Timber in Excavations

Timber in excavations Third edition

7 RA DA

Produced for the Timber Research and Development Association, Stocking Lane, Hughenden Valley, High Wycombe, Buckinghamshire HP14 4ND, by Thomas Telford Ltd, Thomas Telford House, 1 Heron Quay, London El4 9XF First published 1981 Second edition 1984 Third edition 1990

British Library Cataloguing in Publication Data Timber in excavations. 1. Civil engineering. Excavation I. Timber Research and Development Association 624.152 ISBN

0 901348

81 3

(11 Timber Research and Development Association, (c-1This edition Thomas Telford Ltd, London, 1990

High Wycombe,

1981,

1984, 1990

All rights, including translation, reserved. Except for fair copying no part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the Timber Research and Development Association. Typeset by MHL Typesetting Limited, Coventry Printed in Great Britain by Inprint, Hitchin, Herts

Preface

Origins and acknowledgements

This publication stemmed from an initiative by TRADA in 1977 to establish a new committee for timber in temporary works, chaired by J.R. Illingworth, then of George Wimpey Ltd, now Consultant in construction methods and technology. The general brief of the committee was to: act in an advisory capacity to TRADA in all matters concerning timber and wood based materials used in the fabrication of formwork, falsework, and excavation supports. To collate existing information and define areas in which further research and development is required, recommending priorities for a programme of work to this end. The committee was selected to represent both timber trade and construction industry interests. Support for its proposal was sought and obtained from the Department of the Environment in 1978 with TRADA contributing both financially and in providing technical and administrative support. This document is one of several which arose from the projects steered by the above committees and owes much to the efforts of D.J. lrvine of Tarmac Construction Ltd and J.R. Withers of TRADA, chairman and secretary respectively of the working party responsible for its production. This third edition of Timber in excavations has been revised by J.R. Illingworth and D.J. Irvine. TRADA wishes to thank them, and all members of the original committees for their advice, expertise and help in the preparation of this publication. Since the second edition was published, TRADA has undertaken a research project, part funded by the CEC (Commission of the European Communities), entitled ‘More efficient use and re-use of timber in temporary works’. In view of the close links being developed by UK contractors within the European Community, a brief summary of the use of timber in the support of excavations has been included. It is based on an analysis of information obtained by surveys in sixteen West European countries, some of which were outside the EEC.

Contents

vii

List of figures List of tables

ix

Introduction

xi ...

Guidance

XIII

notes

Wide excavations Typical support methods Design methods Safety Engineering a deep and wide excavation Design methods for a soldier pile support

system

Trenches Use of poling boards in trenches Use of runners Use of trench sheeting Use of sheet piling Safety

22 22 26 29 29 29

Shafts Use of poling boards in shafts Use of runners in shafts Struts and walings Chicago method Safety

35 35 38 39 39 40

Headings Supervision Sizes of heading covered by these recommendations Materials encountered in driving the heading Timber to be used in the heading Dimensions and alignment Excavation and timbering Securing the heading Lighting and ventilation Workmanship Safety

41 41 41

Workmanship in excavations Materials Construction and use

50 50 50

42 43 43 44 44 49 49 49

Safety Legislation Responsibilities Check list

52 52 52 55

Support of excavations in Europe Legal requirements and available standards Methods of support used Re-use of timber in excavation support

56 56 58 58

Appendix 1 Timber Stress grading Moisture content Derivation of permissible stresses Duration of load Length and position of bearing Lateral support Depth factor Derivation of permissible stresses supports for trenches Timber requisition

59 59 61 61 61 64 64 64 for timber when used as 65 67

Appendix 2 Check lists for excavation Planning and design Trenching operations Appendix References

3 Glossary

of terms

support

used in timbering

68 68 69 71 75

List of figures

1

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33

2 Battered sided wide excavation 2 Horizontal props and walings 2 Very wide excavations 3 Soldier pile and skin wall 3 Tie back method 4 Raking props 6 Raking struts for soldier pile wall Attaching timbers to H-piles; timber tucked in behind face 13 flange Use of random length timbering with H-piles; clipping 13 method 14 Clipping system to cope with H-pile location errors 14 Detail of clip sizes 20 Alternative strut arrangement for soldier pile wall 20 Alternative strut arrangement for soldier pile wall Initial excavation using poling boards in the 22 middle board method Completed trench using poling boards and the middle 23 board method 23 Typical view of the middle board method 24 Poling boards used in the tucking frame method 25 Typical view of the tucking frame method An alternative use of the tucking frame method using 26 tucking boards Stage 1. Uprights, walings and struts ready for pitching 27 runners 27 Stage 2. Runners pitched ready for excavation Stage 3. Runners driven and trench excavated ready for next frame 28 Stage 4. New runners pitched ready for excavation within the trench supported by the first set of runners 28 New set of runners set inside trench supported by two 29 levels of poling boards used in the middle board method 35 Methods of side supports in shafts Shafts with maximum dimension up to 2.75 m 36 Shafts with maximum dimension up to 5 m 37 Shafts with maximum dimension up to 1.8 m square 38 Chicago method 39 Box setting 45 Poling setting 45 Piling setting 46 Installation of piling settings 46-49

vii

List of tables

1 2 3

4 5 6 7 a 9 10 11

12 13 14 15 16 17

ia 19 20

21

22 23 24 25 26

Flow chart of activities - trenches, wide excavations, shafts and headings Permissible axial load (kN) in struts of SC3 strength class timber Permissible axial load (kN) in struts of SC4 strength class timber Permissible axial load (kN) in struts of SC5 strength class timber Permissible axial load (kN) in struts of Douglas firlarch species Size of strut required for designs in Figures 7, 12 and 13 Summary of methods of side support for trenches Ground conditions and slopes Minimum sheeting requirements Minimum requirements for walings and struts - no water head Minimum requirements for walings and struts - sheets loaded by water head Timber supports for trenches - permissible stresses General duties required under the Health and Safety at Work etc Act Summary of Construction Regulations: excavations, shafts, earthworks and tunnels Frequency of use by percentage of the various support methods for narrow excavations in Western Europe Frequency of use of various support methods for wide excavations in Western Europe Re-uses obtained with components for excavations support Softwood species/grade combinations which satisfy the requirements for strength classes SC1 -SC5 Grade stresses and moduli of elasticity for strength classes: dry exposure condition Modification factor K, by which the geometrical properties of timber for the dry exposure condition should be multiplied to obtain values for the wet exposure condition Modification factor K2 by which dry stress and moduli should be multiplied to obtain wet stresses and moduli applicable to wet exposure conditions Load duration factor K3 Modification factor K4 for bearing stress Maximum depth to breadth ratio Stresses and moduli of elasticity for strength classes: wet exposure condition Timber supports for trenches - permissible stresses

xiv 16 17

18 19 21 30 31 32 33 34 34 53 54

57 57 58

62 63

63

63 63 64 64 66 66

ix

Introduction

Timbering is the term commonly used in relation to temporary works involved in the upholding of excavated faces of wide excavations, trenches, shafts and headings. Modern practice tends to use timber in association with other materials for temporary works, although timber alone is still used economically for smaller or special jobs. The purpose of this book is to provide site staff and planners with quick and convenient methods for assessing site conditions and choosing appropriate excavation supports for trenches, shafts and headings where proprietary systems are unsuitable or are not considered to be cost effective in the circumstances prevailing. Although wide excavations normally require specific design, typical methods of solution are shown, together with example design calculations adequate for planners and site staff to assess feasibility and cost effectiveness at tender stage. Suitable check lists for all methods are provided so that site staff can be sure that any scheme has been properly installed and is safe to use. All ground can be expected to show variations from place to place and it is essential to keep a close watch for any changes and to vary the support system to suit. Any support provided to an excavated face should be regarded as one part of a structure which uses soil as the other part. Arching within the soil tends to modify the forces predicted by classical soil mechanics theories and the support systems suggested for trenches, shafts and headings are those which have been proved by experience to give safe and reliable results where the ground is subject to only visual inspection backed by common sense. These solutions may be termed ‘deemed to satisfy’ and may not in ail cases give the most economic structures. Generally, special design solutions are only justified where adequate soil investigation has been carried out to give information on the engineering properties of the ground. For such cases permissible timber stresses are given. It is recommended that only stress graded timber should be used in excavation support and a brief explanation is given of the history and current situation concerning stress grading, together with a check list of the main information required when requisitioning timber.

xi

Guidance notes

It is important to be systematic in planning and constructing excavations. The flow chart (Table 1) gives the essential steps for each of the general types of excavation, with cross references to the relevant sections in the text. The main sections cover trenches, wide excavations, shafts and headings. They describe methods of support and give advice on the sizes of components or methods to be used for particular circumstances. Detailed consideration of the design of supports to wide excavations is beyond the scope of this book. However, the soldier (H) pile method is so widely used throughout the world that typical methods and details are included, together with an explanation of the basis of design and worked examples. The earth pressures have been derived in accordance with the recommendations of the Civil Engineering Code of Practice No 2 Earth retaining structures, 1951, while taking cognisance of design methods used in the United States of America and the Federal Republic of West Germany. The timber sheeting and struts are designed using the permissible stresses for timber (Table 7) in conjunction with the design method of BS 5268 The slructural use of timber Part 2 Code of practice for permissible stress design, materials and workmanship, 1988. At the time of going to press of this third edition of Timber in Excavations, the Civit Engineering Code of Practice No 2, 1951 (CP2) is in the process of revision. While the final contents of the revised Code are unknown, this document has been based on the existing Code and other international design methods. This is justified in the light of international experience in the use of H-piling and its record over many years. While the safety record may suggest over-design to the purist, it has to be remembered that this is a desirable feature in temporary works of this nature, where earth pressures cannot readily be calculated precisely. In the Trenching section, Table 8 describes the field identification of different types of ground and suggests safe temporary slopes which may be used. These slopes can be used as an alternative to supporting vertical faces in trenches, wide excavations and shafts when space permits and the method can be justified on practical and cost grounds. Where vertical trench sides are required then Tables 8, 9, 10 and 11 can be used for various ground conditions to select the appropriate sheeting arrangement and the sizes of walings and struts. Adequate clearance should be allowed for lowering between struts and generally the designer will first consider the length of pipe, etc to be lowered into the trench and select appropriate strut centres. The vertical centres will tend to be determined by the pipe diameter or by the clearance required for construction purposes. The sections on Shafts and Headings describe the methods of support and scantlings of members which have been found to be satisfactory in use. Where these types of excavation are in use for a long period the risk of wear and tear and accidental damage during service is increased and it may be necessary to add timbers as buffers and rubbing strips to prevent damage to the structural timbers during lifting and mucking out operations. The Workmanship and Safety sections apply to all types of excavation .. .

XIII

Table 1. Flow chart of activities - trenches, wide excavations, shafts and headings

Shafts & Headings

Wide Exca t&ions

Trench es

I

1 information

r

Assemble design data from: Design drawings; Conditions of Contract; Specification; See Appendix 2

etc.

I t

f

Ground information Ascertain ground information from: Trial pits, site inspections, adjacent workings etc. Take particular note of ground water levels and flows. See Appendix 2

I

Consider

the possibility

t

Y

and economics of using an open excavation with side slopes. See Table 8 @I_31)

I

I

,

.

I

1

Sheet@ Choose minimum sheeting required. See Table 9 (p. 32)

I

Wide excavations are usually fully sheeted.

Shafts are usually fully sheeted.

Select the method of construction. See Main Text

T i ’ ’

Choose convenient waling and strut centres. Select appropriate member sizes. See Tables 10 and 11 (pp 33 and 34)

7 Size of Componenls Carry out the design. See wide excavations

stage drawings if necessary. Schedule

xiv

I

Choose sizes of members. See Shafts and Headings

labour, plant and materials.

r

T

Construct

t the Work

Supervise and check workmanship and materials. Ensure safe working practice is used. See Workmanship (p. 50) and Safety (p. 52)

Check

the Work

The Excavation Works Coordinator must inspect the excavation support system before work commences in the excavated area. See Appendix 2

t Use the Works The Excavation

Y

Works coordinator must inspect the excavation at regular intervals and at critical stages. See Appendix 2

t

t Dismantle

support

1

the Works

The excavation support must be removed in a planned manner. The actual method will depend on a number of factors, in particular, the type of construction carried out within the excavated area. Intermediate propping may be necessary. This must be carried out in accordance with the procedures indicated above. i

and should be used in conjunction with the Check lists given in Appendix 2. Appendix 1 on Timber describes the changes which have taken place in the use of stress grading to ensure the engineering qualities of timber. It also gives the derivation of the table of permissible stresses for timber to be used in excavations and sufficient information is provided to allow the designer to modify the stresses for a particular design condition. Having decided that a particular strength class or species/grade is required it is essential to specify it exactly and use of the Timber requisition section in Appendix 1 is recommended.

xv

Wide excavations

A wide excavation is an excavation whose width exceeds 5m. In plan its shape can be infinitely variable from an extra wide trench in open ground to an irregular shape defined by adjacent buildings as in city centre developments. For a given depth, a wide excavation when supported by walings and struts will require heavier struts to the frames than a trench supported in a similar manner. For very wide excavations, ground anchors, tie rods or raking struts should be considered as they will probably be more economic. Wide excavations have to take into account so many site factors that standard solutions are difficult to provide. A design method for raking struts and timber sheeting for a soldier pile system is given. However, it is recommended that wide excavations are subjected to individual design procedures. These designs should be prepared by a suitably qualified engineer previously experienced in this field. Appropriate guidance is available in the following references: Sritish Standard BS 6031: 1981: Code of practice for earthworks British Standard Code of Practice CP 8004: 1986 Code ofpractice for foundations Trenching and shoring manual. Californian Department of Transportation, 1977 Foundation design and construction. Tomlinson, 1976

Typical support methods

Where space is available the most economical method is to slope the sides of the excavation to a point where they become stable. This is known as the battered sided method and is illustrated in Figure 1. For wide excavations where the sides have to be supported, typical methods are illustrated by Figures 2 to 6 (b). Depending on the nature of the site it may well prove economic to use two or more of the methods depicted within the same excavation.

Design methods

Although it is recommended that all wide excavation supports are designed by an experienced engineer, some limited design at an earlier stage is useful for checking purposes or for initial costing. A design method for determining the size of timber raking struts and for the horizontal timber members of a soldier pile support system follows. Any design process must check the stability of the bottom of the excavation against heave or boiling and walls founded in clay must be checked for overall stability.

Safety

All the hazards encountered in trenches are equally applicable to wide excavations. In addition, on congested sites with other buildings close to the edge of the excavation, the effect of a collapse on the stability of adjacent buildings and the resulting consequences must be carefully considered and all necessary precautions taken. See section on Safety (P. 52). 1

Figure 1. Battered sided wide excavation (suitable for open sites with convenient access)

Ground

Slopes dependent on soils

Figure 2. Horizontal props and walings (limited by column buckling of strut)

Note If steel s!ruts or wallngs used. all connectlow should be adequately ;Nelded

Timber struts and wallngs

Steel sheet p,llng

r--..r

excavations

King post

2

level

Figure 4. Soldier pile and skin wall

Steel soldier pile-driven. Penetration fixity as calculation

Timber horlzontal sheeting wedged

for wedged

to soldler

Figure 5. Tie back method

6 :.

::

blocks

3

Figure 6 (a). Raking props

Timber

or steel prop

Steel sheeting (N B Wall to be stable of raking

prop)

II

Figure 6 (b). Raking props

Timber sheeting or concrete

Solder pile Penetration for --+ flxity as calculation

\ IFyb

Engineering a deep and wide excavation

General Deep and wide excavations are frequently required for the construction of building basements, pumping stations and other structures (such as retaining walls). The excavation removes a mass of soil and water and often causes a local lowering of the ground water table. This total stress release within the ground and the change to the water table can result in movement in the surrounding soil. In particular the bottom of the excavation may heave or suffer loss of strength. The degree of movement will have a significant effect both on the loads to be resisted by the system used to support the sides of the excavation, and on the total stability of the ground beneath and adjacent to the excavation. A satisfactory solution to each excavation problem must consider the following general requirements: l

l l

4

the construction method and support system must be safe and economical. there should not be excessive movement of the ground. in some cases the movement must be further limited to avoid danger to surrounding structures and buried utilities.

Movements These (a) (b) (c) (d) (e) (f) (g) (h)

and loads

are affected

by:

the dimensions of the excavation the soil properties ground water control time the type of support system the excavation and bracing sequence the nearby structures and utilities transient surcharge loads.

The larger the excavation the greater will be the change of horizontal stress beneath the base. The degree of horizontal wall movement will depend on the stress changes and the horizontal wall movement will influence the settlement at ground level adjacent to the excavation. The flexibility of the wall has some influence on soil movement although experience suggests that the stiffnesses of ordinary steel sheet piles and soldier piles are not sufficient to have a significant effect. Much of the wall movement occurs below the bottom of the current excavation level and practically ceases when the bracing is installed. Therefore, in order to limit movement, the bracing must be placed quickly and with the minimum of excavation. The use of berms can help to limit (not eliminate) wall movement and improve temporary stability by extending the length of potential failure surface, and inhibiting swelling and softening in clay.

Design The interaction of the soil and the support structure is complex and the design method must take this into account. Usually empirical methods of design are used which make allowance for the interactive effects without requiring them to be analysed explicitly. Consequently there is scope for engineering judgement. The more theoretical methods of analysis suffer from the difficulty of reliably assessing input parameters which limits the accuracy of predictions and hence their use as a design tool. The design method set out in CP2 is derived from the trapezoidal envelopes of strut loads produced by Terzaghi and Peck and allows for the redistribution of force into the supports of multipropped walls. The likely wall movements can be estimated by reference to Peck’s records of monitoring actual excavations (see Tomlinson, 1976). Stability analyses should be done to check against failure of the excavation by heave or base instability. If excavations in clay get too deep relative to the strength of the clay, then the heave may become uncontrollable causing excessive settlements of the surrounding ground and possible collapse of the bracing. Excavations below the water table in sand may give rise to high seepage forces which destabilise the base of the excavation. The designer should also beware of excess hydrostatic head beneath a shallow, relatively impermeable, layer which may cause the bottom to blow.

Design methods for a soldier pile support system

This section gives the design method for raking struts and the timber sheeting for a soldier pile support system and outlines the principles involved. Although the method detailed is specific to a particular application, it can be adapted to cover timber in any strut situation and is suitable for feasibility studies or costing purposes at the tender stage. Strut tables are also included which enable a variety of strut arrangements to be selected and considered, so that the most economic solution is adopted.

5

Design of raking struts for a soldier pile wall Symbols

Unit ‘weight density’ of soil of internal friction 6 of wall friction C Cohesion Ka Coefficient of active earth pressure Pa Thrust due to active earth pressure l-i Depth of excavation Angle 2; Angle

Figure 7. Raking struts for

Surcharge

soldier pile wall (alternative strut arrangements are shown in Figures 12 and

35 KN/rn’

J

T4iIIr

Shear key

13) Compact sand (water table 8 m below ground level)

mrr

_

5500

mm.

Packlw PI 2

_I

I

’ Ground #---

7

slab

SoIdler piles (Ttmber sheeting not shown) Penetration for

J-v-

-cu-

flxlty as caiculatlon Sotl charactenstics 7 = 17.5 kN/m3 I$ = 35O 6 = cJ” c=o Ka = 0.27

Notes and references

1

1 No water pressure considered as the timber sheeting between soldier piles will allow free draining conditions and wfll prevent a building of surface water

Soil Pasoil = Ka. y .H

2 A unllormly distributed live load has been used. Concentrated (line or point] loads must be treated separately where they occur (see also Note 7). 3 The raking strut will be installed while a berm is left on the Inside face of the wall. When the top strut is in position and secure the remamder of the berm is excavated and the horwontal strut is installed Other variations may be used and in some cases the horizontal strut IS omitted and the soldier piles are founded sufficiently deep lor the toe to develop the required passive resistance.

4 Even wtth the arrangement shown it IS essential to prwda sufficient penetralion to resist the upward component of the raking strut thrust (see also Note 18 below). In cases where the ground below formation level IS good the bottom horizontal strut may be omitted and the passive earth pressure at the toe of the pile used to provide the bottom reactlon. When such an arrangement is used the active pressure is assumed to be distributed hydrostatvzally on the wall

Active

on wall

pressure

Surcharge Pasu,cha,ge = 35 x Ka

= 9.45 kN/m*

At ground level

ie equivalent surcharge

Pasoil = 0 At formation level Pasoil = 0.27 = 28.35

X

17.5

x

kN/m*

:. Total soil force /m run Pa,,

= y = 85.05

x

6

kN/m

6

he

=

!k!? 17.5

= 0.54m

height of

5 The quoted clause of CP2 ‘Earth reta,nmQ structures’ requires the eqwalent fluId pressure lo be consrdered for cohesrve sorls However, many engrneers consrder that the mrnrmum pressure should also be used for cohestonless sotls since the coeffictents of aclive pressure may underesfrmate earth pressure in some ctrcumstances

Check

6 Refer to CP2 ‘Earth retaining structures CI.f.4343. The pressure dtstnbutlon dtagram for strut toads IS a maxtmum envelope and allows for the varratton of strut loadmg caused by arching and other effects over a pertod of trme Dependmg on a fudgement of the nature of the retained ground. the loadrng and other factors, the pressure diagram may be reduced when destgnmg the soldrer p&?s and the horrzontal sheetrng. Thus JUdQem@nt should be made only by an experienced desrgner

:. Use soil force for strutted (see below)

Fluid

equivalent density

2

Distribution

(CP2 Cl. 1. 4311)

PaSO,,) excavations

of active pressure

pressure

on strutted

diagram

wall

I 02H 1 = 1200 mm

i

16 y (Pa,,,,+ Pa,.,r,h~& Hi+

pressure

= 4.71 kN/m3

Pa = l x 6’ x 4.71 = 84.78 kN/m (<

7 In this example the surcharge IS added to the trapezium of soil pressure Thts IS because the load IS assumed to be temporary If a permanent surcharge IS expected the” It is suggested that the total lateral force due to surcharge should be added to the total so11 force: the magnitude of the strut pressure envelope would then be

P=

fluid

he

! For

Lateral pressure due to surcharge = 9.45 kN/m c-32.13

9 The IS Ignored

effect when

of the embedded length calculatmg the strut load.

9 A shear key IS needed on the face of the soldler pile since the angle of rnclinatton of the raker exceeds the angle of frictton between steel and trmber when the two surfaces are wet 10 The kickrng post or block I” the slab must be destgned tar the total reactlo” from the struts. Also the concrete slab must be checked to ensure thal there IS no danger of it movrng under the actron of the struts. This is partrcularly Irkely when the slab has been cast on a plastrc membrane If necessary, the use of a downstand key on the understde of the slab must.be constdered to mobrlrse a passive reactron in the u”derlyi”Q ground 11 Unless the struts are posttrveiy frxed to the face of the soldrer pile they should be battened together Lacmg and bracing of adjacent struts rn plan must aiso be consrdered

3

design

assume

kN/m -

all

of

is resisted

by

struts

and

FA

struts

lateral

load

FB

Struts to be at 2.5 m centres horizontally

Loads in struts

Moments 3.5FA

about

= 32.13

FB for horizontal x 4.8

x cl;?

+ 3.8)

= 215.9 + 57.2 FA = 92.3 kN m

FB = 32.13

x

loads and reactions:

22.68 1.4 + ___ 2

+ 9.45

x

x

1.2

1.2 x (3.8

+ 7)

+ 49.9

x 4.8 +

= 86.9 kN m

7

Loads

in strut A

Axial

1

= 92.3

x 2.5 x ~~ cos 300

= 266.5 kN Upwards parallel to face of pile = 92.3 x 2.5 x tan 30° = 133.2 kN Loads

in strut

Axial

4

B

= 86.9 x 2.5 = 217.3 kN

of strut

Design

Symbols %l,g,ll = Grade bending um.adm,ll

=

om,a,II

=

%,g,ll

=

oc,adm.ll

= =

Oc,a,ll

12 855266, Part 2. 1966 CompressIon members - restrained both ends in posltion but not against rotation, length = effective length.

at

13 Stresses gwen III Table 12 for trench supports are based on a load duration of one week and K, = 1.4. (See Table 22). A longer load duration, eg SIX weeks, IS more appropriate for raking shores, thus K, IS taken as up to one year. le K, = 12

stress II to grain Permissible bending stress II to grain Actual bending stress II to grain Grade compression stress II to grain Permissible compression stress II to grain Actual compression stress II to grain

Raking

strut A. Fig. 7

Length

6500mm

= effective

length

L.

Density = 590 kg/m3 and assume perpendicular to member. Assume

300

x 300mm

Assume key.

that vertical

that self weight

acts

section.

load parallel

to face of pile is taken

by shear

bh3 ___ 12bh

asb

= hi = Jg

Slenderness Axial

load

Then cc,a,ll From Table Modify

then

ratio

= Jz

= b

=

= 86.6mm

6500 ~ 86.6

= 75.06

= 266.5 kN 266.5 =

300

x

loo0

x 300

=

2

96

,,,/mm2

.

12 ac,,,ll = 6.9 N/mm2 for SC4 strength

to 6 weeks’

duration

1.2 ~c,a,,,,,rr = 6.9 x Y I .-t

= 5.9 N/mm2

class timber

Consider

bending

Weight

=

590

moment x 9.807

=

*2

o’52 ’ %Oo2=

= ti

8 x 300 From Table Modify

CJ,,~,~~= 8.90 N/mm’

12

to six weeks’

~m,adm,lI

I1

+

where

~

Mm E SC4 Table 12

15

strength

class

Umber

class timber

x

1.2

= 7.63 N/mm*

1.4

factor

(1+

(’

for SC4 strength

load duration

=

Modification

N/mm2

0.61

--

x 3002

8.90

=

x 0.3

x 0.3

6wL2

Gm.a.1~

14 BS 5268 Part 2: 1988 Appendw C Modkatlon factor for compression members

by self weight

1000 run

= 0.52 N/mm

then

induced

K12 for compression

WE

1

2NX*~c,adrn,~~

+

_

members

(1 + q)a2E

~___

-

--__-

2NX2 ac,adm,ll

i

7r2E

112

Nh2fJc,adm,ll

u,adm,N = 5.9 N/mm2

E = 5700 N/mm’ x = slenderness ratio = 75.06 17 = eccentricity factor = 0.005h = 0.005 x 75.06 = 0.375 N = load factor

K,2 = [; i

= 1.5

(1 + 0.375)

+

7r2 x 5700

2 x 1.5 x 75.062

x 5.9 / 7r2 x 5700 -

1.5 x 75.062 = 1.276 = 0.569

-

Compression

[l .2762 member

om,adm.ll

!

1

-

x 5.9

1 .l 28]“2

should

be so proportioned

@m,a.ll

No, a II ’ ’ ue

112

A2--

Qc,a,ll

5

7+ @c,adm,ll

X

that

1.0

K12

X K12)

where 0e=--=

X2E x2

T2 ’ 5700 75.062

= 9.98

9

then 2.96

0.61 7.63 i 1 -

1.5 x 2.96

\

x 0.569 ’

+ 5.9 x 0.569

9.98 0.61 = __ 5.70

2.96 + 3.36

= 0.988

0.988 < 1 .O :. 300 x 300 raking strut A is adequate. For the design of the horizontal strut 6, see Section design of timber struts (Table 3).

16 A simplifying assumpllon has been made to consider a hinge at formation level This does not lead to unsafe design and makes an allowance for the difficultv of assassIng the degree 01 flxlty at the toe of the pile

5 Design Allow 25%

7 Tabulated

of soldier pile reduction

in loading

075

x 9.45z7.1

diagram

to account

for arching

effect

kNIm’

-

Moments FA’

about

hinge

x 4.5 + FB’

x

x

4.8 +

= 376.6 Equate FA’

horizontal

+ FB’=

1.2 -)3 kN m

1.2 x

!

4.8 +

7

> (I)

forces

24.1

x 4.8 +

= 134.4 kNm

10

+ 7.1 x

Subtract

(2) from (I)

3.5 FA’

= 242.2

FA’

= 69.2 kN m

FB’

= 65.2kNm

(2)

Construct

BMD for soldier

Point of zero shear 69.21

C

+ 24.1

=

! 2 = 78.72 + 24.1~ =

. . X

beam

x 1.2 + 24.1

x x

2.lm

ie point of zero shear = 2.1 + 1.2 - 1.5 = 1.8m below FA’ BMAo

=

7.1 x y

+ 17 F

+ 0.3

x 2.5

= (7.98 + 7.8) x 2.5 = 39.4 kN m (hog)

=

BMc

7.1 x 1.5 x + 17 x

1.5 r + 1.81 + 17 x 0.3 x i 0.3 2 + 1.8

'1.2 'L? x ! + 0.3 + 1.8 2 3

1.a2 + 24.1 x ~ 2

- 69.21 x 1.8I x 2.5 = (27.16 + 9.33 + 25.5 = 58.88 kN m (sag)

B&L 17 The 203 x 203 UEP secl~on IS the smallest we usually consldered for soldw piles for pracllcal reasons. However. any suitable secl~an may be used and soIdlers can also be made up using channel seclions back to back wlh spacers. 18 The penelratlon of the soldler pile must be sufficient lo prevenl It moving upwards under the actjon of the raking strut. In cases where the embedded length 01 the soldier pile 1s required to prowde a lateral reactton. the soli should be checked to ensure an adequate factor of safety agamst passwe failure. 19 A rigorous design check would mclude a consideration of shear and web crushing and buckling However, experience shows that flmber struts are not llkeiy lo overload a steel soldwr rn lhls way If good ~~n~lru~l!on procedures are followed. Web sllffeners would be an mconvemence when inslallmg the horizontal members. Care should be taken lo ensure that the raker force IS applied centrally lo the soIdler and no torsional effect IS produced The strut loads determmed m Note 3 should be used m lhts check. Local flange bending should also be consldered where sheeling IS lucked behind the front flange 20 In wsw of the englneermg judgemenls which are necessary lo derive the earth pressure loads it is suggested that the steelwork should be deslgned using BS 449. which uses a permissible stress approach The use of BS 5950, which uses a llmil slate approach lo design, would demand an asses%nenl of the load and serviceabllily factors

= 24.1

x ;

kN m (hog)

203

x 203 Universal

Bearing

(2 = 489.2 cm2)

Allowable

where

i,,

bending

-

124.58)

x 2.5

x 2.5

= 30.13 .__

Assume

+ 39.04

stress

Pile

grade

43

at C

= radius of gyration = 84.68

:. From Table

3A BS 449

Pcb = 165 N/mm* Actual f =

bending

stress

at C

58.88 x 1000 x 1000

b

489.2 x 1000 = 120.35

:. Section

N/mm2

< 165 N/mm2

is adequate

11

Allowable

bending

D

1500

;I=

-=118

T

stress

4.96

‘YY

x 3 x 10

= 90.72 :. From Table P,,

at A

N/mm2

3A BS 449

= 165 N/mm2

Actual

bending

fb = 39.7

x

stress 1000

x

1000

489.2

x

1000

= 81 N/mm’ is adequate

:. Section

retatwely

21 Timber is flexible and tends to deflect under load which enhances the archmg action within the solI. In cases where slifl materials are used for sheeting (eg reinforced concrete) the relieving effect of arching would be

6 Design Allow 50%

reduction

effect

of arching.

32.13 x 0.5 = 16.07KN/m*

I

BM in sheeting

=

=

16.07

x 2.4: 8 11.6 kN m/m height

Try 225

um,a,II

x 150 timber

(Z = 843 cm3)

11.6

x lo3

om,g,ll

x 0.225

=

843 x = 3.1 N/mm2

Permissible =

stress

6.29

Om.adm.lI

=

6.29

5.39 N/mm2 :. 225 x

x

for SC3 timber

22 modify x

lo3

IO3

N/mm2 for 1 week

:. From Table

12

timber sheets

in load as relieving I-

.,

22 When Installing horizontal Umber sheets. care should be taken to avold overdigging. In part~cufar the ground immediately agalrwt the rear flange of the soldier pile should be left undMurbed so that the soil arch is mamtamed. The horizontal timbers can be fixed in a number of ways, Including wedging members behind the front flanges of the soldiers (Flgure 8) or by using special flxlng clips (Figures 9, 10, 11). More detail about the application of the method IS given I” Temporary works lhw role in construction by J R Mngworth, Chapter 7, pages 116-131.

of horizontal

ifz

stress

from Table

12

load duration for 6 week duration

= 5.39 N/mm2

> 3.1 N/mm2

150 SC3 Timber

adequate

in sheeting

225mm

Figure 8. Attaching timbers to H-piles; timber tucked in behind face flange

X 75mm t’mber boards L

5Ai4

PLAN

150mm

X IOOmm X 50~,rrl limberwedges

Figure 9. Use of random length timbering with H-piles; clipping method

4mm

20mln clla.rn~ldsteel bar fl l~/F>rnrrl lcirl(j

UCT/VLOFCLlPPlNG

x 64mm

x 6kg

f

SYSTtM

13

Figure 10. Clipping system to cope with H-pile location errors

2‘25mm X 75mm tlrnber boards Steel scaffold tube offcuts 375mm long

PLAN

Figure sizes

11. Detail of clip

I

14

B

c

D

190

120

165

50

38

210

145

165

50

38

230

C

A

38

173

165

50

Check shear Shear force = 16.07 x 0.225 x ‘$ = 4.34 kN Max shear stress #v,a.

=

4.34 x 1.5 x 1000 225 x 150

= 0.19 N/mm’

I

From Table 12 av,Q,

1

1.34 N/mm*

=

Modify for load duration then

%.adm.

L

= 1.34 x ifz

= 1.15 N/mm”

0.19 < 1.15 :. Section O.K. in shear Check bearing Assume minimum bearing length due to lack of fit = 25 mm Bearing (compression) 0 = c’a’’

perpendicular

to grain

4.34 x 1000 = 0.77 N/mm’ 225 x 25

As bearing occurs at ends of the timber the compressive stresses given in Table 12 do not apply From Table 23 the allowable compressive stress must be divided by 1 .14 and also multiplied by 1.2/1.4 to correct for load duration .‘. &a&,, ,(no wane) =

2.19 x 1.2 1.14 x 1.4

= 1.65 N/mm2

0.77 < 1.65 :. Timber adequate for bearing

7 Tabulated design of timber struts (Tables 2-5) Strut B in Figure 7 and the struts for the alternative arrangements shown in Figures 12 and 13 may be calculated from first principles as shown previously in the section headed ‘Design of strut’. However, for convenience the Strut load Tables 2-5 have been prepared which give the allowable loads in kN for struts of various sections and lengths. The tabulated loads have been calculated in the same manner as that employed when using first principles and include the same load duration factors, densities etc. The tables cover strength classes SC3, SC4 and SC5 and also Douglas fir/larch as large sections are often more easily obtained in this group. The tables are simple to use. The axial load to be carried by the strut is determined by the method shown in the example on page 8 and the length of the strut is obtained from the geometry of the arrangement selected (Figures 7, 12, 13). For the example used the axial strut load = 266.5 kN and the length = 6.5 m. Using Table 3 a load equal to or greater than 266.5 kN is selected from the strut column headed 6.5 and the required section found by extending to the left along the line of figures.

By inspection the smallest adequate strut load is 269 kN which corresponds with the preferred section of 300 x 300 mm, the section determined by first principle calculations. 15

s

Table 2. Permissible axial load (kN) in struts of SC3 strength class timber LOADING

CONDITION

SELFWElGHT= 590kg/r+

WET CONDITIONS

AXIAL LOAD-

BENDING STRESS PARALLEL TO GRAIN = 6.29N/mm2 COMPRESSION STRESS PARALLEL TO GRAIN = 5.94 N/mm2 LOAD DURATION FACTOR K3 = 1.2 MINIMUM MODULUS OF ELASTICITY (E) = 5010 N/mm’

SECTION SIZE

LENGTH 1.25

1.75

1,s

2.25

2. 0

2, 75

2.5

I

3*25

3.0

~

LENGTH t--

M E T K E S

N

3.75

3.5

&4;r.n_

4.25

4.0

4.75

4.5

5.75

5,25

5,O

5 +5

Cl. 2'5 h.75 7 ,';! -> 5 7 *7::a 6 .!I 6 ,5 T+() '7,5 8 t0

200x200

181 176 171 165 i60 154 147 141 134 127 120 113 106

01

7s

70

6s

60

56

200x250

226 220 214 208 201 194 186 178 170 161 153 144 136 127 119 112 104

98

Pi

85

79

74

200X350

318 310 301 292 283 273 263 252 241 229 218 206 195 183 172 162 152 142 133 125 117 109 1.0:~ 96

200x400

,I :,( / :I.o:;9;,< !;p 363 354 344 334 324 313 301 289 276 263 ':'SO237 224 211 I.98 I.86 1.75 I.64 1.54 144 I.55 1.27 I.:13j.1.;]

~5OXT50 L L

?90 '84 378 371 VOY 200 191 182 173 164 Ix!?:? 147 lJ9 131 1.23 Il.6 1.09 1.03 96 L L _ ." 265 258 250 243 235 376 ...L 318 _ L

250X300

348 341 334 326 319 310 302 293 :1'84274 264 254 244 233 222 212 201 191. 181. 171 162 1.53 144 1.36 1.29 I.':'1 I.:\4'I.(.,::!

250x350

406 398 390 381 372 363 353 343 332 321 310 298 286 274 262 250 238 226 31s 203 1Y i3 1. tS 2 l72 I6 3 .l 5 4 1. 45 .l 3 7 1.S 0

250x400

r!3 2 11 200 11i\Y'I. 7'i 3. 69 f6 0 I5 1 465 456 446 436 426 416 405 393 381 369 356 343 3:!Y 316 302 288 275 261 248 T"35 f!

300X300

')33 ._Lh_ ')':i? 424 417 410 402 394 386 378 369 360 351 341 331 321 310 299 :!88 277 266 255 L!44 ,_ "' 1. 3.20 1. 'I, 9 :t 1iii 2 17:: 1C? 3

300X350

495 487 479 470 461 452 442 432 422 411 400 389 377 365 353 340 327 315 302 209 277 264 252 240 229 218 '7()7I';‘.'

300x400

566 557 548 538 528 517 507 495 484 472 460 447 433 420 406 392 378 363 349 335 3E0 306 293 27 9 26 6 254 24:? 2 j 0

350x350

584 576 567 558 '549 540 531 531 511 500 489 478 466 454 442 429 416 403 390 376 36i.J349 336 32::'309 :'96 284 271

350X400

668 658 649 639 629 618 608 597 585 573 561 549 536 532 508 494 480 465 450 43'S 420 405 3YO 3.2:;360 346 331 31:

400X400

769 760 750 740 730 719 708 697 686 674 662 649 636 623 609 SC.'5581 566 551 535 5PO 504 4813
9Y

93

87

5

2

69

48

45

4:!

64

60

56

3

y

3

C?

52

49

'ji ii, I$4 7y

74

j Y 1. :i

Table load

3.

Permissible

(kN) in struts

strength

axial

of SC4 LOADING

c/ass timber

CONDITION

SELF WEIGHT = 590 kg/m3 WET CONDITIONS

BENDING STRESS PARALLEL TO GRAIN COMPRESSION STRESS PARALLEL TO GRAIN LOAD DURATION FACTOR K3 MINIMUM MODULUS OF ELASTICITY (E)

SECTION SIZE

LENGTH

1.25

1.75 1.5

:

2*25 2. 0

2.75 3,5

N

3.75 3.5

AXIAL IL”H” AAn-

8.90 N/mm’ 6.90 N/mm* 1.2 5700 N/mm’ I

3.25 3.0

= = = =

I-

--LENGTH-

METRES

4,25 4.0

+---

4.75 4.5

5.mL 5.0

200x200

210 205 199 193 187 180 173 166 158

200x250

263 257 250 242 234 226 218 209

200X300

316 308 300 291 282 272 262 252 241 229

218 206

200x350

369 360 350 340 330 318 307 294 282

269

255

242 229 216 203

200x400

422 412 401 389 377 364 351 337 323 308

293

278 263 248 234 220

250x250

337 330 324 316 309 301 293 285 276 267

250x300

405 397 389 380 372 362 353 343 333

250x350

473 464 454 444 434 424 413 401 389

250x400

I.50 142 134 127 119 111 104

5.75 5.5

7*75 7.5

8.0

65' 65

61

57

53

50

46

117 110 103

96

90

84

79

74

69

65

61

143 134 126 118 110 104

97

91

86

81

76

115 108 102

90

91

207

194 I.82 171 161 151 j.42 133 126 118 111

105

257 247 237 227 217 207

197

187 177 168 159 151 142

135 127 120 113

107

322

311

299 288 276 264 252

240

228 217 206

lb.6 157 149 141. 1.33

377

364

351 337 324 310 296

283

269 256 243 231 219 208

197 18;' 177 168 159

540 530 519 508 497 485 472 459 446 432

417 403 387 372 356 341

325

310 295 281 267 253 240

228 21.6 "05

300x300

493 485 477 469 460 451 442 432 422

401 390 379 367 355 343

331

318 306 293 281 269 257

246 234 224 21:~ 203

300x350

576 567 557 548 538 527 517 506 494 482

470

389 375 361 346 332 318 305

291 278 266 254

242

300x400

658 648 637 626 615 604 591 '579 566

553

539 525 510 495 479 464

448

337 322 308 294

281

350x350

679 670 660 651 641 630 620 609 598

586

574

495 481 466 451 436 421 407

350x400

776 766 755 744 733 721 710 697 685

672

658 645 630 616 600 585 569

400x400

894 884 873 862 851 839 827 815 802

790

776 763 749 734 719 704

412

170 161 151 142 133 125 19% 184 173 1.62 152 191

458 444 431. 417 403

562 549 536 523 509

85

7.25 7,o

74

180

91

6,75 6.5

80

199 190

98

6,25 6.0

179 168 158 148 139 131 123

19"d 185 175

432 416 399 383 368 352

1?4 3.84

392 377 363 348 335

553 537 520 503 487 470

453 437 420 404 389

689 673 656 640 623 605 588

571 553 536 518 501

18

-&

i

Table 5. Permissible axial load (kN) in struts of Douglas fir-larch species

LOADING

CONDITION SELF WEIGHT = 590 kg/m3

WET CONDITIONS

BENDING STRESS PARALLEL TO GRAIN COMPRESSION STRESS PARALLEL TO GRAIN LOAD DURATION FACTOR K3 MINIMUM MODULUS OF ELASTICITY (E)

= 8.9 N/mm’ = 6.9 N/mm*

AXIAL LOAD -

& f------

= 1.2 = 6480 N/mm*

LENGTH p 1

I__ E N G T H

SECTION SIi!TE 1.75

3. *a

1.5

2. 75

2.5

3.25

3.0

3.75

3 t5

4.25 4.0

4.75 4.5

5.25 5.0

5.75 5.5

6+25 h <‘.I

7 * 25

6.75

6.5

7.0

7.75 8.0

7.5

211 205 200

~OOX~150

264 257 250 243 236 228 220 212 203

lY4

185 176 3.67 3.58 149

140 132 1134

200:x300

jj.7 309 301 L 292 284 274 265 255 245

234

224 213 202 191 181

200x350

370 361 351 342 331 321 31.0 2Y8

287

274

262 250 237 225 2).3 201

190 179

168 159

2Oo:x400

423 412 40':'391 379 367 355 342

328

315

301 286 272 258 244

231

218 206

194 183 172 162 1.53 144 136

250x250

337 i33l 324 317 310 302 295 287 278

370

261 252 242 233 223

21.4 204

250x300

405 397 389 383. 373 364 355 345 336

325

315 304 293 28? 271

260

249 238 227 ~?j.6206

250x350

473 464 455 445 435 425 415 404 393

J81

L36Y 357 344 332 J19

306

293 280 267 255 243 231 220 :?O? 199 189

250x400

541 530 520 509 4Y8 487 475 463 450

437

423 409 395 381 366

353. 337 322 308 2Y4 280 267 254 342 230

219

:'!'!t-! I.?8

300x300

494 486 478 469 461 452 443 434

425

415

405 3Y4 383 372 361

350

338 327 315 303

291 280 268 257 246

235

i ,:_ 7')c ..,2j.5

300x350

576 567 558 548 539 529 518 508

497

406

4.74 462 450 437 424

411

398 385 371 358 344 331 318 305 :?92 "80

300x400

659 648 638 627 616 605 593 581

569

556

543 530 516 502 487

473

458 443 427 412 397 382 367 353 338

350 x350

679 670 661 651 641 631 621 611

600 589

350x400

776 766 755 745 734 722 711 699 687

400x400

894 884 873 862 851 840 828 816

804

L

lY4 188 182 175 168 161. 154

METRES

N

2oox?oo L

;;

2.25 2.0

I

146 139 131 124 1.17 1.10 103

97

9 3. 85

80

75

70

66

61

5 E\ :;4

110 103

37

3j

85

80

'7 "j

7 1. 6 :'

173. 161 152 143 134 126 3.j.Y11:' 105

99

93

88

11.7

5 :L

03

149 141 1.13212'5 13.8 111. 3.05 31;

lY5 185 176 168 159 1.51 3.43 136

128 i:!.L.1.I:< 128

122 11.5

jCb 106 177 1.bCi1~~9 1.5:..I 43

475 461 447 433 419 405 391

18!,

1'1

268 256

324 31.3 297

577 566 553 541 528 515

502 489

377

363 349

674

662 648 635 621 607

592

577 562 547 531 515 500 484 468 452 436

421 406

792

779 766 752 739 725

710

695 680 665 649

633 617 601 584 568 551

534 518

Figure 12. Alternative strut arrangement for soldier pile wall

figure 13. Alternative strut arrangement for soldier pile wall

-

SoIdler0fles

20

Folding wedges

Table 6.

Size of

required

for designs

Figures

strut in

7, 12, 13

Strut Arrangement

Member

Figure

7

Raking strut A Horiz. strut B

Figure

12

Figure

13

Axial

Load

WY

Length

Minimum

Strut Size (mm) _._-.___.

~~.___

(m)

__ Srrenglh class SC3

Strength class SC4

Strength class SC5

266.5 217.3

6.5 5.5

300 x 350 250 x 350

300 x 300 250 x 300

250 x 350 250 x 300

300 x 300 250 x 300

Raking strut A Horiz. strut B

266.5 217.3

7.2 4.2

300 x 400 250 x 300

300 x 350 250 x 250

300 250

300 250

Raking strut A Horiz. strut B

266.5 217.3

7.0 6.0

300 300

x 400 x 300

300 x 300 250 x 350

300 x 300 250 x 300

x 300 x 250

Douglas fir-larch

x 300 x 250

300 x 300 250 x 350

The strut sizes shown in the Table have been selected on a minimum volume basis which will give maximum

economy.

Using the strut load tables suitable struts have been selected for the various strut arrangements shown in Figures 7, 12, 13 and are tabulated in Table 6. It can readily be seen that the strut tables allow the designer to consider very quickly a variety of raking strut configurations and so choose the smallest appropriate section for maximum economy.

21

Trenches

A trench is defined as an excavation whose length greatly exceeds its width. It may have vertical sides, which will usually require strutting from side to side, or battered sides requiring no support. (It is assumed that all trenches constructed in accordance with this guide will be entered by operatives. Therefore, unsupported narrow trenches as defined by CP 2003 are specifically excluded.) Table 8 suggests safe temporary slopes in different types of ground. Table 9 gives guidance on appropriate sheeting arrangements for different types of ground and Tables 10 and 11 sizes for walings and struts.

Use of poling boards in trenches

Poling boards are placed in a vertical position against an excavated face which will stand to a height of 1 metre or more while the boards are set in position (Figure 14). They can also be used in a top waling frame to facilitate the pitching of runners. Timber poling boards range in size from 1 m x 32 mm thick to 1.5 m by 50 mm thick and are approximately 225 mm wide. The boards must be sound and at least of strength class SC3 as defined in BS 5268: Part 2: 1988. The use of poling boards can be divided broadly into two methods the middle board method and the tucking frame method.

Middle board method Operational sequence Excavate to a depth equal to the length of the board. The cut should have roughly vertical faces but with some ground still to be trimmed. Having decided the length of the waling, set the boards to plumb and line at each end of the waling. Install the intermediate boards removing only just sufficient ground to give good bearing against undisturbed material. figure 14. Initial excavation using poling boards in the middle board

method

22

Additional boards where excavation Is cut to Irregular line

Wedges as necessary only between poling board and waling

When both faces have been poled, install waling and struts complete with lip boards at the mid point of the poling boards. Tighten the poling boards to the face of the excavation by driving wedges between the waling and the poling boards so that each board bears firmly against the waling and the work has a neat appearance. As an alternative to timber struts, adjustable metal trench struts can be used. Continue excavation and install the next level of poling boards. When complete, the walings and struts must be ground propped, puncheoned and laced to any frames set above them. Guard and handrails must be fitted and the whole installation should have a neat appearance and be correctly aligned with members squared off to each other. See Figures 15 and 16. figure 15. Completed trench using poling boards and the middle board method

figure 16. Typical view of the middle board method

Faceboard

_

Puncheon or prop (this is often placed against the polmg boards or sheeting)

23

Tucking frame method The tucking frame method differs from the middle board method in that the walings and struts are positioned at the top and bottom of the poling boards. The sequence of operations for an installation by this method is given below: . .

. . .

.

. .

. . . .

Excavate to a depth slightly greater than the length of the board. Dig a roughly vertical face leaving some ground still to be trimmed. Having decided the length of the waling, set the poling boards to plumb and line at each end of the waling with the top edge of the poling boards in line with the top of the excavation. Install the intermediate boards removing only just sufficient ground to give good bearing against undisturbed material. When both faces have been poled, install walings and struts in line with the tops of the poling boards. See Figures 17 and 18. Install walings and struts at the bottom of the poling boards so that the lower ends of the poling boards coincide with the centre line of the waling. Install lip boards and puncheons and tighten poling boards to the face of the excavation, by driving wedges between the waling and the poling boards so that each board bears firmly against the waling and the work has a neat appearance. Continue excavation to a depth equal to the length of the poling board. Install the next level of poling boards by tucking the top end of the board into the gap created between the face of the excavation and the waling. Install lower waling and struts at the bottom end of the poling boards. As an alternative to timber struts, adjustable metal trench struts can be used. The following levels of boards should be installed by the same method. When complete, the walings and struts must be ground propped, puncheoned and laced to any frames set above them. Guard and handrails must be fitted and the whole installation should have a neat appearance and be correctly aligned with members squared off to each other. See Figures 17 and 18.

A variation on the tucking frame method can be achieved by the use of a tucking board (see Figure 19). In this method a tucking board having the same thickness as the poling boards is introduced between the waling and the bottom ends of the poling boards so that its lower edge coincides

Figure 17. Poling boards used in the tucking frame method

Poling board

strut Lacing

are necessary to take up tolerance

Waling

used only between the poling boards

Puncheon

24

Figure 18. Typical view of the tucking frame method -

Handrails

prop(thisis often placed against the poling boards or sheeting)

Foot block

x

with the centre line of the waling. This creates a gap between the installed poling boards and waling into which a lower level of poling boards can be tucked. This method has the advantage that the lower level of poling boards can be seated against a continuous and levelled face instead of butting against individual boards. It also allows the trench to be backfilled leaving the timbered face fully supported as each level of boards is drawn. It does result, however, in a stepped inside face to the trench and an appropriate allowance should be made to the width of the top of the trench to accommodate the reduction at the trench bottom. In all other aspects this method is similar to that given previously and the sequence of operations is the same.

25

Figure 19. An alternative use of the tucking frame method using tucking boards

Second settma of oolina

I/

/II

walinq and first Dolina

Dig to continue

Use of runners

Runners are used in soft or loose ground which requires continuous support at all stages of excavation. They are driven down slightly in advance of the excavation to_ avoid loss of ground from behind the sheeting. Runners are also used in better ground where their employment as open or close sheeting is considered more practicable and economical. Runners may be timber or trench sheeting. Timber runners are at least 50mm thick and approximately 2.5m long and 225mm wide. They can be either square or chisel ended. Steel trench sheets can also be used as runners, particularly if water-tightness is a major consideration, in which case interlocking sheets should be used. The type of runner used and the method of installation will depend on whether the ground can be classified as good or bad. The installation methods and sequence of operations for either of these conditions is given below. Good ground A trench is excavated to a convenient depth not exceeding 1.2 metres and two levels of walings are fixed. See Figure 20. The plan width of the walings must make allowance for the insertion of wedges between the runners and the waling. To facilitate this, spacing timbers are set vertically behind the walings and against the excavated earth face at the end of each strut. Puncheons are set between walings and supported to provide a stable framework during the subsequent driving of the runner. See Figure 21. The runners are then set behind the walings between the uprights, driven until toed in and wedged from the waling. The excavation can then be recommenced at the bottom of the trench.

26

Figure 20. (Left) Stage 1. Uprights, walings and struts ready for pitching runners

Figure 2 7. (Right) Stage 2. Runners pitched ready for excavation

Waling Upright

-

Runners

Strut

Gap for setting runners

Plan on A-A

Plan 3-B

As an alternative to timber struts, adjustable metal trench struts can be used. l An open spacing can be used for the runners and the excavations can be taken below the toes of the runners. When excavating below the runners they should be kept from dropping by tapping the wedges firmly into place. l On completing the excavation to the next level the wedges to each runner are released in turn and the board driven down until it is toed in. See Figure 22. The wedges are then replaced. l To prevent loss of ground beneath the uprights the space is filled with cross poling (shot-t boards) packed behind the adjacent runners. . A second stage of excavation extending below the length of the runners can be carried out by fixing a new set of runners inside the top waling frames and retaining them with new walings fixed on the struts, which may be firmly fixed in place. See Figure 23. The sequence of driving these second stage runners is the same as for the first stage runners. l In planning the dimensions of deep excavations, allowance should be made for setting in the timbering as each new set of runners is installed. l

27

Figure 22. Stage 3. Runners driven and trench excavated ready for next frame (shown dotted)

v-x f--./

-

x

H ‘I

-

l-l-

-

Upright

r’ .P

h

3

1 ; PL

.GL

-

3

-

150mm

-I

.

-_

\A _-

__L

Elevation

-l

-

-

ss;

mi

D

C

Cross poling-,

New uprigh New frame Plan on D-D

Figure 23. Stage 4. New runners pitched ready for excavation within the trench supported by the first sef of runners

2nd stage runners

t stage runners

~

Frnal positlon of head of runners

2nd stage runners Wedges

Plan on E-E

28

Figure 24. New set of runners set inside trench supported by two levels of poling boards used in the Runners

middle board method 0

d-Wedges I-----

q w-------

b

Poling boards

Fmal oosition of ead of runners

Bad ground It will probably be necessary to timber the first trench with poling boards as described earlier. The first stage of runners will then be pitched inside the walings and retained by a second set of walings fixed across the struts. See Figure 24. The toes of the runners must be kept below the excavation level at all times. The runners are driven down 150 mm to 200 mm in stages working along the excavation.

Use of trench sheeting

Trench sheeting is made from steel plate and may be obtained with either interlocking or lap joints. The interlocking sheeting is more watertight than the lap jointed and therefore the possibility of water pressure building up behind the sheets is greater. Trench sheets can be used as runners and are installed in a similar manner to that described for timber runners using timber walings and struts or adjustable metal trench struts. However, they can also be predriven as panels before the excavation commences.

Use of sheet

Sheet piling is rolled steel plate made into individual members of either trough or Z shape in cross section. The members interlock and are stronger than trench sheets and if used with care can withstand driving and withdrawal a number of times without damage. Sheet piling is usually only used in situations where the ground is difficult or water-bearing. In order to make the best use of the strength of the sheet piling an engineer-designed solution is essential. Timber struts and walings can be used particularly in trenches and shafts. Care should be taken when excavating to remove all the material from the troughs of sheet piles so that it does not become a hazard to operatives working at the bottom of the excavation.

piling

Safety

Working in trenches is dangerous and a significant number of deaths and serious injuries result every year from trench wall collapses. Such accidents are avoidable by adopting proven methods and techniques, by the rigorous application of safety rules and by ensuring compliance with

relevant

statutory

regulations.

See

section

on

Safety

(p. 52).

29

Table 7. Summary of methods of side SUPPOSE ’ ’ for trenches

Method

Conditions

Suitability

Poling boards (both middle board and tucking frame methods)

Can only be used where the ground is self-supporting for each level of boards

Use in trenches where there are a number of cross services Use where it is necessary to observe the strata through which the excavation passes (particularly applies to shafts) The tucking board system allows the trench to be backfilled leaving the timbered face fully supported as each level of boards is drawn.

Runners

Can be used in bad ground conditions

Inconvenient where there are a number of cross services. Fewer joints, therefore less risk of loss of ground from behind sheets.

Trench sheeting

Can be used in good or bad ground as an alternative to runners Can be driven in panels before excavating

Sheet piling

30

Used in bad ground conditions and driven before excavating

As for runners Interlocking sheets can be used where exclusion of water is required

Usually used in difficult or water bearing conditions Used where it is necessary to minimise the frequency of struts and walings

Table 8. Ground and slopes

conditions

Ground

Field tests and descriptions

Safe temporary slopes (degrees

type

GRANULAR

(i) Particles visible (ii) Sands feel gritty (iii) Visually assess proportions of boulders/cobbles/sand

COHESIVE

(iI (ii)

Particles not visible When damp silt and fine sand are shaken in the hand water

appears on the surface (iii) Soft clay is easily moulded by fingers (iv) Firm is moulded by strong finger pressure (v) Stiff is indented by strong finger pressure (vi) Fissured clay should be examined for its structure

‘Dry’ site

‘Wet’ site

Boulders Cobbles Gravel Sand

(>200 mm) (60 to 200 mm) (2 to 60 mm) (0.06 to 2 mm)

35145 (6) 3940 (6) 30140 30135

30/40 (6) 30/35 (6) 10130 10130

Silt

(0.002 to 0.06 mm)

20/40

5120

1 Trench depth (m) 1.2-3 Clay (5)

3-6

I

Soft

30145

20130

1o/20

Firm

35145

30140

20125 (6)

Stiff

40145 35145 (see (6) and (7))

25/35 (6)

10120 15125 35/40 (6) 35/45 (6) (7)

5110 10115 20/25 (6) 25/35 (6) (7)

Soft non-fibrous Firm non-fibrous Firm fibrous Stiff fibrous

(0

Rotten or rotting vegetable matter (ii) Smell (iii) Fibrous or non-fibrous (iv) ‘Soft’, ‘firm’ or ‘stiff’

Peat (black, grey or brown clayey or sandy)

FILL

(i) All types of man-placed material (ii) Note constituents including unnatural inclusions

As main soil type (eg clay, sand, gravel, etc)

According to category above

ROCK

(i)

Mass stronger than the above soil types (ii) Important to note cementation, structure and orientation, (fissures, joints, bedding, layers, etc) (iii) Closely jointed rock may act as ‘granular’ and weak weathered rock as ‘clay’

As appropriate to the predominant constituents characteristics

Check orientation planes

(i)

Full description

GROUNDWATER

Levels water encountered Rate of entry (iii) Standing level (iv\ Flood conditions

(1)

from horizontal)

necessary

and

of

See Note 1 (d)

(ii)

Notes These are guide figures to slopes based on and subject to: (a) Temporary conditions (ie generally 1 to 14 days) (b) Field safety and experience (c) A safety limit of 45 degrees (but see Note 7) (d) Water seepage can cause wash out and undermining ‘Dry’ site: minor or no seepage from excavated faces. Minor or no surface run-off. ‘Wet’ site: submerged or widespread seepage from excavated faces. The behaviour of soils is influenced by the grading, particle size, shape and density. In mixed soils the maximum grain size of the smallest 15 per cent by weight of the grading tends to characterise the soil. Normally consolidated clays are usually stronger within a few metres of ground level due to desiccation. However, the clay crust is fissured for the same reason (note the fissures may not be visible) and is seldom greater than 4 metres deep. Classical soil mechanics theory would suggest that clay with a cohesion greater than 30kN/m2 will stand vertically to 6 metres. However, due to the fissuring, the face becomes unstable and lumps fall into the trench. In deeper trenches there is a risk of toe failure due to underlying softer clay. The suggested slopes are intended to avoid danger to workmen from these risks and are offered as a guide only. Flatter slopes may be applicable or required subject to the evidence on: (a) Incipient (structural) failure planes (b) Safety precautions against small or large falling fragments Steeper slopes may be applicable or required and will be subject to the same assessment as in Note 6.

31

Tab/e 9. hfinimum sheeting requirements

Ground

type

Granular Silt Soft Clay Firm and Stiff Clay

Type of sheeting (see key in table below)(‘) Total trench depth: Less than @j 1.2 m

1.2 to 3 m

3 to 4.5 m

4.5 to 6 m

A to D

A

A

A

C to D

B or C

B or C

A or B

As for soft/firm/stiff

Peat Rock

B for incompetent rock

Key to sheering

to

clay D for competent rock (combined with mesh, nets or similar for depths > say 3m)

description

Type of sheeting

Common name

Description

A

Close sheeting

Trench

B

% sheeting

C

% sheeting

D

l/B or no support

sides fully covered by sheeting. 50% of trench sides covered by intermittent sheeting. 25% of trench sides covered by intermittent sheeting. Nominal support by widely spaced sheets or no support.

Notes The vertical spacing of walings in Table 10 is for use with poling boards 32mm thick or with mild steel trench sheets which have a minimum elastic modulus 2 of 35cm3/m. For vertical side trenches less than 1.2m deep no support is required providing method of work is safe. (The Construction General Provisions Regulations 1961.) Competent/incompetent is a combined strength and structure assessment. An incompetent rock is a weak and/or closely fractured rock. A competent rock is strong with widely spaced fractures safely oriented. It is particularly important for support schemes for trenches in rock to be determined by an experienced person. If groundwater is to be prevented from running through the sheeting, interlocking steel sheets may be used. This may increase the pressure on the supports.

Nores lo Table 10 opposite 1 In a shallow trench if 1 Na waler set is used care must be taken against rotation (eg toe-in sheets - Figure 14) 2 Timber to be strength class SC4 or better (see Table 12 - Timber supports for trenches - Permissible stresses) 3 Walings generally to extend at least 2 x thickness beyond strut 4 Walings to be supported by hangers or puncheons and struts by lip blocks or similar (see Figures 14, 15 and 16) 5 AlI support work should be regularly inspected by a competent person and adjusted as necessary 6 Steel trench struts 0130 kN capacity. Struts produced by Acrow. Mabey &Johnson, Mills, RMD and SGB No 0 (0.35 to 0.45m) to No 3 (1.0 to 1.70m) are typical 7 Light Trishores are manufactured by RMD and have a capacity of 170 kN. Other proprietary struts of equal capacity can also be used 6 Maximum surcharge load adjacent to trench is not to exceed 10 kN/m’ (1 tonne/m’) 9 The waling and strut sizes quoted may not be suitable for long term support in clay soils, ie in excess of 4-6 weeks.

32

Table 10. Minimum

requirements

Total trench depth

Timber waling size (b x h)

Vertical spacings of walings

(ml

(mm1 h

Cm)

for walings and struts -

no water head

Timber strut size (mm) for trench width: I-1.5m


1.5-2m

2-2.5m

2.5-3.5m

3.5-4.5m

Single or double proprietary steel struts or right Trishore <2m

Strut spacing (horiz) fm)

2-4.5m

b

Less than 1.2

225x

75

150x

100

3

75 225x 150x100

4.5

225x 150x

1.2

6

Less than

75 100

4.5

6

150x

75

150x100

150x150

1.0 1.2

150x

75

150x100

150x150

150x150

150x200

1.0

15oxtoo

150x150

150x150

150x200

200x200

225 x 75 150x 100 250x 100

0.9

225x 75 150 x 100

One set of walings if sheets are toed-in

150x

Twin 225 x 75 spiked together 200x 100

0.9

150x

1.3

Twin 225 x 75 spiked tðer 225x 150

225x150

1.2 3

One set of walings if sheets are toed-m

300x150 75

1.o

150x150

200x200

225x225

s s

T T

200x200

s S

T T

225x225

D D

T T

D D D

T T T

150x150

150x150

250 x250

75

150x100

150x150

S

7

75

150x100

150x150

S

1

150x100

150x100

150x150

D

T

0.9

150x100

150x100

150x100

D

T

1.5

150x150

150x150

150x150

D

T

1.1 1.5

150x150 200x150

150x150 200x150

150x150 200x150

D T

T T

One set of watings if sheets are toed-in

150x

75

150x100

150x150

S

T

75

150x100

150x150

S

T

1.2

To be designed in accordance with the basic principles given in BS 5266: Part 2: 1988 but with the permissible stresses given in Table 12

1.6

2.5

Less than 1.2

225x

3

Twin 225 x 75 spiked together 225x 150

0.9

150x

1.5

150x150

150x150

150x150

D

T

4.5

225x 150 300 x 150

1 .o 1.3

150x150 150x 150

150x150 150x 150

150x150 150x 150

D D

T T

6

225 x 150 300x 150 250 x 200

0.6 1.0 1.5

150x150 150x150 250x150

150x150 150x150 250x150

150x150 150x150 250x150

D T T

T T T

Less than

200 x 100

One set of walings if sheets are toed-in

150x

75

150x100

150x150

S

T

3

225x 300x

150 150

1.1 1.5

150x100 150x 150

150x100 150x150

150x150 150x150

D D

T T

4.5

300x150 250 x 200

1.0 1.5

150x150 200x150

150x150 200x150

150x150 200x150

D T

T T

6

300x150 250 x 200 250 x 250

0.6 1.1 1.5

150x150 200x150 250x150

150x150 200x150 250x150

150x150 200x150 250x150

T 1 1

T T T

3.0

1.2

3.5

33

Table

11. Minimum

for walings and struts - sheets loaded by water head, ie saturated soil. Ground surcharge 70 kN/m’ (1 tonne/n?) uniformly distributed requirement5

Total

Timber

Maximum

trench depth

waling

(b x

vertical spacings

(m)

(mm)

size h)

Timber strut size (mm) for trench widfh of

1.5-2m

l- 1.5m

tm

2-2.5m

2.5-3.5m

3.5-4.5m

walings (ml

Single or double proprietary steel struts or light Trishore <2m

A

Horirontal spacings of s/ruts

2-4.5m

b u Less than 3.0 _____

225x 150

1.2

150x100

150x100

150x150

150x150

150x200

200x200

D

T

t.Bm

4.5

250 X 200

1.2

150x100

150x150

200x150

200x150

200x200

200x250

T

T

1 .Bm

Notes 1 Timber to be strength class SC4 or better (See Table 12 - Permissible stresses) 2 Walings to extend beyond the face of the strut by not less than twice the thickness of the waling 3 Heave or boilmg at base of trench not considered 4 Walings to be supported by hangers or puncheons and struts by lip blocks or similar (see Figures 14, 15 and 16) 5 All work to be regularly inspected by a competent person and adjusted as necessary 6 Maximum surcharge load adjacent to trench is not to exceed 10 kN/m” (1 tonne/m*)

Table 12. Timber supports for trenches - permissible stresses - no load sharing

Strength

class

SC1 SC2 SC3 SC4 SC5

(N/mm2) (N/mm-?) (N/mm2)

(N/mm*)

Maximum Minimum shear modulus of (iV/mmz) elasticity (N/mmz)

3.32 4.87 6.29 8.90 11.87

2.09(1.20) 2.09(1.59) 2.19(1.69) 2.39( 1.89) 2.79(2.39)

0.9 1.29 1.34 1.34 1.75

Bending parallel to grain

For derivation

Tension parallel

Compression Compression parallel to perpendicular

to grain

grain

2.56 2.91 3.73 5.24 6.99

to grain

3.06 4.63 5.94 6.90 7.60

of stresses see Appendix

3890 4320 5010 5700 6130

1: Timber

Criteria 1 Timber used in trench support 2 Wet conditions 3 Dry sizes in calculations 4 Duration of load - 1 week 5 No wane at bearing (where wane is permitted at bearing use figures in brackets) 6 Bearing not greater than 75mm and not less than 150mm from end of member 7 Effective depth of member not greater than 300mm 8 h/b ratio not exceeding values given in Table 24 Genera/ notes Apart from shear, all stresses are based on those given in BS 5268. The shear values given are based on long experience within the construction industry and are taken from BS 5975 Code of practice for falsework 1982. Maximum

shear

= 1.5 x average =

34

Load Cross sectional

shear where average area

shear

Shafts

A shaft is a vertical excavation of variable depth and section which is normally square, rectangular or circular. Its minimum dimension is not less than 1.2 metres as this is the smallest size in which a man can conveniently work. The digging of deep shafts by hand is an ancient skill which has only partly been superseded by methods employing modern machinery and equipment. Where it is uneconomical to bring a large machine to the site for a small amount of digging or where site access or permissible noise levels are limited then hand digging may be preferred. The methods of supporting the sides of the shafts can depend on whether the excavation is by hand or by machine but as a shaft is, in broad terms, a short trench timbered at the ends, the type of support used is largely determined by the ground conditions. Thus in reasonably good ground, ie ground which will stand for one metre or more, support systems based on the poling board are used, and in bad ground, systems based on runners will be needed.

Use of poling boards in shafts

Figure 25. Methods supports in shafts

of side

The commonly used technique is very similar to that employed in trenching and the same length and section boards are used. The ground is opened up to a depth equal to the length of the poling board and the boards installed and held in position by walings. Walings may be installed at the mid point of the poling board (middle board method) as shown in Figure 25(a) or may be positioned at about the quarter points as shown in Figure 25(b). As in trenching, the waling frames are supported by puncheons which are positioned at the corners of the shaft and may be termed angle posts. Where an intermediate strut is used, then the puncheon is installed as in trenching practice.

(b)

(a)

Deep excavation by middle board system

Strut-,

Excavation struts

using double

35

The waling frames are normally installed by positioning two walings on opposing sides which run the full length of the shaft side. The other walings are cut to length between the inside faces of the walings already positioned and bear on timber stretchers or cleats spiked to them (see Figure 26, plan at B-6). The frame is then supported by angle posts positioned at the ends of the longer members. In square and rectangular shafts the walings act also as struts, and for large shafts it may be necessary to install additional intermediate struts (Figure 27). When the waling frames are positioned and secured and the poling boards tightened against the face of the excavation by wedges between waling and poling boards then the next phase of the excavation can proceed and a lower level of poling boards installed. During the excavation of shafts and at the completion of the installation, waling frames should be secured and supported by lacings, ground props and foot blocks as

figure 26. Shafts with maximum dimension up to 2.75m

225mm x 38mm poling boards 1800mm long

-.-__ 3DOmm x 200mm cross beams

150mm x 150mm uncheon or angle post

150mm X 150mm 225mm laclng

x 75mm 15Omm X 15Dmmframng

Plan at A.A

Frames of 150mm x 150mm

Stretcher _

225mm laclng

x 75mm

Cleats may be used as an alternatlve to stretcher

300mm x 200mm cross beam Plan at R-B

I

II

II

Section

I

I

N&es Method shown IS sultable for depths down to 6m Timber to be strength class SC4. No hydrostatic

head.

Where the shaft IS to be dug in cohesive ground It should be soft or better and should have a mInImum C value of 30 N/mm? A uniformly distnbuted ground load of 10 kN/mZ has been allowed Where higher loads, or point or line loads are expected, then the shaft should be designed by an expenenced person

36

appropriate. This method is used in excavating shallow shafts such as manholes. Proprietary systems are also available for this application and are used where there are few existing services to be considered. A variation in the use of poling boards is shown in Figure 26. In this method the lower level of poling boards overlap the inside face of the upper level by approximately 300 mm and the poling boards are slightly angled from the vertical to maintain the section of the shaft. Waling frames are positioned at this overlap which is tightened by means of wedges. Figure 26 also shows a method of supporting the waling frame against slipping down the shaft during excavation due to ground movement freeing the wedges or before the wedges have been correctly tightened. Substantial ground beams are laid across the mouth of the shaft which are long enough to give a good bearing on the ground on either side of the shaft. The upper waling is puncheoned off the second waling, which is attached to the beams by lacing. The beams then form the upper fixing to the lacings to which all subsequent waling frames are attached. Lacing is necessary since the installation of each new level of poling boards tends to undermine the previously installed support system causing obvious dangers. On very deep shafts a further pair of beams can be installed at a lower level. This is achieved by letting in the ends of four pieces of timber into the sides of the excavation. These are then joined together in pairs to form two beams to which the lower end of the lacing may be fastened. See Figure 26. This gives a very strong and stable support structure.

Figure 27. Shafts with maximum 5m

dimension

(a)

up to

~-__-_______-

20Omm

x 2!1Omm w;lllng

UN

i 20Omm x 75mm corbel head 20Ornin )i 200mm struts 200nim jj 200mm puncheons

1 Im

Note a/l Ivl~:lr:g XIOSS t?e shaft shoulcr be

Notes

4

‘\

\\,

-,

1. _

.’

l-

2OOmm x 2OOmm pllrjcheons

plan

See ‘Use of runners In shafts’ for erection sequence.

Method shown is suitable for depths down to 6m Timber to be strength class SC4. No hydrostatic head. Where the shaft is to be dug in cohesive ground it should be medium soft or better and should have a minimum C value of 30 N/mm’ A uniformly distributed ground load of 10 kN/m2 has been allowed. Where higher loads, or point or line loads are expected, then the shaft should be designed by an experienced person. 37

Use of runners in shafts

Figure 28. Shafts up to I.&n square

Another method is to use runners as shown in Figure 28. In this method runners, up to 3 metres long, are used and are driven to form the lower level support in a manner similar to that described in the trenching section. The ground is first excavated to a suitable depth depending on the type of soil. Vertical corner boards 75 mm thick are positioned and the first pair of walings are cut and installed between them. This is illustrated in the plan view of Figure 28 where A indicates the corner boards and 6 the walings. The other pair of walings, C, is then cut and wedged in position and retained by struts, D to form a rigid frame with a gap between the excavated face and the waling. Runners are then introduced into this gap and are driven to the first level of excavation, usually 1200 to 1500 mm to correspond to half the length of the boards being used, and firmly wedged against the soil from the walings. The shaft is then excavated to the first level of excavation and a lower waling frame installed in a similar manner. The wedges are progressively eased and the runners driven further or allowed to drop as the excavation is continued until the next waling frame can be positioned. This procedure is repeated until the boards are fuity driven and the lower waling frame installed, when lacings can be fixed. For further depths the same procedure is followed. This method has the advantage that virtually continuous support is given to the sides of the shaft and is recommended in situations of variable or unknown soil conditions.

225mm x 38 or 50mm poling boards

225mm

X 75mm

Sectron

Notes 1 Method shown

225mm X 75mm corner posts as long as fbrst depth of poling board

2 Timber

3 No hydrostatic

class SC4.

head.

4 Where the shaft IS to be dug In cohesive ground It should be medium soft or better and should have a mlnimum C vaiue of 30 N/mm? 5 i-1 untformly allowea

Plan

38

IS sultable for depths down ?o 4.5 m

to be strength

dlstrlbuted

ground ioad of 10 kN/m* has been

6 Where higher loads. or point or 11neloads are expected, ihen the shaft should be desrgned by ail expenenced person

Steel sections are often used as runners in the form of trench sheets or sheet piling. As a variation on the above method these may be driven to the full depth of the shaft before excavation and the levels of waling frames installed as the digging proceeds. The frames are usually in timber although proprietary systems or steel sections may be used where the circumstances are favourable or make it necessary.

Struts and walings

Shafts with maximum dimensions up to 2.75m and depth down to 6m The timber sections and methods of fixing as shown in Figure 26 are suitable for shaft sizes in this range. An alternative method is shown in Figure 28. Timber to be strength class SC4. Shafts up to 1.8m square and depth down to 4.5m The timber sections and method of fixing as shown in Figures 27(a) and (b) are suitable. Timber to be strength class SC4. In the case of the large square shaft there may be a need to keep the centre of the shaft clear to allow crane access or similar and the diagonal strutting system shown dotted in Figure 27(a) would be appropriate.

Chicago method

Figure

This method (Figure 29) was evolved for digging deep piers (in excess of 40 m deep) and is particularly suited for clays where there are no significant water bearing inclusions. A circular hole at least 1.4 m in diameter is excavated by hand for a depth varying from 0.6 to 2 metres depending on the strength of the clay. The sides of the hole are then lined with vertical timber boards. These are held in place by two circular steel rings. Excavation then continues to enable another set of boards and rings to be installed. At the founding stratum the hole may be belled out to increase the bearing area. It should be noted that the method has now been largely superseded by machines specially developed for this type of work.

29. Chicago method Lagging

Ring

39

Safety

The restricted working area and the single avenue of escape in shafts means that particular attention should be paid to safety precautions, standards of workmanship and relevant statutory requirements. Amongst others the following hazards must be considered: l l

the danger of gas accumulations at the bottom of the shaft the danger of an inrush of water into the restricted space of the shaft.

It is not suggested that these hazards apply only to shafts or that they represent the major dangers. They are listed only to draw attention to the fact that the geometry of a shaft tends to concentrate the hazard and its effect on the safety of men working within the shaft. As a result, safety precautions become of even greater importance and the following are some of the safeguards that should be considered: l l l

provision of a safety rope provision of a second ladder on the surface a ‘top man’ who should be on duty at all times.

With regard to standards of workmanship it should be borne in mind that shafts are often required as a first stage of driving a heading and are deeper and remain open longer than is normal for trenching. This means that even greater care should be taken to ensure that all components are correctly installed and secured section on Safety, p. 52).

40

against

ground

movements

(see

Headings

Headings are small tunnels cut into the sides of trenches or shafts. They constitute the most hazardous sector of temporary work as, in addition to the dangers associated with trenches or shafts, they introduce the risk of a collapse trapping men with no alternative escape route and with the avenues of rescue difficult and dangerous. The cutting of headings requires a very high degree of careful investigation and consideration. This applies to design, quality of material excavation techniques and workmanship as well as to the more obvious safety and supervision implications. An important difference between open excavation and headings is that in open excavations the temporary works are removed and the excavation backfilled or the temporary works are replaced by permanent supports; whereas in headings the temporary works are usually left in position. It is important therefore in headings that the timbers should be naturally resistant to decay or treated to give decay resistance. This will help to limit subsequent ground movement that would occur as the timber decays and the soil moves into the voids created.

Supervision

The supervision and routine checks involved in the safe operation of temporary works in a heading are basically different from those employed in open excavations in that the routine inspection must be carried out at the point of maximum risk - the advancing face of the excavation. Another important factor is that the speed of the excavation has a bearing on safety as the integrity of the cut face is time dependent and the rapid installation of a support structure is critical. Consequently, inspection and supervision must be carried out by personnel who are experienced in the class of work - preferably in a similar geological context - and who can inspect, approve and authorise continuation without impairing the speed of advance on which the safety of the work may depend. In practical terms the inspection and supervision personnel must be present as the work continues. This places more than usual reliance on the person in charge of the operation and on the skill of the workmen.

Sizes of heading covered by these recommendations

These recommendations apply to headings not exceeding 2 metres either in height or width when measured to the outside of the temporary works. When a heading approaches this limiting dimension it is advisable that a more comprehensive site investigation is made along the heading line and immediately to each side to ensure that there are no unforseen risks in continuing the excavation. Any headings whose dimensions are greater than 2 metres must be designed and supervised by a competent engineer. These recommendations specifically exclude: headings in rock (for this purpose rock is defined as any material beneath weathered rockhead which cannot be efficiently excavated using a pneumatic clayspade) headings which require the use of explosives in their driving. 41

Materials encountered in driving the heading

Soils may be classified in terms of gravel, sand, silt and clay depending on the particle size. The loads borne by the temporary supports are dependent on the materials through which the heading is being driven. A safe approximation (except for swelling ground) sometimes used is that the load carried by the temporary supports closely approaches the weight of the material above the heading as a hydrostatic toad. In other cases, previous local experience provides an average earth pressure. Alternatively, methods of assessing the load on the heading in both the long and short term are discussed in standard text books on soil mechanics and tunnelling such as Tschebotarioff (1973). As has been mentioned the driving of a heading is an activity which is crucially time dependent. The practicability and the resultant safety of the work depends on how long the excavation ahead of the already supported part will stand without support. Experience shows that during excavation the behaviour of the ground can be considered as falling into six broad categories. firm. Where a face can be advanced and a fresh setting erected without any noticeable change taking place in the excavated space. Ravelling. Usually a damp or cohesive sandy gravelly material. Initially, a void can be excavated but immediately pieces start to fall in. Unless this is stopped, the roof caves upward until, in effect, a pair of opposing corbels are formed meeting above the roof of the original excavated volume. This cavity must still be supported. It is to be noted that some materials, for example London Clay, initially appear to be firm but if left unsupported for some time they are converted into ravelling ground as small pieces defined by an intersecting network of fissures start to fall, first from the roof and later from the sides and face, Running. Where dry granular material moves into the tunnel from an unsupported face. Once a slope has formed at the angle of repose of the material, the running ceases. Some lightly cemented or slightly cohesive sands are in the firm category when they are freshly dug. Given time and exposure to air they are rapidly changed into this third class. F/owing. Saturated silts, sands and fine gravels ‘flow’ through gaps in the lining with water seepages. Such ground entry can only be checked by maintaining the end and sides of the heading closely boarded up with timber and packing sufficiently strong to withstand the loads involved. Dewatering ahead of the drive may be necessary and the use of a drive shield may be warranted. The difficulty of excavating a heading depends to a large extent on the position of the groundwater table. If it is possible to lower the groundwater table adjacent to the heading, this may convert flowing ground into ravelling or firm ground. Alternatively, air pressure may be applied in which case the design and supervision must be undertaken by a suitably experienced engineer. Squeezing. Where a clay is soft or of insufficient immediate undrained shear strength to support the overburden load. When exposed, such clay will be extruded through any gaps in the heading support and therefore close boarding, and perhaps back packing, will be required throughout this ‘squeezing’ material. The use of a closed shield may even be necessary at the driving face and even air pressure. Such procedures are outside the scope of this document and in such cases specialist advice

should be taken. Swelling. Some grounds (notably with certain clay minerals) have high swelling characteristics. Heading supports in such ground will have to support high pressure which, depending on time and groundwater conditions, can be well in excess of the simple (and otherwise safe) overburden and surcharge pressure. 42

To summarise, headings should be driven with the greatest possible speed consistent with safety. This reduces the complications brought on by giving the ground a chance to change its nature. Any change is certain to be for the worse. Speed and safety result from the prior experience of the workmen involved and in the use of proven working methods and practices.

Timber t0 be used in the heading

All timber used in the construction of a heading shall be to strength class SC4 or better. All timbers used in the construction of a heading which will be permanently left in situ shall be new material to strength class SC4 or better and shall be protected against decay in accordance with BS 5268 Part 5: 1989, Table 5, Section 1a. This gives a number of options relating to the use and desired service life of the timber. The most appropriate for this application is ‘Timber in bridges, towers, masts, framework piling - in contact with the ground’ with a desired service life of 40 years. The need for and level of preservative treatment depends on the species of timber - guidance is given in the standard. Any timber not complying with these requirements shall not be brought into the heading. When it is necessary to cut a piece of treated timber for use in the heading, the cut face(s) shall be treated by immersion in, or brushing on a copious amount of, preservative supplied by the manufacturer of the preservative used in the impregnation process. All timber shall be inspected for damage or other strength reducing factors that may have occurred after the stress grading operation. Any timber showing such damage shall be indelibly marked as rejected. The minimum dimensions of the timber components illustrated in Figures 30, 31, 32 and 33, shall be Side trees Head trees Side boards Crown boards Waling boards Stretchers Sills Chogs

225 mm 225 mm 150mm 150mm 150mm 150 mm 225 mm 50 mm

x x x x x x x x

75 mm 75 mm 38mm 38mm 38mm 38 mm 75 mm 50 mm

If the span of the head tree exceeds 900 mm then a centre prop must be used or the thickness of the head tree increased. If the unsupported span of a strutted head tree still exceeds 900 mm then the dimensions of the head tree shall be determined by calculation from the depth of the ground above the heading and the stresses appropriate to the strength of class of the timber being used. The section of timber to be used must be determined before work commences.

Dimensions alignment

and

The line and depth of the heading should be shown on a drawing which can be agreed between the contractor, engineer and any other interested parties before work commences. Typically the minimum dimensions of a heading would be: height not less than 1.2 m measured from the top of the sills to the underside of the head tree width not less than 700 m measured between the inside faces of the side trees the length, centre to centre, between one setting and the next is approximately 1.2 m. 43

Excavation and timbering

It has been emphasised that the greatest reliance is placed on the experience and expertise of the men engaged in driving a heading and they will have developed methods and practices to cope with whatever variations of soil and conditions are encountered. It is not therefore considered appropriate in this publication to recommend any sequence of operations and comment is limited to outlining established methods by a few broad statements. The support to headings can be divided into the following main divisions. Box setting This method is illustrated in Figure 30. It is only suitable for small headings in firm ground. Normally roof and side boards are not required, but in localised areas they can be inserted by ‘tucking’ between adjacent head or side trees and the excavated face. Poling setting This method is illustrated in Figure 31 and should be used in ravelling ground. A poling setting is an extension of the box setting with roof and side boards ‘tucked’ between the head and side trees so that both ends are supported. The boards should be in close contact with the top and sides of the heading and may be spaced apart or be in edge to edge contact depending on the nature of the ground. In ground where the floor of the heading may boil or heave then floor poling boards should also be introduced between sills. Piling setting In poor ground which falls into the running or flowing category, continuous support at all stages of the excavation must be maintained and the piling setting method used. The main points of this method are shown in Figure 32 but as it can be made even more complicated when the advancing face of the heading also needs continuous support, an illustrated sequence of operations to cover this eventuality is shown in Figure 33. The leading end of the crown or side piling board should be chisel shaped so that the boards can be driven step by step into the advancing face. The boards are positioned and guided from a fixed setting which has waling boards positioned outside the side and head trees and blocked off from them to form a gap through which the piling boards can be driven. The boards are pitched and driven forward all round the heading and the soil removed to a depth that ensures the board ends are still firmly embedded in the face. A temporary setting is installed to support the board which can then be driven further into the face. Further driving and supporting is carried out until the next permanent setting can be fixed, and the whole procedure re-started. To avoid loss of material from behind the boards, the gaps between the boards should be plugged. Straw may be used for this purpose.

Securing the heading

44

Regardless of which method is used, the excavation of a heading should not cease until a setting under construction is completed with head and side trees in position. The face of the heading should be boarded if the excavation stops for more than two hours. The timber used for this boarding is identical to that used in the crown and side boards and should be firmly wedged to the face from the last pair of side trees.

Figure 30. Box setting 7

Head tree

-

,

/ISide

trees j\

In good rock the settings may be spaced apart with longitudinal struts

Only sultable for small headings In good ground. Roof and side boards may be added where necessary (See

Breasting boards wedged off last setting If required

poling

setting.

Figure

Section through box setting

31)

Figure 31. Poling setting

Yankee

Brob

Poling boards

Stretcher

Stretcher (ai Wlthout

floor

Stretcher tf required

ik) With floor Necessary where bottom 15 soft and has a tendency

(cl Section

through

poling

of head@ to heave

setting

45

Figure 32. Piling setting Wedges between waling and plllng boards If required Top waling bo,trd

Figure 33. (Below, facing page and overleaf) Installation of piling settings

Stage 1 Hoies drllled through sheeting ‘letter

box’

The shaft IS dug to required level. A bent is set up in front of the sheeting with a small gap The bent is securely braced (note: all bracing has been omItted for clarity). Holes are drilled through the sheet to facilitate removal at a later stage.

46

Temporary & wedges

Stage 2

timber

Boards driven through ‘letterbox’

Remove ‘Letter Box’ in sheeting above the headtree. Drove the poling boards Into the ground for about half their length. Boards to have an upward Inclination of 50 mm in 300 mm Wedge ends of boards off a temporary timber.

Stage

3

Break out sheeting down to next line of holes. Carefully rake out ground. Erect a false set about 700 mm from the sheeting. The false set is a “horse head” and is shown below.

Poling

boards

Horse

head

Centre

prop

Sole plate

Stage

4

I

Breasting board wedged under end of poling board

Drive poling boards to their full length. Carefully rake ground into the shaft and as the end of each board is uncovered set up a breasting board about 450 mm long vertically under the ends. This provides support and prevents more ground entering.

47

figure

33 -

continued

Stage 5

DETAIL Showing atternatrve using Yankee Brob.

method

:::B

C

_---

-----_ ------

1

----_ -----------_-----

Set the next centre, line Dnve all the tapered and

Stage

-1

i _I

false set in its correct positlon for centre to and level. It is supported by a centre prop. srde poling boards Note that some boards are drrven big end ftrst

6

_-----_-----__ -------------__------_

Support the new false set on two booms asshown. whrle the remainder of the ground is taken out The booms should be structural hardwood, about 200 x 15Omm and 3 metres long The back of the booms are wedged as shown to give a cantilever support to the false set Remove temporary breasting boards one at a time and rake out

48

i

------

Stage 7

Whrle new false set IS temporarrly supported or two booms, insert new bent Remove booms Drive polrng boards to half way and wedge, then carefully rake out ground Erect a false set about 700 mm In. Dnve polrng boards to therr fuil length Carefully rake ground rnto shaft and as the end of each board IS uncovered set up a board vertrcally under end.

Stage 8

Set the next false set in Its correct positron. supported by a centre prop Drive all the side poling boards Support the new false set on two booms as shown, while the remainder of the ground IS taken out. The back of the booms are wedged as show,n to grve a cantrlever support to the cap. Remove temporarv breasttng boards one at a trme and rake out

Lighting and ventilation

Adequate temporary electric lighting must be installed in the heading to provide illumination for the proper execution of the work. The voltage employed should not exceed 55 volts to earth. This can be achieved by a centre tapped 110 volt system. An efficient air blowing system may be required in lower headings, and if this is necessary it should be provided with fans and ducting as appropriate.

Workmanship

At all stages in the driving of a heading particular attention must be paid to ensure the highest possible level of workmanship is achieved.

Safety

The need for the strict observance of safety precautions and any relevant statutory requirement in the cutting of a heading is self evident and has already been emphasised (see section on Safety, p. 52).

49

Workmanship in excavations

The object of sheeting and strutting excavations is to protect workmen within them and to limit ground movement which may cause damage to adjacent property and services. To achieve these objectives good workmanship is essential at every stage installation, use and dismantling. The work should be planned and all necessary materials obtained before work commences. The operatives engaged in installation should be given clear instructions, preferably in writing and in the form of drawings or sketches. The work must be under the direct control of competent persons experienced in similar work. The supervisor should be present as the work continues since speed of installation is essential and the work should be checked as it goes. The timbering must be inspected regularly and maintained as necessary. Dismantling must be carried out in a controlled manner with proper attention being paid to the quality of the backfill material and the method used for placing and compacting it. Tidy, neat work is easier to inspect and makes it less likely that essential points are overlooked. The use of check lists is strongly recommended.

The frequency of checking must be done with due regard to the circumstances and it is particularly important after holidays and extreme weather

Materials

conditions

as frost or heavy

rain.

All materials should be checked for compliance with the quality specified. Wastage of all materials should be kept to a minimum; when lengths of timber require cutting this should be planned to avoid wastage. Re-use should be considered whenever possible. Tools should be kept sharp and in good order and the cuts made cleanly with the timber properly supported. It should be noted that timbering is an item of temporary works and hence may not be covered by the material and workmanship clauses of the main contract. Therefore main contractors must specify their exact requirements in sub-contract agreements and should not merely refer to the main contract

Construction use

such

and

specification.

The integrity of any excavation support is dependent on the quality of the construction as well as the adequacy of the design. The excavation supervisor and the excavation works co-ordinator should be knowledgeable and experienced in similar work and must be familiar with all aspects of the working drawings. They should be aware of any assumptions made in design and should refer any changes from these assumptions to the designer for his consideration and checking. In particular, the soil and groundwater conditions should be checked to ensure that they are the same as expected by the designer. The face of the excavated ground should be cut as evenly as possible to allow the support system to fit snugly and thus reduce potential

movement of the ground. All operatives should be alert as the excavation progresses for indications of distress such as cracking and subsidence of the ground

50

near the excavation.

It is important that the supporting members are the size and spacing shown on the approved drawings. Any sequence of operations shown on the drawings for installation or dismantling must be followed to prevent the possibility of over-stressing the support system and/or the ground. Full bearing at the end of jacks and struts must be ensured and particular attention should be paid to wane at bearing areas of timber members. All struts must be secured by puncheons, lipping blocks or wires. Members should be ‘eyed-in’ for alignment, ie straight, plumb and perpendicular. Items such as hydraulic and mechanical struts and jacks, when used, should be installed in accordance with the approved drawings and the manufacturer’s recommendations. If a tie back system is used to retain sheeting, the tie-backs should be uniformly placed and evenly adjusted to avoid over-loading individual ties. Timber is a construction material which may be readily altered on site in order to deal with enforced changes caused by unexpected circumstances. No changes should be made without the knowledge and agreement of the supervisor responsible. Considerable care should be taken when altering or dismantling members which are carrying loads. The load should be relieved by loosening wedges (after first ensuring that the member will continue to be held in position) or by cutting out packers specially included for this purpose to avoid cutting the main member. Timber has the valuable characteristic of sustaining considerable deflection before breaking and creaking audibly as it sustains heavy loads. Both these effects should be used for monitoring timber supports and where obvious overload is occurring additional members must be used.

51

Safety

Safety in relation to excavations is particularly important, bearing in mind the variable nature of the ground - often changing significantly in short distances. Because of this and a poor accident report in the past, excavation work has attracted both general and specific legislation. All those concerned with excavations and their temporary support, need to be fully aware of the law and their responsibilities in relation to it.

Legislation

The prime legislation is the Health and Safety at Work etc. Act 1974. This Act came into being following the Robens Report which examined all aspects of health and safety in relation to employed persons. It broke away from the traditional approach, in that it is structured to last a long time by being flexible enough to deal with rapidly changing problems of occupational health and safety. Under the 1984 Act, general obligations (Sections 2-9) are placed on employers, sub-contractors, the self employed, manufacturers and installers of plant and equipment, manufacturers and suppliers of materials and employees. Employers also have duties to safeguard persons other than their employees, eg the public. At the same time those who control premises have a general duty to safeguard the health and safety of all those who may require to make use of or work within such premises. The Act in no way invalidated the existing construction regulations and it is the intention that approved Codes of Practice will gradually be issued, with the authority of the Act, to supplement such Regulations. A summary of the general duties required under the Act is given in Table 13 adapted from RoSPA Construction Regulations Handbook 1976. In addition to the Act, the following Construction Regulations have relevance to safety in excavations: Construction Construction Construction Construction Construction

(General Provisions) Regulations 1961 (Working Places) Regulations 1966 (Lifting Operations) Regulations 1961 (Health and Welfare) Regulations 1966 (Metrication) Regulations 1984

A summary of the items pertinent to excavations in the above is given in Table 14 reproduced from RoSPA Construction Regulations Handbook 1976. The Control of Substances Hazardous to Health Regulations 1988 (COSHH) apply to work on site. An explanation of the ways in which the legislation affects site operations is given in The control of substances hazardous to health in the construction industry, prepared by the Construction Industry Advisory Committee and endorsed by The Health and Safety Executive.

Responsibilities

52

Employers’ general duties, in relation to employees, are covered under sections 2,8, 9 of the Health and Safety at Work Act; to the public under Section 3 and where in control of premises (including sites) Section 4. Employees’ duties come under Sections 7 and 8. Suppliers of plant and equipment, erectors and suppliers are covered in Section 6.

Table

13. Genera/

duties

required

under

the Health

and Safety at Work etc. Act

Duties of employees

Powers of inspectors health and safety

Every person employed has the following statutory obliaations: (4 ‘io comply with the Construction Regulations, and other statutory requirements so far as they affect any act performed by him. (b) To co-operate in the carrying out of all Regulations. To report to the occupier, foreman or other authorized person, any defects he discovers in any scaffold, plant or appliance. (4 To use any equipment, appliance, etc, provided for his health or safety under the Act or Regulations.

The main statutory powers are given in HSA W Section 18, 19 to 26. and 39: To enter, examine and investigate at any reasonable time or at any time in a dangerous situation. To require any premises (or part) or anything in premises within his field of

FA 743

Persons employed must not interfere with or misuse any appliance, etc, provided under the Factories Act or any Regulations.

FA743

Persons must not, wilfully and without reasonable cause, do anything likely to endanger themselves or others.

FA143

An employee who fails in his legal obligations is liable to prosecution. This does not necessarily free the occupier or contractor from the liability to prosecution also.

FA743

of

responsibility to be left undisturbed. (cl To cause any article or substance in the premises to be dismantled or tested. Cd)To take possession of and detain the article or substance in (c) above long enough for examination or for production in legal proceedings. 69 To require a person to give information and sign a declaration. To take measurements, photographs and samples and to require production of relevant documents. (9) To prosecute in a magistrate’s court and issue improvement and Prohibition Notices. (“1 To seize or render harmless any article or substance which is a cause of imminent danger or serious personal injury.

Other relevant provisions of the Act which apply to construction work There

are general

duties

on

HSAW

employers for the health, 2, 3, 3, 9 safety, welfare, training, supervision, etc. of employees. Provision and maintenance of safe plant, the safe use, handling, storage and transport of articles and substances. A duty on employers and selfemployed to protect persons other than employees from risks to health and safety. Duties of designers, manu- HSAWG facturers, importers, erectors and suppliers of articles and substances for use at work. Generally speaking, these should supply articles (including plant and components) which are safe when properly used, and adequate information. In addition to court proceedings, Inspectors can issue Prohibition and Improvement Notices. Provisions for appeals against notices.

HSAW

20-25

HSAW Duties on people who control premises (including 4 sites) where others work. Anybody who has a contract, subcontract including maintenance, repair and other hazards. Power to make regulations, HSAW approve codes of practice 15, 16, 17, 50 and to use codes of practice to prove offences in criminal proceedings. An employee to take reasonable care for the safety of himself or others and co-operate with his

HSA W 7, 3

employer. A duty on all persons not to intentionally or recklessly interfere with or misuse anything provided in pursuance of the requirements for health, safety and welfare. Abbreviations FA Factories Act 1961 HSAW Health and Safety at Work Act 1974 C(GP) Construction (General Provisions) Regulations 1961

C(/fW) C(WP)

Construction (Health and Welfare) Regulations 1966 Construction (Working Places) Regulations 1966

A self-employed

person

to

conduct his undertaking so that neither he nor anyone else is exposed to risks to health and safety.

t/SAW

3(Z), (3)

53

Table 74. Summary of Construction Regulations: excavations, shafts, earthworks and tunnels Inspections and examinations

Materials and equipment Adequate supply of timber or other suitable material to be provided for timbering.

C(GP)B

Machinery to have dangerous parts securely fenced unless safe by position or construction.

C(GP)42 and 44

Certificate of Exemption No. 2 (General) 1966 (Form F2209) exempts crawler tracked shovel excavators and crawler tracked drag-line excavators, adapted for use as cranes, when used for excavating. The exemption is from testing and examination of cranes (Reg. 28): marking of safe working loads (Reg. 29): and automatic safe load indicators (Reg. 30), and is subject to the conditions of the First Schedule of the Certificate.

C(GP)lO(l}

Barriers to be provided round excavations, shafts, pits and openings into which persons can fall a vertical distance of more than 6ft. 6in. (1.980m.)

C(GP) 13

Barriers to be as close as practicable the edge of the excavation.

C(GP) 73

to

Barriers to be erected as soon as practicable after excavating begins.

C(GP)lS

Barriers to be maintained in position except when necessarily removed for the access of persons or material.

C(GP)13

Timbering

* f Adequate and suitable material to be

* Every part of any excavation, shaft, earthwork or tunnel to be inspected by a competent person at least once every day that persons are employed there. * Face of every tunnel and base or crown of every shaft to be inspected by a competent person at beginning of every shift.

Fenong

C(GPJ8

used to prevent danger from falls or dislodgment of sides of excavation, etc. or materials adjacent to it. (Unless nature and slope of sides is such that no dangerous fall is likely to occur). :- Timbering to be done as early as practicable in the course of the work.

C(G P)8

Persons engaged in timbering lo be protected as far as possible from danger.

C(GP)8

Timbering or other supports to be of good construction, sound material, free from patent defect, and of adequate strength for the purpose.

C(GPJlO(2)

Struts and braces to be properly and adequately secured to prevent accidental displacement or fall.

C(GPJO(3)

Competent person to direct the erection and dismantling of all timbering and other supports, as well as any subsequent alterations additions.

C(GP)lO(l)

Experienced workmen to be employed, as far as possible, for the erection, dismantling, etc., of timbering and other supports.

Material for timbering and other supports by competent person before being taken into use.

to be inspected

* Working end of every trench more than 6 ft. 6 in. (1.980m.) deep to be inspected by competent person before beginning of every shift

C(GP)g(lJ

* No shaft, been affect been

C(GP)9(2a)

person to work in any excavation, earthwork or tunnel after explosives have used in or near it, in a manner likely to stability, until a thorough examination has made by a competent person.

* No person to work in any excavation, shaft, earthwork or tunnel unless it has been thoroughly examined by a competent person within the previous seven days.

C(GP)9(2c)

* No person to work in any excavation, shaft, earthwork or tunnel after an unexpected fall of rock, earth or other material, or after substantial damage to timber or other supports, unless the part concerned has been thoroughly examined by a competent person.

C(GP)9(2b)

* Reports of examinations to be made in the form prescribed by the Construction (General Provisions) Reports Order 1962 (SI 1962 No. 224). Certain modifications to these requirements are made when the operation concerned is not expected to last for more than six weeks.

WPW)

on day of examination,

Other precautions

or C(GP)lO(l)

Excavations lighted.

and approaches

C(GP)47

to be well

Materials not to be placed near the edge of an excavation, etc., so as to endanger persons below.

C(GP)l4(1)

No load to be placed or moved near the edge of an excavation, etc., where it is likely to cause a collapse of the side of the excavation and so endanger any person.

C(GP) 14(2)

t If excavation is likely to affect the security of another structure (permanent

C(GP)72(1)

temporary) steps must be taken persons employed from possible structure. I- Means

of reaching

a place

or

to safeguard collapse of that

of safety

C(GP)ll(l)

to be provided (as far as practicable) when there may be danger from rising water or irruption of water or material. Means to prevent overrunning to be taken when vehicle is used to tip material into pit or excavation or over the edge of embankment or earthwork.

C(GP)37

Atmosphere to be well ventilated free from dust, fumes, etc.

C(GP)ZI

and

* These items do not apply to excavations, shafts or earthworks in which no one is liable to be struck or burled by a fall of earth from a greater height than 4ft. (1.210m). Nor do they apply to persons entering an excavahon, etc.. to make it safe or to conduct an examination or inspection, so long as all possible precautions are taken for the safety of such persons (C(GP)S(I) and C(GP)9(4)). : In the case of tunnelling operations in works 01 engineering construction no person can be held to have disobeyed these requirements if failure to comply was due to physical conditions which were beyond his control and against which it was not reasonably practicable to make provision.

54

The above general responsibility is made more specific, as far as employers of labour are concerned, by regulation 3: Construction (General Provisions) Regulations 1961; regulation 3: Construction (Working Places) Regulations 1966; regulation 3: Construction (Lifting Operations) Regulations 1961 and regulation 4: Construction (Health & Welfare) Regulations 1966. Further advice on the relevant legislation and regulations together with good working practice may be found in the National Federation of Building Trades Employers' Safety Manual. However, some specific comment is desirable in relation to Section 4 of the Health and Safety at Work Act. Section 4 imposes on persons duties in relation to those who: (a) (b)

are not their employees: but use non domestic premises made available to them as a place of work where they may use plant or substances provided for their use there.

Section

4 (2) states:

It shall be the duty of each person who has, to any extent, control of premises to which this section applies to take such measures as it is reasonable for a person in his position to take to ensure, so far as is reasonably practicable, that the premises, all means of access thereto or egress therefrom available for use by persons using the premises, and any plant or substance in the premises or, as the case may be, provided for use there, is or are safe and without risks to health. Equally, anyone who has a contract or tenancy in maintenance or repair of any premises or any means of or egress therefrom, or the safety of or the absence of arising from plant or substances in any such premises,

relation to the access thereto risks to health

is also treated as a person who has control of premises and Section 4 (2) will apply. The effect of this section is to impose on the main contractor, or whoever has control of premises, the additional obligation to ensure that all subcontractors, or other firms or persons working on the site, carry on their operations in a manner that does not create any risk to health or safety of anyone on the site. Thus, while any employer on site is responsible for his own men under Section 2, whether or not the hazard has been created by him - and equally to others not in his employ under Section 3 -the person who has control of the premises must ensure that others have fulfilled their legal obligations. The responsibilities of contractors, sub-contractors, employees etc. under the COSHH legislation are explained in The control of substances hazardous to health in the construction industry.

Check list

aides m&moire and help to ensure that nothing is overlooked in relation to safety, check lists of action required before and after excavation commences are provided in Appendix 2. To provide

55

Support of excavations in Europe

A research project undertaken by TRADA in 1985, with support from the Commission of the European Communities (CEC), entitled ‘More efficient use and re-use of timber in temporary works’, included surveys by questionnaire in sixteen West European countries. This showed quite clearly that there is a commonality of methods in use for the support of excavations in Western Europe. However, local variation can be quite wide, primarily due to the local ground conditions in individual countries. Timber accounts for a significant proportion of the materials for excavation support, particularly in some countries, such as Austria, where some contractors seek economy by using for trench sheeting the stronger types of timber based board products which have outlived their usefulness as form liners. In particular, triboards (22 or 27 mm thick), film faced plywood (19 mm thick, or more) and Douglas fir or softwood plywood are used, sometimes doubled up for extra strength. Such an approach may well be worth consideration in other countries; the economics will depend upon the price factors in any individual country.

Legal requirements available standards

56

and

In general in Europe, the contractor responsible for the excavation is required by industrial safety legislation to ensure the safety of his workers and that of the general public. However, the degree of detail and the stringency of any legislation or regulations which apply specifically to excavation work varies widely from one country to another. The strictest requirements appear to be in Austria and West Germany. In Austria, a Federal Decree (DSV) requires the support of all trenches more than 1.25 m deep; while in West Germany, virtually all trenches more than 1.5 m deep must be close sheeted and fully supported. Hydraulic trench struts are forbidden, as they are considered to be potentially unsafe. In Switzerland, regulations for the support of excavations come from the Swiss Insurance Underwriters Institute (SUVA). Contractors using their services must support all excavations deeper than 1.8 m and expect a visit from a SUVA inspector. In the UK, the Construction (General Provisions) Regulations 1961 require that support must be provided to all trenches and other excavations where falls of earth or other material can occur from a height of more than 1.21 m. Legislation apart, some countries have national standards for support work. In Austria, 0 Norm 82503 covers trenches and 82203 deals with tunnelling. In West Germany, DIN 4124: 1981 operates, while in the UK BS 6031: 1981 is relevant. In other countries, such as Luxembourg and the Republic of Ireland, contractors generally use the national standards of neighbouring countries. Contractors in Belgium and Greece have no national standards for the support of excavations. In order to protect their workers they rely on their experience, on the designs of their own engineers, or else on the guidance of the manufacturers of proprietary equipment systems.

Table 15. Frequency of use by percentage of fhe various support methods for narrow excavations in Western Europe

Method Country

Jjmbers and sheeting

H piles with sheeting between

Austria Belgium (2) Denmark FR of Germany France Greece R of Ireland Italy (4) Luxembourg Netherlands Scandinavia Spain Switzerland UK

45 10 20 70 (3) 70 55 5 50

10 5 40 15 15 -

35 25 95 25 15

15 10 10 5 30 5

Steel trench sheeting (1)

Proprietary systems

20

35 15 -

-

-

5 60 20 10 15 45 75 -

Sheet piling

5 -

5

10 50 5 20

10 35 45 20 20

-

10 20 45 -

25 55

20 5

Notes (1) Due to translation difficulties there may be some confusion between sheet piling and trench sheeting (2) The remaining 5 per cent of support is provided by lost concrete sheets (3) Planks are usually placed vertically (4) Figures based on only two questionnaire returns

Table 16. Frequency of use of various support methods for wide excavations in Western Europe

Method Country

H piles with sheeting between

Austria Belgium Denmark FR of Germany France Greece R of Ireland Italy (2) Luxembourg Netherlands Scandinavia Spain Switzerland UK

25 10 20 60 35 50 5 15 5 45 (3) 25

Sheet piling (1)

Steel trench sheeting (7)

Permanent concrete walling

40

35

-

90 80 40

-

-

30

50 15 25

35 50 -

50 100 80 60

-

100

5 35 60

15

90 20 -

Notes (1) Due to translation difficulties there may be some confusion between piling and trench sheeting (2) Figures based on only two questionnaire returns (3) Pre-cast concrete slabs used for sheeting on 95 per cent of sites

sheet

57

Table 17. Re-uses with components for excavations support

Number of replies

Number of reuses Minimum

Maximum

Mean

Median

13

1

180

37

8

13

1

20

7

2

Timber walings narrow/wide excavations

20

1

100

24

9

Timber narrow

12

1

50

14

9

obtained

Timber sheeting generally planks in all-timber systems narrow excavations Timber sheeting generally planks between ‘H’ piles narrow/wide excavations

Methods of support used

struts excavations

reported

In establishing

the methods of support used in any particular country, into two categories: narrow and wide. A series of support types in common usage in Western Europe can then be related to these categories. it is useful to divide excavations

Narrow excavations A narrow excavation is more properly called a trench, ie its length greatly exceeds its width and its width does not exceed 5 m. Unless battered sides are possible, vertical sides requiring support and strutting from side to side will usually be involved. An assessment of the frequency of use of the various types, based on replies to a questionnaire, is given in Table 1.5.

Wide excavations Wide excavations can be defined as excavations whose width exceeds 5 m and in plan are of a shape that can be infinitely variable. In other words, the definition covers situations from extra wide trenches to large area deep basements whose shape is defined by local circumstances. An assessment of the frequency with which the various types are used, based on replies to a questionnaire, is given in Table 16.

Re-use of timber in excavation support

Table 17 summarises the replies received about the number of times that contractors were able to re-use common timber components in excavation support work. As might be expected, they were very wide in their range. Because of the inclusion of some very large estimates in several of the distributions, the median figure is likely to be the most realistic.

Appendix 1: Timber

Timber is a natural material and is subject to a wide range of variations including growth defects which can significantly affect its properties and strength. Obviously very little can be done to control the formation of these defects but careful conversion into sawn lengths and subsequent selection can reduce their effect. Many years of experience and research have resulted in the establishment of proven working stresses for any defect or group of defects. This means that timber can be used in the same predictable, economic and reliable way as any other construction material provided that advantage is taken of the techniques and information now readily available.

Stress grading

Historical Timber without strength reducing defects is known as ‘clear timber’ and is becoming increasingly scarce and costly and therefore no longer viable as a construction material. Over the years an approach has been adopted whereby timber containing strength reducing defects can be safely used providing the defect is specified and controlled. Timber containing such defects is allocated a working stress as a percentage of the ‘basic’ stress appertaining to the ‘clear’ timber. Thus timber containing defects which have been assessed as reducing its strength by half would have a working stress of 50 per cent of the basic. As this method grades timber according to its strength, the process came to be known as stress grading. As the permissible grade working stresses were calculated as a percentage of the basic stress, the percentage figure was used to define the grade. Hence 40, 50, 65 and 75 grades were established with each grade reflecting the percentage of the basic stress as its permissible working stress. Consequently these grades came to be known as numerical grades. Comprehensive grading rules by which defects were specified were also established and, as all grading at that time was entirely visual, were called visual grading rules. The visual grading rules and the associated numerical grade working stresses were incorporated into CP112: Part 1: 1967 The sfructural use of timber. This was subsequently metricated as CP112: Part 2: 1971. Experience showed that too many grades were available and in 1973 BS 4978 Timber grades for structural use was published which basically limited the number of grades to two; Special Structural (SS) and General Structural (GS). New working stress values for the new grades were given and differentiation made between tensile stress and bending stress which previously had been treated as the same. A new method of limiting the extent of knots was introduced. This is known as the ‘Knot Area Ratio’ or KAR method and requires that the extent of the knot within the cross section is estimated so that the ratio of the area of knots to that of clear timber in any cross section can be established and the timber graded according to specific rules given in the standard. Around 1960 it was confirmed that a relationship existed between the modulus of rupture and the modulus of elasticity of timber. This offered the possibility of producing a machine which by applying a small load to successive short lengths of timber and measuring the resulting deflections, allows the strength of the timber to be assessed and the timber 59

to be automatically graded. Several machines were successfully developed and machine grades were also incorporated into BS 4978. These grades are called MSS and MGS and correspond to the visually graded SS and GS. Grades M75 and M50 were also introduced, corresponding to the 75 and 50 grades of the numerical system. All these changes were introduced into CP112 with the adoption in 1973 of Amendment Slip No. 1 (AMD 1265). This amendment continued the concept of ‘Softwood species groups’ which grouped commonly used softwoods into three classifications Sl , S2 and S3 with Sl comprising the stronger timbers. These species groups could be applied to all the stress grading methods so that a grade S2-GS would indicate S2 species group of GS grade. Similarly S2-50 and Sl -MSS indicated both species group and type of grading. As redwood/whitewood was in the S2 species group, S2-50 became a common classification in the construction industry. For an undefined period it was intended that the numerical, SS/GS and machine grades should operate concurrently and that the numerical grades would gradually be phased out. To a large extent the changes in grading rules that have been outlined so far have been concerned with the methods of determining the extent and effect of knots occurring in the timber. Although knots may be considered the major and most frequently occurring strength reducing factor, other natural features such as fissures, that is separation of the fibres caused by drying (also known as splits and checks), slope of grain and rate of growth also affect the strength of the piece. Methods of assessing and limiting these features are given in BS 4978. Limits have also been placed on those features that affect the suitability of the timber in use. These consist of specifying the permitted level of wane, which affects the bearing properties of the piece when used as a beam or joist, and in defining the extent of twist, spring and bow which are distortions that can occur in sawn timber during the drying out process. These limitations are fully detailed in BS 4978.

Current position A revised edition of BS 4978 was published in 1988 under the title Softwood grades for structural use. The major change was the requirement that all timber graded to the standard, whether visually or by machine, must be covered by approved third party quality assurance. Other significant changes were the requirement to include the species or species combination in the marking of stress graded timber and the inclusion of the North American system for control of stress grading machines. BS 5268 The structural use of timber Part 2 Code of practice for permissible stress design, materials and workmanship, first published in 1984, with a second edition in 1988, abolished the numerical grades and introduced a system of ‘strength classes’. There are nine strength classes from SCl, which comprises the weaker softwoods, through to SC9, allowing for the strongest hardwood. The full range of working stresses and moduli of elasticity are given for each strength class. Timbers are allocated into strength classes according to the species, or species combination and grade, ie the weaker grades of strong timber species are grouped with the stronger grades of weak timber species. BS 5268 Part 2: 1988 refers to six sets of stress grading rules: l

For softwoods BS 4978 Softwood grades for structural use (covers both visual and machine grading; also allows timber to be machine graded directly to BS 5268 Part 2 strength class limits) ECE Recommended standard for stress grading of coniferous sawn

60

timber (covers BS 4978)

visual and machine grading; grades equivalent

to

National grading rules for dimension lumber (NLGA), Canada (visual grading in Canada; grades equivalent to USA grades) National grading rules for dimension lumber (NGRDL), USA (visual grading in USA; grades equivalent to Canadian grades) North American export standard for machine stress rated lumber (machine grading in Canada and USA) l

For hardwoods BS 5756 Tropical hardwoods graded for structural

use.

Table 18 shows the species or species groups and grades which satisfy the requirements for strength classes SC1 -SC5 These strength classes cover the softwoods most likely to be used in excavation support. Table 19 gives the physical characteristics for all strength classes.

Moisture content

Timber is a hygroscopic material and the amount of water present within its structure has a significant effect on its structural properties. For convenience and from experience a demarkation point of 18 per cent has been adopted and timber with a moisture content equal to or below this level is considered to be ‘dry’ and timber with higher levels is considered to be ‘wet’. As well as adversely affecting its strength characteristics, timber swells with moisture pick-up and, as 8s 5268 gives both geometric and strength characteristics for the dry condition, compensating correction factors have to be applied to timber when wet. For temporary works situations when timber is exposed and frequently in contact with high water content mixes, it has become the established practice to consider all timber as being in the wet condition. Table 20 gives the modification factor K, BS 5268, by which the geometric properties of dry timber and as measured should be multiplied and Table 21 gives the modification factor K2 by which the dry stresses and moduli should be multiplied to give the equivalent values for wet timber.

Derivation of permissible stresses

The stresses given in Table 19 have been derived from an extensive series of rupture tests. Because of the variable nature of timber, the basic design stresses are derived in such a way that nearly all pieces of timber have a reserve of strength and only the weakest pieces will be fully stressed. Allowance is then made for the effect of defects to derive the grade stresses that qualify different timbers for inclusion in the various strength classes.

Duration of load

The stresses given in Table 19 apply to permanent structures where timber can be expected to carry a given load continuously for up to 50 years. Because timber is known to have the capability of carying higher loads for short periods of time then the allowable stress may be increased depending upon the length of time the stress is applied. The factor by which the stresses may be increased is called the ‘Load Duration Factor’ K3. As temporary works by definition are unlikely to carry long term loads, then tong term allowable stress levels may be increased by the factors

given

in Table

22.

61

Table 18. Softwood species/grade combinations which satisfy the requirements for strength classes SCI-SC5: Timber graded to t3.S 4978; to Canadian NLGA or American NGROL Joist and plank grades and to North American machine stress rated grades Origin

Species

UK

Corsican pine

Grading rufes

Grades to satisfy strength class SC2

SC3

SC4

SC5

BS 4978

GS

M50

ss

M75

GS

SC1

Douglas fir

UK

BS 4978

Douglas larch

Canada, USA

BS 4978 J&P Machine

No 3 900f-l .OE 1200f-1.2E

GS No 1, No 2 1450f-1.3E

GS

Machine

fir-

European spruce

UK

BS 4978

Hem-fir

Canada, USA Machine

BS 4978 J&P

Larch Parana

pine

M50, SS

No 3 900f-l .OE 1200f-1.2E

M50, SS

M75

graded

GS, M50 No 1, No 2 1450f-1.3E

ss Select 1650f-1.5E laOOf-1.6E

to strength ss Select 1650f-1.5E laOOf-1.5E

UK

BS 4978

GS

ss

Any

BS 4978

GS

SS

1650f-1.5E laoof-1.6E 1950f-1.7E 2100f-1.8E class M75 1650f-1.5E laOOf-1.6E 1950f-1.7E 2100f-1.8E

Pitch pine

Caribbean

BS 4978

GS

Redwood

Imported

BS 4978

GS, M50

ss

M75

Scats pine

UK

BS 4978

GS, M50

ss

M75

Sitka spruce

UK Canada

BS 4978 BS 4978 J&P

Southern pine

Spruce-pinefir

USA

Canada

GS No 3

M50, SS GS No 1, No 2

BS 4978 J&P

Machine ss Select

ss

graded to strength

Machine

goof-1 .OE 1200f-1.2E

GS No 1, No 2, No 3 1450f-1.3E

BS 4978 J&P Machine

No 3 goof-1 .OE 1200f-1.2E

GS, M50 Nol, No2 1450f-1.3E

Western red cedar

Any

BS 4978

GS

Western whitewoods

USA

BS 4978 J&P

GS No 3

Whitewood

Imported

BS 4978

ss

SS, M75 Select 1650f-1.5E 1800f-1.6E

class

Select

1650f-1.5E f 800f-1.6E

1650f-1.5E laoof-1.6E 1950f-1.7E 21 OOf-1.8E

1650f-1.5E 1800f-1.6E 1950f-1.7E 2100f-1.8E

ss

___ No 1, No 2

ss Select GS, M50

ss

M75

Notes The machine grades MGS and MSS can be substituted for GS and SS respectively. The S6, S8. MS6 and MS8 ECE grades may be substituted for GS, SS, MGS and MSS respectively. The BS 4978 grading rules apply to timber of a minimum size of 35 x 60mm. The classification of NLGA and NGRDL grades into strength classes applies to timber of a minimum size of 38 x 114mm. Joist and Plank No 3 grade should not be used for tension members. North American machine grades apply to a minimum size of 38 x 63mm. North American machine graded timber is assigned into different strength classes depending upon the section size. See BS 5268 Part 2 for details. BS 5268 Part 2 includes restrictions on fastener loads for:

SC5 timbers (except pitch pine and Southern pine) British grown Sitka spruce and European spruce US western whitewoods Hem-fir and Spruce-pine-fir in strength classes other than SC1 and SC2 Timber graded to North American Structural light framing and Stud grades are included See BS 5.268 Part 2 for details.

in BS 5268 Part 2 but not in this table.

Table 19. Grade stresses and moduli of elasticity for strength classes: dry exposure condition Strength class

SC1 SC2 SC3 SC4 SC5 SC65 SC75 SC85 sc9g

Bending parallel to grain (N/mm2)

2.8 4.1 5.3 7.5 10.0 12.5 15.0 17.5 20.5

Tension parallel to grain (N/mm?

2.2 2.5 3.2 4.5 6.0 7.5 9.0 10.5 12.3

Compression parallel to grain (N/mm2)

3.5 5.3 6.8 7.9 8.7 12.5 14.5 16.5 19.5

Compression * perpendicular to grain

Shear parallel to grain

Modulus of elasticity . Mean

(N/mm’)

(N/mm*) (N/mm21 ~~~~2~ Grade (N/mm2)

2.1 2.1 2.2 2.4 2.8 3.8 4.4 5.2 6.1

1.2 1.6 1.7 1.9 2.4 2.8 3.3 3.9 4.6

Basic

6800 8000 8800 9900 10700 14100 16200 18700 21600

0.46 0.66 0.67 0.71 1 .oo 1.50 1.75 2.00 2.25

4500 5000 5800 6600 7100 11800 13600 15600 18000

Approximate densityf (N/mm2)

540 540 540 590 5901760 840 960 1080 1200

* When the specification specifically prohibits wane at bearing areas the higher values of compression perpendicular to the grain stress may be used, otherwise the lower values apply. t Since many species may contribute to any of the strength classes, the values of density given in this table may be considered only crude approximations. When a more accurate value is required it may be necessary to identify individual species. 5 Classes SC6, SC7, SC6 and SC9 will usually comprise the denser hardwoods.

Table 20. Modification factor K, by which the geometrical properties of timber for the dry exposure condition should be multiplied to obtain values for the wet exposure condition (ie timber with a moisture content over 18 per cent)

Table 21. Modification factor K2 by which dry stress and mod& should be multiplied to obtain wet stresses and moduli applicable to wet exposure conditions (ie timber with a moisture content over 18 per cent)

Table 22. Load duration factor K3

Value of K,

Geometrical propetiy Thickness,

width

and radius

1.02 1.04 1.06 1.08

of gyration

Cross section area First moment of area, section Second moment of area

modulus

Property

Value of K2

Bending parallel to grain Tension parallel to grain Compression parallel to grain Compression perpendicular to grain Shear parallel to grain Mean and minimum modulus of elasticity

Duration

of loading

Modification

factor

0.8 0.8 0.6 0.6 0.9 0.8

1 year

1 month

1 week

1 day

1.2

1.3

1.4

1.5

Care should be taken in applying should be realistic.

this factor that the time scale selected

It should also be noted that the K3 factor only applies to stresses; not apply to values of the modulus of elasticity.

it does

63

Length and

When timber is supported at right angles to the grain, eg when joists are carried on beams, then the bearing stresses induced are resisted by the compressive strength perpendicular to the grain of the piece. When the compressive strength is exceeded then local and limited crushing of fibres within the bearing area will occur. The compressive stress is a function of the bearing area and when wane (ie loss of material at the edges of the piece) is present then higher stresses result. Allowance is made for this by giving two columns of values, for the permissible compression perpendicular to grain stresses in Table 19. If by selection or positioning of the piece of timber, no wane occurs at the bearing area then the values given in the column headed Basic may be used. Where wane is present at the bearing area then the values given in the column headed Grade must be used. Because of the way the stress is distributed, where a bearing is less than 150mm in length and situated not less than 150mm from the end of the member then a bearing stress modification factor K, as given in Table 23 may be used.

position of bearing

Lateral support

For stability against overturning, timber members when used on edge and with the specified degree of lateral support, should not exceed the depth to breadth ratios given in Table 24.

Depth factor

BS 5268: Part 2: 1988 allows the use of the following depth factors for bending and tension members K7 = 1 .17 for solid timber beams having a depth of 72 mm or less. K, =

300 ?O.‘l : Effective depth) for solid timber beams having a depth greater than 72 mm and less than 300 mm.

K, =

o 81 .

(Effective depth)’ + 92300 i (Effective depth)* + 56800

for solid timber beams having a depth greater than 300 mm. Tab/e 23. Modification factor K4 for bearing stress

_Length Value

Table 24. Maximum to breadth ratio

of bearing

(mm)

of Kd

10

15

25

40

50

75

100

150 or more

1.74

1.67

1.53

1.33

1.20

1.14

1.10

1.00

depth Degree of lateral support

No lateral

support Ends held in position Ends held in position and member held in line, as by purlins or tie rods Ends held in position and compression edge held in line, as by direct connection of sheathing, deck or joists Ends held in position and compression edge held in line, as by direct connection of sheathing, deck or joists, together with adequate bridging or blocking spaced at intervals not exceeding 6 times the depth Ends held in position and both edges firmly held in line

64

Maximum depth (h) to breadth (b) ratio_ 2 3 4

5

6 7

K,4 = 1 .I 7 for solid timber K

= f l4

300

ties having

a width

of 72 mm or less.

O”

c width for solid timber ties having than 300 mm.

a width greater than 72 mm and less

The grade bending and tension stresses given in Table 19 apply to material having a depth or width of 300 mm. These values may be multiplied by the appropriate depth factor where the member’s depth or width is less than 300 mm and should be multiplied by the appropriate depth factor when the depth or width is greater than 300 mm. For simplicity, the figures given in Tables 25 and 26 of this publication assume a member depth or width of 300 mm and, as a consequence, are slightly conservative for members of a lesser depth or width. In the rare cases where a beam depth exceeds 300 mm then the design should be checked with the appropriate beam depth factor to ensure that there is no overstressing.

Derivation of permissible stresses for timber when used as supports for trenches

Applying the modification factors previously indicated in this appendix to the stresses in Table 19 the permissible stresses as given in Table 25 may be calculated. The shear values given in Table 25 are based on those derived for permanent structures. For temporary works the use of higher shear stresses has become the accepted practice in the construction industry; these higher values having been proven over many years by the satisfactory performance of a very large number of structures. This fact has been recognised in BS 5975: 1982 Code of practice for falsework where the higher shear values have been quoted. In this publication it has been decided to adopt these higher shear stresses for the design of softwood supports in trenches. These modified shear values for softwoods (SC1 -SC5) have been included in Table 26 which gives the recommended permissible stresses for all strength classes of timber when used as trench supports. These stresses are also given in Table 12 although there the strength classes have been limited to those applicable to softwoods. The stresses given in Table 25 have been calculated as follows: Bending

and tension

(Tm01t. adm,ll

=

urn

or

t CJ.II x K, x K2 x K,

eg for SC1 bending

CJ,,,~,,~,,, = 2.8 x 0.6 x 0.8 x 1.4 = 3.32 N/mm2

Compression

parallel to grain Oc, adm. II = GC.g, II x K, x K2 x K, eg for SC1 %

adm,

Compression 0 c. adm. i =

(T c, g,

eg

@c, adm,

for

SCf

II = 3.5 x

perpendicular 1

x 1

6

1.04

x 0.6 x

1.4 = 3.06 N/mm2

to grain x

K2

x

K3

x

K4

= 2.1 x 1.04 x 0.6 = 2.09 N/mm2

x

1.4 x

1.14

Modulus of elasticity = E x K, x K2 eg for SC1 min E = 4500

x

1.08

x 0.8 = 3890 N/mm2

65

Table 25. Stresses and rnoduli of elasticity for strength

exposure Strength class

SC1 SC2 SC3 SC4 SC5 SC6 SC7 SC8 SC9

classes:

wet

condition Bending parallel to grain N/mm’ 3.32 4.87 6.29 8.90 11.87

14.84 17.81 20.78 24.34

Tension parallel to grain N/mm*

Compression parallel to grain N/mm2

Compression perpendicular to grain N/mm’

2.56 2.91 3.73 5.24 6.99 8.74 10.48 12.23 14.33

3.06 4.63 5.94 6.90 7.60 10.92 12.67 14.41 17.03

2.09 2.09 2.19 2.39 2.79 3.78 4.38 5.18 6.07

(1.20) (1.59) (1.69) (1.89) (2.39) (2.79) (3.29) (3.88) (4.58)

Shear

Modulus of elasficiiy

N/mm2

Mean N/mm’

0.60 0.86 0.88 0.93 1.31 1.97 2.29

2.62 2.95

Minimum

N/mm2

5880

3890

6910

4320 5010 5700 6130 10200 11750 13480 15550

7600 8500 9240 12180 13990 16150 18660

Criteria Timber used in trench support Wet conditions Dry sizes in calculation Duration of load - 1 week No wane at bearing (where wane is permitted at bearing use figures in brackets) Bearing not greater than 75mm and not less than 150mm from end of member Effective depth (width) of member not greater than 300mm h/b ratio not exceeding values given in Table 12

Table 26. Timber supports For trenches - permissible sfresses - no load sharing Strength class

SC1 SC2 SC3 SC4 SC5 SC6 SC7 SC8 SC9

Compression parallel to grain N/mm2

Bending parallel to grain N/mm2

Tension parallel to grain N/mm”

3.32

2.56

3.06

4.87 6.29 8.90 11.87 14.84 17.81 20.78 24.34

2.91 3.73 5.24 6.99 8.74 10.48 12.23 14.33

4.63 5.94 6.90 7.60 10.92 12.67 14.41 17.03

Compression perpendicular to grain N/mm2

Shear

Modulus of elasticity

N/mm2

Mean N/mm2

Minimum N/mm2

2.09 (1.20)

0.9 1.29 1.34 1.34 1.75 1.97 2.29 2.62 2.98

5880 6910 7600 8500 9240 12180 13990 16150 18660

3890 4320 5010 5700 6130 10200 11750 13480 15550

2.09 2.19 2.39 2.79 3.78 4.38 5.18 6.07

(1.59) (1.69) (1.89) (2.39) (2.79) (3.29) (3.88) (4.58)

Criteria 1 2 3 4 5 6 7 8

Timber used in trench support Wet conditions Dry sizes in calculation Duration of load - 1 week No wane at bearing (where wane is permitted at bearing use figures in brackets) Bearing not greater than 75mm and not less than 150mm from end of member Effective depth (width) of member not greater than 300mm h/b ratio not exceeding values given in Table 24

Note Maximum

66

shear

= 1.5 x average

shear where average

shear

=

Cross s~~t~~nal area

Timber requisition

Check list of information

to be provided by site management

Contact and address Job location (main block, pumphouse etc) Timber usage (identify end use, ie permanent, temporary, etc) Drawing reference number (if any) Strength class Species and grade of timber (optional if strength class is specified) Size (minimum sectional dimensions, length(s), and finish (sawn regularised, PAR) 8 Quantity 9 Any special treatment 10 Delivery programme 11 Contact on site and telephone number 12 Means of access 13 Off loading facilities available 14 Address to which invoice should be sent

67

Appendix 2: Check lists for excavation support

Planning and design Item

Comments

Question What are the soil types What

is known

and groundwater

of previous

Is an unsupported

work

conditions?

in this locality?

face able to stand?

(a) Depth to water table? (b) More than one water table? 5

(a) Will dewatering be necessary suitable? (b) Will a water retaining support

6

Should

7

Will drying ground crack? cause ‘block’ falls)

8

(a) Is surface flooding possible? (b) Need for and type of surface

9 10

excavation

Is flooding

be under

from services

Pipe lengths,

diameters

and what will be be required?

water

to prevent

(allowing

water

‘boiling’?

to enter or to

drains

possible? and bedding?

11

Length,

12

Pipe testing

13

Obstructions and all possible services? (water, gas, oil, hydraulic mains, foul sewers, storm sewers, industrial water mains and culverts, electricity and telephone cables)

14

Surcharge

15

In roadway

16

Existing

17

Condition/proximity of existing movement and vibration?

18

Is the timber job?

19

What type of plant

20

Limitations on plant (headroom, lifting capacity)?

21

Hand excavation?

22

Is support

23

Where is the spoil heap to be put? Not within edge of the excavation

24

Is spoil

(a) to be used for backfill? (b) suitable for backfill?

25

Method

of fill compaction?

26

Are bridges

68

breadth

and depth?

requirements?

loads,

vibrations?

or open ground?

structures

condition

or steel piling

(cracks

etc)?

structure

limits,

on the site adequate

ground for the

is to be used? access,

rights

of way,

one off or repetitive?

needed?

1 m of the

Trenching operations

The following is a basic check list. Other items should appropriate for any scheme.

be added as

Date: Checked by: /tern

Yes

Question

1

Is surface material clear of plant, spoil heaps, materials etc for 1 m (or as specified if more) from the edge of the excavation?

2

Are spoil heaps being properly controlled they stay like this in wet weather?

3

Is the trench clear of men while the spoil heap is being worked on?

4

Is the inside of the spoil heap clear of pipes, bricks, stones, tools etc?

5

Is the work properly fenced off and ‘signed’ during the day? Is the work properly fenced off ‘signed’, guarded and lit during the night?

6

Is access adequate without anyone having to jump across trench? Are foot bridges with guard rails available and being used?

7

Are ladders available and being used?

0

Is the supervisor the timber?

9

Is the trench safe from exhaust gases entering from machines working nearby?

10

Does everyone know where the buried services are?

11

Are the men excavating and shoring this trench experiencd in this sort of work?

12

Are they working at safe distances other (ie not too close)?

ensuring

and will

that no-one climbs on

from each

13

Is the ground as design allowed?

14

Is the ground free of deterioration, placing the excavation and adjacent structures at risk?

15

Is the area unaffected by any blasting or other heavy vibrations? Is the ground water level as used in the design? (ie not higher)

16 17

Are there proper sumps?

18

Does the pumping arrangement avoid drawing materials from behind the sheeting?

19

Is the sheeting

20

22

Are unsheeted faces safe with no sign of peeling away etc? Are materials used the correct design sizes and quality? Are wedges tight?

23

Is timbering

21

No. and Action

in accordance

with the design?

free of damage by skips? Continued overleaf

69

/tern

Question

24

Waling and strut spacing specified tolerance

25

Are deflections

26

Are all struts horizontal and positioned the walings? (within 1 in 40)

27

Are frames supported against downward movement? (by hangers or lip blocks, puncheons and sole plates)

28

Have correct

29

Is the method of withdrawing during backfill a safe one?

30

Is work tidy?

31

Back-filling position?

32

Is visibility

33

Are safety helmets

Yes to be within

f 100 mm or

excessive? squarely

pins been used in steel trench

by dumper: adequate

sheeting

are positive

struts?

and support

stops in

in trench? available

to

and being used?

No. and Action

Appendix 3: Glossary of terms used in timbering

The following definitions

refer to excavations

and any associated works.

Term

Definition

back prop

A raking strut used to transfer the weight of timber to the ground in deep trenches; usually placed below every second or third frame.

batter (rake)

An artificial, uniform steep slope. In trenching, be less than the angle of repose of the soil.

bent

A trapezoidal

berm

A horizontal slope.

biat (byatt)

A timber bearer giving support to guard rails, decking, walkways, etc.

bitch

A fastening of iron or steel used for securing heavy timbers which cross each other and which is similar to a dog but has one of its ends at right angles to the other.

blocking, chock or chog

A timber block used as a distance piece or packing, eg between a waling and the temporary or permanent lining of the excavation, to permit the insertion and erection of vertical reinforcement in retaining walls or other permanent construction.

blow or boil

A displacement of sand, silt or gravel in the bottom of an excavation upward flow of water.

boom

A length of large section hardwood used in driving a heading.

box heading

A heading in which walls and roof are close-timbered.

bracing

A diagonal member used to stiffen or brace the timbering.

breasting board

Used as temporary support to face of heading.

brob or nail spike

A fastening

cap, capping piece or distributor

A piece of timber placed over the joint where two walings butt to take the thrust of the strut.

chock or chog

See blocking. A term used for heavy close sheeting shafts.

cladding

or cleading

the angle of batter must

timber frame. ledge in an earth cutting to ensure the stability of a steep

by

of iron with its head bent at right angles to the shaft.

in connection

with the sinking

of

cleat

A block of timber fixed to a member to prevent the movement of other abutting timbers.

sheeting or close timbering

Vertical or horizontal boards placed in close formation to hold up the face of an excavation. A block of timber fixed to a member to prevent movement of abutting struts. Short lengths of poling boards placed horizontally across a gap between runners or sheeting and tucked in behind them and used where runners or sheeting cannot be driven continuously and vertically.

close

corbel head cross poling

cutting-out distributor

piece

A short piece of timber which may be cut out to facilitate the striking of timbering. See cap.

71

dog

A fastening of iron used for spiking large timbers together and having both ends bent down and pointed.

dumpling

The ground temporarily left in the middle of an excavation serve as an abutment for the timbering to the surrounding

face waling or face piece

A waling across the end of a trench supported by the ends of the side walings and which, together with the end strut also acting as a waling, supports the end face of a trench.

facing wall

The lining, usually of reinforced concrete, either precast or cast in situ, constructed against the face of an excavation in place of timber sheeting, and supported by the main timbering. Facing walls are left in after construction and are frequently used to receive asphalt waterproofing. Wedges used in pairs, overlapping each other and driven in opposite directions in order to hold or force apart two parallel surfaces.

folding wedges

which may trenches.

foot block

A timber pad used to spread a load from a ground prop or side tree.

formation

The finished level of the excavation at the bottom of a trench or heading prepared to receive the permanent work.

frame

In a trench, any pair of walings on opposite sides of the trench together with all struts that separate them. In a shaft, all the walings and struts at the same level. The word ‘frame’ is often regarded as including boards supported by these timbers.

the setting of poling

normal poling frame

A frame in which the walings support the poling boards at their mid-points.

tucking frame

A frame in which the walings support the poling boards at their ends.

ground or top frame

A timber frame of walings and struts laid a foot or so below ground level, used as a guide for the first ‘setting’ of runners or trench sheeting.

guide frame

A timber frame erected above ground level to act as a guide for runners or trench sheeting, and as a staging from which they may be driven.

ground prop

A prop or puncheon placed between the lowest frame and a foot block on the bottom surface of an excavation and used to support the dead weight of the timbering.

guide runner

A runner driven ahead to form a guide for driving intermediate

hanger

See tie-rod.

runners.

head boards

Boards at the roof of a heading which are in contact with the ground above.

heading

Excavation

head tree

A horizontal timber in the roof of a heading which rests on the side trees and supports the head boards.

in a tunnel.

heave

Rising of the floor of a deep excavation

interlocking pile kicking piece

See steel sheet piling. A length of timber spiked to a waling to take the thrust from the end of a strut which is not at right angles to the waling.

in soft silt or clay.

kicking post

A post set in a ground slab to take the thrust from the end of a raking strut.

king piles

A line of piles to the full depth of the excavation, driven at strut intervals in the body of a wide trench before excavation, serving as supports and abutments for struts shorter than would otherwise be required.

lacing

A vertical timber spiked to the sides of struts or walings and tying them together to carry the weight of the lower frames as excavation advances. A timber driven between the ends of opposing members of a frame to lock them in position and spiked to a member against which it rests.

liner or stretcher lip, lipping block lipping piece

72

A short length of timber fixed and spiked to the top of a strut, and projecting sufficiently beyond its end so as to rest on a waling. It supports the weight of the strut while wedges are being driven.

open sheeting

Used in excavation in which the sides are reasonably firm and not likely to crumble. Generally consists of vertical poling boards, spaced at intervals and supported by walings and struts; or horizontal sheeting openly spaced and held in position by soldiers and struts.

page

A small timber wedge. A pair of poling boards strutted apart to support the trench wall in good ground.

pinchers pitching

The process of raising runners or piles into vertical position ready for driving into the ground.

poling back

The operation of excavating behind timber supports already in position and timbering the new face.

poling boards

Timber boards generally from 1 m to 1.5 m long and from about 25 mm to 50 mm thick, used in supporting the faces of an excavation.

puncheon

A vertical prop to support a higher waling or strut from the one below.

or prop

raker

An inclined compression

runners

Long vertical timbers at least 50 mm thick with their lowest end chiselshaped; used in unstable ground instead of poling boards, and driven downwards in advance of the excavation.

scantling

A term used to denote the breadth and thickness

setting

All boards held in position by one frame of timber, or in the case of tucking or piling frames, by two adjacent frames.

sheeting

Boards, planks or timbers used in conjunction with walings or soldiers and struts to support the sides of an excavation.

sheet piling

Vertical members of timber, reinforced concrete or steel driven into the soil in a row to retain soil during excavation and to assist in the exclusion of water. A steel or iron attachment suitably shaped to fit and reinforce the cutting edge of a runner or sheet pile.

shoe

member.

of a piece of timber.

side boards

Boards forming the sides of a heading.

side trees

Timbers which support the head trees and side boards in a heading.

sill (or till)

A timber laid across the bottom of a heading or trench and carrying at its ends the feet of the side trees. A liner or chogs may be used on the sill to keep the side trees or ground props apart.

soldier

A vertical timber taking the thrust from horizontal and supported by struts across the excavation.

staging

A working platform supported

steel sheet piling

Sheet piling formed of rolled steel sections with interlocking joints and used principally for excavations in difficult or water bearing soils.

stemming

To prevent the loss of soft or loose ground, such as dry sand, through joints of openings in timbering; hay, straw or similar material may be pushed into the crevices to stop or ‘stem’ the flow.

sheeting or walings

on the main framing of trenches.

stretcher

See liner.

strut

A horizontal member in compression resisting lateral thrust from the sides of an excavation. Open-jointed or perforated pipes laid in the trench at the bottom of excavations to drain the ground as the work proceeds.

sub-drains sump

An excavation below the level of the bottom of a trench into which water drains and from which it may be baled or pumped.

swinger

A pointed iron bar about 1 m long, used as a lever for moving runners.

tie rod or bolt

A steel rod or bolt sometimes used instead of lacings between successive frames to take their weight and prevent movement of the timber.

73

tucking board

A narrow timber used behind walings in tucking frames.

tucking frame

See frame.

waling

A horizontal excavation.

wedge

See folding wedge and page.

weephole

In wet ground a hole sometimes provided through sheeting to allow the discharge of water so as to prevent the development of a dangerous hydrostatic head of water.

Yankee brob

A Z-shaped metal strap.

74

member supporting

the poling boards or sheeting

in an

References

British Standards Institution. Specification for the use of structural steel in building. British Standard BS 449. Part 2 Metric units. London, BSI. 1969. British Standards Institution. Softwood grades for structural use. British Standard BS 4978. London, BSI. 1988. British Standards institution. The struc&ural use of timber. British Standard BS 5268. Part 2 Code of practice for permissible stress design, materials and workmanship. London, BSI. 1988. Part 5 Code of practice for preservative treatments for constructional timber. London, BSI. 1989. British Standards Institution. Code of practice for falsework. British Standard BS 5975. London, BSI. 1982. British Standards Institution. Code of practice for earthworks. British Standard BS 6031. London, BSI. 1981. British Standards Institution. Code of practice for foundations. British Standards BS 8004. London, BSI. 1986. Construction Industry Advisory Committee. The confrol of substances hazardous to health in the construction industry. London, HMSO. 1989. Health and Safety Commission. Control of substances hazardous to health and confrol of carcinogenic substances. Approved Codes of Practice. London, HMSO. 1988. Illingworth, J. R. Temporary works: their role in construction. London, Thomas Telford Ltd. 1987. Institution of Civil Engineers. Earfh retaining structures. Civil Engineering Code of Practice No 2. London, ICE. 1951. National Federation of Building Trades Employers. Construction safety. Part 4 Excavations, shafts and tunnels. London, NFBTE. 1972. Royal Society for the Prevention of Accidents. Construction Regulations Handbook 10th Edition (Metric). Birmingham, RoSPA. 1976. Thomson, G. W.(Editor). Trenching and shoring manual. Sacramento, California, Department of Transportation. 1977. Tomlinson, M. J. Foundation of design and construction. London, Pitman Publishing Ltd. 3rd Edition, 1976. Tschebotarioff, G. P. Foundations, retaining and earth structures. Tokyo, London, etc. McGraw Hill. 1973. UK Parliament. Control of Substances Hazardous to Health Regulations 7988. Statutory Instruments No 1657. London, HMSO. 1988. UK Parliament. Health and Safety at Work etc Act 7974, Chapter 37. London, HMSO. 1974. UK Parliament. Factories: The Construction (General Provisions) Regulations. Statutory Instruments No 1580. London, HMSO. 1961. UK Parliament. Factories: The Construction (Lifting Operations) Regulations. Statutory Instruments No 1581. London, HMSO. 1961. UK Parliament. Factories: The Construction (Working Places) ffegulations. Statutory Instruments No 94. London, HMSO. 1966. UK Parliament. Factories: the Construction (Health and We/rare) Regulations. Statutory Instruments No 95. London, HMSO. 1966. UK Parliament. Factories: The Construction (Metrication) Regulations. Statutory Instruments No 1593. London, HMSO. 1984.

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