The Design and Constructions Ofr Shett-piled Cofferdams - Special Pubblication 95

July 21, 2017 | Author: Giorgio Riva | Category: Deep Foundation, Soil Mechanics, Concrete, Geotechnical Engineering, Mechanical Engineering
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Special Publication 95




.. CONSTRUCTION INDUSTRY RESEARCH ANO INFORMATION ASSOCIATIO)\j 6 Storey's Gale Westminster London SW1 P 3AU Tel 071-222 8891 tbftot ce Fax 071-222 1708


THOMAS TELFORD PUBLICATIONS Thomas Telford House, 1 Heron Quay London E14 4JD Tel : 071 -987 6999 Fax: 071-538 4101

Centra Ingegneri



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.

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.

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.

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

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

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

Research Team

Reader lnterest

B P Williarns D W ai te CEng

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

Steering Group

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. CLASSIFICATION CIRIA ISBN O 86017 361 5


Thomas Telford ISBN O 7277 1980 7


Guide to design and construction

© CIRIA 1993


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.


Committee guided


Engineers involved in design, construction or management of cofferdams



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 MICE P E Wilson BSe CEng MICE

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

Research Managers A R McAvoy


J H Sakula MA CEng MIStrnetE MICE

(To July 1991) (July 1991 onwards)


Sponsors 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.


OON34839 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 2

CIRIA Special Publication 95

CIRIA Special Publication 95




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©


CIRIA Special Publication 95


List of Figures List of Tables Notation


l INTRODUCTION 1.1 Cofferdarns 1.2 Temporary works 1.3 Scope of the report 1.4 Supervision of design and construction 1.5 Causes of failure

13 13 14 14 16 16

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

17 17 17 17 17 18 18 19 20

3 CONCEPTUAL DESIGN OF SHEET PILE COFFERDAM 3.1 Appraisal of information 3.2 Lirnitations on design 3.2.1 Availability of plant and materials 3 .2.2 Extemal supports 3.2.3 Internai supports 3.3 Ground pretreatment 3.4 Choice of pararneters for design and driveability

21 21 22 22 23 23 25 26

4 DESIGN OF WALL 4.1 Water and earth pressures on wall 4.1.1 Total and effective stress 4.1.2 Water pressures 4.1.3 Earth pressures 4.1.4 Earth pressure coefficients 4.1.5 Choice of ground pararneters 4.1.6 Layered ground 4.1.7 Sloping ground surface 4.1.8 Check Iist for calculating active and passive pressures 4.1.9 Variation of ground pressare due to wall flexibility 4.1.10 Earthquake Ioads 4.2 Other loads on cofferdarn during construction and use 4.2.1 Construction plant 4.2.2 Spoil heaps 4.2.3 Adjacent structures 4.2.4 Other loads 4.2.5 Latera! pressure on the wall due to Ioads other than uniform surcharge 4.3 Analysis of wall 4.3.1 Methods of analysis (cantilever or single prop walls) 4.3.2 Strength factored 4.3.3 Moments factored 4.3.4 Choice of method of analysis 4.3.5 Design for free or fixed earth suppor! conditions 4.3.6 Use of computers for design

27 27 27 28 35 38 38 41 41 44 44 45 45 45 45 46 46 46 47 47 48 48 51 51 54

Cl AIA Special Publication 95



4.3.7 Multi-prop walls 4.3.8 Soldier piles 4.3.9 Design stresses in steel sheet piles 4.4 Factors of safety 4.4.1 Cantilever walls and propped walls with free earth suppor! 4.4.2 Propped walls with fixed earth suppor! 4.5 Overall stability 4.5.1 Sloping sites 4.5.2 Circolar slip instability 4.5.3 Bottom stability 4.5.4 Pressure due lo river or tidal flow 4.5.5 Wave forces 4.5.6 Overtopping 4.5.7 Scour 4.5.8 Protection from vessel impact 4.6 Choice of pile section 4.6.1 Section profùe 4.6.2 Choice of section to suit driving conditions 4.7 Generai layout of sheet piling 4.8 Check lists 4.8.1 Design of cofferdam 4.8.2 Check list for analysis of cantilever wall (simplified method) 4.8.3 Check list for analysis of a propped wall with free earth suppor! 4.8.4 Check lisi for analysis of a propped wall with fixed earth suppor! (simplified method) 4.8.5 Check list for analysis of a multi-prop wall (using the stage-by-stage method) 5 DESIGN OF SUPPOR T SYSTEM 5.1 Generai 5.2 Walings 5.2.1 Straight walings 5.2.2 Circolar walings 5.3 Anchorages for external suppor! 5.3.1 Walings 5.3.2 Anchors 5.3.3 Passive anchors 5.3 .4 Ground anchors 5.4 Struts for internai suppor! 5.5 Double-skin, earth-filled cofferdam

64 64


65 65 65 65 66 67 70 70 73 74 75 76

6.8 Monitoring of the cofferdam 6.9 Removal of the cofferdam

105 105




111 111

111 112 115

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

116 116

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

121 121 170 185

Appendix C Safety regulations


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


118 118



78 78 78 78 82 84 84 84 85 85 86 89

6 CONSTRUCTION, MAINTENANCE AND REMO VAL OF THE COFFERDAM 6.1 Contro! of work (including design) 6.1.1 Generai 6.1.2 Contractual requirements 6.1.3 Statutory requirements 6.1.4 Other legai liabilities 6.1.5 Organisation of temporary works contro! 6.1.6 Responsibilities of the temporary works coordinator 6.2 Installation of sheet piles 6.2.1 Types of pile driving equipment 6.2.2 On si(e storage of piles 6.2.3 Pitching and driving 6.2.4 Order of driving 6.2.5 Safety 6.3 Excavation 6.4 Sealing of leaks and separated interlocks (or isolated piles not driven to leve!) 6.5 Contro! of water 6.6 Access and safety 6.7 Maintenance of the cofferdam 6

55 58 59 60 60 61 61 61 62 62

91 91 91 91 91 92 92 93 93 93 94 96 100 101 101 103 103 104 104

CIRIA Special Publication 95

CIRIA Special Publication 95


list of Figures Figure l Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure lO Figure 11 Figure 12 Figure 13 Figure 14 Figure 15

Types of single sldn sheet pile cojferdams Double wa/1 earth-filled cofferdams Movement of ground around the cojferdam Externally suppor/ed cofferdams Typica/layouts of internally supported cojferdams with straight sides Types of interna/ support [or cojferdams with straight sides Types of circular cofferdams Water pressures - hydrostatic F/ow net diagram Water pressures on cofferdams - typica/ cases Typica/ cofferdam f/ow nets Water pressures - simplijied methods Effect of width of a cofferdam on the flow net Pressure diagram [or mixed rota/ and ejfective stress design 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

Figure Figure Figure Figure Figure Figure Figure Figure Figure

51 52 53 54 55 56 57 58 59

Figure Figure Figure Figure Figure Figure Figure Figure Figure

60 61 62 63 64 65 66 67 68

Circular reinforced concrete walings Types of anchor Strut details Direction of accidental /oad on struts Effective length of struts Double-sldn, earth1illed cojferdams Pressure diagram [or a double-s/dn, earth-filled cofferdam Construction of a double-s/dn, earth-filled cofferdam F/ow chart of principal activities and responsibilities [or design and construction of a temporary cofferdam On sile storage of piles Recommended storage procedure [or steel sheet piling Genera/ arrangement and detail of the buti we/ding /empiate The 'Pane/' driving technique - guide frame Detail of hammer leg guides The 'Pane/' technique - stages in driving Pitching piles C/osing inter/ocked sheet piling Excavation under water

83 84 87 87 88 89 89 90 92 95 96 97 97 98 99 100 101 102

39 39 40

List of Tables

41 42 43 44 46 47 49 49 50 52 53 53 56 57 58

Table Table Table Table Table Table Table Table Table Table Table Table Table

l 2 3 4 5 6 7 8 9 lO 11 12 13

Cofferdams without sheet piling (after Packshaw">) Cofferdams dependent on sheet piling (after Packshaw'. 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



total stress - pore water pressure



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.


CIRIA Special Publication 95

CIRIA Special Publication 95


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.

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

Time period after loadlng

Clays (coheslve soils)

Granular solls (non·coheslve solls)

Short term

- undrained - total stress

- drained




- effective stress - f (c'= O)

- water pressures unknown

- water pressures known

-drained - effective stress

-drained - effective stress - f (c'= O)

-c' and cf~' - warer pressures known

u i.e. where: 'Yw

= =




density of water (fresh water 9.81 kN/m') 3 (salt water 10.00 kN/m ) depth below water table

- water pressures known

Note: For temporary cofferdams c' is nonnally taken as zero for


clays as well as for sands and gravels. Permeable ground

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.


lmpermeablel ground

U ~ Yw(d- i) U ~ Yw(h +d-j) (a) Gross water pressures


a Water pressures -

(b) Net water



The maximum net pore water pressure, u, = y.(h + i - j). 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, King13", Dickin and King'"i, Kaiser and Hewitt13'' and Cedergren 110'.

Figure 1O Water pressures on cotferdams - typica/ cases

32 CIRIA Special Publication 95

Cl AIA Special Publication 95


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 Netwater pressure


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 angle of wall friction, and c. the wall adhesion.

o the

Assumption (l}

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.

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


2(d + h - j)(d -i) 2d +h- i- j


= Assumption (2)

Average hydrostatic pressure at toe 2 U -_u,+u - 2 - = Yw [ d


(h-i-J)] --2--

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

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/m 2 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 Steady state Fine sand

Wall adheslon c..

Angle of wall friction O

seepage Netwater pressure

(b) Layered graun d- high aver low permeability








O.Se' 0.5cQ 25 kN/m 2



but max

50 kN/m 2

Figure 12 Water pressures - simpfified methods Note: c' nonnally taken as zero

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 and cr,'

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


CIRIA Special Publication 95

CIRIA Specia/ Publication 95

= The 'at rest' pressure coefficient = Effective vertical stress


For nonnally consolidate' 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.

Angle of shearing resistance, ·;n w ro




1. 5


~-- )-

df:-c :S

-ìlr+veL . -- _





V --


...... -~





c' ~
















.-' ~


2.6 ~

0.4 1.


5~ 1 10







o8 ,c-


:-- tc---


o O.









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,.

~t=--~ ~ -/-/


) w w ro




--+ _ _ _ o(+ve)


-- -

1 10







5- _--


o u

v--- _-.... -

/ /7 V / ........


)' > 30°


= =

1.2 1.2 - 1.1 Figure 34 Overa/1 stability with difference in ground leve/

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. 60

CIRIA Special Publication 95

CIRIA Special Publication 95


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).

' /

if":~==fl"- Raking



/ Tie rods and





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/m 3 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



_; , whtch approxlmates to -·~ ~

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

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:

4.5.2 Clrcular sllp lnstablllty

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

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).


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.



Hl===H -------------- --

= /'






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.

CIRIA Special Publication 95

Depth of cut-off




_Dc H

4.5.3 Bottom stablllty

Wìdth of cofferdams



Figure 36 Circu/ar s/ip instability



H 0.5H

0.4H 0.5H 0.7H

Figure 37 Bottom stability (after BS 8004 141}

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 CIRIA Special Publication 95


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.

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

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.

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.

4.5.4 Pressure due to rlver or ti dal flow

4.5.8 Protectlon from vessel lmpact

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.

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.

: l l


--- r:

il l l l


l l l l l0m l


l l l l

Wl Wl 77'


l l l



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:

m; "ll l l l

l l

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

lengths of pile shaft not driven below soil surface leve!

4.5.5 Wave forces

lengths of shaft retaining water only

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 I 1"l. It is a complex subject and it is advisable to seek the advice of an expert if significant wave loading could occur.

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.


CIRIA Special Publication 95

CIRIA Special Publication 95


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, 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.

so of rotation

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.

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

4.6.2 Cholce of sectlon to sult drlvlng conditlons

Clay descriprlon

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

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

BSEN lO 025 Grade SIOA, BS 4360 Grade SOA

Maxlmum length

450 600-700 700-1500 1600-2500 2500-3000 Not recommended

400 450-600 600-1300 13000-2000 2000-2500 4200-5000

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.

Mlnlmum wall modulus cm 3/m ofwall

Domlnant SPT

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

Minimum wall modulus cm 3/m

(N value) BSEN lO 025 Grade 430A, BS 4360 Grade 43A O- IO Il- 20 21-25 26- 30 31-35 36-40 41-45 46-50 51-60 61 -70 71-80 81 - 140

BSEN lO 025 Grade SIOA, BS 4360 Grade SOA




Grade FE510A for lengths greater than 10 m

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.

450 850 850 1300 1300 2300

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 •

2300 3000 3000 4200 4200

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 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


CIRIA Special Publication 95

CIRIA Special Publication 95


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 ± 50mm Verticality after pitcbing l in 200 Verticality after driving 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 corner can plug up toe of pile causing spreading the clutches

Gap between guide walings

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.


Welded junction pile -------~--..l


...----------Guide waling




l l l l l



Larssen section 9W

Sheet piles 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.

Double bent corner pile

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 wbile 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.

Welded junction pile

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

Frodingham section 2N

Figure 40 Layout of stee/ sheet piling


CIRIA Special Publication 95

CIRIA Special Publication 95


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.

• restraints an construction, e.g.:

- access - working area and obstructions

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).

- 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:

Gaps fil led in with light section pile if necessary



• 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.


Originai penetration


_ Effective



-t penetration increase in

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).

Figure 42 German waterfront method of driving 5. Decide an use of effective or total stress parameters far each ground stratum.


6. Decide an values for ground parameters.

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

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

4.8.1 Design of cofferdam l. Collect ali information available an:

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:

• structure to be built within the cofferdam • drawings and other documents

• pore water pressure, active an d passi ve

• ground and soil

• net water pressure for design

• water levels in the ground

• horizontal earth pressure, active and passive.

• if cofferdarn is in water then:

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

- variations of tide levels and currents

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

- variations of river levels and flows

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

- wave climate - any water traffic


CIRIA Special Publication 95

CIRIA Special Publication 95


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

any special instructions e.g.:

- penetration

- accommodation of services

- prop load

- pipes or accesses at lower levels.

- leve! and value of maximum bending moment

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.

modify or factor the pressure diagrams as required by the method of analysis analyse for design penetration at the chosen faclor of safety

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

• 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

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


c) Nel water pressure


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

= 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



Earth pressures

procedures for excavation and erection of supports details of groundwater contro! or dewatering if required together with any fai! safe procedures or standby plant

Net water pressure

Figure 43 Analysis of a cantilevered wa/1


procedures for construction of permanent work including any alterations to supports

l l 'l

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.

procedures for backfilling and remava! of supports any checks- required during !ife of cofferdam remava! of cofferdarn


:l 72

CIRIA Special Publication 95

CIRIA Special Publication 95


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

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

Moment of PA + moment of Pw - moment of P, 4. Calculate the value of the prop force 'T' which equals PA +Pw- 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 Pw. 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).

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 PAand Pw as in Step 2 above. If the sum of the areas PAand Pw 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. F FP, Fnp or Fr. 3,

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 +Pw.· 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) GL Prop T -->·l\

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 ---A


Pw P,


Earth pressures


Net water pressure



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

b) Active earth pressure


Earth pressures

Net water pressure

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

PA 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.

c) Net water pressure





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



CIRIA Special Publication 95

CIRIA Special Publication 95


4. Totalloads on supports are:

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

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

a) Passive earth pressure P, b) Active earth pressure


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

c) Net 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• + Pw - P,) divided by distance 't'.

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 on each support and the largest bending moment at any stage of construction.

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.+ Pw 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.

Support no.

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.

Total active pressure including net water pressure

Exc. L

BA _L.,._4-:'-'-'"' 8 (b)

Passive ---pressure

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): (c)

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).

Figure 46 Multi-prop wa/ls

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.


CIRIA Special Publication 95

CIRIA Special Publication 95


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).

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.

l Waling

Larger gaps filled with (1) Wedge to bring its face parallel to waling -----(2) Pairs of folding wedges to close gap

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



5.2.1 Stralght wallngs

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

!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.

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).

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

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 it is not necessary, in theory, to have 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.

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 ry

Acc1dentalload Worst direction horizontal



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.

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



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.


CIRIA Special Publication 95

Figure 54 Direction of accidenta//oad on struts

CIRIA Special Publication 95


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).

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).






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.








lxx= L lyy :o= L



[ r




y L/2



n Strut


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

lxx= l lyy = L/2

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).


i Strut


lxx= L/2 lyy = L/2


Figure 55 Effective /ength of struts

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.

Additional pressure to resist tie force

Active pressure

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.



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


CIRIA Special Publication 95

CIRIA Special Publication 95


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).

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





.,. ~









t'il z:..


All uni~s l?. L.


kN ond rn.





Paro rne~erS


verhce; 5 = 20·3




o o

o o





4b 4



Kp=- 4-5


,? Cla~



~ Sand



Cu""40 kp= 1·0



"(5"5-= 20·3


Sub. Arh~!.sion Head


t-C! ·O

56 4 lib-3


Kpc= 2-4

& ::; 15° Kp· 5·8


= 20

K=· Cu- 96 o




Cw= 25·0

Kpc>< Cu- 144-0





5 ,zo-3











"' "' "' Cl.~(O


Pu rnpin'J t"-o


156 3 349 7




--Yzx2xZ·5 -Yz.xl3xZ5


Lever ~rm

2.5·0>< 3·41

Momen~ +85·3


x z s) +0·41


(Y3x2.5) +0·41







v3 x


-O· l




::E L cr w








0- fN

----or \()







'3x15)+ 1·0 (!3x3oJ+ l·5 (53 d O) + 2 5 (!3x20) +55 2 ( 13x2 O) + 5·5

( iZ xOoB)+

7 5

l~xlo)+zs (>'3xlo)+Z·5 (Yzxzo)t35

= 14·"l4 kN

Ear 1-h


449·5 844 ·8 1209·5

(i3xzo)+5·5 ('Y3X2o)+55 ( ~ x 0·08) + 7 5

283 ·5

Mp =

8~ inspechon Ma>3 x 3·0) + (%xzo)+ (Y3 x2o) t

l ·O


x 40 y, 1·5 " 30· o x 29 x 3 o " 43 5

!i !2 x

39 x 3·0 ; 58 5 Z· O "103 o Yz x 143 x z·o "143· o

Yz x 103 x 55


O·OB "



b·58 b 58 5 08 5 08 Z ·og


08 0·08 MA



zero shear i.e. where orea


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 Pqssive




(̧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


Yzxl37 x Z o " 137 o izxl7ìx2·0 " 177 o 470 x o 08 " 37 6

io l-o l moment "MA+


Mw- Mp

= 1050 9


114·3 167·4 177 5 1602 145 ·6 106 ·13 0·2 1050·9

+ 513


Yz x"

Ach've Eari-h

Yzx Vz


Yz. x

zo 2.5 25 40 2.9

Reachon See

poge 13

t or %5

or Prop no.2

x 1·5 = 30·0 x o 79 = zz·9 l ·O 3-3 X0·79z = 95 2


129 · 5


IlO 3 308 o 513·2




95·2 -t 34·7 = 129·9 = 12.9·5

Readion ar pcop.

near enou:3h Ok:

255·4 193 ·6 132·2 15 582· 7

So level

oF z.ero

Pro p

Eacrh Achve

t 36 ·8


os a bave

= 166·3 kN/m run

CIRIA Special Publication 95

CIRIA Special Publication 95

31 b 3· l

+ 4Z6·1


12.9 5

IZ · 5 1/3 8 30·0 22· 9 l· o


Lever qrm

Area -

2- 582· 7


sheor = t5·21

Take nnomen~s abouf- and above fhis leve\




Yz x 10 x 0·7"\ 2 = _3_·1_


Tora\ lood on A-op no.2

x 1·0 = IO·O x l o = IZ. · 5 x [·5 = 18· 6

,... 9_8_1_·_4 " 129·5 7· 58

oç pressure


94 9

= 981 ·4 "

( conhnued)

Tr:J leve l a t t 5 ·2.1

40 X0·7"\; 31


Merhod - Fp

ce equa l ro the reqchon oF the pro p.

8b 4

y2. ){ o. 08


G-ross Pr-essure

72· 5





Calculafe Maxirnum BM in sp"'n pelow Pr-op no.2



Prop no.1 @ RL t 13·0 Prop no [email protected] RL t 8·5 RL t 5·0 E>
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