Modular Gravity Retaining Walls Design

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Licensed copy:Trans4m Ltd, 19/11/2007, Uncontrolled Copy, © CIRIA

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Cover photograph: Porcupine block wall used for bank protection (courtesy MMG Civil Engineering Systems Ltd).

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ClRlA C516 London, 2000

Modular gravity retaining walls - design guidance

Tim Chapman BE MSc CEng MICE MlEl

with contributions from

Howard Taylor BSc PhD FEng FlCE FIStructE

Duncan Nicholson BSc MSc DIC CEng MICE

sharing knowledge building best practice

6 Storey’s Gate, Westminster, London SW 1 P 3AU TELEPHONE 020 7222 8891 FAX 020 7222 1708 EMAIL [email protected] W EBSITE www.ciria.0rg.uk


Summary

This report provides guidance for specifiers, designers, manufacturers, installers and owners about the design of low-height gravity retaining walls composed of different modular units. It presents consistent methods for the design of systems.

The report’s coverage includes an introduction to low-height modular retaining wall systems, factor in selecting an appropriate system and an explanation of the principles and other considerations of wall design. It deals with the engineering properties of soil and fill and the choices of design values and external loads. While covering general design applications, such as the assessment of external stability, there are design calculations for specific wall types, particularly for external stability. There are notes on specification and quality control, and the concluding chapters examine the performance requirements for low-height modular retaining walls, including the need for regular maintenance, with suggested schedules for inspection visits.

Worked examples are included in appendices.

Modular gravity retaining walls - design guidance

Chapman T with contributions from Taylor H and Nicholson D

Construction Industry Research and Information Association

Publication C5 16 0 CIRIA 2000 ISBN 0 86017 516 2

Keywords Low-height wall types, selection, design, construction, durability, maintenance. ~ ~~

Reader interest

Geotechnical and civil engineers, contractors, consultants, clients, wall owners and suppliers.

Classification

AVAILABILITY Unrestricted

CONTENT Design guidance

STATUS Committee-guided

USER Geotechnical and civil engineers, contractors, consultants, clients, wall owners, wall manufacturers

2

Published by CIRIA, 6 Storey’s Gate, Westminster, London SWlP 3AU. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, including photocopying and recording, without the written permission of the copyright-holder, application for whch should be addressed to the publisher. Such written permission must also be obtained before any part of this publication is stored in a retrieval system of any nature.

ClRlA C516


Acknowledgements

I

T h s report, part of CIRIA’s ground engineering programme, was produced as a result of Research Project 546, Low-height modular retaining walls - design guidance. It was carried out under contract to CIRIA by Ove Arup & Partners, in collaboration with Tarmac Precast Concrete Ltd.

ClRlA C516

The report sets out good practice in the planning, selection, design, installation and maintenance of low-height modular retaining walls. Whle it is primarily concerned with gravity walls of retained heights up to 3 m, many of the principles also apply to higher walls.

The report accords with design to Eurocode 7 Part 1 and to BS 8002: 1994. It is also consistent with design to Highways Agency Standards.

Following CIRIA’s usual practice, the research was guided by a Steering Group whch comprised:

Mr B S Chang (chairman) Dr K C Brady Dr E R L Cole

Mr K M Dight

Mr N Finegan

Mr G Horgan Mr B T McGinnity

Mr D P McNicholl

Mr D R Moreland Mr M Stevenson

Mr S Willoughby

Mr P B Woodhead

Alfred McAlpine Civil Engineering

Transport Research Laboratory Lancashne County Council (representing the County Surveyors’ Society)

Henry Boot Construction (UK) Ltd

Highways Agency

MMG Civil Engineering Systems Ltd London Underground Limited

Wardell Armstrong

Galliford plc Gibb Ltd

Thorburn Colquhoun Ltd

Department of the Environment, Transport and the Regions.

CIRIA’s research manager for the project was Dr A J Pitchford.

The project was funded by the Construction Directorate of the Department of the Environment, Transport and the Regions and CIRIA’s Core Programme sponsors. In addition, CIRIA gratefully acknowledges the following organisations, which provided funding for the project:

Maccafem Ltd Marshalls Mono Limited

Netlon Limited P h Group Limited

TWIL Limited.

3


4

CIRIA and the authors would llke to give special acknowledgements to the individuals listed below who contributed to the production of this report.

Ove Arup & Partners

Mr R Armitage Professor J Atkinson Mr R Rudrum Mr D Twine Mr P Cracknell Mr J Shillibeer Mr H C Yeow

Mrs L V Jhoke

Dr P R J Morrison Dr B Simpson

Tarmac Precast Concrete Ltd

Mr N Moxon

Mr A Hewett

They would also like to thank all those who responded to questionnaires sent out as part of this project and to the following for supply of photographs and figures:

Maccaferri Ltd (Figures 2.4a and b) Tinsley Wire (Sheffield) Ltd (Figure 2 . 4 ~ )

Rutbin Concrete Ltd (Figure 2.6a)

MMG Civil Engineering Systems Ltd (Figures 2.7 and 2.8b)

Marshalls (Figure 2.8a) Brick Development Association (Figure 2.10)

Tarmac Precast Concrete Ltd (Figures 2.1 1 and 2.12).

ClRlA C516


Contents

Summary .......................................................... ............................................. 2 Acknowledgements .................................................... ............................................ 3

Boxes ................................................................................................................................ 7 Tables ............................................................. ............................................... 7

Figures ........................................ ............................................................................ 8 Notation .......................................................................................................................... 10 Glossary .......................................................................................................................... 12

Introduction ......................................................................................................... 17 1.1 Background to the project ................. ....................................... 17 1.2 Objectives for the report ................... ....................................... 17 1.3 Scope of the report ......................................................... ............. 17

Layout of the report ...................................................................................... 18 1.5 Design assumptions ...................................................................................... 19

Categorisation of retaining walls .................................................................. 21 1.7 Design responsibility .................................................................................... 23

Summary of design methodology ................................................................. 26

1.4

1.6

1.8

Review of types of low-height modular retaining walls ................................... 28 2.1 General ...... ............................................................................

2.3 Crib walls ...................................................................................................... 33 2.4 Drystack masonry ....... ............................................................................. 36

2.6 Precast reinforced concrete stem walls ....... ............................................. 40 2.7 2.8

n walls ..........................................................

2.5 Masonry ........................................................................................................ 38

Other modular retaining wall systems .......................................................... 41 Walls and vegetation .................................................................................... 42

Selection ............................................................................................................... 44

3.2 Appearance ................................................................................................... 44 3.3 Durability ...................................................................................................... 46

3.1 Construction considerations .......................................................................... 44

3.4 Economy ....................................................................................................... 47 3.5 System selection ........................................................................................... 47

Design concepts .................................................................................................... 49 4.1 Design process ..................................................... ................................... 49 4.2

4.4 Comparison of retaining wall design methods ..........................

4.6 4.7 Movements and damage predictions for neighbouring structures ................ 65

Limit state design .......................................................................................... 52 4.3 Design life ................... ..................................................... 54

4.5 Strength for design .................................... .......................................... 57 Selection of geotechnical parameters ........................................................... 62

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5

6

7

8

9

10

11

Material properties ............................................................................................. 67 5.1 Measurement ................................................................................................ 67 5.2 Design values ................................................................................................ 68 5.3 Geotechcal design reports .......................................................................... 75 5.4 Contaminated ground .................................................................................... 75 5.5 Selection of backfill ...................................................................................... 77

Loads .................................................................................................................... 81

6.2 6.3 Earth pressures .............................................................................................. 84 6.4 Compaction of backfill ................................................................................. 88 6.5 Water pressures and drainage ....................................................................... 91 6.6 Trees ............................................................................................................. 99 6.7 Other load cases .......................................................................................... 100

6.1 Surcharges .................................................................................................... 81 Unplanned.and planned trenches and excavations ........................................ 84

Design applications . general ........................................................................... 101 7.1 Sliding ......................................................................................................... 101

Bearing capacity and overturning ............................................................... 106 Overall instability including deep slip surfaces .......................................... 113

7.4 Internal stability .......................................................................................... 114

7.2 7.3

7.5 Corner details., ............................................................................................ 114 7.6 Design examples ......................................................................................... 114

Design applications . specific wall types ......................................................... 115 8.1 Gabion and bastion walls ............................................................................ 115 8.2 Crib walls .................................................................................................... 117 8.3 Drystack masonry walls .............................................................................. 119 8.4 Masonry walls ............................................................................................. 120 8.5 Precast reinforced concrete stem walls ....................................................... 121

Specification and quality control ..................................................................... 122 9.1 Specification ............................................................................................... 123 9.2 Specification details .................................................................................... 123

Maintenance schedule . . . . . . I .......................................................................... 125 9.6 Commercial issues ...................................................................................... 125

9.3 Quality control ............................................................................................ 124 9.4 Inspection during construction .................................................................... 124 9.5

Inspection and maintenance ............................................................................. 126 10.1 Inspection ................................................................................................... 126 10.2 Maintenance ................................................................................................ 131

Concluding remarks .......................................................................................... 134

Appendix A1 Active and passive pressure coefficients ............................................... 135 Appendix A2 Design examples ................................................................................... 148

6

References .................................................................................................................... 196

ClRlA C516


Boxes

1.1

4.1

5.1

5.2

6.1

Roles used in gravity retaining wall design ........................................................... 23

Highways Agency checking categories ................................................................. 51

Highways Agency material classes used for backfilling retaining walls ............... 77

10.1 Possible inspection frequencies ........................................................................... 127

Definition of uniformity coefficient ...................................................................... 73

Definition of overconsolidation ratio .................................................................... 87

Tables

2.1

3.1

4.1

4.2

5.1

5.2

5.3

5.4

5.5

5.6

5.7

5.8

6.1

6.2

6.3

6.4

6.5

7.1

Obstacles to vegetation establishment ............................................................. ..... 43 Modular wall selection .......................................................................................... 48

Notional classification of design working life ....................................................... 54

Partial factors - ultimate limit states in persistent and transient situations ........... 56

Unit weights of soils (and similar materials) ......................................................... 68

Weight densities and angles of repose for different materials ............................... 69

Undrained shear strength ....................................................................................... 71

@&it for clay soils ................................................................................................... 72

@'for siliceous sands and gravels .......................................................................... 73

@'for rock .............................................................................................................. 74

SPT descriptions .................................................................................................... 74

Fill materials grading limits .................................................................................. 78

of active and passive failures in soil ...................................................................... 85

Design compaction pressures ................................................................................ 90 Highways Agency compaction requirements for different material classes .......... 91

Suggested material grading for a Type A filter drain material .............................. 94

Approximate values of retaining wall movement for the development

Design water table level ........................................................................................ 92

Bearing capacity factors for drained conditions .................................................. 109

ClRlA C516

10.1 Inspection details ................................................................................................. 128

10.2 Inspection types ................................................................................................... 130

7


Figures

G1

G2

1.1

1.2

1.3

2.1

2.2

2.3

2.4

2.5

2.6

2.7

2.8

2.9

T e r n used in gravity retaining wall design .......................................................... 12 Nomenclature for wall corners .............................................................................. 14

Layout ofrepo rt ..................................................................................................... 20

Examples of walls in EC7 geotechnical categories ............................................... 22

Design process for a low-height modular retaining wall ....................................... 27

Retaining wall classification ............................................................... Structural action of modular wall types .................................... Details of gabion walls ......................................................... Photographs of gabion walls and bastion ....................... Typical single thickness crib wall (fill not shown) .................................... Photographs of crib walls .........................................................

Typical single width drystack masonry wall (Porcupine wall)

Photographs of drystack masonry retaining walls ...............................

Typical masonry retaining wall ................................................ . .

2.10 Photograph of masonry wall forming a bridge wing wall ..................................... 39

2.1 1 Typical precast reinforced concrete stem wall ...................................................... 40

2.12 Photographs of precast concrete retaining walls ................................................... 41 2.13 Other types of modular retaining wall ................................................................... 42

3.1 Proposed procedure for the outline design of earth-retaining walls ...................... 45

4.1 4.2

4.3

4.4

4.5

Example of a wall design check certificate ........................................................... 50 Mobilisation of different soil strengths ................................................................. 60 Relationship between stress, strength and water content in a soil at failure .......... 61

Interaction between a retaining wall and another structure ................................... 66

Decision-making process of site investigation ...................................................... 64

5.1 Correlation factor5 between undrained shear strength and SPT blowcount

Single page geotechnical design report content .................................................... 76

(c. /SPT N) in clays ......................................................................................................... 71

Grading envelopes for Highways Agency fill materials ....................................... -79 5.2

5.3

6.1

6.2

Layout of Section 6 ............................................................................................... 81

Examples of load cases for a uniformly distributed surcharge for the design of retaining walls ........................................................................................ 83

6.3 Effects of trench excavation on passive resistance ................................................ 85

6.4 Approximate values of retaining wall movements for the development of active and passive failures in soil ...................................................................... 86

6.5 Assessment of compaction pressure on retaining walls ........................................ 89

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6.6

6.7

6.8

7.1

7.2

7.3

7.4

7.5

7.6

7.7

7.8

8.1

8.2

Typical drainage schemes for reinforced concrete retaining walls ........................ 95

Typical drainage schemes for other types of gravity retaining walls ................... 96

Evaluation of water pressure on potential failure surfaces .................................... 98

Layout of Section 7 ............................................................................................. 101

Sliding failure ...................................................................................................... 102 Examples showing how back excavation angle can dictate active pressures ...... 103

Bearing capacity and overturning ............................................

Bearing pressure area .............................................................. Types of typical slip failure ................................................................................. 113

Sliding surface in case of inclined formations and shear keys

Bearing capacity coefficients ..................................................

Settlement induced by loss of fines caused by flowing water ............................. 115

Forces normally acting on a crib wall ................................................................. 119

10.1 Wall inspections . typical danger areas .............................................................. 127

10.2 Scope of inspection- critical areas ..................................................................... 131 10.3 Typical inspection record .................................................................................... 132

ClRlA C516

Al. l K. factors ............................................................................................................. 140

A1.2 Kp factors ............................................................................................................. 147

9


Notation

Roman upper case letters

accidental action B 2 ‘the design effective foundation area width design effective foundation width effect of an action Force, action design value of an action permanent action specific gravity of soil grains horizontal force, action active earth pressure coefficient active earth pressure coefficient for cohesion Coefficient of horizontal earth pressure at rest coefficient of horizontal earth pressure at rest for a retained slope angle passive earth pressure coefficient passive earth pressure coefficient for cohesion the design effective foundation length mobilisation factor bearing pressure coefficient bearing pressure coefficient bearing pressure coefficient SPT blowcount effective line load per metre width of roller variable action soil uniformity coefficient vertical force, action design value of a material property characteristic value of a material property

Roman lower case letters

design value of a geometrical property characteristic value of a geometrical property cohesion intercept in terms of effective stress undrained shear strength wall adhesion relationship factor between SPT and c, depth where compaction pressures remain significant inclination of load inclination factor inclination factor inclination factor active earth pressure passive earth pressure overburden or surcharge pressure effective overburden pressure

ClRlA C516 10


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4u uniaxial compressive strength of soil or rock

0 1 + U 3

2

I I

S

SC shape factor sq shape factor -5 shape factor

, I

0 1 - 0 3

2 t

U pore water pressure W water content Wf water content at failure zcr critical depth for compaction pressures

Greek lower case letters

angle retained slope angle bulk unit weight design effective unit weight dry density partial factor (material) moist weight density partial safety factor (force) water density interface angle of wall friction design value of interface friction angle of convexity total stress effective stress major principal stress minor principal stress effective stress at failure maximum compaction pressure vertical stress effective vertical stress total shear stress effective shear stress shearing resistance at failure angle of shearing resistance effective angle of friction critical state angle of friction drained angle of friction peak angle of friction undrained angle of friction

11


\ '

I b Base

Glossary

TERMS USED IN GRAVITY RETAINING WALL DESIGN

The following terms for parts of retaining walls are used frequently in this guide and are illustrated in Figure G1:

0

0

0

0

0

0

0

0

0

0

0

0

0

0

active side

angle of temporary cut, etc

backfill drainage

heel

key overdig

passive side

retained height

stem

surcharges

toe

virtual back

weephole.

Surcharge I &

12

Figure G1 Terms used in gravity retaining wall design

ClRlA C516


t

The following terms are often ;sed in masonry and other wall types:

0

0

fin - an outstanding rib or buttress to a masonry wall

header - a brick or block laid with its long axis perpendicular to the face of the wall

perpend - a vertical masonry joint

reveal - a portion of wall between fins stretcher - a brick or block laid with its long axis parallel with the face of the wall.

The terms for wall corners, salient and re-entrant are shown in Figure G2. Similar terminology is used for corners in buildings.

0 Active pressure: when a retaining wall moves away from a soil mass, the coefficient of earth pressure reduces so that the horizontal effective stresses fall to a limiting value known as the active pressure.

Passive pressure: when a retaining wall moves into a soil mass, the coefficient of earth pressure increases so that the horizontal effective stresses rise to a limiting value known as the passive pressure.

0

DEFINITIONS

A selection of relevant definitions from EC1 (Part 1, Clause 1.5) and EC7 (Part 1, Clause 1 S . 2 ) are given below. The actual definitions are given in normal text and notes made in the Eurocodes are given in smaller text. Parallel references from the CDM regulations are made in italic script.

Accidental action (A) (ECI) Action, usually of short duration, which is unlikely to occur with significant magnitude over the period of time under consideration during the design working life. Note: An accidental action can be expected in many cases to cause severe consequences, unless special measures are taken.

Accidental design situation (ECI) Design situation involving exceptional conditions of the structure or its exposure, eg fire, explosion, impact or local failure.

Action (F) (ECI) Force (load) applied to the structure (direct action). An imposed or constrained deformation or an imposed acceleration caused, for example, by temperature changes, moisture variation, uneven settlement or earthqu;..e (indirect action).

Action effect (E) (ECI) The effect of actions on structural members, eg internal force, moment, stress and strain.

Characteristic value of an action Fk (ECI) The principal representative value of an action. Insofar as this characteristic value can be fixed on statistical bases, it is chosen so as to correspond to a prescribed probability of not being exceeded on the unfavourable side during a reference period, taking into account the design working life of the structure and the duration of the design situation.

Characteristic value of a geometrical property ak (ECI) The value usually corresponding to the dimensions specified in the design. Where relevant, values of geometrical quantities may correspond to some prescribed fractile of the statistical distribution.

ClRlA C516 13


t-

14

Salient corner

-.r -

Passive soil

Figure G2 Nomenclature for wall corners

Characteristic value of a material property Xk (ECI) The value of a material property having prescribed probability of not being attained in a hypothetical unlimited test series. This value generally corresponds to a specified fiactile of the assumed statistical distribution of the particular property of the material. A nominal value is used as the characteristic value in some circumstances.

ClRlA C516


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Comparable experience (EC7) Documented or other clearly established information related to the ground being considered in design, involving the same types of soil and rock and for which similar geotechnical behaviour is expected, and involving similar structures. Information gained locally is considered to be particularly relevant.

Construction works (ECI) Everythmg that is constructed or results from construction operations. Note: This definition accords with IS0 6707: Part 1. The term covers both building and civil engineering works. It refers to the complete construction works comprising structural, non-structural and geotechnical elements.

The Construction (Design and Management) Regulations 1994 have a specific definition for ‘construction work’ that relates to the legal application of the CDM Regulations. However, in the context of this guide, the two definitions can normally be taken to be the same.

Design situations (ECI) Those sets of physical conditions representing a certain time interval for which the design will demonstrate that relevant limit states are not exceeded. They can be transient, persistent or accidental. A transient design situation is relevant during a period much shorter than the design working life of the structure and has a high probability of occurrence. It refers to temporary conditions of the structure, of use, or exposure, eg during construction or repair. A persistent design situation is relevant during a period of the same order as the design working life of the structure and generally refers to conditions of normal use.

Design value of an action Fd (ECI) The value obtained by multiplying the representative value by the partial safety factor B.

Design value of a geometrical property ad (ECI) Generally a nominal value. Where relevant, values of geometrical quantities may correspond to some prescribed fractile of the statistical distribution.

EC7, Clause 1 S.2 , contains the following additional definitions:

Design values of a material property Xd (ECI) Value obtained by dividing the characteristic value by a partial factor yM or, in special circumstances, by direct determination.

Design working life (ECI) The assumed period for which a structure is to be used for its intended purpose with anticipated maintenance but without substantial repair being necessary.

Execution (ECI) The activity of creating a building or civil engineering works. Note: The term covers work on site; it may also signify the fabrication of components off site and their subsequent erection on site.

Ground (EC7) Soil, rock and fill existing in place prior to the execution of the construction works.

15


16

Hazard (ECI) Exceptionally unusual and severe event, for example an abnormal action or environmental influence, insufficient strength or resistance, or excessive deviation from intended dimensions. This definition does not correspond with the use of the word ‘hazard’ in the CDM Regulations.

Limit states (ECI) States beyond whch the structure no longer satisfies the design performance requirements.

Maintenance (ECI) The total set of activities performed during the working life of the structure to preserve its function.

Permanent action (G) (ECI) Action whch is llkely to act throughout a given design situation and for which the variation in magnitude with time is negligible in relation to the mean value, or for whch the variation is always in the same direction (monotonic) until the action attains a certain limit value.

Representative value of an action (ECI) Value used for the verification of a limit state.

Serviceability limit states (ECI) States which correspond to conditions beyond which specified service requirements for a structure or structural element are no longer met.

Strength (ECl) Mechanical property of a material, usually given in units of stress.

Structure (ECl) Organised combination of connected parts designed to provide some measure of rigidity. Note: IS0 6707: Part 1 gives the same definition but adds ‘or a construction works having such an arrangement’.

The Construction (Design and Management) Regulations 1994 have a specific definition for ‘structure’ that relates to the legal application of the CDM Regulations. However, in the context of this guide, the two definitions can normally be taken to be the same.

Structure (EC7) As defined in ENV 199 1- 1, Basis of design, including fill placed during execution of the construction works.

Ultimate limit states (ECI) States associated with collapse, or with other similar forms of structural failure. Note: They generally correspond to the maximum load-carrying resistance of a shvcture or structural part.

Variable action (QJ (ECI) Action which is unlikely to act throughout a given design situation or for which the variation in magnitude with time is neither negligible in relation to the mean value nor monotonic.

ClRlA C516


I I Introduction I I

1 .I ~

BACKGROUND TO THE PROJECT

This research was commissioned by CIRIA in 1996 with the objective of providing guidance on the selection, design, installation and operation of low-height modular retaining walls. The project was undertaken by Ove Arup & Partners and Tannac Precast Ltd, designers and suppliers respectively of modular retaining wall systems. The steering group that guided the work represented clients, specifiers, designers, suppliers and installers. The authors consulted widely via questionnaires and a literature search in order to collect together the information required to produce a consistent and coherent design, covering all types of modular retaining wall systems.

OBJECTIVES FOR THE REPORT

The aim of this report is to produce a guide for the design of low-height retaining walls composed of proprietary modular units that provides a unified approach for the designers of all proprietary systems. The guide facilitates a simple design approach for a wide spectrum of site conditions by drawing from a range of references and summarising the various code requirements for design.

The report is for structural and civil engineers who are not necessarily specialists in the geotechnical aspects of gravity wall design and who need to design or specify such walls as part of an overall development. The guide will also be of use as a reference for engineers more familiar with geotechnical design, for students of civil and structural engineering and for others who want to gain an appreciation of the issues to be considered in selecting a low-height gravity retaining wall.

The report accords with design to Eurocode 7 Part 1 (DD ENV 1997-1: 1995), referred to hereafter as EC7, and to BS 8002: 1994. It is also consistent with design to Highways Agency Standards. Highways Agency Standards are written for the design of permanent structures for the UK’s trunk road and motorway network but their requirements are adopted by most highway authorities and are useful for many other situations.

SCOPE OF THE REPORT

The guide covers the geotechnical design of walls as follows:

gravity retaining walls composed of modular units, retaining a soil height up to about 3 m. Many of the principles also apply to higher walls, although higher walls would often also use other means of support such as props, ties or soil reinforcement

the design of retaining walls specifically in the UK, but many of the recommendations could also be applied in other countries

the design of permanent structures, although much of the guidance could also be used for the design of temporary structures.

17


1.4

The guide does not cover several areas.

1. Embedded retaining walls, even those composed of modules such as sheet piles or lungpost walls. For the design of such walls, reference should be made to Padfield and Mair (1984), as well as to EC7 and BS 8002. Reinforced soil walls are also not considered explicitly. Guidance for the design of reinforced soil is given by Jewel1 (1996) and BS 8006.

The design of propped walls or walls which rely on ties or anchorages for stability. The use of piles to assist stability is also not covered.

Walls such as cast-in-place concrete walls are not covered. As they share many characteristics with modular concrete walls, many of the guide’s recommendations could also apply to them.

The guide does not address the design of retaining walls to resist tides, wave forces and scour. Further guidance on these matters is given in BS 8002, Clause 4.7.

2.

3.

4.

Care is required where a series or tiers of low-height retaining walls are used to retain a substantial height. In this case, the active pressures from the retained soil behind one wall can interfere with the passive resistance of higher walls. Such situations are complex and outside the scope of this design guide. Specialist advice should be sought.

This guide follows the geotechnical categorisation system proposed by EC7 (see Section 2 of EC7). The guide has been written to give walls with factors of safety appropriate for geotechnical categories 1 (small and relatively simple structures) and 2 (conventional types of structures and foundations with no abnormal risks or unusual or exceptionally difficult ground or loading conditions). It does not address the design of structures described under geotechnical category 3 (very large or unusual structures, structures involving abnormal risks, or unusual or exceptionally difficult ground or loading conditions and structures in highly seismic areas). More details on categorisation are given in Section 1.6.

The guide does not address the structural design of wall elements. These will normally be proprietary systems provided by specialist contractors and suppliers. It does provide general advice, however, on the forces for which they should be designed.

Although the guide does not include design of ancillary structures such as parapets, the retaining walls must be able to withstand horizontal forces such as those a parapet could exert on the wall.

LAYOUT OF THE REPORT

This first section states the objectives of the report and its scope. Important concepts such as the EC7 categorisation of retaining walls and design responsibility are introduced in Section 1 and then firther developed in later sections.

Section 2 reviews the available systems of low-height modular retaining walls and explains their similarities and differences. Photographs and line drawings illustrate how the finished products can look.

Guidance is given in Section 3 on the selection of an appropriate retaining wall system for a particular set of circumstances and requirements. The advantages and limitations of different systems are compared, to allow the most advantageous system to be chosen.

18 ClRlA C516


1.5

The important design principles and considerations are set out in Section 4. This section concentrates on the engineering decisions that need to be made before a design can be attempted. Responsibilities are further developed and the design methods based on EC7 and BS 8002 are compared.

Material properties of soil and fill are considered in Section 5 and values for use in design are presented. The report also covers the information which should be presented in a geotechnical design report.

Section 6 presents information on the external loads for which the retaining wall should be designed. A broad definition of ‘load’ is used, analogous to EC7’s use of the term ‘action’. It includes data for assessing surcharges, earth pressures, water pressures and compaction pressures, as well as the effects of excavations and trees.

General design applications are addressed in Section 7, which covers calculations that are applicable to all wall types. Procedures and data for the assessment of external stability are presented.

Design calculations necessary for specific wall types are covered in Section 8. This section provides greater emphasis on the calculations that would be carried out for internal stability.

Specification and quality control are discussed in Section 9.

Section 10 examines the performance requirements for low-height modular retaining walls and gives guidance on their maintenance. It also provides guidance on the inspection and assessment of retaining walls to check that they continue to fulfil their function over their design life.

The layout of the report is summarised in Figure 1.1.

DESIGN ASSUMPTIONS

EC7 Clause 1.4 makes a series of assumptions which designs to that code should fulfil. Designers should confirm that those assumptions apply or should communicate their different assumptions to all relevant parties. The assumptions are as follows:

that data required for design are collected, recorded and interpreted

that structures are designed by appropriately qualified and experienced personnel

that adequate continuity and communication exist between the personnel involved in data collection, design and construction

that adequate supervision and quality control are provided in factories, plants and on site that execution is carried out according to the relevant standards and specifications by personnel having the appropriate skill and experience

that construction materials and products are used as specified in EC7 or in the relevant material or product specifications. (In fact, materials and products are not covered explicitly by EC7 so this EC7 requirement is equivalent to stating that products and materials should be used as intended by the manufacturers)

that the structure will be adequately mahtained

that the structure will be used in accordance with the purpose defined for the design.

ClRlA C516 19


selection criteria

Section 1

assumptions wall category design responsibilities

-n Section 6

surcharge ( i trenches compaction ,) water pressures

Section 7 - general requirements for all wall types 9 Section 8 - requirements for specific wall types

Build

Section 9 - specification and construction control

n Operate

Section 10 - inspection and maintenance

I I

Figure 1.1 Layout of report

20 ClRlA C516


1.6

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CATEGORISATION OF RETAINING WALLS

The EC7 geotechnical categorisation system can be used to divide the walls described in this guide into geotechnical categories 1 or 2. EC7 recommends that the designer should initially allocate the retaining wall to a particular category. This selection should be reviewed at each stage of the design and construction process and changed where necessary.

Category 1 structures should possess the following characteristics:

the difference in ground level on either side of the wall, including any slopes, does not exceed 2 m

the ground conditions are known from comparable experience to be straightforward

the wall construction does not involve excavation below the ground water table or comparable local experience (defined below) indicates that such excavation would be straightforward

experience indicates that a qualitative geotechnical investigation (normally examination of the subgrade during construction) will be sufficient

there is negligible risk to property or life should such a wall fail.

This last requirement means that retaining walls withm a distance of a highway boundary equal to twice the retained height cannot be considered as category 1 structures.

EC7 Clause 1 S.2 defines ‘comparable experience’ as ‘documented or other clearly established information related to the ground being considered in design, involving the same types of soil and rock and for which similar geotechnical behaviour is expected, and involving similar structures. Information gained locally is considered to be particularly relevant’.

The only walls whose height is within the limit covered by this guide which would require to be designed under geotechnical category 3 are those in highly seismic areas or walls which support petrochemical, nuclear or other hazardous facilities, where the consequences of a wall failure could be catastrophic. Such walls should be designed by specialists in the relevant fields. The factors of safety suggested by this guide are not appropriate.

All other walls fall into geotechnical category 2. Category 2 structures require that:

0 the wall is to be designed for normal loading conditions with no abnormal risks due to failure

quantitative geotechnical data and analyses are carried out. 0

Design of walls to geotechnical category 2 could be used to justify a more economical design for structures which would otherwise be classified as category 1. For this particular example, the increased investigation and design costs of a geotechnical category 2 wall should be balanced against the potential savings in materials and construction over a geotechnical category 1 design.

Examples of walls in the different categories are illustrated in Figure 1.2.

21


A :

Geotechnical Category 1

- A

1.5m

I


T

Road

I

2.5m Tm

Petrochemical

2m


Figure 1.2 Examples of walls in EC7 geotechnical categories

EC7 permits the geotechnical investigation for category 1 structures to be delayed until the actual construction of the works, to verify all the design assumptions. This requires that the person supervising the construction of the wall is competent to assess the adequacy of the foundations and has a thorough understanding of the requirements of the design. This approach carries the risk that adverse conditions may be encountered, which need to be remedied during construction. For geotechnical category 2 structures, a desk study and ground investigation should be carried out in advance of design and construction. More details on particular requirements for the desk study and ground investigation are given in Section 4.6.

22 ClRlA C516


1.7

Category 1 structures may be designed by ‘prescriptive measures’, described by EC7 as conventional and generally conservative details in the design, and attention to specification and control of materials, workmanship, protection and maintenance procedures. Higher category structures should be designed to satisfy the appropriate limit states by calculations, apart from measures to ensure durability against frost or chemical attack where direct calculations do not exist. For these instances, prescriptive measures need to be used.

The other design methods permitted by EC7 are:

0 load tests and tests on experimental models

0 the Observational Method (explained by Nicholson et al, 1999).

These methods will seldom be economical for walls of the height covered by this guide. An approach that can be used in some circumstances is where the wall is built on the basis of inadequate design data but the possibility of failure is acceptable to the client. This approach can only be followed when the risk of a failure causing injury is negligible. It might be economical to follow this approach, for instance, on a farm where a low-cost storage facility is required.

DESIGN RESPONSIBILITY

The CDM Regulations impose a wide definition of ‘design’. It includes drawing, design details, specification and bill of quantities (including specification of articles or substances) in relation to the structure.

Clear allocation of design responsibility is always important and definitions of the various roles are given in Box 1.1. This issue is particularly important in the case of proprietary walling systems installed on a site about which the walling system supplier, often acting as the designer, initially knows nothing.

Box 1.1 Roles used in gravity retaining wall design

Eight roles used in gravity retaining wall design are identified in this guide.

Client - the person who is promoting the development. Under the Construction (Design and Management) Regulations 1994 (CDM Regulations) the client must select and appoint a competent planning supervisor (where so required by the CDM Regulations) and principal contractor. The client must also be satisfied as to the competence of the designer.

The client must be satisfied that adequate resources have been allocated for health and safety, and ensure that construction work does not start until the principal contractor has prepared a satisfactory health and safety plan. After project completion, it is the client‘s responsibility to ensure that the health and safety file is available for inspection.

Specifier - the person who identifies the requirements for the retaining wall and who produces documents to arrange its procurement. If the retaining wall forms part of a larger development, the specifier could be the architect or firm of consulting engineers designing the rest of the development. A retaining wall required for a minor redevelopment, however, could be purchased directly by the client.

ClRlA C516 23


Designer - the person who confirms that the chosen wall system has an acceptable margin against internal and external failure and who produces documents to communicate the design intent to a supplier via drawings and a written specification. Design responsibilities for internal and external aspects of the wall are often split. If this is the case, both designers have a responsibility to check that their part of the design is compatible with other parts. In most cases, the designer is also the specifier or supplier (or a separate design consultant employed by the specifier or the supplier).

The CDM Regulations have a wide definition of a ‘designer’, which encompasses any person who carries on a business where designs are prepared or who arranges for any person under that organisation’s control to prepare a design relating to a structure or part of a structure.

The designer, under the CDM Regulations, has specific key duties which include, so far as reasonably practicable, alerting clients to their duties; considering the hazards and risks which could arise during the construction and maintenance of the structure; designing to avoid and reduce risks to health and safety; making sure that the design includes adequate information on health and safety; and to co- operate with the planning supervisor and any other designers. Information on safety at the design stage should be given on drawings and specifications etc, and the designer is responsible for passing this information to the planning supervisor so it can be included in the project‘s health and safety plan.

Checker - a person or organisation employed to confirm the adequacy of the designer’s design and that it is accurately communicated by the specification and drawings. The checker may be part of the designer’s team or from an outside body, depending on the type and location of the wall.

CDM planning supervisor - under the CDM Regulations, this person (where required by the Regulations) has to co-ordinate the health and safety aspects of the project design and the initial planning. Their role is to ensure, as far as is reasonably practicable, that designers comply with their duties, that a pre-tender health and safety plan is prepared before appointment of a contractor and that the project is notified to the Health and Safety Executive. The planning supervisor also has to ensure that the health and safety file is prepared and delivered to the client at the end of the project.

Principal contractor - this person may be the installer of the retaining wall system or the main contractor. Under the CDM Regulations, the client must appoint a principal contractor to undertake the specific duties allocated by the Regulations. The principal contractor is responsible for developing and implementing the health and safety plan, arranging for competent subcontractors, the co-ordination of contractors, obtaining risk assessments from contractors and control of the general site safety. The health and safety file is compiled with information supplied by the principal contractor.

Supplier - usually a producer of a range of retaining wall products or the producer’s agent. The supplier should confirm that the purchased retaining system complies with the design details that have been provided. The supplier should notify the specifier if the design details appear to be inadequate for the particular situation or if no design has been provided.

installer - the contractor who installs the wall modules on the site so as to construct a retaining system. The installer should notify the specifier if conditions mean that the requirements of the specification cannot be complied with or if a situation is encountered which is different from that assumed in the design.

24 ClRlA C516


In broad terms, four situations can be considered. They are detailed below, in order of decreasing input by specifier.

1. Specifier designs everything The specifier is the designer who designs the wall modules and the supplier provides the modules which fulfil the specification. The units will be produced as a one-off run and therefore lose the benefits of standardisation at large scale. The specifier also designs the retaining wall that will be composed of the modules, so that it will be stable in the particular situation and ground conditions. This approach is unlikely to be economical, except for in the largest contracts where one-off production costs are offset by the volume required.

Specifier selects a standard product, but checks its external stability and internal strength for particular conditions In this situation, the specifier selects a standard proprietary product, taking on the responsibility to check that the product is sufficiently strong and stable under the forces imposed on it by the ground. The specifier, although carrying out the stability calculations, nevertheless relies on whatever structural capacity has been assigned to the system by the supplier.

Specifier provides supplier with an interpretative geotechnical report and requires the supplier to confirm strength and stability of the product in the particular conditions The specifier describes the site and the proposed development and provides the supplier with an interpretative geotechnical report. The supplier provides a product suitable for those conditions, after having carried out whatever analyses would be required. The supplier should confirm that the product will be adequate for the design situation, but should inform the specifier of any deficiencies in the geotechnical interpretative report or of any reservations about the extent or quality of investigatory work relevant to the design.

Specifier requires supplier to confirm stability of the product in the particular conditions The specifier indicates the site to the supplier and describes the design requirements. The supplier provides a product appropriate to those conditions, after having carried out whatever desk study, ground investigations and analyses are needed.

2.

3.

4.

When design responsibilities are split, one body should still take responsibility for the overall design and check that all elements of the design have been satisfactorily completed and are mutually consistent. The identity of that body will depend upon the circumstances of the particular retaining wall.

The design of the retaining wall system to resist a given set of forces is usually referred to as the ‘internal stability’ design, ie the structural strength of a reinforced concrete wall or the stability of the assemblage of modular blocks or units for other types of retaining wall. The design to confirm the appropriateness of those forces for a particular set of site and ground conditions is referred to as the ‘external stability’ design, ie the overall stability of the gravity wall formed by the modular structure in the particular situation and ground conditions. Design for external stability should make sure that there is an adequate margin against failure at both the serviceability and ultimate limit states (see Section 4.2), and that the forces for which the internal stability design was carried out are appropriate. All design requires satisfaction of the internal and external parts of the design. Responsibility for each part should be clearly assigned. In many instances, different parties are responsible for internal and external design. In such cases the body responsible for the overall design, often the specifier, should check that the internal and external stability designs are compatible.

ClRlA C516 25


1.8

1.8.1

1.8.2

1.8.3

26

The Highways Agency (BD2/89 Part 1) uses Approval in Principle (AIP) documents to define the checking responsibilities, to set out what is required by way of soil design parameters and design assumptions, and to confirm that they are reasonable. More details are given in Section 4.1.2.

SUMMARY OF DESIGN METHODOLOGY

Initial identification of wall needs and requirements

The wall specifier should first identify the key requirements for the wall, eg what height of soil needs to be retained, how close to the wall any surcharge load would be and what measures could be taken to fit the wall into the overall development, such as wall aesthetics or the need to preserve any existing trees. The specifier should also identify any constraints on the wall, such as not being able to excavate far behmd the proposed wall line or the presence of any particularly corrosive soil.

It greatly assists wall selection if some form of desk study and ground investigation has been carried out at this stage. As a low-height retaining wall will usually be a small part of a larger development, much of the information gathered for the main development will be useful for scheme design of the retaining wall.

Selection of wall type

Using the details in Section 2 and guidance set out in Section 3, it will generally be of benefit to select a preferred wall type early in the design process. As the design progresses, other wall types might be found to be more economical for overcoming problems with the wall’s situation or particular aesthetic requirements. The more flexible the specification is in permitting alternatives, the more suitable the eventual solution will be.

At this stage the initial EC7 wall category should be chosen, based on the wall requirements (see Section 1.6).

Design, specification and tendering

Using one of the suggested specification and procurement strategies in Section 1.7, the specifier should consider how best to distribute responsibilities for design of the wall. Depending on which design approach is to be followed by the specifier or a specialist contractor, procurement may take place before or after detailed design.

The specifier should make sure that:

0 responsibility is clearly allocated for each part of the design process

an appropriate desk study and ground investigation have been carried out to the satisfaction of the wall designer

the wall design is satisfactory in terms of external and internal stabilities

the design details are hlly communicated to the contractor.

0

ClRlA C516

There should be a requirement in the construction contract that significant variations in either construction or the ground conditions found during construction are fed back to the designers to check that the design takes account of them. T h s process is shown in Figure 1.3.


I 1 Define Geometry Refer to:

and design life and site and programme constraints

Section 3

+ Obtain site specific data

(topography, ground conditions Sections 4 and 5

likelihood of future trenches, etc. Section 6

Select preferred wall

Analyse wall for external stability

Section 3

Sections 7 and 8

Analyse wall for internal stability

Sections 7 and 8

Revisit design to check it is buildable and compatible with overall design and whole life

costing

1

- Construction

- Drainage - Environment - Ground movements

and damage - Corners

considerations Section 3.1

Section 6.5 Section 6.3 Section 4.7

Section 7.5

Figure 1.3 Design process for a low-height modular retaining wall

Section 9

Section 9

Section 10

ClRlA C516 27


2 Review of types of low-height modular retaining walls

2.1 GENERAL

Modular walls may be constructed using a wide variety of techniques and materials. They share the common feature that their construction involves the erection of a number of mainly similar modular elements.

A large variety of retaining systems exist. The types covered by this guide are illustrated in Figure 2.1, which shows an overall classification,of retaining walls.

Retaining walls

I

not tied back

I Embedded retaining walls

I

I I secant diaphragm

I I sheet pile contiguous

wall bored pile bored pile wall wall wall

Gravity retaining walls Hybrid e g gravity

wall supported partially by piles

tied back e.g. anchored

wall

I

; reinforced masonry drystack crib gabion ; concrete stem wall masonry wall wall j wall wall

; soil

!

Figure 2.1 Retaining wall classification

The types of wall covered by this guide can be subdivided into two principal classes:

0 those that rely on structural strength of a reinforced concrete stem

those where stability is maintained by gravity action - no (or minimal) tension is allowed to develop.

28

These classes are shown in Figure 2.2.

ClRlA C516


Reinforced concrete stem walls

a) Internal stability maintained by structural strength of reinforced concrete stem

Masonry walls Gabon walls

b) Internal stability maintained by gravity action - no (or minimal) development of tension

Figure 2.2 Structural action of modular wall types

Each wall type has its particular uses. In some applications, one wall system alone would be the most suitable; in other applications several wall systems could be suitable. The choice of system for a particular application involves a wide range of considerations, including the following:

0

0 appearance durability

construction considerations and site limitations

0 economy

0 maintenance. , I

These considerations are dealt with in more detail in Section 3.

Modular walls have basic common features that distinguish them from in situ walls and recognition of these distinctions forms the first stage in the process of their selection. Modular walls can usually be installed without a specialist labour force (carpenters, steel fixers, concreters, etc) and the construction process is generally quite fast. The modular units are factory-made and can be checked for quality before incorporation into the wall. Specialist labour is required for masonry and dry stone walling. In situ walls require a range of site craft slulls, but can be more effective in difficult situations, eg where the supporting foundation soil is poor or where interaction with other structures is required.

- ClRlAC516 29


2.2

30

Modular walls can be constructed in a wider range of weather conditions than in situ walls, particularly in the winter when the temperature may be too low for satisfactory concrete work. In very wet weather modular walls can be constructed, but the control of water penetration into the foundation soil and the moisture content of the retained material should be kept within the design requirements.

GABION AND BASTION WALLS

Gabions are wire mesh boxes filled with stones which, when placed side by side, constructed in stepped course and laced together, form a single gravity structure. Their form of construction is illustrated in Figure 2.3 and photographs showing a completed wall are shown in Figure 2.4.

Gabions are supplied in a flat-pack arrangement, usually in bundles with all lacing wire, spirals or fixing rings to secure the gabion boxes together. In the UK, the mesh used in the manufacture of the gabions is either formed from woven continuous wires or welded mesh. The steel mesh is protected against corrosion with a zinc or zinc-aluminium coating. For greater durability or application in marine or polluted environments an additional bonded plastic, thermoplastic or epoxy resin polymer coating can be provided. The fill should be durable.

Gabion walls can be built with either the front face or rear face stepped. It is desirable, where possible, to incline the wall at least 6-8" from the vertical towards the retained fill material, as in Figures 2.3b and 2 . 3 ~ .

Gabion walls are constructed typically in 1 .O or 0.5 m high courses, using standard size gabion units. In order to facilitate construction, the backfill can be placed and compacted, keeping it to the same levels. Gabion walls are permeable and will allow retained fill to drain freely. Where water has to be prevented from seeping through gabions, eg adjacent to a road or footpath where it could promote growth of algae or allow ice to form, a small (say 0.5 m hgh) impermeable wall and drainage system should be provided at the toe of the gabion wall, or an effective drainage system should be provided to intercept seeping water.

With correct detailing, gabion walls can be designed to support vegetation using growing pockets. Root growth within or near a gabion structure is not normally detrimental.

Gabions are suitable for underwater uses. Construction is by prefilling the gabion and lowering it to place by crane, using a lifting frame. This process also may be adopted for other areas with poor access.

Gabions can look attractive in rural locations or where they can fit in with a rocky landscape and vegetation. The appearance of the front face of the gabions can be controlled by hand placing selected stone fill to suit the surroundings. The specifier of gabion walls should detail the quality of finish required.

A recent innovation, developed primarily for the military, is the bastion. Bastions are collapsible wire or geotextile mesh multicellular structure systems. They are designed for rapid erection from a flat-pack delivery, making them ideal for emergency works. The assembled size of a bastion is typically up to 10 m long, 1 .O m wide and up to 1.4 m high. Bastions are manufactured from the same steel mesh combinations as gabions and are available with the same corrosion protection options.

ClRlA C516


ClRlA C516

a) Typical gabion wall without stone filling (after BS 8002)

b) Gabion wall with a stepped face front

Figure 2.3 Details of gabion walls

c

c) Gabion wall with a stepped rear face

31


I .

-%--

a) Gabion wall retaining a road

b) Gabion wall retaining a motorway side slope

c) Bastion filling

Figure 2.4 Photographs of gabion walls and bastion

32 ClRlA C516


2.3

ClRlA C516

When assembled, the bastion forms a series of individual cells which are lined with a non-woven geotextile. At construction, the cells can be filled with suitable granular material for retaining structures and almost any material for flood prevention works: sand, ballast, earth, stone, concrete, even snow depending on availability. Figure 2 . 4 ~ shows a bastion being filled.

Bastions, as intended, are most often used by the military or for emergency flood protection works. They should be considered only as temporary retaining structures, except where they incorporate measures to prevent loss of fill material through the mesh and use some form of facing to protect any geotextile mesh against ultraviolet degradation. Such a facing is also shown in Figure 2.4. It should be noted that if the individual bastions are not laced together, then they do not form a single structure when a number of individual units are joined together.

CRIB WALLS

Crib walls comprise a grillage of header and stretcher units placed on a firm foundation, usually of mass or reinforced concrete. For low walls it may be possible to use compacted hard core. The spaces between the crib grillages are filled with a free- draining, non-aggressive coarse granular material (see Figures 2.5 and 2.6). Without interceptor drains, this open structure could allow water to flow across adjacent pavements, leading to growth of algae or icing in winter. Planting is possible in the front face and the walls provide suitable anchorage for climbing or cascading vegetation. Crib walls are often laid to a batter, an angle corresponding to 1 horizontal to 12 vertical being typical.

The header and stretcher elements may be made of reinforced concrete or timber and are designed to be interlocking, to give the wall continuity. Walls can be made with multiple rows of cribs at the base although, for very low walls, a single row is likely to be adequate. The reinforced concrete elements are usually designed for manual handling. Some more complex cellular systems exist, where the headers and stretchers are integrated, such that they require a crane for safe lifting.

Durability of crib walls is provided by appropriate concrete cover to the reinforcement in the header and stretcher elements, or by treatment to timber elements. The infill material should be frost-resistant. It is possible to vandalise a crib wall because of the small cross section size of the headers and stretchers. Timber and reinforced concrete are susceptible to fire, although in a wall situation it is unlikely that fires will be intense enough to cause more than superficial damage. Crib walls, particularly those with a batter, are easily climbed which can also encourage vandals.

Once the headers and stretchers have been erected, it is possible to fill a crib wall with lean mix concrete making it more akin to a masonry wall. In this case, the free-draining nature of the wall is lost and a drainage system may have to be incorporated behind the wall to prevent the build-up of water.

Crib walls can look very attractive in rural or other planted locations, and merge well with the countryside.

33


Figure 2.5 Typical single thickness crib wall (fill not shown) (after BS 8002)

and gravel

34 ClRlA C516


Y

a

a) Concrete crib wall face

.

E

b) Partially dismantled timber crib wall showing nature of infill material and retained soil

Figure 2.6 Photographs of crib walls

* ClRlA C516 35


2.4 DRY STACK MASONRY

Drystack masonry walls consist mainly of precast concrete special blocks and occasionally bricks that are designed to interlock with each other and produce a solid wall face (see Figures 2.7 and 2.8). A drystack wall relies upon gravity to support the retained material and will have an interlock shear resistance between each layer of blocks. The interlock also assists accurate placing of successive layers of blocks.

The wall will require a foundation of mass, reinforced or possibly precast concrete construction, similar to that required by crib walls. It is possible with some systems to build the walls thicker at the base to provide additional stability. It is also prudent to speclfy the top layers of blocks to have some form of mortar pointing, adhesive or capping to avoid them being dislodged by vandals as the individual blocks are usually designed to be light enough to conform to manual handling regulations.

Most of these walls have open perpends and are relatively free draining, but some may require granular backfill and a drain to avoid the build-up of hydrostatic forces. This drainage can allow water to flow across adjacent pavements, leading to the growth of algae or icing in winter. Walls can be constructed vertically or with a batter on an inclined foundation, to provide better stability for greater heights.

Drystack walls will often have a distinctive appearance defined by the shape of the proprietary block used. Walls may be constructed to gentle horizontal or vertical curves by adjustment to the perpends, sometimes also with partial removal of shear keys. Wide perpends may also be used as planting locations. These walls look attractive in many locations and may be used in association with top, bottom or wall-face planting. Wall-face planting may be in spaces provided by open joints filled with a suitable growing medium.

Valying inclinations of front face

90' 82.5" 75' 6 7 9 We

Figure 2.7 Typical single width drystack masonry wall (Porcupine wall)

36 ClRlA C516


a) Revetlok gravity wall system

ClRlA C516

b) Porcupine block wall used for bank protection

Figure 2.8 Photographs of drystack masonry retaining walls

37


2.5 MASONRY

Masonry walls are made with brick, block, natural or manufactured stone conventionally bedded with mortar. These walls are built on a mass or reinforced concrete foundation (see Figures 2.9 and 2.10).

Masonry walls should be provided with a coping or other details to avoid water saturation and possible frost damage. Frost resistance is determined by two factors:

the degree of protective detailing

the frost resistance of the masonry unit mortar used.

The use of copings, drainage, damp-proof courses and waterproofing will help prevent saturation of the wall and improve durability. Details are given in BS 5628 Part 3. Advice should be sought to check that the chosen materials have sufficient frost resistance.

These walls can be provided with fins or reveals to improve their overturning resistance and may conveniently be built to curves and irregular plan forms.

Brickwork should be constructed in panels, measuring 10-15 m in length between movement joints, which should be properly detailed to prevent vertical lines of unattractive seepage appearing in the wall face. Blockwork constructed in panels of this length may require horizontal reinforcement.

The maximum height of masonry that should normally be built in one day is 1.5 m and backfill pressures can only be added when the mortar has had time to gain strength. Mortar type should be chosen for durability.

It is possible to provide horizontal reinforcement in the joints and vertical reinforcement inside small cells that are filled with mortar during construction. Walls with more significant reinforced infill are considered later.

Masonry walls can be provided with integral patterns or other decoration. They can look particularly attractive in urban locations, where they can be detailed to be in sympathy with the local buildings. In urban and rural locations they can be built using local natural stone if suitable. Examples are shown in the Brick Development Association publication by Haseltine and Tutt (1991).

I 38 ClRlA C516


5 f- !w

C20 mass concrete min. cement content 220kg/m3

E U (U .c .-

8 z

E E C 5

at 2.50m centres - -

Figure 2.9 Typical masonry retaining wall (after Atkinson, 1994)

Figure 2.1 0 Photograph of masonry wall forming a bridge wing wall

ClRlA (2516 39


2.6 PRECAST REINFORCED CONCRETE STEM WALLS

Precast reinforced concrete stem walls, sometimes called cantilever walls or gravity cantilever walls, comprise reinforced concrete L-shaped or inverted T-shaped walls, which rest on the ground or on a concrete foundation and act in conjunction with the mass of the retained fill (see Figures 2.1 1 and 2.12).

The units are manufactured to a range of heights, from 1 m to more than 3 m, and are of conventional reinforced concrete designed to BS 81 10 or, on occasions, to the water- resisting concrete code, BS 8007. The units typically weigh about 2 tonnes and require the use of a crane for offloading and installation. Special units can be provided to form returns, whether right-angles or more gradual curves. The walls are often designed on the assumption that they will not have to resist water pressures. A suitable drainage system is therefore a requirement of the design.

These walls are also useful for materials storage in construction and other industries, as well as for the storage of crops.

In permanent situations these systems may be made more attractive by a facing which may be of brickwork, blockwork or a timber or other grillage to support climbing plants.

Wall unit Corner units

1500 i

0 0

0 0

0 0

Figure 2.1 1 Typical precast reinforced concrete stem wall

_Lc__.

0 0

0 0

0 0

40 ClRlA C516


2.7

a) Used as a retaining wall

' b) Used for materials storage

i . Figure 2.12 Photographs ofprecast concrete retaining walls

OTHER MODULAR RETAINING WALL SYSTEMS

Walls may be made from stacked car tyres filled with soil, sand bags, logs, etc. and will be similar in concept to many of these wall types but are often used for temporary applications. A hybrid wall type, whch uses two masonry skins as formwork for a site- poured reinforced concrete stem, is sometimes used. This wall type is often known as a permanent formwork or a grouted cavity wall. Examples of these systems are shown in Figure 2.13.

ClRlA C516 41


2.8

U

- - .(iii) Reinforced concrete stem poured - -

(iv) Fill placed behind wall

a) Walls composed of old tyres infilled with soil

(ii) Masonry placed at front and back faces as permanent formwork

u u

/' I

/ (i) Base poured first

b) Hybrid masonry retaining wall

Figure 2.1 3 Other types of modular retaining wall

WALLS AND VEGETATION

Planting of vegetation to provide a pleasing face has been mentioned in the description of many of the wall systems. It should be recognised, however, that the face of a wall that is deliberately drained to avoid ground moisture build-up for structural reasons is not the most suitable place for planting.

Advice on planting vegetation in civil engineering can be found in Coppin and Richards (1990) and may also be sought from the manufacturers of the various systems. Care should be taken both in the choice of plants suitable for locations within, above or below the wall and for the aspect, and also regarding the suitability of the growing medium (usually loose topsoil or growbags) which may require special water retention measures.

It is not realistic to assume vegetation will flourish in small pockets in the face of a drained wall. In permeable walls, the vegetation and topsoil can be washed out by percolating water. In urban environments, they can be removed by vandals. Next to highways, de-icing salts can kill plants. In places where an attractive vegetative cover is essential, an irrigation system should be considered.

ClRlA C516


Care should be taken to check that the subsequent migration of the topsoil would not reduce stability. Planting can bring problems with respect to root damage and from wind loading on large trees (see Section 6.6).

A schedule of obstacles which can hinder the establishment of vegetation is reproduced in Table 2.1

The establishment of vegetation benefits wall stability, but the effects are usually shallow and should not be relied upon in design.

Table 2.1 Obstacles to vegetation establishment (after Barker, 1997)

1 Selection: 0 wrong plants 0 wrong planting method 0 wrong planting season

2 Basic plant needs not catered for: 0 light (shaded location) 0 water (drought or waterlogging) 0 nutrients (eg infertility or toxicity from heavy metals)

gases (over-compaction removes air and water voids; presence of methane, carbon dioxide, carbon monoxide)

3 Difficult sites: 0 estuarieslsea coast

pipelines 0 industrial waste 0 exposed steep slopes on uplands

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4 Maintenance: wrong treatment, eg overcutting broken fencing allowing grazing or erosion by walkers

5 Vandalism

43


3

3.1

3.2

Selection

The previous section presented the different types of modular walls described in this report and illustrates their essential features. Ths section will examine the various retaining wall characteristics, presenting them in a form that will aid the choice of the appropriate system or systems for a particular location. The section does not provide technical detail and is intended for the guidance of the owner and non-specialist specifier. It should be read in conjunction with the suppliers’ brochures and guides for the particular proprietary products under consideration.

Oliphant (1997) presents a number of case histories where a choice was made between different types of retaining walls. His flow chart, which is reproduced as Figure 3.1, leads to selection of wall type.

CONSTRUCTION CONSIDERATIONS

These are important in the choice of wall type. The location of the site and its accessibility are relevant, as are the ground conditions where the wall will be built and the type of material to be retained.

A site could have special requirements with respect to construction noise and limited hours for working, particularly if it is in a residential area. A rural site, perhaps with difficult access for road vehicles, might need a temporary hardstanding for trucks and cranes. There may also be difficulty with headroom for cranes below overhead wires on any site.

Compaction of the retained fill will be necessary and there will have to be access for suitable equipment. Compaction could be by rolling or ramming (a technique that is useful in very restricted urban locations).

A restriction that severely limits the use of any gravity wall system occurs where ground movements have to be very closely controlled next to an adjacent structure. In this situation the excavation to allow the wall to be installed could lead to ground movements large enough to cause damage. Wall types requiring minimal excavation are likely to be more suitable in urban situations, whle in other sites the amount of excavation required is unlikely to be a constraint.

Some systems are reusable and can be safely dismantled and re-erected in a different location. Precast reinforced concrete stem wall systems are used for raw materials retention in this way.

APPEARANCE

44

Modular walls offer a very wide choice of external finish. In towns hard rigid facings may be the most appropriate, particularly if the public has access right up to the wall. Facings can be in brick, block or profiled or exposed concrete, all of which are vandal- resistant although a medium for graffiti. Where separate facing walls are used, careful detailing is required, for example the facing wall sharing a common foundation, the provision of adequate ties, leaving a sufficient gap to allow deflection of the retaining wall, and measures to prevent the build-up of water in the gap.

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Obtain site and project information and define design criteria

v Carry out desk site surveys

Conceptual design

Compile catalogue of alternative wall types. l- Preliminary design

Step 1

Reduce catalogue to a few alternatives based on selection criteria and preliminary assessment of stability

Preliminary design Step 2

Carry out comparative cost study and environmental assessment

Figure 3.

SELECT RETAINING WALL

Proposed procedure for the outline design of earth-retaining walls (after Oliphant, 1997)

In the countryside and in urban areas where there is soft landscaping walls can be chosen for their compatibility with their surroundings, which will often involve planting.

A further environmental issue that relates to appearance is noise. Modular walls can be made with irregular facings, which scatter reflected sound. They can also have a voided front face to act as a sound absorber. Crib and gabion walls require special attention to detail for situations at the side of public walkways, where poorly turned wire or poorly treated timber could catch clothing. Unless they are provided with an impermeable stub wall at ground level and a drainage system, their free-draining properties might lead to an adjacent footpath becoming persistently wet and causing algae growth or icing in winter. Dry stone walls may also have this problem.

Modular systems are versatile and a wide range of facings is possible. Manufacturers of proprietary systems can often modify their products to provide a particular effect. In addition to the modular walls covered by this guide, reinforced soil systems can provide comparable facings.

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3.3

3.3.1

3.3.2

3.3.3

3.3.4

DURABILITY

Corrosion and degradation

The modular systems described in this guide are all potentially durable for long design lives. Correct standards of design for internal and external stability, manufacture, construction and maintenance should be used, as appropriate for the location.

Reinforced concrete should be designed and produced to the relevant codes and with a sufficient cover of good quality concrete to embedded reinforcement steel. Units placed close to carriageways should be designed to cope with water spray carrying chlorides from road de-icing and, in some arduous locations where very cold winters are experienced, air entrainment should also be considered. Units near carriageways may also have to be resistant to vehicle impact. Safety barriers can protect wall faces from accidental damage.

The foundation soil and backfill should be compatible with the wall materials used. For instance, sulphate-bearing soil would require concrete of adequate sulphate resistance.

Concrete should be dense, well compacted and properly cured to maximise its potential strength. Timber should be treated to prevent rot or insect infestation, and particular attention should be paid to re-treating any faces cut during construction. The timber supplier should provide a guarantee for the service life of a treated timber retaining wall, which should exceed the design life. Wire and mesh used in gabion manufacture should be zinc or zinc-aluminium coated, or they may be protected by plastic or thermoplastic polymer coatings.

Certain stored materials (such as manure or fertilisers containing phosphates and nitrates) can affect the durability of some of the wall components. Special measures may be required to provide unimpaired use over the wall’s design life.

Fire

Resistance to fire is a factor in assessing durability and should play a part in the choice of retaining wall materials. It s e e m unlikely, unless a fire is extremely intense, that a wall would fail. Timber, steel or concrete would all suffer to some extent in a fire. Some highway authorities preclude the use of timber crib walls close to highways, due to concerns about having to replace a wall following a fire.

Fill materials

Fill materials in crib and gabion walls should be resistant to erosion and frost.

D ra i nag e

46

If drainage is provided, measures should be put in place to prevent clogging of drainage systems and to allow clogged systems to be unblocked. More information on drainage is given in Section 6.5.2.

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3.4 ECONOMY

Modular gravity retaining walls are usually chosen because they are an efficient means of providing an attractive facing. From the fact that all the wall systems covered by this guide are sold in relatively large volumes, it follows that each holds a suitable balance of economy for appropriate applications. Yonan (1993) has presented data which show that for a particular application, gabions and reinforced soil were approximately two thuds of the cost of an in situ reinforced concrete retaining wall and only about one third of the cost of providing sheet piles.

Modular wall systems are essentially to be assembled on site. The assembly may require a manual approach (as in crib walls, for example) or they may require large cranes as in the case of reinforced concrete stem walls. Intensive manual methods may take more time than the automated methods of erection, which could lead to a trade-off between the construction programme and availability of heavy construction plant. The potentially rapid speed of erection of modular systems makes their use attractive in applications such as motorway widening, where the costs of delay can be prohibitive. Some systems, such as masonry walls, require a planned approach for fill placing and compaction.

The labour issues and the relative cost of the components, some of whch can be relatively inexpensive because of mass manufacture make it very difficult to make any general comparisons on cost. The local market situation also has a significant effect.

For larger projects, knowledgeable engineers or quantity surveyors can provide data on the retaining wall system which is most economical for a particular situation. For smaller schemes where such advice is not available, manufacturers can be asked to indicate the suitability of their product. When a number of manufacturers are advising, the specifier should check that the proposed schemes are of similar extent and any differences in schemes for competing products should be clarified.

Modular retaining wall systems all offer economies due to their prefabrication and standardisation of components. They are often cheaper than in situ forms of retaining wall construction, although they are sometimes less versatile in particular circumstances.

SYSTEM SELECTION

Table 3.1 summarises the advantages, limitations and other characteristics of the systems described in Sections 2 and 3. It is an aide mdmoire for selection.

3.5

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48

. . . .

. . . .

. . . . . . U

5 .o .o m m c c

- 0 -

. . . .

. . .

. . . . 3 E C .- m

. . . .

. .

. . . .

. . . .

. . . . . .

. .

. . . .

. . . .

8 Y

0 -0 -

. .

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. . .


4

4.1

4.1 .I

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I

Design concepts

DESIGN PROCESS

Designer’s technical responsibilities

It is important that responsibility for all parts of the design is clearly assigned before the design process is started. This includes interpretation of the ground investigation and the carrying out of calculations for external and internal stability. The timing for the ground investigation might depend on the geotechnical category selected for the structure (see Section 1.6).

The different parts can be broken down into three common elements.

Ground investigation

A desk study (see Section 4.6.1) followed by a ground investigation (see Section 4.6.2) will normally have been executed. For geotechnical category 1 walls, the ground investigation may be delayed until construction starts. Although the ground investigation may not have been principally aimed at the retaining wall site (these walls are usually ancillary to a larger development), it should provide sufficient information to allow the wall to be designed and constructed. If site-won materials are to be used as backfill or for filling gabions, the ground investigation should include testing to assess their suitability. Responsibility for the scope and interpretation of the ground investigation will depend on how the contract for the ground investigation is let. Outputs include the (factual) ground investigation report (Section 4.6.2), from which a geotechnical design report should be prepared (Section 5.3).

Wall design

The responsibilities of the designer under the CDM Regulations are outlined in Box 1.1.

The retaining wall has to be designed for internal stability and external stability. Strategies allowing different parties to undertake internal and external design are outlined in Section 1.7.

Wall construction

Either through the specification or by other means, the essential assumptions underlying the design should be communicated to those supervising construction on site. Any circumstances encountered on site, such as a different backfill, a different overall slope angle or poorer ground which could produce an adverse situation for the wall design, should be reported by the installer to the designer to make sure that they have been accounted for by the design.

49


4.1.2

Name of scheme

Checking and approval

Project reference

Depending on the situation of a retaining wall and the consequence of its failure, some checking and formal approval that an appropriate design has been carried out should be made. The concept of geotechnical categorisation of retaining walls following EC7 is explained in Section 1.6. For category 1 walls, the design check may be rudimentary. It may solely rely on a check on the approach followed and be carried out by the particular design team leader. For category 2 retaining walls, a formal check should be carried out by another engineer to confirm that the input data are sufficient, that the internal and external stability are consistent and complete and that the design intent has been adequately communicated by means of the drawings and specification. An example of such a design check certificate is given in Figure 4.1.

Name of structure I Date of certificate

Key data

Surcharge

Drainage

Backtill

Retaining wall type

Source of data - give reference

Site investigation

Internal stability design calculations

External stability design calculations

Specification and drawing references

Subject to the comments given below, we certify that reasonable professional skill and care has been used in the design of this retaining wall and that the: 0 site investigation information is adequate

scheme requirements have been adequately described internal and external stability designs are complete and consistent design has been accurately described in the contract drawings and specification.

Signed

.............................................. ...... ...... .............................................. . . . . . . . . . . . . Design office team leader Date Principal Date

Comments

50

Figure 4.1 Example of a wall design check certificate

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The Highways Agency has a well-developed checking and approval system which is set out in Box 4.1. Some highway authorities impose more onerous checking requirements.

Box 4.1 Highways Agency checking categories

The Highways Agency, as an expert client, gives much advice on the checking of designs and their requirements are often used more generally. BD2/89 Part 1 divides structures into Categories 0-111 according to cost, complexity and the consequences of failure. Basic details for the structure are summarised in a standard format in an Approval in Principle (AIP) document, which is vetted and approved by the client's Technical Approval Authority (TM). Structures with a retained height greater than 1.5 m and with a front face angle of 70-90" qualify as structures that require AIP. Lower face angles are classified as slopes and require geotechnical certification instead.

The AIP is used to check the appropriateness of design assumptions. Check certificates for the design require signatures according to the category of the structure. These categories are different from the geotechnical categories proposed by EC7.

There are four categories.

Category 0

Minor structures which conform in all ways to Departmental Standards. They correspond in many cases to EC7 geotechnical category 1 structures, but would also include some geotechnical category 2 structures. They do not require AIP, but do require certification. Checking should be carried out by another engineer in the design team. The example given is retaining walls with less than 3 m retained height

Category I

Simple structures which can be analysed by 'static methods' and where all aspects of design are in accordance with current Departmental Standards. AIP and certification are required. Checking should be carried out by another engineer within the design team. Retaining walls of 3-7 m retained height are cited as examples.

Category I1

Intermediate structures which have redundant features and may contain departures from, or aspects not covered by, current Departmental Standards. The checking team may be from the same office but should be independent of the design team. Separate design and check certificates are required. Difficult foundation problems are given as an example.

Category 111

Complex structures which require sophisticated analysis of highly redundant features, and where consequences of failure would be severe. They correspond in many cases to EC7 geotechnical category 3 structures, which are not covered by this guide. The checking team should be selected for their special knowledge and experience and should be from a different organisation to the design team. Separate design and check certificates are required.

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4.2

4.2.1

4.2.2

LIMIT STATE DESIGN

Philosophy

When a retaining wall, or part of it, fails to satisfy any of its performance criteria, the wall is deemed to have reached a ‘limit state’. This design guide follows the limit state design method in which various limit states are considered separately in the design and their occurrence is either eliminated or is shown to be sufficiently unlikely. The ultimate limit state and serviceability limit state should be separately considered. Definitions for each are given in Sections 4.2.2 and 4.2.3. Once it has been established that the retaining wall has an adequate factor against the ultimate limit state, the likely wall deflections should be calculated and they should be compared to the deflections permitted for the serviceability limit state. Methods for calculating wall deflections are given in Section 6.3 and settlements in Section 7.2.3.

EC7 requires the following limit states to be considered for gravity retaining walls:

e

e

0

e

e

e

e

e

e

e

bearing resistance failure of the soil below the base

failure by sliding at the base of the wall

failure by toppling of the wall

loss of overall stability

failure of a structural element such as a wall, anchor, wale or strut or failure of the connection between such elements

combined failure in ground and in structural element

movements of the retaining structure which may cause collapse or affect the appearance or efficient use of the structure, nearby structures or services which rely on it

unacceptable leakage through or beneath the wall

unacceptable transport of soil grains through or beneath the wall

unacceptable change to the flow of groundwater.

The designer has to judge whether additional limit states should be taken into account in respect of the particular site conditions or other specific requirements not covered by this guide.

EC7 Clause 2.4.2 (14) permits partial factors for actions (forces) to be reduced to unity for accidental situations, the accidental limit state.

Ultimate limit state

The ultimate limit state is defined as the state beyond which a failure mechanism can form in the ground or in the retaining wall, or severe structural damage occurs in principal structural elements. Annex B of HA document BD41/97 separates the ultimate limit state into that for the structural elements and that for overall stability of the structure and surrounding soil. Additionally, the ultimate limit state of other structures supported by the surrounding ground should be considered. As BD 4 1/97 covers the design of reinforced brick retaining walls, its explanation of limit state is not directly applicable to more flexible types of retaining walls.

52

For simplicity in design, states which could lead to collapse are often considered in place of the collapse itself, such as forward sliding of a gravity retaining wall. These states are also classified and treated as ultimate limit states.

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4.2.3

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EC7 Clause 8.4(4) requires that, as far as possible, retaining walls should be designed in such a way that adequate warning of danger is given by visible signs. The design should guard against the occurrence of brittle failure, such as sudden collapse without obvious preliminary deformation. The retaining wall system should be designed to exhibit sufficient ‘ductility’ in approaching geotechnical limit states to give visible warning of failure and no additional provision should be necessary,

Designing against the ultimate limit state requires the likely hazards to be identified. For analysis purposes, these hazards are related to particular postulated failure mechanisms. These can be summarised as:

external stability hazards

- bearing capacity failure - overturning - sliding - overall instability including deep-seated slips

0 internal stability hazards - failures of the wall components, modules or system.

External stability calculations are addressed in Sections 7 and 8 and are ultimate limit state checks. It is necessary that calculation checks are carried out for each mode of external stability failure as it is often the case that alteration of wall geometry will have a beneficial effect for one mode of failure, while having a detrimental effect on another mode of failure.

Internal stability is also covered in Section 7 and 8.

Serviceability limit state

The serviceability limit state is defined as a state at which specified serviceability criteria are no longer met. Annex B of BD 41/97 separates the serviceability limit states into that for the structural elements and that for overall stability of structure and surrounding soil. Additionally, the serviceability limit state of other structures supported by the surrounding ground may have to be considered.

Serviceability limit states include strains or movements in a retaining wall which could affect the visual appearance of the wall, result in unforeseen maintenance or shorten its expected life. They also include deformations in the retained ground which could affect the serviceability of any adjacent structures or services. Where appropriate, numerical values should be assigned to the permissible deformations. These will depend on the sensitivity and condition of the structures. Further details are given in Section 4.7.

EC7 Clause 8.7.2(4) requires that where the estimated displacements exceed 50 per cent of the limiting values (selected by the designer to represent serviceability limit state), the design should be justified by calculations for the following situations:

where nearby structures and services are unusually sensitive to displacement

where more than 3 m of soil of high plasticity (liquid limit of more than 50 per cent) is retained where the wall foundation is on soft clay

where comparable experience is not well established.

53


4.3

4.4

4.4.1

Little advice is available in the choice of limiting values. Designers are cautioned against selecting very onerous values unless they are required for a particular reason. Limiting values on a differential movement will often be more relevant because where a long line of retaining wall is visible, distortions along its length will affect its appearance.

DESIGN LIFE

The UK National Application Document (NAD) for EC1 Part 1 replaces that of Eurocode’s Table 2.1. It is reproduced here as Table 4.1.

Table 4.1 Notional classification of design working life (after UK NAD for EC7, Part 1, Table 2.7)

Class Notional design Examples working life (in years)

1 1-5

2 25

3 50

4 100

5 120

Temporary structures

Replaceable structural parts

Buildings and other common structures, other than those listed below

Monumental buildings and other special or important structures

Bridges

Some retaining walls, such as those adjacent to highways, require design lives equivalent to those for bridges.

COMPARISON OF RETAINING WALL DESIGN METHODS

Genera I req u i remen ts

Traditional retaining wall design methods (eg CP2) relied on the provision of adequate restraining forces to resist a set of disturbing forces. The movements required to generate those restraining forces were usually ignored and generally no guidance was given on what soil parameters should be used for the calculation of both disturbing and restraining forces.

The approach was based on a simple definition of factor of safety (defined as restraining forces or moments/disturbing forces or moments), which could be quoted as having been achieved. Values of 2 were usually sought for bearing capacity, sliding and overturning and 1.3-1.5 for overall stability. Sometimes a factor or safety of 3 on bearing capacity was used to limit settlements.

The traditional approach was not compatible with moves to fit geotechnical engineering design into a limit state framework and it has been abandoned by contemporary design guides (such as GEO, 1993; BS 8002: 1994; EC7, 1995).

54

The aim of the new approach to retaining wall design remains the same as before: the design of retaining walls which are serviceable and safe and which continue to remain so for the design life of the structure.

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Two similar approaches are presented in t h s design guide: those of BS 8002 and EC7. A main difference exists between them, however. BS 8002 assumes a ‘worst credible’ set of design assumptions and then seeks only a factor of unity between restraining and disturbing forces (ground and load combinations). EC7 uses ‘characteristic’ (moderately conservative) design assumptions but then uses hgher factors of safety for a number of distinct cases. Designers should be cautious about mixing the two approaches. Where BS 8002 factors are to be used, that code’s onerous assumptions should be applied. If EC7 factors and cases are to be used, the use of BS 8002 design assumptions could result in an overly conservative retaining wall design.

For low-height retaining walls, the forces involved are relatively small and in many cases a conservative design may not cost much extra. However, the application of an unexpected load case (such as an increase in the surcharge loading applied behind the retaining wall or the excavation of a trench in front of it) may result in the wall failing one of its limit states. Great care is therefore required to define the design cases that need to be considered.

EC7 Clause 8.4(4) suggests that, as far as possible, retaining walls should be designed in such a way that adequate warning of danger is given by visible signs. The aim that a wall should fail in a ductile rather than a brittle manner should apply to both EC7 and BS 8002 retaining wall design approaches.

4.4.2 Use of EC7 in design

Application of EC7 involves the selection of a geotechcal category for the structures or part of a structure under consideration, the selection of appropriate design parameters and the carrying out of a design. For a category 2 structure (see Section 1.6 for the requirements of the various geotechmcal categories), this will require quantitative geotechmcal data and analysis.

The Eurocodes have identified three design cases (A, B and C) for which each design should be verified, as relevant. EC7 makes it clear that all structures must satisfy all three cases, both structurally and geotechnically. Case A primarily relates to buoyancy problems, where water uplift forces comprise the main unfavourable action, and so would not be relevant for the vast majority of low-height modular retaining wall designs. Case B is often critical to the design of the strength of structural elements. It is not applicable to problems where there is no strength of structural materials involved. Case C is generally critical for geotechnical stability of the structure (bearing capacity, sliding and overall stability) and it is therefore often critical for the sizing of structural elements in geotechnical problems such as retaining wall design.

The partial factors that should be applied in each of the cases are shown in Table 4.2. It is important to note that the partial factors on actions are multipliers and increase the applied forces (actions), while those on ground properties are dividers and reduce the ground strength. Reducing the ground strength also has the effect of increasing the active pressures.

For accidental design situations, all numerical values of partial factors for actions should be taken equal to unity.

Design values of ground properties xd should be derived from the characteristic value xk (EC7, Clause 2.4.3) using the equation

x d =xkl h

where yn, is the relevant partial factor for the ground property.

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4.4.3

56

Table 4.2 Partial factors - ultimate limit states in persistent and transient situations (after EC7, Table 2.1)

Actions Ground properties

Case Permanent Variable tan 4 c’ cu q u Unfavourable Favourable Unfavourable

Case A 1.00 0.95 1 S O 1.1 1.3 1.2 1.2

CaseB 1.35 1 .oo 1 S O 1.0 1.0 1.0 1.0

Case C 1.00 1 .oo 1.30 1.25 1.6 1.4 1.4

Design action = action x factor Design property = property I factor

EC7 Clause 2.4.3(5) warns that when selecting characteristic values for soil and rock properties, they should represent a ‘cautious estimate’ of the values which will govern the behaviour of the ground.

The simplest approach to the use of EC7 for retaining wall design problems is to solve first for case C which uses factored design strengths for soils but which leaves permanent (dead load) actions unfactored and variable (live load) actions with a factor of 1.3. This usually gives the size of the retaining wall that is required. For integral walls where the strength of structural elements is important, case B then needs to be analysed where soil strengths are not factored but actions (including earth pressures calculated for newly placed fill) are factored. Forces from soil such as earth weight and earth pressure in virtually all cases can be considered as permanent, while surcharges can be considered as variable. This initial design should then be checked again to confrm that cases A, B and C have all been satisfied. Design of the structural elements should then be carried out with compatible codes (eg EC2 for reinforced concrete design).

Use of BS 8002 in design

The main British Standard used for the design of low-height modular retaining walls is BS 8002: 1994 Code ofpractice for earth retaining structures. It gives guidance on how the design data should be gathered and used for various situations.

The code (Clause 3.1.8) gives guidance for the selection of soil strengths to be used in design. The strength value is chosen on the basis of:

(i) limitation of deformation (serviceability limit state) - apply mobilisation factor M to peak strength, ie

whereM= 1.2

If a total stress design is being carried out, the mobilisation factor M applied to the representative undrained shear strength should be at least 1.5 if displacements are required to be less than 0.5 per cent of the wall height. Clays requiring large strains to mobilise peak strength will need larger factors than 1.5.

(ii) limitation of strength (ultimate limit state) - choose k 5 &it to maintain ductility and thereby avoid a brittle collapse.

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4.5

4.5.1

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BS 8002 Clause 3.1.9 tends to produce earth pressures which overestimate those on the active side of the wall and underestimate those on the passive side of the wall, for the small wall deformations that occur in the working state. By following a ‘worst credible’ design approach, considering:

0 highest credible water table

0 onerous surcharge

0 maximum over dig 0

0 allowance for corrosion,

BS 8002 claims that the design method produces ultimate forces for input into designs using other British Standards. Unfortunately not all other British Standards follow the same approach and some require further factoring (eg BS 81 10 for concrete walls and BS 5628 for masonry walls).

allowance for scour or service trenches

A general introduction to BS 8002 is given by Akroyd (1996) and the subtleties of that code’s design approach are described by Bolton (1996). Cole and Watt (1994) suggest that additional factors are required for the internal design of all masonry walls. Puller and Lee (1996) compare BS 8002 to previously-used design approaches and were critical of the logic behind mobilisation factors.

STRENGTH FOR DESIGN

Effective stress principle

The external stability of a wall depends on the strength of the soil retained and the strength of the soil below and in front of the wall. It also depends on the pore pressures in the groundwater and in water which may fill cracks in the ground. Soil strength depends on a number of factors, including whether the soil is drained or undrained, the I

displacements allowed and the pore water pressures.

The following introduction to soil strength is necessarily simplified. It covers the basic considerations for choice of strength parameters for the simple design of walls. Following these principles will lead to safe but conservative designs. The selection of less conservative soil parameters and the use of more complex analyses should only be undertaken by those with specialist knowledge in geotechnical design.

A clear understanding of effective stress is necessary for successful design of retaining walls. The principle of effective stress, discovered by Terzaghi, is very simple but absolutely fundamental. It says that all soil behaviour, including strength, is governed by the effective stress #‘given by

#I= CT- U (4.3)

where CT is the total stress, the prime represents effective stress and U is the pore pressure. The total stress (T arises from the bulk unit weight of the soil, external loads and from any free water. This means that water in cracks in the ground applies a total stress to the sides of the crack. In saturated soil the pore pressure U is the pressure in the pore water.


4.5.2

58

Due to the fact that water cannot transmit shear stresses, total and effective shear strengths in the ground are equal and

r’= r (4.4)

Equations (4.3) and (4.4) define effective stresses. The principle of effective stress says that it is these which govern soil behaviour in general and soil strength in particular.

Drained and undrained conditions

Both the soil grains and pore water are virtually incompressible and so, in saturated soil, volume changes can occur only if water drains from the pores. The speed at which water drains depends on the pore pressure gradients and especially on the permeability of the soil. Permeability can be very large (ie water can drain quickly) or very small (ie water can only drain slowly).

Permeability of soil depends principally on the grain size and especially on the sizes of the smallest grains. Relatively coarse-grained soils, clean sands and gravels have high permeability and drain quickly. Fine-grained soils, those containing clay and fine silt sized grains, drain slowly.

If the soil can drain quickly, drainage of the soil (in the soil mechanics sense) will occur during construction and the pore pressures are always in equilibrium with the groundwater and drainage conditions. In this case the conditions are called ‘drained’. If the soil drains slowly, there may not be time during construction for drainage to occur. In t h s case the volume and water content of the soil does not change and the conditions are called ‘undrained’.

If soil is loaded or unloaded, undrained pore pressures will change and they will not be in equilibrium. With time, the out-of-balance pore pressures will change until they reach the equilibrium condition corresponding to the drained condition. This process of changing pore pressure at constant total stress is called ‘consolidation’. During consolidation the effective stresses change because the pore pressures change. The final condition of undrained loading followed by consolidation is equivalent to drained loading. Consolidation is also used for the specific case of an increase in total stress leading to excess water pressures which, as they reduce, lead to a reduction in soil volume. The opposite process of a reduction in total stress leading to an increase in soil volume is called ‘swelling’.

Where a retaining wall cuts into the ground, its construction reduces total normal stresses (but increases shear stresses), and so undrained construction reduces the pore pressures. Consequently, during swelling, pore pressures rise and so effective stresses reduce. This means that during the time after completion of construction of a retaining wall, the strength of the soil reduces as the pore pressures rise.

The principal differences between drained and undrained conditions and the implications for design calculations are described below.

Drained conditions

Water drains during construction so water contents change. The pore pressures are in equilibrium and can be determined. Calculations are done in terms of effective stresses. The calculations are called ‘drained’ or ‘effective stress’ analyses.

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4.5.3

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Undrained conditions

No water drains during construction and so water contents remain unchanged. Pore pressures are not in equilibrium and cannot be calculated easily. Calculations are done in terms of total stresses. The calculations are called ‘undrained’ or ‘total stress’ analyses. In virtually all cases, the soil will have drained before the design life of the wall is reached and analysis should also be carried out for drained conditions.

Soil strength

In general terms, soil strength is the maximum shear stress which the soil can safely sustain. T h s depends on the effective stress, the water content and the displacement or strain which is allowed.

Figure 4.2 illustrates typical behaviour of soil during shearing. If the soil is undrained the volume remains constant but pore pressures change as shown. If the soil is drained the pore pressures remain constant but the volume or water content changes in the manner shown.

As shown in Figure 4.2, there are three characteristic points at which the shearing resistance is a strength.

Peak strength

This is the maximum shear stress. It occurs at relatively small strains (often about 1 per cent in a laboratory test) and therefore at relatively small displacements (1 per cent strain corresponds to about 1 mm for a typical laboratory specimen). At the peak state the pore pressures or volumes are changing.

Constant volume strength (also called critical state, fully softened or ultimate strength)

This is a strength when the soil continues to strain at constant shear stress and constant volume and pore pressure. The constant volume strength is reached after moderate strains in a laboratory test (often about 10 per cent in a laboratory test) and after moderate displacements (often about 10 mm).

Residual strength

This is the very smallest strength a soil can have. In clays in which the grains are flat and platy the residual strength may be as little as half the constant volume strength. In other cases in which the soil grains are more rounded or angular the residual strength is the same as the constant volume strength. Lupini et aZ(l98 1) show that residual strength becomes significant in fine-grained soils with plasticity index greater than 20 per cent. In clays the residual strength is only reached after relatively large displacements.

59


4.5.4

60

Shear stress

~ Peak

Constant volume (Critical state)

Residual - - - - _ (Clay only) I-- - -

1 I I * - 1 O h - 1 0% >100mm

-1 mm -10mm Strain or displacement

Change of pore pressure &change of volume

_ - - - - I Strain or displacement

Figure 4.2 Mobilisation of different soil strengths

Drained and undrained soil strength

The designer must choose whether to carry out analyses in terms of effective stresses (for drained conditions) or in terms of total stresses (for undrained conditions). The choices are considered firther below. The strength parameters required will be different in each case.

Figure 4.3 illustrates, in simplified form, the general relationships between soil strength, effective normal stress and water content. This diagram applies generally to peak, constant volume and residual strength but the strength parameters will be different in each case.

At failure at a particular water content wf , the effective normal stress is df and the shearing resistance rf is the strength. The soil will have different strengths at different water contents and different normal effective stresses.

In terms of effective stresses, the strength is given by:

q = c’ + a; tan4 (4.5)

where c‘ is called the cohesion intercept and #‘is called the angle of friction or the angle of shearing resistance. Equation (4.5) is in terms of effective stresses and so it is for drained conditions. It can only be written in terms of effective stresses.

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Strength T' or t

t Strength t' or t

t

U' I f I Effective normal stress U' I I

Water content I

t I I I

I

W F

f Water content

I I I I

F

Effective normal stress (7' f

0'

Figure 4.3

For undrained conditions the water content does not change (by definition) and, at a particular water content and for undrained conditions, the strength is given by:

Relationship between stress, strength and water content in a soil at failure

rf = c, (4.6)

where c, is called the undrained shear strength. It is essential to emphasise that equation (4.6) applies only for undrained loading when the water content remains constant and so the strength remains constant too.

The soil parameters required for design calculations, and which must be determined by laboratory testing, by in situ testing or by some other means, are:

e for drained conditions: c' and 4' 0 for undrained conditions: undrained shear strength c,.

Sometimes a parameter $,, is given. Because undrained strength depends only on water content, it is independent of the total normal stress at failure of. Consequently $,, must be equal to zero. If values for $,, are given which are not zero then either the samples were unsaturated when they are tested or there was something wrong with the sampling and testing procedures. In such a situation, the soil parameters should be reappraised by a specialist before using them in calculations.

61


4.5.5 Choice of strength for design

In choosing parameters for the design of retaining walls, the designer is faced with two basic questions. Firstly, are the calculations going to be made for undrained conditions in terms of total stresses using the undrained strength c, , or for drained conditions in terms of effective stresses using the effective stress parameters c’and qY? Secondly, which strength, peak, constant volume or residual is appropriate for the design?

These are choices which the designer has to make. They will depend on circumstances such as the drainage conditions in the soil, whether the wall is a temporary or a permanent structure, and what the factors of safety, risks and consequences of malfunction will be. These choices are ultimately for the designer but some general guidelines can be set.

Normally designs should be for drained conditions taking the worst set of pore pressure conditions envisaged. These will give safer designs than undrained analyses in most cases. If undrained analyses only are used, the designer should be satisfied that there will not be any significant drainage during the design life of the wall. The decision to undertake only a total stress analysis requires a very thorough understanding of the drainage behaviour of the soil and is not generally recommended.

If an undrained analysis is used, consideration should be given to the development of vertical cracks which may fill with water during rain. The design should be checked for the case where there is a full hydrostatic water pressure distribution of the active side unless it can be shown that this is impossible.

BS 8002 and EC7 give advice on the choice of parameters that should be used (see Section 4.4).

If investigations reveal the presence of old landslides with slip planes in directions favourable for slipping the residual strength should be considered for these slip planes. ,

4.6

4.6.1

62

SELECTION OF GEOTECHNICAL PARAMETERS

Desk study

The site investigation should be appropriate to the category of retaining wall to be built. A site investigation should consist of both of the following:

0 deskstudy

ground investigation.

Guidance on what should be covered by a desk study is given in BS 5930 and EC7 Clause 3.2.1 (2). The following should be taken as a minimum:

0 walk-over survey

0 inspection of geological maps inspection of topographical maps, including historical maps and photographs

collection of easily accessible previous geotechnical data for the site

discussions with local people with experience of construction in the area (the local authority building control office or county council materials laboratory is often helpful in this regard).

ClRlA C516


r

It will often be necessary to check old maps and aerial photographs for a complete picture to be formed of the likely problems on the site. Particular features whch would be identified by a good desk study include:

existing trenches for drains or other services

historical features such as old borrow pits or buried channels

geological features such as a subgrade clay exhibiting residual shear surfaces or a chalk containing solution features

the possibility of the structure being affected by subsidence from old mines ( h p Geotechcs, 1990 gives details of areas of the country affected by old mines while Atkinson, 1993 and BD 10 give guidance on measures to avoid damage due to the collapse of old mine workings)

contamination of the ground or groundwater, based on previous historical use of the site.

BS 5930, HA34 and Atkinson (1993) give more comprehensive lists of data sources whch may be useful for particular circumstances.

It is very important that the desk study is properly carried out as it identifies the hazards that will be investigated in the ground investigation. An inadequate desk study leads to an inadequate ground investigation, which increases the risk of ground-related problems arising during construction.

4.6.2 Ground investigation

The ground investigation for a geotechnical category 2 retaining wall in relatively uniform ground conditions, where no abnormal features were revealed by the desk study, would typically involve exploration points at approximately 20-40 m centres (EC7 Clause 3.2.3) with a minimum of two exploration points being required for the shortest walls. BS 8002 Clause 2.1.2 suggests a borehole spacing along the retaining wall of 10-50 m centres, depending on the geological conditions. For a backfilled gravity wall the borehole depth should be at least twice the proposed wall height whde for a wall formed by excavation, the borehole depth should be at least three times the proposed retained height. Trial pits can provide useful information on ground and groundwater conditions. They can either be used to supplement boreholes or, in some cases, to replace some of the boreholes where the depth of the trial pits will provide equivalent information. For linear features, such as retaining walls, there is often a tendency to position all the investigation points along the line of the retaining wall. However, some points should be positioned both behind and in front of the retaining wall to give a clear indication of variation of ground conditions across the retaining wall section as well as along it. EC7 recommends that the depth of exploration should extend beneath the founding level by 1-3 times the width of the foundation. Where locally won fill is to be used as a backfill, the ground investigation should also collect information on its properties and variability. Recommendations for the execution of the ground investigation are given in BS 5930 and for the soil testing in BS 1377.

Irrespective of the method used to procure the ground investigation (possible methods are given by SISG, 1993), the designer co-ordinating the design of the retaining wall should check that an appropriate ground investigation has been carried out. The SISG documents recommend that a client’s principal technical advisor (PTA) should appoint a geotechnical advisor (GA) to advise on the extent of ground investigation work required. Figure 4.4 shows the decision-mahng process that should be followed during a site investigation.

ClRlA C516 63


RESPONSIBILITY ACTION

STAGE I: Initial engineering assessment _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ - - - - - - - - - - - - - - - - - - - -

PTNGA: (Geotechnical Advisor)

PTA (Principal Technical Advisor) Arrange meeting to appoint GA I Initiate engineering assessment: - establish scale of engineering project and

performance expectation or requirements; - define Project Work Plan

_ _ _ _ _ _ _ _ _ _ _ _ - - - - - - - - - - - - - - - - - - - - - - - - - - - - . STAGE 11: Preliminary site appraisal

GA:

PTA Authorise GI

4 PTNGA: Appoint GC

GNGC: (Geotechnical Contractor) +

Control, supervise and adjust GI as conditions emerge

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ - - - - - - - - - - - - - - - - - - - - - . STAGE IV: Interpretation and reporting

GNGC: (Geotechnical Contractor) v

Prepare and agree factual report of all data

Initiate site appraisal through - deskstudy - walkover survey

A L

PTNGA:

PTNGA:

ground and vice versa; identify

Send preliminary report to client

1

GA:

PTNGA:

Establish scope of Ground investigation (GI); desian GI in detail

I Question if client’s brief has changed I

GA:

PTNGA:

GA:

is required

Produce full Geotechnical Report

64

Figure 4.4 Decision-making process of site investigation (after SISG, 1993)

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4.7

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It is bad practice to invite competition for the extent of ground investigation, as this rewards the smallest proposal with the least number of exploratory holes, thereby increasing the risk of unforeseen ground conditions being encountered during construction.

The results should be summarised into a ground investigation report (see EC7 Clause 3.4 for suggested content).

MOVEMENTS AND DAMAGE PREDICTIONS FOR NEIGHBOURING STRUCTURES

If a structure is built on the ground behind a retaining wall, the retaining wall should be reappraised for the new surcharge conditions. The surcharge could cause the retaining wall to displace laterally, resulting in settlement of the retained ground and damage to the new structure.

Where a new retaining wall is built to create a cutting beside an existing structure, a number of aspects of stability should be checked in addition to the checks carried out for a normal design. These are:

the bearing capacity of the existing structure for the temporary slope formed to allow the retaining wall to be built (this temporary condition is often the most critical)

settlement of the newly placed fill as the retaining wall deflects under the active pressures

the effect of vibration from construction activities (these are rarely critical unless plant that causes significant vibration is used).

The wall could cause differential settlement to the structure. Burland et aZ(l977) explain the different modes of movement and their structural significance. Boscardin and Cording (1989) give data to allow the significance of movements to be assessed. These problems are complex and should be analysed by a specialist.

In some cases, underpinning of the neighbouring structure will be necessary. Various underpinning systems are summarised by Thorburn and Littlejohn (1993). Examples are shown in Figure 4.5.

65


Virtual back of

wall New structure i

Additional surcharge on active wedge

I

a) New structure built behind existing retaining wall

Temporary case

IL Permanent case

Temporay excavation for retaining wall

b) New retaining wall built in front of existing structure

Figure 4.5 Interaction between a retaining wall and another structure

66 ClRlA C516


5 Material properties

5.1

5.1 . I

5.1.2

MEASUREMENT

Unit weight

The unit weight of coarse-grained materials cannot be reliably measured using simple ground investigation techniques. Sand replacement tests in the base of a (safely shored) trial pit can give usefkl data for dry coarse-grained materials but are slow. The unit weight of stiff saturated clay can be determined from the weight and volume of soil in a U102 sample.

Strength

A clear distinction has to be made between the assessment of undrained and drained soil parameters. The strength of any particular soil layer should be described either by drained parameters (c; 4') or undrained parameters (cu, q5,, = 0), and never by a combination of the two. An explanation of the relevance of the types of parameters is given in Section 4.5. Testing methods are given in BS 1377.

'Quick' unconsolidated undrained (UU) triaxial tests can only be used for measuring the undrained shear strength c,. For the reliable measurement of drained strength requirements, specimens must first be consolidated. Even with the use of side drains, drained testing rates for clays can be very slow so clays often have their drained parameters measured in consolidated undrained (CU) tests, with pore water pressures being measured while the sample is sheared undrained. In coarse-grained materials, consolidated drained (CD) triaxial tests are often feasible on recompacted disturbed samples.

The use of multi-stage triaxial tests where the strength is repeatedly measured on the same specimen at successively higher cell pressures is not recommended.

Undrained strength

Undrained strength determinations can only be made on fine-grained materials. Hand vane tests in trial pits provide a rapid and economical method of measuring undrained shear strengths up to 120 kN/m2. Pocket penetrometer results are less reliable but can be used to estimate undrained shear strengths up to 250 kN/m2. Undrained shear strengths measured from U1 02 samples are only reliable if:

metal sample tubes are used (plastic liners have a poor sampling ratio and allow the sample to be damaged prior to testing)

the sample is hlly sealed with wax to prevent it losing its moisture which modifies its strength prior to testing.

0

As the undrained shear strength can vary in a single soil layer, both spatially and in depth, a number of measurements should be made.

ClRlA C516 67


5.2

5.2.1

Drained strength

The drained strength is normally measured either in the triaxial apparatus or in the shear box. The triaxial strength is often about 2’ lower than the shear box strength due to the different conditions of testing. The shear box strength conditions are those most relevant to retaining wall design.

When specifying drained strength testing, at least three strength measurements should be made and they should be made over a similar normal stress range to that mobilised by the wall on the particular soil layer (for the design of retaining walls covered by this guide, normal stresses higher than 100 kNlm2 are unlikely to be needed). Drained strength measurements are more susceptible to variations from the quality of sampling so better sampling techques should be used. For tests on coarse-grained materials, the target density for each test should also be specified as it has a very significant influence on strength.

DESIGN VALUES

Weight density

EC7 Clause 3.3.3( 1) requires that the unit weight be determined with sufficient accuracy to establish design values of the actions which derive from it. The unit weights shown in Table 5.1 are recommended for use in design, in the absence of reliable site-specific data. Often the values of unit weight given in Table 5.1 are more reliable than measured data. ‘Moist’ weight should generally be used for partially saturated soils above the water table, whle ‘saturated’ weight should be used for soil below the water table.

Table 5.1 Unit weights of soils (and similar materials) (after BS 8002, Table 1)

Material ymoist moist weight saturated weight density (kN/m3) density (kN/m3)

Loose Dense Loose Dense

A Coarse-grained

Gravel Well-graded sand and gravel Coarse or medium sand Well-graded sand Fine or silty sand Rock fill Brick hardcore Slag fill Ash fill

16.0 19.0 16.5 18.0 17.0 15.0 13.0 12.0 6.5

18.0 21.0 18.5 21.0 19.0 17.5 17.5 15.0 10.0

20.0 21.5 20.0 20.5 20.0 19.5 16.5 18.0 13.0

21.0 23.0 21.5 22.5 21.5 21.0 19.0 20.0 15.0

B Fine-grained Peat (very variable) Organic clay Soft clay Firm clay Stiff clay Very stiff or hard clay Stiff or hard glacial clay

12.0 15.0 17.0 18.0 19.0 20.0 21.0

12.0 15.0 17.0 18.0 19.0 20.0 21.0

68 ClRlA C516


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The use of 'buoyant weights' in calculations where the total stress is reduced by an equivalent water pressure often leads to error and should be avoided. Instead of using this approach, it is recommended that total stresses, water pressures and effective stresses are separately evaluated and used in calculations as appropriate.

Care should be taken when extracting values of density or unit weight from ground investigation reports. The dry density is often reported, particularly from the results of compaction tests. This value is for saturated soils for oven-dried soil devoid of water.

It can be converted into a bulk unit weight for saturated soils using the following expression:

Y= Yd ( l -t w> (5.1)

The dry density is calculated from

where G , the specific gravity, normally equals about 2.60-2.75 for most soil particles (Bolton, 1981).

ENV 1991-2-1: 1995 gives properties for various materials. Values for angle of repose, which may be useh1 for the design of low-height modular retaining walls, are given in Table 5.2. The angle of repose is approximately equivalent to the critical state friction angle h i t .

Table 5.2 Weight densities and angles of repose for different materials (after ENV 1991-2-1: 1995, Tables 4.1, 4.3, 4.4, 4.6 and 4.7), (continued overleaf)

Material Weight density y Angle of w / m 3 repose (")

Normal weight concrete 24 Masonry

basalt dense limestone granite sandstone hollow glass blocks solid terracotta

Steel

27-3 1 20-29 27-30 21-27 8 21 77

Timber 2.9-9 .O Stored construction materials Aggregates

lightweight normal heavyweight

20 30 20-30 30 >30 30

Bulked sand and gravel 15-20 35 Brick sand (crushed or broken bricks) 15 35 Bulk cement 16 28 Fly ash 10-14 25 Water 10 -

69


5.2.2

Table 5.2 Weight densities and angles of repose for different materials (after ENV 1991-2-1: 1995, Tables 4.1, 4.3, 4.4, 4.6 and 4.7), continued

Material (continued) Weight density y Angle of kN/m3 repose (")

Stored agricultural materials Farmyard

manure (with dry straw) dry chicken manure

Fertiliser, artificial NPK, granulated phosphates, granulated potassium sulphate urea

Fodder, green, loosely stacked Grain

general barley herbage seeds maize inbulk oats oilseed rape wheat in bulk wheat in bags

Grass cubes Peat

dry, loose, shaken down dry, compressed in bales wet

Silage Straw

inbulk(dry) baled

Coal block briquettes, tipped block briquettes, stacked coke

Firewood Peat

black, dried, firmly packed black dried, loosely tipped

manure (minimum 60 per cent solids)

slurry (maximum 20 cent solids)

whole (514 per cent moisture content, unless indicated otherwise)

7.8 9.3 6.9 10.8

8-12 10-16 12-16 7-8 3.5-4.5

7.8 7.0 3.4 7.4 5.0 6.4 7.8 7.5 7.8

1 5 9.5 5-10

0.7 1.5

8 13 6.5 5.4

6-9 3-6

-

45 45 -

25 30 28 24 -

30 30 30 30 30 25 30

40 35

-

- - -

-

- -

35

35 45

-

- -

Undrained shear strength

70

In the absence of site-specific data, the undrained shear strength can be deduced from physical descriptions. These are summarised in Table 5.3. They can also be correlated from Stroud's (1988) correlation with SPT blowcounts, shown in Figure 5.1. T h e 5 factor is chosen depending on the soil's plasticity index and is multiplied by the SPT blowcount N to give the undrained shear strength. In some low plasticity soils, the value of undrained shear strength deduced from SPT data may be more reliable than directly measured values.

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Table 5.3 Undrained shear strength (after BS 5930, Clause 41.2.5)

Term Undrained shear Tactile description strength c, (kN/m*)

Very soft <20 Exudes between fingers when squeezed

Soft 20-40 Moulded by light finger pressure

Firm 40-75 Moulded by strong finger pressure

Stiff 75-150 Cannot be moulded by fingers; can be indented by thumb

Very stiffhard >150 Can be indented by thumb nail

BS 5930 also gives further subdivisions of these strength ranges which are widely used, as follows:

Soft to fm 40-50

Firm 50-75

Firm to stiff 75-100

Stiff 100-150

In undrained analyses, & always equals zero.

10

8

6 - N E . B 7 .I-

4

2

I I I I I I

\ 0 0 0 0\O0

ooo\ 0 0 0 o-o=\ @ O 0

o C b - o - - - O p ; o o 0 0

0 0 0 0

,--

0

0

PLASTICITY INDEX f%\

Figure 5.1 Correlation factor f1 between undrained shear strength and SPT blowcount (Cu/SPT N) in clays

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5.2.3

72

Drained strength parameters

The friction angle to be used in design in the absence of site specific data can be deduced from basic properties, following the guidance of Tables 5.4, 5.5 and 5.6.

Table 5.4 q5kr,t for clay soils (after BS 8002, Table 2)

Plasticity index (percentage) + L i t (degrees)

15 30

30 25

5 0 20 80 15

Care should be used if the presence of pre-existing shear surfaces is suspected. These are caused by historical movement of the clay and residual friction values should be used, which are much lower. Such situations require specialist design input.

The friction angle for sands and gravels mainly composed of silica minerals can be calculated from the following expressions:

4 e a k = 3 0 + A + B + C (5.3)

h i t = 3 0 + A + B (5.4)

Care should be used in applying these formulae for well-graded materials with a clay content greater than about 10 per cent. Such a soil might appear to be, for example, a silty fine sand. A more cautious approach should be adopted as the material could behave more like a finer-grained material, ie as a sandy silt.

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Table 5.5 #‘for siliceous sands and gravels (after BS 8002, Table 3)

A Angularity (’) Rounded Sub-angular Angular

A (degrees) 0 2 4

B Grading of soil (’) Uniform Moderate Well graded

B (degrees) 0 2 4

C N’ (3) (blows per 300 mm) <10 0 20 2 40 6 60 9

C (degrees)

(’) Angularity is estimated from visual description of soil (*) Grading can be determined from grading curves by use of the uniformity

coefficient (see Box 5.1).

Grading Uniform Moderate Well graded

Uniformity coefficient <2 2-6 >6

A step-graded soil should be treated as uniform or moderately graded soil according to the grading of the finer fraction.

(3) Ilr from results of standard penetration test modified where necessary to correct

Intermediate values of A, B and C by interpolation.

for overburden.

Box 5.1 Definition of uniformity coefficient

Uniformity coefficient (U,)

Uc is defined as the ratio of the particle size where 60 per cent of soil grains by weight are finer (D60 ) to the particle size where 10 per cent of soil grains are finer (DIo). Uc = D60/D10.

ClRlA C516 73


74

Table 5.6 4'forrock (after BS 8002, Table 4)

Stratum #'(degrees)

Chalk

Clayey marl

Sandy marl

Weak sandstone

Weak siltstone

Weak mudstone

35

28

33

42

35

28

Note 1 The presence of a preferred orientation of joints, bedding of cleavage in a direction near that of a possible failure plane may require a reduction in the above values, especially if the discontinuities are filled with weaker materials. For instance, weathered coal measures can have #'as low as 12" on planes parallel to the bedding.

Note 2 Chalk is defined here as unweathered medium to hard, rubbly to blocky chalk, grade I11 (see Clayton, 1990)

The values given in Table 5.6 only apply if the rock joint angles have the same orientation as the place on which the coefficient of active pressure K, is being evaluated. If the joints are filled with clay, lower angles may need to be used.

Equivalent correlations from field observations in coarse-grained soils with SPT N value can be made from Table 5.7. The terms and field correlations are taken from BS 5930.

Table 5.7 SPT descriptions

Term SPT N value Field correlation

Very loose 0 4 -

Loose 4-10 can be excavated by a spade; 50 mm peg can be easily driven

Medium dense 10-30 -

Dense 30-50 requires pick for excavation; 50 mm peg hard to drive

Verydense >50 -

For the drained strength, be wary of ground investigation reports whch give high values of c', say greater than 10 kN/m2. These are usually due to the soil testing laboratory using statistical techniques to define values of c' and q4 over the stress range over which the laboratory tests were carried out. This stress range is usually far in excess of the range which is applicable around a low-height gravity wall, leading to an unsafe set of strength parameters for the wall design. Even if the soil is so heavily overconsolidated to give a h g h value of c 'before construction, construction of the retaining wall will modify the in situ stresses and swelling of the clay could result in its value of c' falling to a small value.

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5.3

For these reasons, c' should normally be assumed to be zero and a straight line drawn through the origin and the strength points. If the plot is drawn so that the scales on the

axes are the same, the friction angle can be calculated from the angle ci drawn between a design line through the data points and the s axis, using the expression Q = sin-' (tan a) if triaxial tests were used or #'= a if shear box tests were used.

GEOTECHNICAL DESIGN REPORTS

It is recommended that a geotechnical design be summarised into a geotechnical design report following the format required by EC7 (Clause 2.8). The content of the geotechnical design report will vary with the scale of the project but, for simple projects, EC7 acknowledges one page as being sufficient.

The following should be included in the report:

0

0

0

0

0

a description of the site and surroundings

a description of the ground conditions

a description of the proposed construction

design values of soil and rock properties

brief description of codes and standards applied

statement of level of risk (this will affect the geotechcal category selected)

geotechnical design calculations and drawings

a note of items to be checked during construction or requiring maintenance or monitoring after construction.

An example of a single page geotechnical design report is given in Figure 5.2

5.4 CONTAMINATED GROUND

The desk study may have identified potentially polluting activities as having occurred on or near the site. These may have polluted the soil or water beneath the site.

The effects that should be considered include:

0

0

0

the safety of construction workers coming into contact with the soil or water

the safety of people in the vicinity of the completed structure

the disposal of surplus spoil

deleterious effects on buried materials, such as sulphate attack on concrete, chloride attack on steel and hydrocarbon attack on plastic.

Reference should be is made to CIFUA Special Publications 78 and 10 1-1 12 for the assessment of ground contamination. Part 4 of SISG (1993) gives guidance on the safe drilling of contaminated sites. 4

Even if no pollution has been identified on the site, the sulphate content of the soil and groundwater should still be measured where concrete will come into contact with soil or groundwater (see BRE Digest 363, 1991).

ClRlA C516 75


Job name: I Job no: I Sheet no ..... of ......

Made by: Date: I

Structure reference: I Checked by: Date:

Reports used:

Ground investigation report (give ref. date)

Factual:

Interpretation:

Codes and standards used (level of

acceptable risk):

Description of site and surroundings:

Calculations (or index to calculations)

Approved by: Date:

Section through structure showing actions:

Assumed stratigraphy used in design, with properties:

Information to be verified during construction

Notes on maintenance and monitoring

'igure 5.2 Single page geotechnical design report

76 ClRlA C516


5.5 SELECTION OF BACKFILL

5.5.1 Types of backfill

The wall specification should stipulate the backfill to be used behind the wall. The properties that should be considered will depend on whether or not locally-won backfill is to be used, and if the material is required to be free-draining. The optimum backfill is:

0

of consistent properties (and therefore needing minimal control testing)

easy to compact, giving high strength and stiffness (thus limiting the active pressures on the retaining wall while providing a good founding material for any structures behind the retaining wall)

free draining to minimise the build-up of water pressure. 0

It is seldom that such a good material is readily available for incorporation in the works. Therefore a balance has to be struck between the better performance of a good imported fill and the lower cost of using site-won material.

The Highways Agency Specification for highway works (1994), Series 600, gives the specification requirements of earthworks for incorporation into highways works. A companion advice note, HA 4419 1 Design and preparation of contract documents, gives advice on what particular properties should be specified. The Department of the Environment, Transport and the Regions material Types 1 and 2 are not included under the Earthworks series, but exist in Series 800 which covers unbound sub-base materials. The closest HA earthworks materials to Types 1 and 2 are types 6F2 and 6F 1 respectively.

BD 30/87 cautions against the high cost of importing good quality fill. It suggests that, apart from some special circumstances, it is more economical to reuse material won on site. It recommends a number of material types as being most suitable for backfilling retaining walls. Details of these are given in Box 5.2.

Box 5.2 Highways Agency material classes used for backjilling retaining walls

~~ ~

Class 6N Selected well-graded coarse-grained material This can consist of natural gravel, natural sand, crushed gravel, crushed rock, etc. It is a mainly frictional material with less than 15 per cent passing the 63 prn sieve and with a minimum uniformity coefficient of 10.

Class 6P Selected uniformly-graded coarse-grained material This is similar to 6N but accepts instead a minimum uniformity coefficient of 5. It also allows chalk with a saturation moisture content of 20 per cent or less. (For details on the determination of the saturation moisture content, see TRRL Report LR 806.)

Class 7A Selected fine-grained material This is intended to include any acceptable material available on site (see Section 5.5.2 for unacceptable materials). It can consist of sand, silt and clay with 15-100 per cent passing the 63 prn sieve. Overconsolidated material can be used provided its liquid limit does not exceed 45 and its plasticity index does not exceed 25.

Class 76 Selected conditioned pulverised fuel ash (PFA) PFA characteristics vary from source to source and from time to time. Therefore only conditioned PFA from a single source should be used for any structure in order to provide consistent properties. Where PFA is used as fill, the drainage system at the base of the structure should be resistant to the effects of sulphates and should be designed to resist the infiltration of fines. PFA can be particularly susceptible to erosion by rainwater. This can include faulty drainage systems.

ClRlA C516 77


Annex B of BD 4 1/97 suggests that a friction angle of 30" for PFA can be used for preliminary design provided it is confirmed by subsequent testing. It cautions against using c' >5 W/m2 for PFA.

Allowable grading limits for the material types in the box are given in Table 5.8 . These are also shown in Figure 5.3.

Table 5.8 Fill materials grading limits

Material Description Typical Uniformity Percentage by mass passing sieve size shown (mm) class use coefficient

(see Box 5.1)

UC 125 90 75 37.5 10 5 0.6 0.063

6F1 Selected capping - 100 75- 40-95 30-85 10-50 <I5 granular 100

material

(fine -grading)

Selected capping - granular

material

(coarse-

grading)

Selected fill to >I0 well-graded structures

granular

material

Selected fill to >5 granular structures

material

Selected fill to -

cohesive structures material

Selected fill to - conditioned structures

PFA-cohesive and to

material reinforced

earth

6F2 100 80- 65- 45- 15-60 10-45 0-25 0-12

100 100 100

6N 100

6P

7A

7B Not applicable

100

100 15-

100

78 ClRlA C516


ClRlA C516 79


5.5.2 Excluded materials

Structural fill should not include any of the following material types:

natural or contaminated soil which will be chemically aggressive

frozen material 0 degradable material such as topsoil, peat, wood, vegetation or any other

organic or otherwise perishable material 'pe of

0

material which could be toxic, dangerous or prone to spontaneous combustion

soluble material or collapsible soils.

Care should be exercised if the fill contains metal, rubber, plastic or synthetic material as they could interfere with compaction or subsequent performance of the fill. The use of clays prone to swelling should be carefully considered as they can exert very high pressures on the back of retaining walls (see Carder, 1988 for details of how London Clay spoil recovered from depth exerted twice the pressure over the top 2 m of a retaining wall that would be calculated by other means). For thls reason, the Highways Agency Specification for highway works prohibits the use of clay fill with a liquid limit exceeding 90 or a plasticity index exceeding 65, although studies currently being carried out on Lias Clay may,allow some exceptions to be made in the hture.

BD 30/87 does not permit the use of fill derived from argillaceous rocks such as shale and mudstone within 5 m of a retaining wall. This is because:

0 they can increase earth pressures by swelling

they may release sulphuric acid which can adversely affect concrete

they can cause drainage materials to become blocked by the formation of crystalline sulphates.

ao ClRlA C516


6

6.1

LOADS

The layout of this section is illustrated in Figure 6.1.

6.6 Trees

6.1 Surcharge 6.1 Surcharge

1 1 1 1 1 1 6.4 Compaction of (1 backfill I \ - I \

I \ \ 6.3 Earth pressures

\ i6.5 Water pressures and drainage

\

U 6.7 Other load cases

\ I - - - - - - -

6.2 Unplanned and planned trenches and excavations

Figure 6.1 Layout of Section 6

SURCHARGES

All walls should be designed to allow for a surcharge on the surface of the retained ground. This allowance includes:

0 variation in ground levels

. . permanent surcharges such as buildings or road or rail construction

construction surcharges such as plant or stacked construction materials

temporary surcharges such as vehicle loading, crowd loads (eg for sports stadia or shopping centres) or stored goods.

ClRlA C516 81


82

BS 8002 recommends that ‘all walls should be designed for a minimum design surcharge loading of 10 kN/mz ’ and adds that ‘more adverse values [should be] adopted in particularly critical or uncertain circumstances’. EC7 allows the designer to choose a value but cautions that repeated crane rail loading on a quay wall can be particularly onerous.

The value of surcharge chosen should be appropriate for the situation of the retaining wall. Therefore if vehicular access to the top of the retaining wall is possible, it should be assumed that such access will occur at some stage in the retaining wall’s life. The value of surcharge should be carefully chosen to reflect the maximum surcharge that would be applied. For flat ground, it is recommended that the surcharge should never be less than 5 kN/m2 (equal to only 0.25 m of surcharge soil), even if other codes suggest a lower figure, eg BS 6399 Part 1 which allows 2.5 kN/m2 for car parking for cars and light vehicles not exceeding 2500 kg gross mass.

Designers should recognise that temporary loading due to stored or stacked materials during construction often provides a more onerous case than the wall’s subsequent use. The surcharge value chosen should also allow for reasonable possible future changes of land use behnd the retaining wall. The surcharge that can be applied to sloping ground behmd a retaining wall is likely to be less, provided the slope angle used for assessment of K, takes into account any variation in ground levels.

Vehicle loads are covered in the Highways Agency publication BD 37/88, Loads for highway bridges, which is applicable to all highway bridges in the UK. It gives guidance on how loading from road and railway vehcles should be calculated. HA loading is the normal design loading for UK road carriageways while HB loading is an allowance for exceptional industrial loads which may use the road.

The values that should be used are quoted in the Approval in Principle (AIP) document for the particular highway scheme. HA and HB loads are not additive, the more onerous load should be chosen. RU loading represents the loading that should be applied to the areas occupied by tracks for main railways while RL, loading is a reduced value for lines carrying only rapid transit passenger rolling stock, such as London Underground lines. However, in Clauses 5.8.2.1 and 6.5.1 of BD 37/88, it simplifies the calculations to give nominal loads as follows:

0 cycleway/footbridge = 5 kN/m2 0 HA loading = 10 kN/m2

HBloading

+ 45 units = 20 kN/m2 + 30 units = 12 kN/m2

RUloading = 50 kN/m2 (on areas occupied by tracks) RLloading = 30 kN/m2 (on areas occupied by tracks).

There may also be a need to consider horizontal surcharge loading, eg from vehcle impact. Guidance on the assessment of these forces is beyond the scope of t h s guide.

The beneficial effect of surcharges should not be used in design. Examples of when to consider surcharges are shown in Figure 6.2.

Also shown in Figure 6.2 is a horizontal crash load. This is normally considered only for internal stability calculations. Unless the horizontal load is very large compared to the sliding resistance, such loads are seldom taken into account for external stability calculations.

ClRlA C516


Parapet

Uniformly - distributed surcharge

a) Load case 1: often critical for bearing pressure calculations and design for internal stability

Horizontal load due to crashing vehicle

Uniformly - distributed surcharge

'Virtual back' of wall

~

b) Load case 2: often critical for sliding stability and bearing pressure calculations

Figure 6.2 Example of load cases for a uniformly distributed surcharge for the design of retaining walls

ClRlA C516 83


6.2

6.3

The spread of surcharge loading between wall modules depends on how the modules have been connected together. Where there is little connection (such as along a crib wall), a concentrated surcharge can produce an unsightly bulge along the retaining wall. For modular retaining walls, concentrated loads such as wheel loads can be very onerous.

If connection between modules is poor, each module should be designed to withstand the full effect of such concentrated point loads. Alternatively, a safety barrier can be used to avoid the retaining wall module being subjected to such a high surcharge over its design life.

UNPLANNEDANDPLANNEDTRENCHESANDEXCAVATIONS

For ultimate limit state calculations for walls whch depend on the passive resistance of the ground in front of the structure, EC7 (Clause 8.3.2.1) requires that the ground level of the passive soil be lowered by an amount equal to 10 per cent of the wall height to a maximum limit of 0.5 m. BS 8002 (Clause 3.2.2.2) has similar requirements, but regards the height limit of 0.5 m as a minimum value. BS 8002 emphasises that the value should be reviewed for each design and more adverse values adopted in particularly critical or uncertain circumstances. The requirement for an additional or unplanned excavation as a design criterion is to provide for unforeseen and accidental events.

Planned or foreseeable excavations should also be considered. Wherever a service or drainage trench is likely to be dug in front of a retaining wall, its effect on loss of passive resistance should be allowed for. Some guides did not allow the use of passive resistance in design against sliding for this reason. Such a prohibition is arbitrary as a trench dug to beneath the founding level has the effect of further increasing the total active force on a potential sliding surface (see Figure 6.3) with no corresponding increase in passive force. Some designers ignore the upper 1 m of ground in front of a wall because, if it is poorly compacted, the strains required to mobilise its passive resistance are larger than would normally be tolerable. T h s could be avoided if it is specified that this material be properly compacted. A trench excavated immediately in front of a retaining wall can also reduce the allowable bearing pressure, which could lead to a bearing capacity failure.

The effect of digging a service or drainage trench in front of a retaining wall can be mitigated in some circumstances by only excavating short lengths at any one time. However, for modular retaining walls, even a short trench can have a serious influence on the stability of an individual module.

EARTH PRESSURES

Lateral movement of a retaining wall causes the earth pressure in the retained soil to fall towards its active value and the pressure in the soil in front to rise towards its passive value. The wall movements required to mobilise active and passive pressures are summarised in Table 6.1 (after Hong Kong Geoguide 1, GEO, 1993), the deflection ratio is the wall top movement (combined wall translation and rotation) normalised by the retained height of the wall.

84 ClRlA C516


Case 1: Trench excavation to underside of base

1 I

I I

Unplanned I Active soil wedge

trench

/

- in this case, the trench removes all passive resistance and illustrates the reason why many codes prohibit the use of passive resistance in stability calculations

Case 2: Trench excavation beneath underside of base

- in this case, a new deeper failure plane becomes more critical because the active pressure increases with the square of the retained height while the sliding resistance increases only proportionally with height

- such a trench is likely to invalidate sliding and bearing calculations which did not take it into account

Figure 6.3 Effects of trench excavation on passive resistance

Table 6.1 Approximate values of retaining wall movement for the development of active and passive failures in soil (after GEO, 1993)

Soil type Deflection ratio (wall top deflection/wall height) for development of failure condition

Active case Passive case

Dense sand

Loose sand

Stiff clay

Soft clay

0.001

0.005

0.01

0.02

0.02-0.06

0.06-0.3 5

0.02

0.04

Note: applicable to rigid retaining wall with soil KO < I , therefore does not apply to very heavily overconsolidated clays. Its results should be applied with caution to flexible retaining walls

85 ClRlA C516


86

10

8 -

6 -

2 W 4 - [12 3 v) v) W 0:

I I- [12

a 2-

4 2 b 0.8 a b 0.6 I- z

0.4

1 -

0 LL LL W 0 0

0.2

0.1

The movements to mobilise these pressures are illustrated in Figure 6.4. For the case of a medium dense coarse-grained material, EC7 (Clause 8.5.4) gives wall deflection ratios of 0.005 and 0.001 as being necessary for development of the active limit state for rotation about the wall toe or for translational movement respectively. Reliance on passive pressure should be treated with caution. It can be seen from Figure 6.4 that the movement to mobilise the passive resistance relies on far greater movements than those to mobilise active pressure. Often, for design purposes, only half of the passive resistance is used in calculations, talung into account the need to limit wall deflections.

I 1 1 I I I I I l l I I I I I I II I

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

I I

I

- I 1 I

I -

Legend: - -- . I

Dense sand

Loose sand - - - - -7

I I I I I 1 I I I I I

Active case 4 Passive case WALL MOVEMENT,

H

10

8

6 Y"

4 ;

E

3 v) v) W

2 1 I- : W

1 5

0.8

0.6 8

0.4 y I- z

LL LL W s

0.2

0.1

Active case Passive case

Figure 6.4 Approximate values of retaining wall movements for the development of active and passive failures in soil

ClRlA C516


ClRlA C516

BS 8002 Clause 3.1.9 warns about assessment of active earth pressures for the particular condition when a retaining wall is built on a very compressible formation. Settlement beneath the retained fill causes the retaining wall to rotate backwards into the fill. This can lead to higher earth pressures from the backfll than would otherwise be expected.

EC7 (Clause 8.5.2) suggests that where the deflection ratio on the active side of the wall is less than 0.0005, at-rest (KO) conditions apply. In this case, reduction of earth pressures to their active value should not be assumed. It uses the following relationship for the evaluation of KO for a horizontal ground surface:

K, = ( I - sin 4) JOCR (6.1)

The overconsolidation ratio (OCR) can be calculated from its definition given in Box 6.1. Estimation of the overconsolidation ratio requires knowledge of the geological history of the site.

Box 6.1 Definition of overconsolidation ratio

Overconsolidation ratio (OCR)

OCR is defined as the ratio between the maximum vertical effective stress that the soil has experienced during its lifetime and its current vertical effective stress. For normally consolidated deposits where the current state of stress is the highest, OCR = 1. Soils that are overconsolidated because a surface surcharge has been removed have a high value of OCR at the surface, which reduces with depth as the current vertical effective stresses in the soil increase.

KO also increases as the retained slope angle behind the wall increases. The at-rest earth pressure for a retained slope angle /3 may be calculated using the expression

KO = KO (1 + sin p) (6.2)

Annex B of BD 4 1/97 suggests that for external stability calculations, it can be assumed that earth pressures have fallen to their active values. However, for the internal stability calculations for calculating the necessary strength of structural elements (such as reinforced concrete stem walls), at-rest earth pressures should be used multiplied by a partial factor of 1.2 for ultimate limit state calculations (or 0.67 for relieving effects) and by 1 .O for serviceability limit state calculations. This is to allow for the higher earth pressures developed behind the stem by compaction and to be sure that the wall would suffer a sliding failure before the wall stem fails.

Numerical values of K, and Kp are given in Appendix A 1 and are based on theory presented in EC7. They rely on an analogy with bearing capacity theory, rather than Rankine’s or Coulomb’s formulae. Also included in Appendix A1 are graphs which take into account the roughness of the rear of the wall 6, which acts to reduce the coefficient of earth pressure. For walls where the virtual back is unrestrained fill to fill (ie restraint could be provided by a mesh binding one side of the fill together, as in a gabion wall), the interface angle of wall friction 6 should be considered to be zero. Only when the retained soil at the virtual back of the wall is restrained or is a structural member should roughness be considered. Typically, fill to concrete may be considered to have an interface roughness of 6= O S @ . Only when the virtual back of the wall is notably rough (fill to a gabion wall) can 6 be assumed to equal @.

a7


6.4

88

The following Rankine relationships can be used for approximate calculation of active and passive limit pressures where p is the slope of the ground behind the retaining wall and the roughness 6 is assumed to be zero (ie smooth). For drained materials, c' should be taken as zero for design purposes. These relationships take into account the effect of sloping ground behind the retaining wall (4. K, = tan2(45-qY2)(1 + s i n 4

K~ = t a n 2 ( 4 5 - ~ 2 ) ( 1 + s i n ~

K,, = 2 dKa (1 + cWlc')

Kpc = 2 dKp (1 + c,,,')

These equations may be invalidated in certain circumstances (see for instance GEO, 1993). The use of the graphs and tables in Appendix A1 is preferred.

The active and passive earth pressures acting on the wall are given by

p i = K, 0,' - K,,.c'

pp' = Kp ov'+ KPcc'

Total soil pressures are calculated by adding the effective horizontal soil pressure Ko,' to the water pressure, which is usually denoted as U. The water pressure should be calculated from the design water table, as explained in Section 6.5.

If undrained conditions are being considered for a fine-grained material, this can imply a negative active force for a low-height retaining wall as K,, x c, is greater than K, x 0,.

In this case it should be assumed that the implied tension in the ground allows a crack to form which fills with water. The design case should therefore allow for a water-filled tension crack on the active side of the wall exerting hydrostatic pressure to a depth equal to the base of the wall.

COMPACTION OF BACKFILL

The Highways Agency Specification for highway works, Table 614, gives details of the amount of compaction required from different types of plant for different fill types. Smaller plant is generally more efficient for walls of this height and offers the following additional advantages:

0 the horizontal pressures locked into the retained ground by compaction are smaller

it is less easy to damage or overload individual wall elements (especially in crib walls) if lighter more manoeuvrable plant is used.

The method used for the calculation of compaction pressures was originally proposed by Ingold (1 979), but has been restated more recently by Symons and Clayton (1 992). The pressures calculated are most relevant for the internal design of retaining walls with reinforced concrete stems. The stem deflection is often too small for these locked in pressures to be relieved by its forward deflection.

Compaction pressures need not be used for external stability calculations where forward movement of the retaining wall is possible (ie it is not propped by a structure in front of it) and acceptable (the deflection would not exceed a serviceability limit state).

ClRlA C516


They report pressures greater than those calculated from KO d, to a depth they called h,, defined as

KO

where P is the effective line load per metre of the roller, and y is the density of the material undergoing compaction.

The maximum pressure dh can be calculated from

and this occurs below a depth z,, where

z,, = KO - 4: These terms are illustrated in Figure 6.5.

Rigid wall

hC

Depth

\ \ \ \ \ \ \ \ \ \ \ \ \ _ - - - - - -

(6.9;

(6.10)

(6.1 1)

Figure 6.5 Assessment of compaction pressure on retaining walls

For light plant, the depth z,,is often small (if KO = 0.4, P = 8 kN/m and y= 19kN/m3, then z,, = 0.2 m) and for low-height retaining walls h, will sometimes be relatively large (1.3 m using similar data, 2.0 m if P = 20 kN/m). Therefore it can be assumed that the compaction pressures can be simplified to a rectangular stress distribution. In the absence of more precise data, the compaction pressures in Table 6.2 can be used for design.

ClRlA C516 8!


90

Table 6.2 Design compaction pressures

Mass per Effective line Critical Depth where Horizontal metre width load per depth for compaction pressure from of roll metre of compaction pressures remain compaction

roller pressures significant (Wm) P (kN/m) zcr (m) hc (m) Q’hrm W l m 2 )

500 5 0.16 1 .oo 8

800 8 0.21 1.29 10

1000 10 0.23 1.45 11

2000 20 0.33 2.05 16

3000 30 0.40 2.51 19

5000 50 0.52 3.24 24

10 000 100 0.73 4.58 35

Notes: 1. For dead weight (non-vibratory) rollers, the effective line load is the weight of roller divided by roll width.

For vibratory rollers it may be calculated as the weight of roller plus the centrifugal force operating. Where no information is available as to the centrifugal force, it may be calculated as twice the dead weight (as stated by GEO, 1993). For vibrating plate compactors, it may be taken as twice the dead weight of the compactor divided by the width of the base plate.

2. Values based on material density, y= 19 kN/m’.

The Highways Agency Specification for highway works restricts the plant types that can be used within 2 m of a structure to:

vibratory rollers with a mass per metre width of roll not exceeding 1300 kg and a total mass not exceed 1000 kg

vibrating plate compactors having a mass not exceeding 1000 kg

vibro-tampers having a mass not exceeding 75 kg.

Vibrating plate compactors and vibro-tampers are typically 0.3-0.5 m wide.

Where compaction is used, it is recommended that a horizontal pressure of at least 8 kN/m2 is allowed for in the retaining wall design.

At the design stage the type of backfill will be known but not the compaction plant. An appropriate type of plant should be chosen to meet the compaction requirements without producing excessive wall design pressures. A note should be added to the drawings stating that heavier plant is not to be used in the 2 m zone immediately behind the wall.

The higher of the pressures calculated using earth pressure coefficients or compaction pressures should be used in design. At any particular level, their effects are not additive. Therefore for flat ground the compaction pressures will usually dominate, while for steeply sloping ground behind the retaining wall, earth pressures may be larger.

The Highways Agency Specification for highway works gives specification requirements for materials. Materials with a closely defined grading envelope, such as Classes 6F1 and 6F2, have a method specification for compaction (ie a set number of passes of particular types of plant) while materials with less specific grading requirements require performance specification for compaction. Compaction requirements are summarised in Table 6.3.

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6.5

6.5.1

ClRlA C516

Table 6.3 Highways Agency compaction requirements for different material classes (after the Highways Agency Specification for highway works)

Material class Highways Agency compaction requirement

6F 1 6F2

6N 6P

7A

Specification for highway works, Series 600, Table 614, Method 6

End product: 95 per cent of maximum dry density of BS 1377: Part 4 - vibrating hammer method

End product: 100 per cent of maximum dry density of BS 1377: Part 4 - 2.5 kg rammer method or a dry density corresponding to 5 per cent air voids at field moisture content, whichever is lower

End product: 95 per cent of a maximum dry density of BS 1377: Part 4 - 2.5 kg rammer method

7B

Further details on compaction is given in HA 70194.

WATER PRESSURES AND DRAINAGE

Rationale for drainage system

The use of wall drainage is recommended if the wall backfill is relatively impermeable or if infiltrating water has nowhere else to go. If no drainage is provided, water pressures should be calculated assuming the highest water table level that is physically possible (eg before overtopping of a wall element occurs or some other direction of water flow controls the water table level). A rise in water table level from the toe of a retaining wall to the top can more than double the horizontal pressure on the retaining wall. This can happen relatively suddenly and is the reason why water is one of the main causes of retaining wall failures. It is therefore important that the water table level, direction of water flow and seasonal variations of water in the ground are well understood before construction starts. Rainstorms or burst water mains can create particularly severe conditions. Free-draining wall types such as gabions can be appropriate in situations where water flows are anticipated to be high and where maintenance would be infrequent.

If water is not allowed to drain away, it could act to collect road de-icing salts which can cause deterioration of materials such as reinforced concrete.

Excess water pressures can force reinforced concrete wall modules apart, particularly at re-entrant corners, and lead to loss of fines through the wall. This would cause settlement of the retained fill. The wall modules should be adequately tied together to prevent this happening.

A good ground investigation will incorporate standpipes and piezometers following the advice of BS 5930 Clause 20.1, which cautions against reliance on water observations made during the excavation of trial pits and boreholes. Standpipes and piezometers should be read until the observed water levels reach equilibrium and this equilibrium level should be confirmed, usually in permeable ground by pouring more water into the standpipe or piezometer and checking that the water level returns to the same equilibrium value. The investigation should also take account of seasonal variations.

91


6.5.2

92

EC7 requires that where the retained soil is of medium or low permeability (silts and clays), the water table level for general geotechnical design purposes should be taken at the surface of that soil layer.

Phenomena such as underdrainage (downwards water flow for hydrological reasons giving a water pressure variation with depth which is less than hydrostatic) and, more importantly, artesian conditions (water flowing upwards trying to create a water table level above the ground level) could have an impact on the design where those conditions occur.

The construction of the retaining wall and other structures in the vicinity can have a marked effect on water flows and water table levels. It is for this reason that Clause 3.4 of BS 8102: 1990 Code ofpractice forprotection of structures against waterfiom the ground recommends minimum water table levels that should be allowed for in the design of basements, in the absence of specific site data. These are summarised in Table 6.4 and are recommended for the design of retaining walls unless site-specific data suggest higher levels or appropriately reliable drainage measures would permit lower values to be used.

Table 6.4 Design water table level (after ClRlA Report 139, 1995). ~~

Basement depth (m) Design water table level

0-1.3

1.3-4.0

1 m above base level

1/4 of basement depth below ground level

>4.0 1 m below ground level

Drainage systems

Drainage systems reduce the water pressures that act on the wall and thereby allow a more economical wall design to be made. However, if a design relies on the benefit of a drainage system, the system has to be reliable. This requires that:

the drainage system would be appropriate to the ground conditions into which it is to be placed and for the volumes of water it is required to handle its continued successful operation would be easily observable in a planned maintenance cycle of sufficiently frequency for any impairment to be detected before a problem occurs

the design life of the drainage system would be compatible with the design life of the retaining wall and be easily repairable if a problem is detected. However emphasis should be placed on designing a drainage system which would not need repair, but could be cleaned and maintained.

The anticipated flow volume should be calculated and should be within the capacity of each part of the drainage system. This means knowing the permeability of each part of the drainage system. BD 30/87 recommends that drainage pipes should be formed from a continuous system of porous or perforated drainpipes not less than 150 mm diameter throughout. Except where the backfill material is very permeable, a vertical drainage layer is recommended.

EC7 allows maintenance to be omitted only if the wall does not rely on the drainage system or where it can be demonstrated from comparable experience that the drainage system will operate adequately without maintenance.

ClRlA C516


BS 8002 Clause 3.2.2.3 requires the water pressure regime to be used in design to be ‘the most onerous that is considered to be reasonably possible’. A severe water regime only has to occur once and for a relatively short period of time for the retaining wall to be affected. Clause 3.3.5.3 requires allowance to be made for possible changes in the groundwater levels as result of temporary or permanent modification which may be made to the water conditions by the earth retaining structure itself, both permanently and during construction. For cohesionless backfills of medium to low permeability (2 x 10‘’ m / s or lower) and for fine-grained soils, it is usual to provide a drainage layer behind the retaining wall to prevent the build-up of hydrostatic pressure.

The drainage layer can be composed of:

cast-in-place porous no-fines concrete

blanket of rubble or coarse aggregate, clean gravel or crushed stone

hand-placed pervious blocks as dry walling (checking that these are permeable to water, a high pore space by itself is not sufficient)

0 a graded filter drain

a geotextile filter in combination with a permeable coarse-grained material (to prevent fines migrating into the filter layer)

0 a geotextile composite/fin drain.

The first three types of drainage layers are described in Clause 5 13 of the Specification for highway works (permeable backing to earth retaining structures). Care should be taken in the choice of drainage materials when the backfill is susceptible to piping, such as chalk, PFA or Class 7A selected cohesive material, when a 300 mm thick vertical drainage layer of sand complying with grading C or M of BS 882 Table 5 should be used as a drainage layer behind the wall.

In addition, BD30/87 Clause 6.3.9 requires that cohesive material, PFA or chalk backfill should be capped with a 500 mm thick layer of granular material to limit infiltration and should be underlain by a 450 mm thick graded filter. These requirements effectively prohibit the use of material susceptible to piping as backfill to low-height walls on highway schemes.

For materials which are not susceptible to piping, the Specification for highway works requires that a graded filter drain should be at least 300 mm thick and should satisfy the following inequalities, for satisfactory long-term performance.

15% size of the drainage material 85% size of the backfill material

Piping ratio: < 5

15% size of the drainage material 15% size of the backfill material

Permeability ratio: > 5

A suggested material grading for a Type A filter drain material is given in Clause 505 of the Specification for highway works, as shown in Table 6.5.

The water entering the drainage layer should drain to a system that will allow free exit of the water either by the provision of weepholes, or by porous land drains and pipes laid’at the bottom of the drainage layer and led to sumps or sewers via catchpits. Even if weepholes do not form the main drainage system, they can provide a useful back-up in case the main system fails, both as a check on the operation of the system and to limit the rise in water level by pressure relief.

ClRlA C516 93


94

Table 6.5 Suggested material grading for a Type A filter drain material

BS sieve size Per cent by mass passing

37.5 mm

20 mm

10 mm

5 m m

1.18 mm

600 pm

150 wm

100

85-100

50-100

35-90

15-50

5-3 5

0 - 5

Weepholes should not be used where they may cause unacceptable disfigurement of the front face of the wall. They are usually laid to have a fall back into the retained soil so they act as a pressure overflow only, thereby to minimise staining by trickling water on the face of the wall. They should not discharge to a pavement where the water is likely to cause a slip hazard if it freezes. Where weepholes are used, they should generally be 50 mm in diameter, depending on the permeability and at an appropriate horizontal spacing. Concrete or other impermeable material should be placed immediately below the weepholes or drainage pipes and in contact with the back of the wall, in order to prevent the water from reaching the foundations. Care should be taken to prevent the drainage pipework being damaged during compaction of the drainage layer and backfill. Particular problems affecting drainage systems include silting and biofouling. The usual measures used to counteract these problems are a system of manholes which allow the drainage ducts to be rodded.

It is important that the drainage system is operational where the water pressures that act on a wall occur. For a retaining wall with a long heel, these can be some distance back from the stem, as shown in Figure 6.6. Typical drainage details for gravity walls are shown in Figure 6.7.

Drainage systems are reviewed by Bird (1992). He found that while water was a contributory factor in the failure of old walls, no evidence could be found for any failures of walls designed to recognised codes of practice as a result of water pressures. Water level rises which caused failures in retaining walls were most often blamed on factors such as blocked culverts or burst water mains. Some retaining walls had suffered problems in service such as staining of faces. There were also problems of access for inspection and maintenance. He concluded that the cost of providing drainage measures was low compared with the consequent adverse effect on maintenance and stability if they were inadequate.

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Lower permeability fin to limit infiltration

Drainage channel

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75mm diameter I

Drainage channel

uraina e pipe cOnnectlOn to a suit& wttet

(a) Wall with sloping badmll

Drainage channel

7En Paved surface to limit infiltration

Filterldrainage materials

, . Blinding layer

(b) Wall retaining level ground

Figure 6.6 Typical drainage schemes for reinforced concrete retaining walls

95


Lower permeability fill to limit infiltration 1

Drainage channel

75mm diameter Filteddrainage materials

Drainage pipe connection to a suitable outlet

Concrete to prevent water erosion of foundation

(a) Wall with sloping backfill

I Drainage channel

Filter/drainage materials

Paved surface to limit infiltration

75mm diameter PVC DiDe

! Drainage pipe connection to Blinding layer a suitable outlet ' Concrete to prevent water

erosion of foundation (b) Wall retaining level ground

Figure 6.7 Typical drainage schemes for other types of gravity retaining walls

96 ClRlA C516


6.5.3 Design water pressures

Where the wall is designed to accommodate water pressures, consideration should be given to the water pressures for which the wall should be designed to resist. If there is no groundwater flow, the water table level behind the retaining wall will be constant and the pressures are simple to evaluate. Where flow towards a drainage layer (drainage boundary) is occurring, the water pressure that should be used is that acting on the critical failure surface, not that on the back or the virtual back of the retaining wall. Some guidance on the calculation of these water pressures is given in Figure 6.8. For most walls of the height covered by this guide, this calculation will be unnecessarily elaborate and use of conservative water pressures will suffice.

Even where a drainage system has been designed to remain operational over the entire design life of retaining wall, sometimes the water pressures on the critical failure surface will be significant and should be accommodated by the design. In these cases, an inclined drain like that shown in Figure 6.6a or 6.7a is likely to be far more effective than a drain immediately behind the retaining wall like that shown in Figure 6.8b.

6.5.4 Frost susceptibility

Frost can destroy a foundation soil, subgrade or backfill by progressive freeze-thaw action. Frost-susceptible soils include:

fine-grained soils with a plasticity index of less than 15 per cent or less than 20 per cent if the water table is within 0.6 m of formation level (liability to frost heave decreases with increasing compaction however)

coarse-grained soils or crushed granites with more than 10 per cent of soil retained on a No. 200 BS sieve (this sieve size has an equivalent particle diameter of 75 pm; the closest modern sieve size is 63 pm or 0.063 mm)

crushed chalks some limestones (generally those with an average saturation moisture content in excess of 3 per cent)

burnt colliery shales

pulverised fuel ash with more than 40 per cent passing the No. 200 BS sieve.

TRRL Laboratory Report 90 (Croney and Jacobs, 1967) provides guidance and recommends that for frost-susceptible soils the foundations are at least 0.5 m below surrounding ground level.

BS 8004 Clause 3.2.9.1 suggests that within the British Isles a depth of 0.45 m is sufficient to prevent frost-induced heave in sensitive soils. It adds that thin concrete rafts or slabs should not be left exposed to long periods of frosty weather where frost- susceptible soils are within 0.45 m of the concrete surface.

This advice could also apply to the base of a concrete retaining wall. Frost-susceptible backfills should be protected by a 0.45 m layer of non-susceptible material, where they would otherwise be exposed to the elements when the wall was completed.

ClRlA C516 97


Critical failure surface

impermeable boundary

(a) Groundwater seepage condition

Critical failure surface

Note increase in water pressure on potential failure surface due to surface infiltration.

ce

Note non-linear water pressure distribution on potential failure surface due to steady seepage

\ impermeable boundary

(b) Surface Infiltration

N I =I v! 2 0

I1 ZI

y=0.008aa sec aa

aa (degrees)

(c) Variation of U with aa for condition (b) above

Figure 6.8 Evaluation of water pressure on potential failure surface (after Navfac, 1982)

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ClRlA C516

TREES

Trees can have a number of effects on retaining walls which may have to be accommodated by the wall design and specification. Modular retaining walls, with their additional longitudinal flexibility, are particularly susceptible to these effects. They are given below.

In certain circumstances, trees are protected by Tree Preservation Orders. Guidance is given in BS 5837: 1991, Guide for trees in relation to construction. Information on water demand for different tree species is given by NHBC (1995) and Atlunson (1993).

Damage caused by trees in fine-grained soils

Trees remove water from soils and in fine-grained soils this leads to consolidation of the soil. This consolidation process results in settlement. The consolidation process can take longer than 10 years to occur and so is not easily accommodated by the wall design. Similarly, the removal of a tree results in swelling of the soil and heave. These effects can be particularly severe if they occur on the passive side of the retaining wall as the settlement or heave will occur in the soil under the retaining wall. Guidance on these processes is given in BRE digest 4 12 (1 996).

Roots

In addition to ground movements in fine-grained soils caused by trees abstracting water from the soils, tree roots can be very intrusive and can cause direct damage both to retaining walls and drainage systems. The problem is best avoided by planting trees a safe distance from retaining walls. Guidance is given in BS 5837: 1991 Guide for trees in relation to construction. It recommends that a young tree should be planted so that the centre of its trunk is at least 1 m from a masonry boundary wall if the mature tree height is less than 15 m, and 2 m away if the mature tree height is greater than 15 m. Information on mature tree heights is given by Atkinson (1993). It is emphasised that this information refers to direct damage caused by the tree, and excludes the effects of consolidation and swelling in fine-grained soils.

Where there are trees and shrubs close to the retaining wall, it may be necessary to prune and coppice than to keep them (and their roots) under control.

Tree weight

A tree can act as a particularly heavy surcharge and can also impose a high horizontal force from its acting as a ‘sail’ moment in high winds. Main tree roots are generally less than 600 mm deep and this is where the weight of the tree can be assumed to act. Again this problem is best avoided by planting trees at least as far back from the active side of the retaining wall as the retaining wall is high, and even hrther back on slopes.

Tree damage

Trees can also be damaged by construction work. BS 5837 gives guidance on the safe distances for fencing to be established around trees.

99


6.7

100

OTHER LOAD CASES

The retaining wall should be designed for the most onerous load case which could be applied over the life of the structure. Carehl consideration may be required to devise all the load cases for which the wall should be designed. Some forces such as horizontal collision loads on parapets may have most effect on internal stability. Vertical loads from parapets may also need to be considered, particularly when parapets are being fitted to existing walls. Others, such as the removal of vertical load from the wall, for example during demolition of a structure above, are less obvious but could still result in active soil forces overcoming the resisting forces.

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7

7.1

7.1 . I

Design applications - general

The layout of this section is illustrated in Figure 7.1.

7.5 Corner details

7.4 Internal stability

7.2 Bearing capacity and overturning

- -7

7.3 Overall stability

Figure 7.1 Layout of Section 7

SLIDING

Factors in assessing sliding force

This mode of failure occurs when the active force behind the wall exceeds the sliding resistance beneath and any passive pressure in front of the wall so that the wall moves forward. The mode of failure is illustrated in Figure 7.2.

The sliding stability of retaining walls is often the most critical when the subgrade on which the wall is to be built is competent. Wall failures by sliding are often triggered by a rise in water pressure increasing the total horizontal force on the active side of the retaining wall.

The disturbing action is the force from the retained fill and water. If the ground behind the retaining wall is flat, the active wedge is inclined at an angle of 45 - 412 to the vertical. This force pushes the wall forward and as the wall moves, the soil pressures fall towards their active values. This movement also relieves any increased horizontal pressure remaining from compaction. If the wall is restrained from moving by a significant amount (this can be calculated from Table 6.1 which gives the amount of wall rotation necessary for active conditions to pertain), the higher at-rest pressures should be used.

ClRlA C516 101


Passive force

I / I / I /Active wedge

I / = critical failure

1 surface I

Virtual back of wail/; ,/I’ I /

L I, I Passive wedge = critical failure surface +

Sliding resistance

Figure 7.2 Sliding failure

Where a steep angle is cut in the natural ground behind a retained wall, depending on the relative friction angles of the fill material and natural ground, it could be the natural ground that generates the active pressure. The circumstances leading to this pheno- menon are shown in Figure 7.3. It is particularly found in clays where the undrained shear strength allows relatively steep faces to be cut, but the drained strength (defined by its effective angle of shearing resistance 4’) is lower than that of the fill, giving an increased K, value and hence increased active pressure.

The active force can be reduced by:

0 flattening any slope behind the retaining wall

use of high strength or lightweight fills

stabilisation of the cut face behmd the wall, in extreme circumstances, by soil nails.

The use of concrete backfill can be beneficial, but in many cases, its becoming in effect part of the wall, means that the vertical plane on which active pressures are evaluated is further back. This does, however, offer the advantage of lengthening the plane on which sliding resistance occurs.

The base of the block of soiVstructure beneath the retaining wall generates the sliding resistance needed for sliding stability. It is therefore important that this excavated formation surface is carefully protected so that its strength is not lost.

Loosening of coarse-grained soils or water-softening of fine-grained soils can result in far lower sliding resistance than calculations might indicate. For this reason, EC7 requires that the formation surface should be quickly blinded and water be prevented from ponding on it. Placing a separator such as a geotextile between the formation and the wall base can reduce the sliding resistance and should only be used with expert advice.

102

The effect of surcharge pressures immediately behind the retaining wall, but in front of the vertical face on which the active pressure is evaluated, is usually beneficial as it increases the sliding resistance of the block. However, the surcharge pressures behind the vertical face contribute to the active force and are adverse. These surcharge pressures are illustrated in Figure 6.2.

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Beneficial forces should not be used 4 design unless they can be relied upon to act.

Passive resistance forces should not be used unless their effect can be relied upon. Trenches and other excavations can destroy this resistance force, as discussed in Section 6.2. If the ground in front of the retaining wall is flat, the passive wedge is inclined at an angle of 45" + 472 to a vertical surface.

Case 1 S h a k back angle

AGtitrepregswe from granular fill

Active pressure from cohesive soil

lower friction i n u w d length

fill should be

Case 2 Steep back angle (facilitated by short-term undrained strength of clay)

Active pressure Active pressure from granular from cohesive fill soil

-Active pressure from cohesive soil should

- Sliding resistance can be taken as far back as

be used in design

the vertical active plane intercept

Figure 7.3 Examples showing how back excavation angle can dictate active pressures

ClRlA C516 103


7.1.2 Use of shear keys, dowels and inclined formations

Inclined formations and shear keys can be used to enhance the sliding resistance of walls, instead of selecting a wall with a wider base. As such, modules are not standard shapes, and are likely to be more expensive. Alternatively they can take the form of cast-in-place concrete beneath a modular wall. Inclined formations are most easily analysed by considering failure along the plane ABCD in Figure 7.4 and using the extra passive resistance to enhance the wall stability. The effect of an inclined formation is therefore to deepen the sliding plane without having to replace all the soil above the plane with concrete.

--I Inclined formation I I I

I /

I I

I

I I I I I I I

I /

I I /

Shear keys A

I \\\-\\\ I I

/ I

I I I

I I I /

I I I I

I I I I

/ /

/

J E D I

Figure 7.4 Sliding surface in case of inclined formations and shear keys

Shear keys can also be used to enhance wall stability. Their physical effect is locally to deepen the failure plane. The wall should be analysed by consideration of the passive and active forces on either side of the shear key, but taking an appropriately reduced length of sliding resistance, as shown in Figure 7.4. Lines CD and EF can be fixed by assuming them to form active and passive wedge angles respectively. Thus the sliding surfaces are BC and FG (and DE, although its contribution will be small), and shear key resistance is the difference between the passive and active forces acting on either side of the shear key. Where the soil strength reduces with depth or where a particularly weak layer of soil is expected to occur at the bottom of the shear key, the plane AIJK should also be checked, taking into account the increased active pressures on AI and passive pressures on JK. The joint between the wall’s heel and the shear key should be reinforced to provide an adequate structural resistance.

104 ClRlA C516


7.1.3

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Dowels nailed through the base of a precast reinforced concrete module can assist stability. If sufficient dowels are used, they reinforce the underlying soil to the extent that the shear plane on which sliding is evaluated is pushed deeper to the base of the dowels.

Hambly (1 979) gave the following guidance on construction problems with inclined foundations and shear keys.

I . The process of constructing a shear key frequently loosens and softens the material on which reliance is placed for passive resistance.

Shear keys are generally not placed at the front of footings because of the risk of the lateral resistance being removed by excavation for services. A shear key beneath the middle of the footing is usually easiest to construct as the excavator can straddle the trench. A shear key at the back of the base obtains most passive resistance because of the increased vertical pressure acting above the passive wedge, but may require more excavation of the wedge of soil behind the wall.

Inclined formations require deeper excavation at the back of the wall but are less prone to softening than shear keys. However, control of the more complicated excavation shape is time consuming and it is often more economical to construct the full width at the lower depth. Adequate tolerance and cover should be specified.

2.

3 .

Shear keys and inclined formations are difficult to implement with modular retaining walls. If modular elements incorporate a shear key downstand or an inclined base, it is llkely that very tight construction tolerances will be required. Careful attention should be paid to the shear key being in intimate contact with the soil against which it will react for passive resistance.

Design

The design sliding resistance should be based on the interface strength between the base of the wall and the soil beneath. This is normally calculated using the drained strength as o’tan 6 where 6 is the interface friction between the base of the wall and formation material, even for fine-grained soils. Drained cohesion c’ is seldom used for sliding resistance and should only be used with caution. For precast concrete wall bases the interface angle 6may be taken as 2/34: where #’ is the internal angle of friction of the formation material (see comments in Section 7.1.1 regarding protection of the formation surface). The value of 6 will vary depending on the wall and formation material and will typically be between %#‘and #!

Sliding stability requires that the following inequality must be satisfied:

Vtanad 2H

where Vis the total vertical load on the wall base H is the total horizontal load acting on the rear of the wall 6 d is the design value of interface friction.

Where there is passive resistance at the toe of the wall the component of design horizontal resistance may be added to the left hand side of the inequality, provided it can be relied upon to act at all times (see Section 6.2).

105


7.2

7.2.1

106

BEARING CAPACITY AND OVERTURNING

Principles

The bearing capacity of the foundation strata is the ability of the ground to withstand the combination of actions (vertical, horizontal and moment loading) that the wall transfers to the ground with respect to vertical stability of the wall. When the combination of actions exceeds the bearing capacity of the ground, large displacements (settlements and rotations) of the wall will occur possibly leading to total collapse of the structure. The mechanism of bearing capacity failure is shown in Figure 7.5a.

Prior to examining the methods for calculation of the bearing capacity of the ground, it is worthwhile mentioning two combinations of actions which have special significance when considering wall stability.

Overturning

\

This is the limiting condition for bearing capacity and is a result of the resultant force vector crossing the base of the wall outside the plan area of the wall. The result of thls occurrence is for the wall to rotate about its toe and topple over. The check for overturning is only relevant for walls that are constructed on a very strong formation (rock or concrete) because, for other types of foundation, bearing capacity failure will have occurred prior to overturning. In order to provide a safe design it is recommended that the resultant load should not act w i h the 10 per cent of the retained wall height from the front of the wall. An example of overturning failure is shown in Figure 7.5b.

Middle third

When the resultant force vector lies within the middle third of the wall base the heel of the wall will not lift off the foundation strata. It should be noted that if the force vector lies withm the middle third of the base there is no guarantee that the bearing capacity failure will not occur and hence it should not be considered to be a stability check. Where the resultant of load acts within the middle thrd of the wall base the whole of the wall base will be in compression. When the resultant load acts outside the middle third the rear of the wall will tend to detach from the formation strata as the reliance on tensile forces between wall and formation cannot be relied upon.

The effect of high horizontal forces is to reduce the vertical bearing capacity. For these cases, inclination factors should be used (see Section 7.2.2).

For calculation of bearing capacity, it is important to distinguish between two states:

the maximum safe bearing pressure - the calculated bearing pressure which has an adequate factor against failure (ultimate limit state)

the allowable bearing pressure - the bearing pressure whxh results in acceptable settlements or tilts (serviceability limit state).

0

Usually the maximum safe bearing pressure is calculated first, and that value is reduced where necessary to give the allowable bearing pressure. The maximum allowable bearing pressure will be calculated after consideration of both allowable movement (from settlement analysis) and the maximum safe bearing pressure.

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When calculating the maximum safe bearing pressure, consideration of the state of soil drainage should be made. Hence, the design should be carried out for undrained or drained conditions as appropriate, or both when both conditions apply at different stages of the wall’s design life.

Surcharge

a) Bearing capacity failure - tilting induced by failure of soil under toe of wall

Surcharge

b) Over turning failure - wall rotates on toe bearing on very strong subsoil

Figure 7.5 Bearing capacity and overturning (after GEO, 1993)

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7.2.2

108

Calculation of maximum safe bearing capacity after EC7

Equations for bearing capacity can be found in Annex B of EC7. The equations most relevant to design of walls are reproduced below for both undrained and drained conditions and the values for drained bearing capacity factors are listed in Table 7.1 and shown graphically in Figure 7.6. The EC7 method for calculation of bearing capacity considers only the soil that is beneath the wall base. Where there is soil in the passive zone in front of the wall the mobilised passive force may be used to reduce both the net moment loading and net horizontal force acting at the base of the wall, although in most cases the passive component of soil resistance should be conservatively ignored.

For a retaining wall it is often the case that the wall will be significantly longer than it is wide (L’ >> B’) and that stability will be calculated for a unit length of wall. Where this is the case the shape factors (s, , sq and s.,) will have values of 1 .O. Wall stability will then be calculated assuming that B’ is equivalent to A’ and the results will then be applicable to allowable bearing capacity per metre run of wall.

Undrained conditions

The design bearing resistance is calculated from

V/A ‘= (2 + r) c, s, i, + q

with the design values of dimensionless factors for:

the shape of the foundation s, = 1 + 0.2 (B’/L’) s, = 1.2

the inclination of the load Z caused by a horizontal load H:

i, = 0.5 . (1 + ,/-)

for a rectangular shape for a square or circular shape

Drained conditions

The design bearing resistance is calculated from

V I A’ = C’ N, s, i, + q’ N, sq i, f 0 .5~’ B’ N., s., i.,

with the design values of dimensionless factors for:

0 the bearing resistance (see Table 7.1 and Figure 7.6) N, = e

N., = 2 (N, - 1) tan #’when 62 412 (rough base)

the shape of foundation (all equal to 1 .O when (B‘<<L‘) sq = 1 + (B’IL’) sin#‘ sq = 1 + sin#’ sY = 1 - 0.3 (B’/L’) sy = 0.7 S, = (s, . N, - 1) / (Nq - 1)

the inclination of the load, caused by a horizontal load H parallel to B’ i , = ( l - 0 . 7 H / v 3 i.,=(l - H / v ) ~ i c = ( i q . N q - l ) / (N,- 1)

tan2 (45 + $12) N, = (N, - 1) cot #’

for a rectangular shape for a square or circular shape for a rectangular shape for a square or circular shape for rectangular, square or circular shape

0

Values are also given in EC7 where H i s parallel to L‘, but these are not relevant for retaining wall problems.

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Table 7.1 Bearing capacity factors for drained conditions (after EC7, Annex 6 )

0

12

14

16

18

20 22

24

26

28 30

32 34

36 38 40

1 .o 3.0

3.6

4.3

5.3

6.4

7.8 9.6 11.9

14.7 18.4

23

29

38

49 64

0

0.8

1.3

1.9

2.8

3.9

5.5 7.7 10.6

14.6 20

28

38

53

75 106

5.1

9.3

10.4 11.6

13.1

14.8

16.9 19.3

22

25 30

35 42

51 61

75

100

50

20

10

5

2

1 10 15 20 25 30 35 40 45

@' ("1

Figure 7.6 Bearing capacity coefficients (after EC7)

ClRlA C516 109


7.2.3

The additional influences of embedment depth, of inclination of the foundation base and of the ground surface should also be considered.

EC7 does not differentiate between flexible and rigid foundations when calculating the maximum safe bearing capacity. This is considered to be generally correct for geotechnical design. However when the stress distribution beneath the wall base is required for basic structural design, there will be a differentiation between flexible and rigid walls. Typical idealised stress distributions are shown in Figure 7.7 for flexible and rigid walls for a vertical load per metre run of wall ‘V’ acting at the wall base. The distinction between rigid and flexible bases depends on the relative width, height and nature of the foundation. As a general principle, walls with reinforced concrete bases will be rigid while wide gabion bases will be flexible.

Calculation of settlement

Placing a 3 m high retaining wall on flat ground will increase the vertical stresses by about 55 kN/m2, a similar stress increase to that beneath a four-storey building founded on a raft. Total settlement beneath the retaining wall can therefore be significant. However, when setting settlement limits, total settlements will seldom be critical for a low-height modular retaining wall. It is the difference in settlements along a retaining wall or between adjacent units which impairs the wall’s appearance or ability to retain water or, in cases of large differences in settlement, its durability or ability to fulfil its function. It is therefore suggested that if movement limits are to be set, they should be set in terms of differential movement or tilt (differential movement per unit length), unless there are obvious reasons for also limiting total settlement.

Movements between the back and front of the retaining wall caused by the overturning moment also cause tilt, but in this case the tilt is across the retaining wall section. Tilting in this direction may be important if a facing wall is to be placed in front of the retaining wall.

Settlements are notoriously difficult to predict as they depend on the situation of the wall. The more important of the aspects which affect settlement are outlined below. In each case, where relative comments are made it is assumed that all other factors remain the same.

1.

2.

3.

4.

5.

Normally consolidated soils are likely to have lower stiffness than overconsolidated soils and therefore will give higher settlements.

Coarse foundation soils usually give the lowest settlements.

With fine-grained soils, a significant proportion of the final settlement will occw after construction of the retaining wall has been completed. The proportion will depend on the profile of increase in stress with depth and the consolidation characteristics of the soil.

A formation containing peat or organic matter will tend to suffer much greater settlements than other materials. As much of this additional settlement occurs after completion of construction, the differential movements it causes tend to be particularly damaging. Walls founded on these soils should therefore be treated with great caution and either expert advice should be sought or other solutions considered.

The height of the retaining wall and the width of the retaining wall and backfill strip will affect the magnitude of stresses at depth, which in turn influences the relative contribution to the total settlement of those deeper deposits.

110 ClRlA C516


Flexible retaining wall

Bearing area

(a) For flexible walls

I r----

Where e

I I

h V 6e q=- I+- B( B )

(b) For walls with rigid bases

Figure 7.7 Bearing pressure area

\\\m\\\

B Wheree>

i

2v q= - ? - s e

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112

In terms of differential settlement, the following factors should also be considered:

the stresses in the ground will be higher beneath the central portion of the length of retaining wall rather than at the ends. Therefore tilts are likely to be most pronounced near to the ends of the retaining wall length

a constant rate of differential movement, such as that occurring under a wall of linearly increasing height, should maintain straight lines in the structure for an observer looking along the wall sharp increases in differential settlements along a retaining wall are likely to be most noticeable. These can be due to effects such as sudden differences in retained height, eg where the original topography changes suddenly but the wall top remains at the same level, or sharp changes in geology, eg where the thickness of a very compressible near-surface deposit suddenly increases.

0

For foundations on fine-grained soils, the principle of ‘floating’ the wall foundation (remove load then replace with equivalent load) can be used to minimise settlement of the wall on compressible materials, provided the wall and backfill are placed quickly enough so that the underlying clay doesn’t have time to heave in response to the initial excavation.

Where excessive differential settlements are expected, it is normally cheapest to design around the problem rather than try to reduce settlements. Settlements can be reduced to some extent by using lightweight fills such as PFA. In many cases, the only way to reduce settlements will be to pile the structure. Measures such as preloading, possibly accelerated by vertical drains, can also be used to induce the settlements before the construction of the wall. If instead, the wall can be designed to accommodate the differential settlements, this is likely to result in a much more economical w a w c - t u r e . Tlus can best be achieved by avoiding long straight lines wluch accentuate the effects of differential movements. This can be achieved for instance by allowing vegetation to overhang the top of the wall.

Where settlements are likely to be significant, it is recommended that expert advice be obtained to consider the interaction of soil and structure.

Simple appraisal of foundation settlement can be made on the basis of the following assumptions:

0 that the wall imposes strip loading on the underlying ground, with a wall length at least four times the wall width that there is an infinitely deep soil layer of constant stiffness below the wall. 0

For coarse-grained materials which are normally consolidated and with no subsequent- time related settlement, Burland and Burbidge’s (1985) equations, summarised by Tomlinson ( 1995), reduce to the following expression:

2.59 BO.’ settlement (in mm) = ,,4

For overconsolidated clays, the long-term settlement based on Butler (1 975) and also reported by Tomlinson (1995) can be reduced to the following expression:

159 B settlement (in mm) = - C”

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7.3

Stresses q and undrained shear strength c, should be given in kN/m2 and foundation width B should be given in rn. Values of c, and N should be appropriate for the zone of soil stressed by the new retaining wall. Settlements in normally consolidated clays would be greater than the values calculated using the equation for overconsolidated clays. Settlements calculated by talung the actual wall width as B and ignoring the contribution to settlement of the weight of any backfill behind will also tend to be underestimated.

OVERALL INSTABILITY INCLUDING DEEP SLIP SURFACES

Ths mode of failure is overall failure of the soil around and below the retaining wall. Consideration of this type of failure is unnecessary where the ground on either side of the retaining wall is nearly flat and the soil strength increases with depth. Examples are shown in Figure 7.8. Global slip surfaces can be subdivided into rotational failures (slip circles) and translational slips (a wedge formed by the intersection of two or more failure surfaces.

(a) Overall circular slip failure

I I

/ /

/ /

/ /

1 / '

0 0 , .

c c - _ _ _ - - -

(b) Failure along a weak layer

/ I

/

Figure 7.8 Types of typical slip failure

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7.4

7.5

7.6

Non-circular slip surfaces should be considered where a particularly weak soil layer passes beneath the foundation and for certain wall geometries. This is equivalent to consideration of a deeper sliding surface to that used in assessing the sliding stability.

Guidance on slip surface analyses is given by Bromhead (1992).

INTERNAL STABILITY

The wall design should include checks for internal stability, ie that the retaining wall retains its shape and continues to fulfil its function when external loading is applied. It involves assessing the forces acting in failure modes that pass through the wall or between wall elements and confirming that the strength or resistance of the retaining wall and of components is sufficient. Design for internal stability for each of the wall types covered by this guide is covered in more detail in Section 8.

CORNER DETAILS

Corner details need care. With many types of wall, special details are required for corners calling for a higher standard of workmanship to install properly. For salient corners (see Glossary for definition) on concrete stem walls, many precast concrete corner units allow each side of the corner to prop the other side. However, for many other wall types, a higher partial factor of safety against failure should be applied to the soil strengths, as a greater volume of soil is actively pushing against a reduced volume of passively resisting soil.

Re-entrant corners (see Glossary for definition) involve a reduced volume of active soil and an increased volume of passive soil, so no extra factors are required against external modes of failure. However, the active soil forces are trying to separate the two corner modules so sometimes increased measures are required to tie the two corner modules horizontally, in order to prevent an internal separation failure.

The stiffening effect of corners, and salient corners in particular, can prevent KO pressures reducing to active values. This can be particularly important for the internal stability design of stresses in concrete stem walls.

Drainage measures around corners should be carefully considered. If rodding is planned as a means to prevent failure of the drainage system, acute corners may need to accommodate a manhole.

DESIGN EXAMPLES

114

Worked examples illustrating the design methods given by this guide are included in Appendix A2.

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8.1

Design applications - specific wall types

GABION AND BASTION WALLS

The nature of the gabion filling material usually results in a very permeable wall. This is useful where the retained material is saturated or in locations prone to severe rain. Where used next to rivers and tidal waters the water in the backfill can drain out easily when the water level falls. However, fluctuating water levels such as those from tidal action can increase the settlement of poorly compacted infill and could also lead to fines being washed out of the retained soil. An example is shown in Figure 8.1. This situation could be avoided by use of an appropriate filter behmd the retaining wall.

Water ..-w through wall

/

Figure 8.1

Structure

7= (ii) Settlement

/

loss of fines caused by flowing water

of foundation

Gabions are flexible, accommodating larger total and differential settlements than other wall types. They are therefore often used where the subgrade soil is poor, as is frequently the case near rivers. In these situations, the grading should be carefully selected so that the finer proportion is not washed out by the flowing water. Coppin and Richards (1990) explain how gabions can be vegetated to integrate the walls into their surroundings. However, the high permeability of the fill material is not conducive to holding the water in the soil which is essential for plant growth.

8.1 .I Durability of the basket material

Both the durability of the basket and the fill need to be considered.

Although BS 8002 indicates that gabions can also be made from nylon, polypropylene, wickerwork or bamboo slats, these materials are seldom used as they are easily damaged by fire. They are almost invariably made with steel.

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116

The following paragraphs describe the options that exist for protecting steel mesh.

Unprotected

Uncoated wire can be used for short-term temporary works. There may also be some circumstances where uncoated wire can be used in permanent works provided the wire thickness contains an allowance for corrosion and the consequences of a failure are not serious.

Galvanised wire

All wire should conform to BS 1052: 1980 (1991). Woven wire mesh should be galvanised to BS 443 (coating weight typically 240 g/m’ but depends on wire thickness), and welded mesh should be hot-dip galvanised to BS 729 (coating weight typically 460 g/m’ but depends on wire thickness) after welding. If the soil or groundwater is aggressive (determined from soil resistivity, redox potential, dissolved salts, eg chloride ion content, total sulphate content, pH value and moisture content), then PVC coating or other form of protection should be considered instead. Coatings such as Galfan (a 95 per cent zinc-aluminium alloy, available under trade names BezinalTM or GalMacTM) have been found to give greater resistance in aggressive environments than traditional (zinc coated) galvanised wire. Coatings are prone to abrasion from shingle in flowing water.

Clause 626 of the Specijkation for highway works requires all wire to have been both galvanised and coated with PVC.

Plastic or thermoplastic coated wire

Plastic or thermoplastic coatings such as PVC (polyvinyl chloride) should conform to BS 4102. The radial thickness of the coating over the galvanised wire core should be at least 0.25 mm according to BS 8002 or 0.25 mm for bonded coatings and 0.55 mm of PVC for extruded coatings according to the Specification for highway works, Clause 626. The coating should be capable of resisting the effects of immersion in sea water, exposure to ultraviolet light and abrasion, when tested for a period not less than 3000 hours in accordance with BS 2782: Part 5: Method 540B (IS0 4892). It should be sufficiently bonded to the wire to prevent a capillary flow of water through the annulus between the wire and coating.

Care should be taken to avoid any protective coating being damaged during construction or during the service life of the structure, eg by the abrasion from moving shingle or stones next to rivers or beaches.

If a gabion wall is shaped so that the stone filling remains stable after failure of the cage, it can continue to fulfil its function, even after the cage has failed. Alternatively, if a rigid and impermeable structure is acceptable, the wall can be repaired by grouting the gabion fill with a cement grout. Provision should be allowed for wall drainage for such walls.

Binding wire should also be treated for durability in the same manner as the gabion mesh. Where the groundwater is particularly aggressive, (eg moorland springs with a low pH), special consideration should be given to the durability of the gabions.

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8.1.2 Durability of the filling materials

Stones for filling gabions should conform to BS 5390 for hardness, crushing strength and resistance to weathering, frost susceptibility in particular. The Specification for highway works Clause 626 requires that Class 6G material should be used for filling gabions. The material criteria are:

its constituents are natural gravel, crushed rock other than argillaceous rock, crushed concrete or any combination of these

its 10 per cent fines value, tested in accordance with BS 812: Part 11 1 with soaked samples, is at least 50 kN

its grading should be determined using BS 812: Part 103, such that both of the following are fulfilled: - the maximum size of fill material should not exceed two thirds of the minimum

dimension of the gabion compartment or 200 mm, whichever is smaller the minimum size of fill should not be less than the size of the mesh opening. -

The stone should be as small and as uniform as possible, consistent with the requirement that it be retained by the mesh. BS 8002 Clause 4.2.6.3.7 suggests that 5 per cent of the stone may be as small as 50-80 mm with a maximum stone size of 200 mm. However for marine structures, 175 mm is taken as the usual minimum stone size.

8.1.3 External sta bi I ity

Gabion walls should be designed as gravity mass walls. Sliding stability under the base of the wall should be calculated using the strength of the foundation soil only, ie 6= q$crit of foundation soil, provided no geofabric separator is used (see Section 7.1.1). It can be assumed that full friction will develop between the retained backfill and a gabion wall, giving a roughness angle of 6= bCrit.

Bastions which often incorporate a geotextile at their back should use a lower roughness angle, typically 6= The virtual back of the wall is taken as a line joining the top back corner of the uppermost gabion to the bottom back corner of the lowest gabion.

8.1.4 Internal stability

Gabion walls should be proportioned so that the resultant force at any horizontal section lies within the middle third of that section. No allowance should be made in the design for the strength of the wire.

Analyses should be made on horizontal sections above the base of the wall to check that there is adequate resistance to sliding using a design friction angle for the gabion fill sliding against itself, ignoring the effect of the wire mesh.

8.2 CRIB WALLS

The Highways Agency design note BD 68/97 sets out the requirements for crib walls on the trunk road and motorway network. The accompanying advice note BA 68/97 provides advice on the internal and external stability design of crib walls.

Past problems with crib walls have usually been caused by poor specification or construction techniques. It is therefore important that these aspects are carefully controlled.

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8.2.1

8.2.2

118

In particular, the placing of backfill should be carried out carefully to avoid causing damage to the crib modules. Crib wall elements are usually small, so it can be difficult to achieve adequate cover to reinforcement in concrete sections.

Vegetation growing in the fill through the modules can be used to disguise the nature of a crib wall. Information on the use of vegetation for this purpose is given in Coppin and Richards (1990). Vegetation can also control washout of fill by flowing water. Often vegetation fails to establish itself properly in cribs because the relatively steep front face quickly drains water from the fill. Where vegetation is required to grow, consideration should be given as to how water essential for plant life can be kept in the fill.

Crib walls are usually built to a batter, which should not be steeper than 1 horizontal to 4 vertical. Lower height walls, generally less than 2 m, are sometimes built with a vertical face provided their width exceeds their height.

While crib walls can carry sloping backfills, their use is not recommended by BS 8002 Clause 4.2.7.2.3 for situations where surcharges from building foundations or other structures could impose loading on the crib wall or its foundation. This is because crib walls have little stiffness. BS 8002 Clause 4.2.7.4.1 proposes that vehicular traffic should be kept 4.5 m or the height of the wall, whichever is the greater, away from the wall and therefore suggests that provided no roadway is within that distance, the effect of wheel surcharge loading need not be considered. BD 68/97 Clause 6.23 calls for the use of barriers to make sure that this condition is met.

If the crib modules are to be placed manually, the modules should be appropriately sized for the number of people who will be available to lift them.

Durability

Timber crib modules should be treated with appropriate preservative, including any timber faces cut during construction. Concrete should be properly detailed and constructed to provide good cover to reinforcement. These walls can be susceptible to damage by vandals. Exposed crib modules can be broken and wooden elements can be set on fire. The fill can be removed by hand. Wooden elements can also be set on fire by a crashed car thus their use is forbidden by some highway authorities in cases when they could be hit by traffic.

External stability

The forces for which a crib wall should be designed are shown in Figure 8.2.

BS 8002 Clause 4.2.7.2.3 warns against the use of crib walls to retain unstable slopes. This is because the crib wall will not offer much resistance to failure planes passing through it. The excavation for the foundations below the toe or into an existing slope may precipitate the slip and it is impractical to extend the cribwork below the level of the potential slip planes.

If impermeable fill or lean mix is used as infill either in the crib wall or in the space behind, drainage should be provided to prevent the build-up of water pressures.

A crib wall should be designed as a gravity mass wall, with its dimensions defined by the area enclosed by the crib modules. Full wall friction can be taken on the back of the wall to assist stability.

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Resultant force acting on the rear of the crib structure

8.2.3

8.3

8.3.1

Figure 8.2 Forces normally acting on a crib wall

Internal stability

The main aspect of internal stability that will concern designers who are not suppliers of crib units is checlung that sliding and overturning failures cannot occur at various levels within the wall.

The detailed design of reinforced concrete crib modules will normally be undertaken by specialist suppliers and is beyond the scope of this guide. Information on the forces for which the crib modules should be designed is given in BS 8002 Clause 4.2.7.4.2 and in greater detail in BD 68/97.

Care should be taken that the crib infill material cannot escape from the crib wall. Sometimes it needs to be retained using a geotextile.

DRY STACK MASONRY WALLS

There are a number of proprietary systems of precast drystack walls. They usually consist of staggered interloclung blocks which can be placed on top of one another, usually by hand. Examples include Porcupine walls and natural stone walls.

Dura bi I ity

The wall units are essentially durable and no further treatment is normally required. The coping blocks often are attached with adhesive to prevent damage to the wall by vandals.

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8.3.2

8.3.3

8.4

8.4.1

8.4.2

External stability

They should be designed as gravity mass walls. The frictional resistance of the wall base should be designed using a friction angle appropriate for the interface between the blocks and the soil. For a rough base, the friction angle of the soil can be used.

Where a soil-to-structure interface is to be provided, masonry walls should be designed using a friction angle Sof 0.5q5crit on the back of the retaining wall. However the installation of a membrane at the wall back, for instance in a drainage layer, should trigger the designer to adopt the use of a lower friction angle.

Internal stability

Precast dry stack walls should be proportioned so that the resultant force at any horizontal section lies within the middle third of that section.

Checks should be made of horizontal sections above the base of the wall that there is adequate resistance to sliding.

MASONRY WALLS

Masonry retaining walls, both plain and reinforced, have a long tradition in the UK. Their design is covered by BS 8002 and BS 5628.

By far the most common form of masonry wall is the simple brick wall. The design of unreinforced brickwork retaining walls is addressed by Haseltine and Tutt (1991). BS 8002 Clause 4.2.4 suggests that simple stem walls are suitable for retained heights up to about 1.5 m while greater heights can be accommodated by stepped or buttressed walls. BD 41/97 gives the Highways Agency requirements for the design of reinforced pocket type and grouted cavity brickwork walls. Reinforced and prestressed walls are covered by NCMA (1993).

Thomas (1 996) gives clear guidance on the design and specification of all types of masonry walls. The publication also gives details from the relevant standards.

The types of unit which can be used in masonry walls are covered by the following British Standards:

BS5628 Code ofpractice for use of masonry

+= Part l : 1992 Structural use of unreinforced masonry

+= Part2: 1995 Structural use of reinforced and prestressed masonry

+= Part3: 1985 Materials and components, design and workmanship

Durability

Durability of masonry retaining walls is addressed in BS 5628 and Thomas (1996). Particular attention should be paid to sulphate attack.

External stability

120

Where a soil-to-structure interface is to be provided, masonry walls should be designed using a friction angle Sof on the back of the retaining wall. However the use of membrane at the wall back, for instance in a drainage layer, should trigger the designer to use a lower friction angle.

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8.4.3 Internal stability

The design of masonry walls to resist horizontal loading is covered by BS 5628 Parts 1 and 2, and simple design rules are given by Haseltine and Tutt (1991). Walls should be designed to resist overturning; buttresses and fins can be used to provide resistance in circumstances where their use would not conflict with other requirements, eg buttresses might conflict with a preferred facing.

8.5 PRECAST REINFORCED CONCRETE STEM WALLS

8.5.1 Joints

Retaining walls composed of precast concrete elements have vertical joints in the face between each pair of elements. BS 8002 Clause 4.3.1.4.6 recommends that where necessary (generally where water penetration from the retained fill through the wall joints would be unsightly or could damage a facing structure), ‘joints should be lined with a resilient jointing material about 10 mm to 20 mm thick and sealed with a proprietary sealing compound. Dependent upon the ground water present, waterbars may also be required’. Differential settlement of the subgrade across these joints could lead to cracking of jointing material, probably losing the joint’s impermeability.

8.5.2 Dura bi I i ty

Design should be to BS 81 10 or EC2 or to BS 5400 Part 4 for l-ughway schemes. Standard precautions for ensuring the durability of reinforced concrete should be followed.

8.5.3 External stability

The vertical plane on which the active forces are evaluated is that through the back of the heel, not the stem. If the wall is to rely on drainage for stability, it is important that the effects of the drainage are felt on that vertical plane and on the potential slip planes beyond it. This vertical plane is known as the ‘virtual back’ to the wall (see Glossary).

There should be no friction taken on a virtual wall back when horizontal pressure factors K, are being evaluated ie 6= 0.

8.5.4 Internal wall stability

The forces from the external stability analysis should be used for the design of the stem and heel, following the guidance of BS 8002 Clause 4.3.1.4.3 (see Section 4.4.3) and DD ENV 1992-1-1 (EC2) or BS 8110.

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9 Specification and quality control

This section does not attempt to restate the established procedures for specifying civil engineering work, but to identify areas of special attention for the procurement and installation of low-height modular retaining walls. A further aim is to remind clients of statutory and other duties required in the procurement chain. Certain clients, such as the Highways Agency, have their own specifications for use on their schemes.

A method specification is a series of instructions which defines how the work is to be carried out. It may include:

communication of specifier’s intent

design basis and calculations

record of ground investigation report and any interpretative information including, for example, the need for verification of anticipated ground conditions for geotechnical category 1 walls

record of design and construction reference standards

description of all bought in items including relevant standards

description of materials on site to be reused including all relevant standards

test panels

breakdown of all work stages

drawings which unambiguously illustrated the works required

photographs illustrating any special finish required

special construction risks anticipated by the wall designers

quality control and inspection regimes

safety and welfare plans

maintenance plan

statement of any special rules of workmg on site, working hours, noise, security, etc

means of establishing payment and coping with variations as work progresses.

A different approach from providing a method specification defining all aspects of the wall is that the client and specifier may agree that a performance specification is appropriate. Such a specification defines the position, uses and requirements for the wall and allows suppliers to put forward their own solution to the performance brief. This process may be considered appropriate for modular retaining walls as it allows suppliers to use their particular expertise more fully. It can only be followed when the wall requirements are sufficiently flexible for them to be described simply and for several competing systems to provide an acceptable end product.

122

However it will still be necessary for many of the topics listed above for a method specification to also be stipulated for a performance specification. In this case a successful design can be achieved by co-operation between the designer and the successful supplier. The performance specification could also exclude solutions that would not be acceptable for particular reasons.

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The client has a particular statutory responsibility under the CDM Regulations. That is the appointment of a planning supervisor at the start of the design process. The assembly of a small modular retaining wall could require a site establishment that would be on the borderline of the minimum requirements and the requirement for application of CDM rules should be reviewed in this light.

The client normally has no CDM duties with respect to the manufacture of components of a modular wall at a factory off site as they are discharged by the directors of the company and factory management under the Factories Acts.

The designer also has particular statutory responsibilities under the CDM regulations. Designers’ decisions may be audited so full records should be kept of decisions, risk assessments and actions. CIRIA Report 166 (Ove Amp & Partners, 1997) gives relevant guidance.

9.1 S P E C I F I CAT I 0 N

9.1 .I EC7 Category 1 structures

In these cases the geotechnical investigation can be qualitative and the designer may use relevant local knowledge in preparing the calculation for external and internal stability (see Section 4.2).

The specification should allow for these aspects but further information, including internal and external stability calculations carried out by the responsible body (see Section 1.7), should nevertheless be complete and thorough. The content of the specification would therefore be:

statement of specifier’s intent

design basis and calculations

nature of qualitative assessment of geotechnical issues (including those that need to be verified during the construction process)

description of bought-in materials including relevant standards

description of materials on site to be reused including all relevant standards drawings which unambiguously illustrate the works required

photographs illustrating any special finish required

other points listed in Section 9.1 as appropriate.

9.1.2 EC7 Category 2 structures

These structures would require the specification information identified in Section 9.1.

9.2 SPECIFICATION DETAILS

Standard specification clauses and methods of measurement already exist in a number of sources, eg Highways Agency Specification for highway works. The first stage in preparing the detailed specification is to review the clauses for earthworks, concrete work, brickwork, etc and whenever possible, to adopt relevant clauses. The next stage is to consider the construction process, step by step, for the wall systems that may be built and list any activities and materials that are not covered in standard clauses. In each case the work should be defined and the material specified in terms of current standards or relevant additional tests.

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9.3

9.4

The manufacturers can be consulted at any stage about model specification clauses and details of materials that the designer might wish to review and either incorporate or modify.

Matters of particular relevance to modular wall construction to which attention should be paid include:

control of site conditions in care of foundation excavations, preventing them being open for long periods prior to placing of the base concrete, not allowing water to collect on a freshly excavated formation, care to prevent compaction plant hitting the wall elements during compaction, etc

weather conditions when construction work is permitted

temporary stability during construction

compliance tests and frequencies of testing for backfill materials

requirement for inspection before buried work can be covered

specification of finished appearance, building of mock-up, reference samples, etc

cleanliness of the site during construction to achieve the correct standard of finished work

drainage requirements.

QUALITY CONTROL

The specifier should, after consulting the designer, define the quality control regime for the project. This usually takes the form of a quality plan, which defines the relevant measures and responsibilities of those operating the quality system and would lead on to a list of quality procedures.

It is likely that the manufacturers of retaining wall materials and systems, like any other major suppliers to the construction industry, would have some form of quality assurance scheme in place. A purchaser is fully entitled to question this scheme, request an audit and ask for further relevant measures to be taken if necessary.

A good quality control scheme should include reference to basic standards for wall design such as BS 8002 and a basic test of the competence of a contractor and supplier is to question knowledge of the construction aspects of this standard.

The quality control system defines procedures (written and physical processes and tests) that are to be carried out. It is the function of the inspection process to check that this is done.

INSPECTION DURING CONSTRUCTION

124

The inspection process during construction should cover all relevant aspects of the earthworks and other engineering works that is carried out. Experienced inspectors should be aware of the standard procedures and tests with respect to excavation, testing and storing reused material, condition of the site concrete works, bricklaying, drain laying, backfilling and consolidation, etc. They also need to understand the design requirements and the specifier’s intent.

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9.5

9.6

The inspector should alert the specifier and designer should unexpected non-compliant conditions or details arise. The designer can then carry out any further design checks and the specifier be made aware of any potential changes to cost and programme through extra work, savings, etc.

Modular walls may involve the use of proprietary materials or construction procedures. The inspector should be briefed by the designer or supplier of these areas and should be given appropriate inspection requirements.

A good inspector should nevertheless question and refer back to the designer any practices or materials that are not described in the specification or which do not appear to conform to the design intent.

The inspector should make sure that the installer and supplier produce sufficient and accurate records to confirm that the wall has been built to fulfil the requirements of the design. Non-compliances should be noted together with details of how they were overcome. Once built, the completed construction records should be incorporated in the principal contractor’s health and safety file.

MAINTENANCE SCHEDULE

Sufficient records should be kept and, if required, a schedule of lifetime inspection and maintenance for the finished work should be prepared. This is important not only to check that the durability of the structure is maintained, but also to identify warning signs of failure through abuse or changing site conditions. A further benefit of the maintenance records is to assess the performance of the wall in carrying increased loads or other known changed conditions. This subject is described in more detail in Section 10. The maintenance requirements may be described in the health and safety file.

COMMERCIAL ISSUES

The commercial aspects of the specification may be special because of size and value of the manufactured and supplied element of the final works. The supply of proprietary components can form the major part of the cost of a project. It is important for the purchaser to read any standard terms and conditions of sale that the supplier may propose and resolve problems of understanding, interpretation and disagreement before any order is placed, or commitment to use a system is made.

125


I 0

10.1

10.1.1

Inspection and maintenance

This section gives recommendations for inspection and maintenance regimes that would be appropriate to walls covered by this guide after they have been built. Inspection is usually simple to carry out and inexpensive. Problems which are not corrected by subsequent maintenance can be expensive to remedy later and can be hazardous, even in the case of relatively simple geotechnical category 1 and 2 structures covered by this guide. Maintenance should be distinguished from repair. Repairs are carried out when some defect is detected which had not been prevented by the inspection and maintenance regime.

The inspection regimes and suggestions for routine maintenance that are recommended should be based on the designer’s intent. The designer should check that an inspection and maintenance regime consistent with the design and the required design life is specified. The designer should also confirm that the client understands the need for inspection and maintenance if the wall is to fulfil its design requirements over its required design life. Ensuring that any inspection and maintenance requirements are written into the health and safety file goes some way towards accomplishmg this aim.

INSPECTION

Scope

The completed structure should be inspected regularly in accordance with a schedule prepared by the designer. The need for inspection depends on the degree of risk and the planned design life of the structure. The inspection should consider the condition of the structure and its ability to sustain the external forces and fulfil its internal stability requirements. In the case of the latter, the supplier should be consulted during the definition of the schedule.

The consideration of stability under external forces requires a review of the area around the structure, observing the condition of the ground above and beneath the wall, at each end and at any corners. The level, line and state of the retained material and the material at the foot should be observed as should the line and slope of the wall. Any signs of disturbance or new building near to the wall should also be sought. Table 10.1 gives a more complete list of the critical areas and the reasons behind their importance. The main features of Table 10.1 are illustrated in Figure 10.1.

The internal stability of a wall is related to the materials and soundness of the construction of the wall itself. The inspection techniques relevant to the particular modular wall materials should be used.

126 ClRlA C516


In front of the wall The wall Behind the wall

boggyareas - - heave of ground new trenches seepages leaking from weepholes

movement horizontal vertical tilting

cracking loose capping blocked weepholes impact damage seepage parapets I fences

heavy trees new buildings new surcharge roads signs boggy areas fissures ground movement ponding of water

Figure 10.1 Wall inspections - typical danger areas

It is usual for inspection regimes to require one or more of three kinds of inspection, defined below as:

type 1 inspection

type 2 inspection

type 3 inspection.

A type 1 inspection is an informed look at the wall and its surroundings. It need not be carried out by a qualified engineer, but the person making the inspection should be aware of possible defects and their consequences. The type 1 inspection is a means of checking that no obvious problems are developing. It is useful in de tekning whether foliage needs to be trimmed and to see whether changes of use, for example new surcharges, have been imposed. A type 2 inspection looks at the general wall condition and may trigger the need for a more detailed type 3 inspection. Possible inspection frequencies are suggested in Box 10.1.

Box 10.1 Possible inspection frequencies

For a structure covered by this report it is suggested that a type 1 inspection is carried out every four months. Type 2 inspections are carried out after the first winter and summer of the life of the structure and then after two years and at two-yearly intervals thereafter. A type 3 inspection would replace the type 2 inspection at the end of the contract maintenance period and then be carried out at six-yearly intervals, or more frequently if a need is indicated by a type 2 inspection

ClRlA C516 127


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128 ClRlA C516


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ClRlA C516 129


10.1.2

130

The scope of the types 2 and 3 inspections are similar except that the type 2 inspection looks at sample areas, apart from a global check of line, level, etc. The type 3 inspection involves close inspection of all areas. Table 10.2 summarises the features of the three types of inspections.

Table 10.2 Inspection types

Inspection Possible frequency Purpose

Type 1 . Every four months . To note obvious changes of state of

. to determine need for trimming

. to detect new changes of use of the

. to check whether water is leaking

the wall

foliage

wall - new surcharge, etc

from weepholes

Type 2 . First autumn and spring . every two years if defects To act as a means of defining

. as a sample of a large installation to maintenance

check for underlying problems become obvious

Type 3 . Prior to end of contractor’s

. every six years if need is

. To investigate in detail and find any

to define need for unusual maintenance or removal and replacing of suspect areas

maintenance period underlying problems

indicated by simple survey

The Highways Agency design and advice notes BD and BA 63/94 have established procedures for these kinds of inspections and the Institution of Structural Engineers publication, Appraisal of existing structures (1980), is another useful source of guidance.

The inspection of a retaining wall should extend beyond the area of the wall itself to the ground on each side where the influence of the wall is felt, both in terms of the loading or load resisting zones. Figure 10.2 shows the areas that may be considered for a wall with and without sloping backfill, for walls of height up to 3 metres. Confirming that the surcharge behind the retaining wall has not increased is particularly important.

Records

The importance of records is to allow previous and future inspection reports to be read together to observe a pattern of changes. The value of good records, prepared in a standard way, cannot be overemphasised. Records should pinpoint the structure exactly, by map references, and include written and photographic material. Figure 10.3 gives suggestions for the content of a record sheet.

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r

c Wall with no surcharge

10.2

Wall with surcharge or retaining sloping ground

Figure 10.2 Scope of inspection - critical areas

MAINTENANCE

10.2.1 Rationale

Maintenance is generally carried out for two reasons. It is necessary to make sure that the original design intent is maintained with respect to groundwater levels, loading, etc and also it is necessary to remedy problems which could interfere with the continued satisfactory operation of the wall.

10.2.2 Loading

The forces that a wall is carrying will change if loads are applied to the zone of influence behind the wall as illustrated in Figure 10.2. These loads may be permanent material surcharges, a new building or the use of heavy mobile machinery behind the wall. A maintenance requirement could be to remove and prohibit such loading.

10.2.3 Water pressure

Noting that water pressures have not increased above the values used in design is crucial for the long-term stability of the wall. Significant increases in the design water table level require the design of the retaining wall to be reassessed. Maintenance aspects that are important in this area are cleaning drains and weepholes, checking for burst services and removal of unplanned vegetation that could clog any drainage.

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132

Structure

Map reference

Address

Client Name/address

Inspector Name/address

Date (Date of last survey)

Current and recent weather conditions

Type of inspection 1/2/3 (delete as applicable)

Notes

Type 2 inspection Areas surveyed

Conditions above wall

Conditions below wall

Condition of wall

Drainage/weepholes

Notes/sketches/photos (refer)



Type 3 inspection Layout drawing with grid

Conditions above, below and on wall tied to the structure grid references, noted as above

Maintenance recommendations

Other recommendations

Figure 10.3 Typical inspection record

ClRlA C516


10.2.4

10.2.5

Wall materials

Wall materials of concrete, metal or timber, can all be checked by conventional means. Where they have not been covered by facing or filling, remedying defects is straightforward. Maintenance by removal and replacement should not be attempted without design advice.

Monitoring

An aspect of maintenance which unites both the survey and maintenance phases is monitoring. If it is thought that movement (either global or local) is taking place, a period of monitoring is often useful so that the underlying cause of the problem can be identified and remedied.

Before starting to monitor a wall, it is important to establish the purpose of the monitoring. It may be practical to measure global movements by simple survey methods. A steel tape, spirit level and plumb bob as well as a surveyor’s level are all simple to use and sufficiently accurate to detect differences, provided the measurements are made to a static datum. Local movements across cracks may be checked with tell- tales or crack microscopes and at a larger scale can be checked using a steel tape to indelible marks or lines. Even if only simple measurements are made, records should be kept and attention should be placed on making and repeating measurements at exactly the same place as that used originally and in exactly the same way.

It is possible to carry out simple measurements of water table level, but they should be carried out and assessed by someone experienced in geotechnical investigation.

ClRlA C516 133


11

134

Concluding remarks

This project report was prepared for the following reasons:

0 little design guidance is available in the UK for the design of gravity retaining walls and for gravity walls formed from modular components in particular

failures of gravity retaining walls can occur because often responsibility for their design is muddled

many designers avoid particular modular systems because of perceptions about their past performance and lack of knowledge about how to avoid those problems.

It is therefore recommended that the following steps should be taken for the successful construction of low-height gravity retaining walls formed from modular units:

0

0

design responsibilities should be clear and explicit

design for internal and external stability should both have been carried out and should be mutually compatible

an appropriate site investigation should have been carried out

feedback should be passed from designers to contractors so that particular details are correctly built and back again to confirm that the design can accommodate any unexpected conditions that are encountered

a programme for inspection and maintenance is established to help the retaining wall to remain serviceable over its design life.

0

0

ClRlA C516


A I

ClRlA C516

Active and passive pressure coefficients

This Appendix includes tables and graphs giving active and passive pressure coefficients, calculated according to the numerical procedure proposed in Appendix G of EC7 (ENV 1997-1: 1994).

A key assumption of this procedure is that what EC7 defines as the angle of convexity v should be positive. In certain circumstances for the range of values calculated for this Appendix, the angle of convexity is less than zero. No values are presented for this case and other methods are recommended for the calculation of active and passive pressure coefficients.

Information is presented on the following:

sign convention

0

tables for active pressure coefficients

graphs for active pressure coefficients

table for passive pressure coefficients

graph for passive pressure coefficients.

135


Table A I .I

e= -30 -20

6= 0 . 0.54' 1.04' 0 0.54' 1.04'

4' 10 0.585 0.553 0.537 0.623 0.588 0.571 15 0.445 0.411 0.394 0.488 0.451 0.433

20 0.335 0.304 0.288 0.380 0.345 0.327

25 0.249 0.223 0.209 0.293 0.262 0.246 30 0.182 0.161 0.149 0.223 0.197 0.183

35 0.130 0.114 0.105 0.166 0.146 0.134

40 0.090 0.079 0.071 0.121 0.105 0.096 45 0.060 0.052 0.047 0.085 0.074 0.066

Sign convention 0,6$ positive

-1 0 0

0 0.54' 1.04' 0 0.54' 1.04'

0.662 0.625 0.607 0.704 0.665 0.646 0.536 0.495 0.475 0.589 0.544 0.522 0.432 0.392 0.371 0.490 0.445 0.422

0.345 0.308 0.289 0.406 0.363 0.340 0.272 0.241 0.223 0.333 0.294 0.273 0.212 0.186 0.171 0.271 0.237 0.218 0.162 0.141 0.128 0.217 0.189 0.172 0.121 0.105 0.094 0.172 0.149 0.134

Sign convention 0 negative 6,P positive

Sign convention e,s,p = 0

Passive

Sign convention e,s,p = 0

P //A\\W

c)

value (after EC7)

136 ClRlA C516


I I -1 0

~~~~

-30 -20

6= 0 0.54' 1.04'

0.630

0.491

0.378 0.286

0.212

0.152

0.106

0.071

0 0.54' 1.04'

0.670 0.632 0.614

0.540 0.498 0.478 0.429 0.389 0.369 0.337 0.301 0.282

0.259 0.229 0.212

0.195 0.170 0.156

0.142 0.124 0.112

0.100 0.087 0.078

0.594

0.454 0.343 0.256

0.187

0.133

0.092

0.061

0 0.54' 1.04'

0.712 0.672 0.653

0.593 0.547 0.525 0.488 0.442 0.420 0.396 0.354 0.332

0.317 0.280 0.260 0.249 0.218 0.200 0.191 0.166 0.151

0.142 0.123 0.111

0.578

0.436 0.325 0.240

0.173 0.122

0.084

0.055

e=

6=

4' 10

15

20

25

30

35 40

45

Table A I .3 Ka value (after EC7)

-30 -20

0 0.54' 1.04' 0 0.54' 1.04'

0.715 0.675 0.656 - 0.718 0.697

0.587 0.542 0.520 - 0.595 0.571 0.472 0.428 0.406 0.536 0.486 0.461

0.370 0.330 0.310 0.435 0.389 0.365 0.282 0.249 0.231 0.344 0.304 0.282

0.207 0.182 0.167 0.265 0.232 0.213 0.147 0.128 0.116 0.196 0.171 0.155

0.098 0.085 0.077 0.140 0.121 0.109

-1 0

0

0

0 0.54' 1.04'

- 0.715 0.695

- 0.601 0.577 - 0.502 0.476 - 0.416 0.391

- 0.342 0.317

- 0.278 0.255

- 0.223 0.202

- 0.175 0.157

3 0.54' 1.04'

- - 0.742

- 0.654 0.627

- 0.551 0.523

- 0.457 0.429

- 0.372 0.345

- 0.296 0.272

0.229 0.208

- 0.172 0.154

-

0 0.54' 1.04'

- - 0.789

- - 0.689

- - 0.594

- - 0.505

- - 0.422

- - 0.347

- 0.279 - - - 0.218

Table A I .4 Ka value (EC7)

e=

6=

4' 10

15

20

25 30

35 40

45

10

D 0.54' 1.04'

0.707 0.597

0.505 - - - - -

0.687

0.573 0.479

0.400

0.334 0.278 0.230 0.189

20 30 1 D 0.54' 1.04' 0.54' 1.04'

0.731

0.629

0.544 0.471

0.409

0.355 0.309

0.268

0.777

0.691 0.617

0.554

0.500 - - -

ClRlA C516 137


Table A1.5 Ka value (after EC7)

P / + ’ 0.4

e=

6=

4’ 10

15

20

25

30 35

40

45

Table A I .7 Kp value (after EC7)

e=

4‘ 10 1.420

15 1.699

20 2.040

25 2.464

30 3.000

35 3.690

40 4.599

45 5.829

138 ClRlA C516


KEY TO APPENDIX A I FIGURES

Figure A I .I

Page 140 B=+ 30" a) p= 0" b)p=0.4qP c )p=0 .8qP

B= + 20" d) p= 0" e) p= 0.4 qP f ) p= 0.8 qP

Page 142 8=+ 10" g) P = 0" h) p= 0.4 @' i) p= 0.8 qP

Page 143 B=+ 0" j ) p= 0" k) p= 0.4 qP 1) p= 0.8 qP

Page 144 B = - 10" m) p= 0" n) p= 0.4 @' 0) p= 0.8 qP

Page 145 8= - 20" p > P = O " q) p= 0.4 qP r) p= 0.8 f

Page 146 B= - 30" s) p= 0" t) p= 0.4 qP U) p= 0.8 qP

Figure A I .2

Page 147 values for 0 = 0", p= 0", 6= 0"

ClRlA C516 139


c C a, 0

0 0

3 m 0

Q c r m a, a, >

.- %

2

2

.- c 2

E 0.9 1

'G 0.8 ~

0.7 8 2 0.6

g 0.4 r 0.3 a, 0.2

= 0.1

0.0

1 - L - a,

1 I

.-Ap 5

I

-

I ! .- 3

0.5 -_ ~

I

I c L . - . - I ___-___ I m

a, >

I

I ! !

~ - I

0.9 I I

6 = 0,0.5 & 1 .O $" (u<O)

, _ _ _ _ - _ ~ - I I I

I i 0.1 0.0

10 15 20 25 30 35 40 45 00

a) Values for 0 = +30", 0 = 0" T

E 0.9 'G 0.8 0 0.7

a,

5 0

3

U)

2 0.6 0.5

g 0.4 c m r 0.3

0.2 '= 0.1

0.0

>

I

I

= 0,O.SV ( W O ) = I.OI$"

(WO, $>30°)

15 20 25 4' 30 35 40 45

, 6 = 0,0.5, 1 .O$" (u<O)

b) Values for 0 = +30" , 0 = 0.44"

140

c) Values for 0 = +30" , p = 0.84"

FIGURE A l . l Ka value after EC7

ClRlA C516


( W O )

- r p__i_-p- o.3

a, 0.2 'F. 0 1

__ - - ~

5

2 a, I >

1 I I

O.Ol0 15 20 25 30 35 40 45 4 O

e) Values for 8 = +20° , p = 0.44'

6 6

= 0 , 0 . 5 1 $ O (u<O) = 1 .OV (u<O, go>4O0)

> .- - 0.0 I I 2 10 15 20 25 $0 30 35 40 45

f) Values for 8 = + Z O O , p = 0.84'

6 = 0,o.s & 1 .o go ( W O )


ClRlA C516 141


2 o 1 - - c.

10 15 20 25 30 35 40 45 4 O

g) Values for 8 = + l o o , p = 0"

z 0.9 a,

0 8 ~

.-

6 = 6 =

a, o,2 .- :-I

______.-___ __- a, > Y

2 O.Ol0 15 20 25 30 35 40 45

4 O

h) Values for 8 = + l o o , p = 0.44"

0,0.5g0 1 .Ob"

Y c a, 0

0 0

3 U) U)

n

.- E 2

2 5 L m 0

0.9 I 1 0.8 0.7 0.6 0.5 0.4 0.3

6 = 6 =

0,0.51$O (u<O)

1 .ago - _ _ _ ~

o,, 0.0

- ___-- ____

10 15 20 25 go 30 35 40 45

i ) Values for 8 = + l o o , p = 0.84'


142 ClRlA C516


c C a, 0 .- 5 s 2

2 5

3 U) v)

Q

L m a, a, > .- c 8

c C a, 0 .- 5 8 2

2 5

3 v) U)

Q

L m a,

.- 9 2 c

CI C a, 0 .- 5 s !??

!??

3 v) v)

a c r m a, a, > .- c

8

A A

o, . -.. I 0.7

0.6 0.5 0.4

0.3 0.2

u.u 10 15 20 25 30 35 40 45

4 O

j) Values for 0 = 0" , p = 0"

6 = oo 6 = 0.51$O 6 = 1 .O$O

0.9

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1

I

15 20 25 30 35 40 45

4" k) Values for 0 = 0" , p = 0.44"

6 = oo u0<O) s = o.5$ 6 = 1.04"

0.8 0.7 0.6

0.5 0.4 0.3 0.2

10 15 20 25 30 35 40 45 4 O

s 6

= 0,0.51$~ = 1 . o y

I) Values for 0 = 0' , p = 0.84'

FIGURE Al.1 Ka value after EC7

ClRlA C516 143


c K a, 0 .- 5 8 E 2 E!

3

a r m a, a, > .- U

8

c S a, 0

0 0

3 v) v)

Q

.- 5 E

E! 5 L m a, a, > .- c.

8

c)

K a, 0 .-

8 E!

E!

3 U) cn Q .K

a, a, >

5

8 .- c

6-0' 6 = 0.5' 6 = 1 .OI$'

10 15 20 25 30 35 40 45 4 O

m) Values for 8 = - l o o , p = 0"

6 = 0' 6=0.5 ' 6 = 1 .O$'

I 15 20 25 30 35 40 45

oo n) Values for 8 = -10" , p = 0.44"

6 = 1 .OI$' o, 0.0

~ ~ ~ ~ ~ p p ~ L ~ ~ -

I

10 15 20 25 30 35 40 45 4 O

0) Values for 8 = - l o o , p = 0.84'

144


ClRlA C516


0.0 ' I

6 = 0" 6 = 0.5go 6 = 1 .O$"

10 15 20 25 30 35 40 45 4 O

p) Values for 8 = -zoo, P = 0"

0.6 0.5

0.4 0.3

0.2 0.1

15 20 25 30 35 40 45 4"

q) Values for 8 = -20°, p = 0.44'

0.9

0.8 0.7 0.6 0.5 0.4 0.3

0.2 0.1 0.0

10 15 20 25 30 35 40 45 b0

r) Values for e = -2O", P = 0.84"

NGURE A l . l Ka value after EC7

= oo ""<O, g a o y = 1.0 0 " = 0.5

ClRlA C516 145


E 0.9 :P 0.8 8 0.7

g 0.6 gj 0.5 g 0.4

a,

t

0

3

L

m r 0.3 E 0.2 > = 0.1

0.0 t !

10 15 20 25 $0 30 35 40 45

s) Values for 0 = -30", p = 0"

1 1 ~

, ~

V."

1 +- 0.8) ~ ! 1 1 1 0.7

40 t) Values for 0 = -30" , p = 0.44"

0.9 0.8 0.7 0.6 0.5 0.4 0.3

0.2 0.1 0.0

6 = oo 6 = O S + " 6 = 1 .Ob"

6 = oo 6 = 0.51$O 6 = I.OI$"

I 1

6 = 00 6 = OSb0

10 15 20 25 30 35 40 45 40

U) Values for 8 = -30", p = 0.84"

146

FIGURE Al.1 Ka value after EC7

ClRlA C516


10 15 20 25 30 35 40 45 oo

Values for 8 = 0" , p = O", 6 = 0"

FIGURE A1.2 Kp value after EC7

ClRlA C516 147


A2 Design examples

EXAMPLE 1

Design of a precast reinforced concrete stem wall to EC7 and BS 8002.

EXAMPLE 2

Design of a gabion retaining wall to EC7.

148 ClRlA C516


$ 2 3 3

d

s" d

cp J, d)

8

4

! 2 i a

5

a

F

n 2 0

J S J

3

ClRlA C516 149


150 ClRlA C516


4

Q U A

i s

d

t H1 0

3 W

0 0 II

W b

3 Y,

W

\r

m 64 c! & d

ClRlA C516 151


v, r

m

152 ClRlA C516


ClRlA C516

b b b . .

c‘c - 4- Q

I

153


-i 4 0' 7 UI J c 1

w 5

154

P .! 3- Y m 4 k W W J, U

W 2 (P 4 d 3

1 L

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3 W d 3' >

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L 2 0 Y 0 IH

i I f C

3

____

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a i as ? W

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!k

d 5 J J 0

k 2 d, W s W d 3 a tb

3 2 - A J J)

2

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a w 3 n

n 3 W n d Q

5 s

2

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F

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CL D

5 J

Is 3 0

156 ClRlA C516


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Y 3 2

9

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a 0 T II

11 I' J P .A .A

ClRlA C516 157


.

Jl Id

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II

y 11

b O

l 0 J t

e

158 ClRlA C516


4 2

O r

i d

d U

a i d W

5 d

s

ClRlA C516 159


0

s

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5

0

$ i

4 1 f "

P4

II

a

160 ClRlA C516


* 0 z

ClRlA C516

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f U

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162

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ClRlA C516


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164 ClRlA C516


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ClRlA C516 165


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166 ClRlA C516


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s

w ; il 3 w

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ClRlA C516 167


4 0 z 2 Lc r(

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fn

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168 ClRlA C516


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/

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ClRlA C516 169


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cn

170 ClRlA C516


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J 4

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t - . -

ClRlA C516 171


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II

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3- 4 5

172 ClRlA C516


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0-

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?J W

ClRlA C516 173


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174 ClRlA C516


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ClRlA C516 175


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W

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176 ClRlA C516


ClRlA C516 177


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d h

In

0

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a 4 W

s cn

A A

s"

f l n Y II 00 ;; 9 w HI F A = k

5 L ul Q)

d

w -P d F 9

2 2

ClRlA C516 178


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x

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s 3-

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tl

ClRlA C516 179


180

0 0 r(

0


ClRlA C516

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182 ClRlA C516


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ClRlA C516 183


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9

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S

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0

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184 ClRlA C516


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ClRlA C516 185


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186 ClRlA C516


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190 ClRlA C516


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ClRlA C516


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192 ClRlA C516


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ClRlA C516 195


References

AKROYD, T N W (1996) Earth retaining structures: introduction to the code of practice The Structural Engineer, Vol74, No 21, pp 360-364

ARUP GEOTECHNICS (1 990) Review of mining instability in Great Britain Amp Geotechcs for the Department of the Environment

ATKINSON, M F (1 993) Structural foundations manual for low rise buildings E & FN Spon

BARKER, D (1 997) Planting with a purpose Highways, 24 April, pp 24-26

BIRD, R W (1992) Review of drainage to earth retaining structures for highways TRL contractor report 324

BOLTON, M D (1991) A guide to soil mechanics

BOLTON, M D (1996) Geotechnical design of retaining walls The Structural Engineer, Vol74, No 21, pp 365-369

BOSCARDIN, M D and CORDING, E J (1989) Building response to excavation induced settlement ASCE Journal of Geotechnical Engineering, Vol 11 5, No 1

BROMHEAD, E (1 992) The stability of slopes Blackie Academic and Professional, 2nd edition

BUILDING RESEARCH ESTABLISHMENT (1991) Sulphate and acid resistance of concrete in the ground BRE Digest 363

BUILDING RESEARCH ESTABLISHMENT (1996) Desiccation in clay soils BRE Digest 4 12

BURLAND, J B , BROMS, B B and DeMELLO, V F B (1977) Behaviour of foundations and structures 9th International conference on soil mechanics and foundation engineering, Tokyo

BURLAND, J B and BURBIDGE, M C (1985) Settlement of foundations on sand and gravel Proceedings of Institution of Civil Engineers, Part 1, 78, pp 1325-1381

196 ClRlA C516


ClRlA C516

BUTLER, F G (1975) Heavily over-consolidated clays Review paper: session 111, Conference on settlement of structures Pentech Press

CARDER, D R (1988) Report on ICE meeting on earth pressures on retaining walls and abutments Ground Engineering, July, pp 7-1 0

CLAYTON, C R I (1990) The mechanical properties of the chalk Proceedings of international symposium, Brighton, pp 2 13-232 Thomas Telford

COLE, E R L and WATT, P (1994) Gravity earth retaining structures, Section 4 of BS 8002: 1994 Symposium on earth retaining structures BS 8002 Institution of Structural Engineers

COPPIN, N J and RICHARDS I G (eds) (1990) Use of vegetation in civil engineering CIRIA/Butterworths, CIRIA Book 10

CRONEY, D and JACOBS, J C (1967) The frost susceptibility of soils and road materials TRRL Laboratory Report 90

GEOTECHNICAL ENGINEERING OFFICE, HONG KONG GOVERNMENT GEO (1 993) Guide to retaining wall design Geoguide 1,2nd edition

HAMBLY, E C (1979) Bridge foundations and substructures Building Research Establishment

HARFUS, M R , HERBERT, S M , SMITH, M A et a1 (1995-1998) Remedial treatment for contaminated land, Volumes I-XI1 CIRIA Special Publications 101-1 12

HASELTINE, B A and TUTT, J N (199 1) The design of brickwork retaining walls The Brick Development Association Design Guide No. 2, revised edition

INGOLD, T S (1979) The effects of compaction on retaining walls GCotechnique Vol29, No 3, pp 265-283

INGOLDBY, H C and PARSONS, A W (1977) The classification of chalk for use as afill material TRRL LR 806

INSTITUTION OF STRUCTURAL ENGINEERS (1980) Appraisal of existing structure

197


JEWELL, R A (1 996) Soil reinforcement with geotextiles CIRIA Special Publication 123

JOHNSON, R A (1995) Water resisting basements - a guide CIRIA Report 139

LEACH, B A and GOODGER, H K (1991) Building on derelict land CIRIA Special Publication 78

LUPINI, J F, SKINNER, A E and VAUGHAN, P R (1 98 1) The drained residual strength of cohesive soils Geotechnique Vol31, No 2, pp 181-213

NATIONAL CONCRETE MASONRY ASSOCIATION (1993) Design manual for segmental retaining walls (modular concrete block retaining wall structures) NCMA Publication No. TR 127, 1st edition

NAVFAC (1982) Naval facilities engineering command design manual; 7.1 Soil mechanics

NHBC (1 995) National House Building Council, Standards Volume 1

NICHOLSON, D, TSE, C-M and PENNY, C (1999) The Observational Method in ground engineering: principles and applications CIRIA Report 185

OLIPHANT, J (1 997) The outline design of earth retaining structures Ground Engineering, September 1997, pp 53-58

OVE ARUP and PARTNERS (1997) CDM Regulations - Work sector guidance for designers CIRIA Report 166

PADFIELD, C J and MAIR, R J (1984) Design of retaining walls embedded in stiffclay CIRIA Report 104

PULLER, M and LEE, C K T (1996) A comparison between the design methods for earth retaining structures recommended by BS 8002: 1994 and previously used methods Proceedings o f Institution of Civil Engineers Geotechnical Engineering, 1996, Vol 1 19

SITE INVESTIGATION STEERING GROUP (SISG) (1 993) Part 1 - Without ground investigation, ground is a hazard Part 2 - Planning, procurement and quality management Part 3 - Specification for ground investigation Part 4 - Guidelines for the safe investigation by drilling of landfills and

Site investigation in construction Thomas Telford

contaminated land

ClRlA C516 198


STROUD, M A (1988) The standard penetration test, its application and interpretation Geotechnology conference on penetration on testing in the UK Institution of Civil Engineers

SYMONS, I F and CLAYTON, C R I (1992) Earth pressures on backjWed retaining walls Ground Engineering, April, pp 2 6 3 4

THOMAS, K (1996) Masonry walls, specification and design Butterworth Heinemann

THORBURN, S and LITTLEJOHN, G S , (eds) (1 993) Underpinning and retention Blackie Academic and Professional, 2nd edition

TOMLINSON, M J (1 995) Foundation design and construction Longman Scientific and Technical, 6th edition

YONAN, S J (1 993) Geotextile retaining walls for motorway widening Retaining structures conference, pp 541-548 Thomas Telford

BRITISH STANDARDS

BS 443: 1982 Specijkation for testing zinc coatings on steel wire and for quality requirements

BS 729: 1971 Specification for hot dip galvanised coatings on iron and steel articles

BS 812: Part 103: 1989 Testing aggregates, method for determination ofparticle size distribution

BS 812: Part 11 1: 1990 Testing aggregates, methods for determination of ten per cent fines value (TFV)

BS 882: 1992 Specification for aggregates from natural sources for concrete

BS 1052: 1980 Specification for mild steel wire for general engineering purposes

BS 1377: 1990 British Standard methods of test for soils for civil engineering purposes Parts 1-9

BS 2782: Part 5 Method of testingplastics

BS 4 102: 1990 Specijications for steel wire and wire products for fences

BS 5400: Part 4: 1990 Steel, concrete and composite bridges, code ofpractice for design of concrete bridges

BS 5628: Part 1: 1992 Code ofpractice for use of masonry, structural use of unreinforced masonry

BS 5628: Part 2: 1995 Code ofpractice for use of masomy, structural use of reinforced and prestressed masonry

ClRlA C516 199


BS 5628: Part 3: 1985 Code ofpractice for use of masonry materials and components, design and workmanship

BS 5837: 1991 Guide for trees in relation to construction

BS 5930: 198 1 Code ofpractice for site investigations

BS 6399: Part 1: 1996 Loading for buildings, code ofpractice for dead and imposed loads

BS 8002: 1994 Code ofpractice for earth retaining structures

BS 8004: 1986 Code ofpractice for foundations

BS 8006: 1995 Code of practice for strengthenedheinforced soils and otherfills

BS 8007: 198 1 Code ofpractice for design of concrete structures for retaining aqueous liquids

BS 8102: 1990 Code ofpractice for protection of structures against water from the ground

BS 81 10: Part 1: 1997 Structural use of concrete, code ofpractice for design and construction

BS 81 10: Part 2: 1985 Structural use of concrete, code ofpractice for special circumstances

BS 81 10: Part 3: 1985 Structural use of concrete, design charts for singly reinforced beams, doubly reinforced beams and rectangular columns

HIGH WAYS AGENCY STANDARDS

Highways Agency Specification for highway works (1994)

BA 63/94 and BD 63/94 Inspection of highway structures

BA 68/97 Crib retaining walls

BD 10197 Design of highway structures in areas of mining subsidence

BD 21/93 The assessment of highway bridges and structures

BD 30187 Design (substructures and special structures), materials. Section I : Substructures; Section 2: Bac&lled retaining walls and bridge abutments

BD 4 1/97 Reinforced clay brickwork retaining walls ofpocket-type and grouted cavity type construction

HA 34/87 Ground investigation procedure

200

HA 4419 1 Earthworks - design and preparation of contract documents (1 995)

ClRlA C516


ClRlA (2516

EUROCODES

DD ENV 199 1 Eurocode 1 - Basis of design and actions on structures Part 1 Basis of design (together with UK National Application Document): 1996

Part 2-1 Actions on structures - densities, sewweight and imposed loads (together with United Kingdom National Application Document): 1996

DD ENV 1992 Eurocode 2 - Design of concrete structures Part 1-1 General rules and rules for buildings (together with UK National Application Document): 1992

DD ENV 1997 Eurocode 7 - Geotechnical design Part 1 General rules (together with UK National Application Document): 1995

20 1


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E N Y l R O N M f N T

TRANSPORT

The Construction Directorate of&e DETR supports the programme of innovation and research to improve the construction industry's performance and to promote more sustainable construction. Its main aims are to improve quality and value for money from construction, for both commercial and domestic customers, and to improve construction methods and procedures.

This publication sets out good practice in the planning, selection, design, installation and maintenance of low-height modular retaining walls, composed of different modular units. It will be valuable for specifiers, designers, manufacturers, installers and owners.

While primarily concerned with gravity walls of retained heights up to 3 m, many of the principles also apply to higher walls.The report accords with design to Eurocode 7 part I and to BS 8002: 1994, and also is consistent with design to Highways Agency Standards.

The report reviews low-height modular retaining wall systems, provides guidance for their selection, and explains the principles and other considerations of wall design. It deals with the engineering properties of soil and fill, and the choices of design values and external loads. While covering general design applications such as the assessment of external stability, there are design calculations for specific wall types, particularly for external stabi1ity.Thet-e are notes on specification and quality control, and the concluding chapters examine the

, performance requirements for low-height modular retaining walls, including the need for regular maintenance, with suggested schedules for inspection visits. Worked examples are included in the appendices.

ISBN 0 86017 516 2


Modular Gravity Retaining Walls Design

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