persons actually engaged in lhe conslruction, placing, repairing or alteration of the cofferdam or caisson or carrying oul inspections or examinations required by this Regulation if appropri~ le precautions are. laken 10 ensure lheir safety as rar as carcumstances perm11.


(1) No person may work in a cofferdam or caisson unless it has been inspected by a competent person on that or the preceding day. No record need be kept of these inspections.

In addition they must be thoroughly examined: (a) whenever explosive charges have been

fired,

(b) whenever any damage has occurred, and

(c) in any case, every seven days.

A record of these thorough examinations must be made in the official register: Form 91 Part 1 Seclion B.

(2) This Regulation does not apply to persons en· gaged in the construction, altering, etc., or to the inspection or examination of cofferdam s and caissons, but precautions must be taken to ensure the safety of persons carrying out such work.

195

196

Appendix D Dimensions and properties of steel sheet piles manufactured in the United Kingdom

These tables, covering Frodingham and Larssen steel sheet piling, are reproduced from pages 8 and 10 of 'Steel Sheet Piling Products', June 1992, with the permission of British Steel pie, Sections, Plates & Commerciai Steels.


la§§. i" Ji, !§ Hf i Maximum

' ~

liieq~llìs for ,,

Driving

Frodingham Steel Sheet Piling

r·~

b Section m m

(nom.)

1BXN 476

IN 483

2N 483

3N 483

3NA 483

4N 483

5 425

Frodingham Section

1N 111XN

2N 3N 3NA 4N

5

Section (Tongue)

1N 1BXN 2N 3N 3NA 4N 5

"' h d

m m m m (nom.)

143 12·7

170 9·0

235 9·7 -

283 11.7

305 9·7

330 14·0

311 17·0

Approx. Max. Length (metres)

11 "14 14 18 18 23 24

lnterlocks with (Groove)

2N,3NA 3N,4N 1BXN, 3N, 3NA 4N 3N, 111XN, 2N 5 NONE

f, b

if/j~..:; + x • d

Sectional l ft f2 Area

m m m m m m sq.cm (nom.) (nom.) (nom.) permetre

ofwall

12·7 78 123 166·5

9·0 105 137 126·0

8·4 97 149 143·0

8·9 89 145 175·0

9·5 96 146 165·0

10·4 77 127 218·0

11.9 89 118 302·0

The maximum length for each piling section depends upon the type of strata encountered, penetration required, and the type of construction for which the piling is designed. The tab!e is provided as a guide only.

Section lnterlocks with (Groove) (Tongue)

1N NONE 1BXN 2N, 3NA 2N 1N,3NA 3N 1 BXN, 2N, 3NA 3NA IN, 2N 4N 1BXN 3N 5 4N

Mass Moment ,_

kg per kg per linear sq. metre me tre ofwatl

of lnertia cm' pe•

me tre

Section Modulus

cm' Il pe• me tre

62·1 130·4 4919

47·8 99·1 6048 :j 54·2 112·3 13513 1150

66·2 137·1 23885 1688

62·6 129·8 25687 1690

82·4 170·8 39831 24i4 j l

100·8 236·9 49262 .. ~

In hard driving conditions it may be necessary to move up a section size to achieve the required penetration. Alternatively Grade Fe 510A(50A) steel may be uS€d.

.. Interlocking inPairs Frodingham pii es are normally

supplied interlocked in paìrs, which saves t ime in handling and pitching. Single piles can be delivered to suit customers' requirements.


Larssen Steel Sheet Piling

r-

' 1-

'd

-l t h

l. .lr t b

-

Dimensions Sectional Mass Comblned b h d t t Area Moment Sectlon

and J '

properties ~

Sectlon m m m m m m m m Flatof Pan cm' kgper kgper of lnertia Modulus (nominai) (nominai) (nominai) m m permetre llnear sq. metre cm• cm'

ofwall me tre ofwall permetre per metre

6W 525 212 7·8 6·4 331 108 44·7 85·1 6459 610

9W 525 260 8 ·9 6·4 343 124 51·0 97·1 11726 902

12W 525 306 9 ·0 8 ·5 343 147 60 ·4 115·1 18345 1199

16W 525 348 10 ·5 8·6 341 166 68·3 130·1 27857 1601

20W 525 ·400 11·3 9·2 333 188 77·3 147·2 40180 2009

25W 525 454 12 ·1 10·5 317 213 87·9 167·4 50l'Z7 2499

32W 525 454 17·0 10·5 317 252 103·6 197·4 73003 3216

GSP3 400 250 13·0 8·6 271 191 60·0 150;0 16759 1340

4A 400 381 15·7 9·6 219 236 74·0 185·1 44916 2360

6 420 440 22·0 14·0 248 370 122·0 290·5 92452 4200

6 420 440 25·4 14·0 251 397 131 ·0 311 ·8 102861 4675

6 420 440 28·6 14·0 251 421 138·7 330·2 111450 5066 --

Recomm~nded ,. ... :; .

Maximum ~~:"!··-; ~~-

Lengthsfor

Driving ~

Larssen Approx. Max. The maximum length for each

Sectlon Length (metres) piling section depends upon the type of strata encountered,

6W 9 penetration required, and the type 9W 14 of construction for which the piling Wt' 12W 17 is designed. The table is provided

16W 20 as a guide only.

20W 23 In hard driving conditions it l 25W 23 may be necessary lo move up a l

sec1ion size to achieve the required l

32W 26 GSP3 18

penetration. Alternatively Grade BiblfotecR ,... .... , ~ l• Fe 510A ( SOA) steel may be used.

4A 23 ecolté l

6 30

•.' ., ~·:

. ,. · ..

198 34~'/!j t, . u CIRIA Special Publication 95

the design and constructions ofr shett-piled cofferdams - special pubblication 95

Documents

detailed design

structural design

successful design

design offices

construction committee

thomas telford isbn

management of cofferdams

jd ciria special publication