BS-8002

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BRITISH STANDARD

1

BS 8002

:

1994

Code of practice for

Earth retaining

structures

British Standard

The preparation of t h s Brltlsh Standard was entrusted by the Techrucal Sector Board for BLuldlng and Cml Engmeenng (BI-) to Techlucal Cornm~ttee Blb26,

upon whlch the followmg bodles were represented Assoc~at~on of Consulting Engmeers

Assoc~atlon of Geotechrucal Speaahsts

Department of the Environment (Construct~on D~rectorate) Department of Transport

Federatlon of Clwl Engmeenng Contractors Federatlon of P h ~ g Speclalets

Inst~tut~on of Clvd Engineers

Inst~tution of Structural Engmeers

1

BSI 1994

Tms Bntish Standard, hav~ng

5

been prepared under the

i

directmn of the Technical Sector Board for Bmldmng and Clvll

Ensneenng (B1 ), was Amendments issued since publication

pubhhed under the authonty

I

I

1 The followmg BSI references

1

relate to the work on thls

! standard

Committee reference 91526 Draft for comment 87114383 DC

I

I

I

i ISBN 0 580 22826 6

Rxt affected

of the Standards Board and Amd No

1

comes Into effect on

15 Apnl1994

Contents

Committees responsible

Page Inslde front cover

Foreword 5 Code of practice

_-

Section 1 Introduction 1.1 Scope 1.2 References 1.3 Defin~t~ons 1.4 Major symbols

1.5 Selection and types of structure

1.5.1 General

1 5.2 Selection of type 9

Section 2. Data for design 2.1 S ~ t e and geotechrucal data

2.1.1 General

2.1.2 Site lnvestlgations

-

2.1.3 Ground water

2.1.4 Flood tldes and waves

2.1 5 Chmate

2.1.6 Trees

2.2 So11 properties

2.2.1 General

2.2.2 Select~on and evaluation of soil parameter values

2.2.3 Clay soils

2.2.4 Cohes~onless solls

2.2.5 Silts 2.2.6 Rock

2 2.7

I

W

2.2.8 Wall fnct~on, base f n c t ~ o n and undramed wall adheson

2.3 Externally applied loads

Section 3. Design philosophy, design method and earth pressures

Desgn ph~losophy General

Lun~t state d e s w Ultimate hmlt states S e m c e a b ~ t y 1m1t states

Lunlt states and compat~b~hty of deformations Desgn values of parameters

Applied loads Desgn sod strength Desgn earth pressures Design method

Equ~l~bnnm calculat~ons Desgn s~tuations

Calculations based on total and effectwe stress parameters Deslgn usmg total stress parameters

3.3.3 Active earth pressures 3.3.4 Surcharge loads 3.3.5 Water pressure

3.4 Resistance to movement 3.4.1 General

3.4 2 Passive earth resistance 3.4.3 Weak rocks

3.4.4 Layered solls

3.4.5 Water pressures and seepage forces

Section 4. Design of spec~fic earth retaining structures

4.1 Interrelation of sectlon 3 and sectlon 4 4.1.1 General

4.1.2 Deslgn 4.2 Gravlty walls 4.2.1 General 4.2.2 Foundations

4.2.3 Mass concrete retamng walls

4.2.4 Unremforced masonry retanlng walls 4.2.5 Remforced so11

4.2.6 Gab~ons 4.2.7 Cnbwork

4.3 Reinforced concrete and reinforced masonry walls on spread foundations

4.3.1 Remforced concrete walls (other than basement walls) 4.3.2 Basement walls, excavation, support and retent~on systems 4.3.3 Remforced and prestressed masonry retalnmg walls

4.4 Embedded walls 4.4.1 General

4.4 2 Types of wall and apphcabdity 4.4.3 Design

4.4.4 Steel sheet phng 4.4.5 T~mber sheet pdes

4.4.6 Remforced and prestressed concrete sheet plles 4.4.7 in sltu concrete plle walls

4.4.8 Diaphragm walls 4.4.9 Soldier/lung p~les

4.5 Strutted excavations and cofferdams 4.5.1 General

4.5.2 Struts, tles, waling3 and anchorages 4.5.3 Cellular cofferdams

4.6 Anchorages 4.6.1 General 4.6 2 Equllibnum 4.6.3 Ground anchorages 4.6.4 Tens~on p~les 4.6.5 Deadman anchorages 4.7 Waterfront structures 4.7.1 General

4.7 2 Concrete and remforcement

4.7.3 Des~gn

4.7.4 Construction

Annexes

A (normatwe) Graphs for

K,

and Kp

B (~nformative) Trad~t~onal des~gn methods for embedded walls 102

1 U ~ u t weghts of solls (and s~mllar materials) 11

2 p',,, for clay sods 13

3 p' for s~l~ceous sands and gravels 15

4 p' for rock 15

5 Select~on of p ~ l e slze to suit dnvmg cond~t~ons m granular soils usmg

lmpact hammers 69

6 Select~on of ode slze to sult d n v m ~ cond~t~ons m cohes~ve sods 70

Figures

Strength envelopes for a gwen pre-consohdat~on Denvat~on of N' from SPT value N

Llm~t states for earth retammg structures Pressure d~agrams

Graphcal detenrunat~on of actlve earth pressure for cohes~onless solls

Graph~cal determmatlon of actwe earth pressure for coheswe soils Construct~on of earth pressure d~agrams for earth retalrung structures m multi-layered sod

Flow net determ~natlon of pore water pressure Lmear vanahon

m

hydraul~c head

Graded filter d m n

Foundat~ons of gravlty walls Bas~c forms of mass concrete walls

Masonry clad mass concrete wall wlth cavlty

Stepped and buttressed retamng walls in unremforced masonry Hexagonal woven mesh gab~on cage (typical)

Welded mesh gabion cage (typ~cal) Examples of gab~on reta~rung walls Sect~on and elevat~on of typ~cal cnb wall Examples of timber cnbwork

Examples of remforced concrete cnbwork

Further examples of reinforced concrete cnbwork

Open cut

Remforced masonry grouted-cavlty constructlon Remforced masonry Quetta bond constructlon Reinforced masonry pocket-type constructlon Remforced hollow blockwork constructlon

Post-tens~oned masonry d~aphragm wall constructlon Types of embedded retainmg wall

Actwe pressure dlagrams relatmg to mmmum strut loads rn braced earth retammg structures (Terzagh~ & Peck)

Illustrat~on of method of calculation of bendmg moments and frame loads by successwe stage analysls in cofferdams

Typ~cal sections of t~mber sheet piles Detad of dnvmg edge

Honzontal sheetmg (laggmg) Vert~cal sheetmg (lagg~ng) Compos~te steel sold~er piles

Vanous methods of locat~ng the sheetlng (laggng) Cofferdam for nver crossmg

Cofferdam m water

Types of cellular cofferdams Types of anchorage

Non-mterference of zones for anchored wall Double wall constructlon where zones mterfere

Actwe pressure - Honzontal ground surface behmd wall Values of

K, (honzontal component)

Passwe pressure

-

Honzontal ground surface behind wall Values of

K p (honzontal component)

Actwe pressure - Sloping ground surface behmd wall Values of K, (honzontal component) (based on Kensel and Absl, 1990)

Actwe pressure

-

Sloplug ground surface behmd wall Values of K, (horuontal component) (based on Kensel and Absl, 1990)

Actwe pressure - Slopmg ground surface behmd wall Values of K,

(hor~zontal component) (based on Kensel and Abs~, 1990)

Passwe reslstance - Sloping ground surface behmd wall Values of Kp (honzontal component)

Passive reslstance - Sloping ground surface behind wall Values of Kp (honzontal component)

Passlve reslstance

-

Slopmg ground surface behmd wall Values of Kp (honzontal component)

Different methods of assessing the ratlo of restonng moments to

61 62 62 62 62 63 65 66 67 72 72 76 76 77 78 79 81 85 87 88 89 94 95 96 97 98 99 LOO 101 overturning moments 102 Index 107 List of references 110

- 8

BS 8002

:

1994

.

.

Foreword

BS 8002 has been prepared under the d~rectlon of the Techn~cal Sector Board for Budding and Cwll Engmeenng

T h ~ s code of practlce IS a complete revwon of the C~vd Engmeenng Code of

Practice No 2, w h c h was ~ssued by the Institution of Structural Engmeers m 1951 on behalf of the Clv11 Engmeenng Codes of Pract~ce Jomt Committee A draft of t h ~ s code of practlce, was ~ssued in 1988 for publ~c comment and In 1992 a new commlttee rewewed and rewsed the text

The m a n changes m the deslgn of earth retanmg structures m t h ~ s code of practlce are

a) the recognltlon that effective stress analys~s 1s the mam bas~s for the assessment of earth pressures w ~ t h total (undraned) stress analysls bemg important for some walls dunug or ~mmedlately following construction,

b) the need to take account of the effect of movement (or lack of it) upon the resultmg earth pressures on the wall The largest earth pressures wh~ch act on a retanung wall occur dunng workmg cond~t~ons These earth pressures do not Increase

d

the wall deforms suffic~ently to approach falure cond~t~ons T h ~ s code of practlce takes Into account that for small movements of a wall the shear strength developed m the so11 1s less than the mammum shear strength measured in a convent~onal tnax~al test and furthermore that when large strans occur m the soil, the shear strength may reduce to the res~dual shear strength value

It has been assumed m t h ~ s code of practlce that desgn of retammg walls IS

entrusted to chartered structural or chartered c ~ w l e n p e e r s who have sufficient knowledge of the pnnc~ples and practlce of so11 mechan~cs as well as the

principles and practlce for the use of the appropnate structural matenals, 1 e

masonry, concrete, steel or t~mber

T h ~ s code of practlce does not restnct des~gners from applymg the results of research nor from takmg advantage of special sltuatlons or prevlous expenence m the des~gn of retaming structures

In t h s code of practlce references have been made to non-BSI pubhcat~ons The t~tles of these pubhcat~ons are a v e n m annex C

The llst of those engmeers who have partlc~pated m the preparation of the ~ m t ~ a l draft, m the spec~ally convened panel and

m

the more recently formed

commlttee mcludes the majonty of engmeers who have a speclal mterest m retalnmg walls The Cha~rman throughout the long process of draftmg, revlewmg and complete red~aftmg, has been Mr Thomas Akroyd, M Sc Tech, LL B (Hons), C Eng

,

a former Pres~dent of the lnst~tution of Structural Engmeers

DrBS~mpsonPh D

,

C Eng

,

F I C E

MrKWVlckery BSc , F G S

1) Mr D Walte C Eng

,

M I C E

,

F I Struct E

As a code of practrce, thls Brltlsh Standard takes the form of guldance and recommendat~ons It should not be quoted as if it were a speclficat~on and

particular care should be taken to ensure that clalms of compliance are not m~sleadrng

Compliance with

a

British Standard does not of itself confer immunity from legal obligations.

.

.

Section

1.

Introduction

1.1

Scope

The subject of thls code of practlce IS the des~gn

and constructlon of structures to retam soils and matenals w ~ t h sunilar engtneenng propertles, at slopes steeper than those w h ~ c h they would naturally assume The code of practice provides gu~dance for a desgner, conversant w ~ t h theoretical and apphed so11 mechamcs and expenenced in structural design and construction The code ls

pnmanly apphcable to small and medium walls \nth a retamed he~ght of up to about 8 m, although many of the recommendat~ons are more generally appl~cable Spec~alrst advlce should be sought w11h regard to the demgn and constructlon of larger structures and for those where movement of the retaned soils requlres close control

The code 1s divlded Into four sectlons

Sect~on 1 explans the terms used m the document and summarizes the factors ~nfluencmg the choice

of a retamng wall

Sect~on 2 descnbes the slte and geotechn~cal data that IS reqwed together w ~ t h material propertles

It gwes gu~dance on the determmation of the values of representatwe soil strength necessary for des~gn purposes

Sect~on 3 identifies the des~gn pmosophy and the des~gn methods for earth r e t m m g structures, mcluding the determmation of earth pressures and the analysis of overall stable equ~l~bnum It defines des~gn so11 strength and cons~ders the loads on retauung walls and the forces avdable to atlam equhbnum with tolerable displacements Gu~dance 1s gwen on methods of slmple practical des~gn and on the mfluence of ground conditions

Sect~on 4 cons~ders m detml vanous md~vldual types of structure and apphcat~on of earth pressure theory together u l t h matters of constructlon and mamtenance

1.2

References

1.2.1 Normative references

T h ~ s Bnt~sh Standard incorporates, by dated or undated reference, provlslons from other

publications These normative references are made at the appropnate places in the text and the c ~ t e d pubhcat~ons are hsted on page 110 For dated references, only the e d ~ t ~ o n c ~ t e d apphes, any

subsequent amendments to or revmons of the c ~ t e d pubhcation apply to this Bntlsh Standard only when mcorporated in the reference by amendment or revxion For undated references, the latest e d ~ t ~ o n of the c ~ t e d pubhcat~on apphes, together w ~ t h any amendments

1.2.2 Informative references

This Bnt~sh Standard refers to other pubhcations that pmv~de mformation or guidance Edit~ons of these publ~cations current at the time of issue of thls standard are listed on the ins~de back cover, but reference should be made to the latest editions

1.3 Definitions

For the purposes of this Bntish Standard the followmg defimtlons apply and are l ~ i n ~ t e d to words used w t h spec~al meanmg in t h ~ s document

Normal so11 mechanics terminology is not defined

1.3.1 active earth pressure

The earth pressure exerted on the retauung wall by the retamed soil It may be greater than the fully actlve earth pressure (see 1.3.11 and 3.1.9)

1.3.2 conservative values

Values of sol1 parameters wh~ch are more adverse than the most hkely values They may be less (or greater) than the most hkely values They tend towards the lunit of the cred~ble range of values

1.3 3 design situation

A set of phys~cal cond~aons for w h ~ c h it should be demonstrated that a h tstate (see 1.3.13 and 3.2.2) wdl not occur

1.3.4 design soil strength

Sod strengths wh~ch are assumed will be mobhzed at the occurrence of a h ~state (see 1.3.13) The t desgn value of sod strength is the lower of either the peak sod strength reduced by a mob~l~zat~on factor (see 1.3.14) or the cnt~cal state strength

1.3.5 design surcharge load

Loadmg wluch 1s assumed to occur at some tune dunng the Me of the structure and for w h c b the des~gn should provide The m m u m requlred value IS 10 kN/m2 See 3.3.4

1.3.6 design value of a parameter The value of the parameter entered mto eqwhbnum calculatrons

1.3.7 design value of wall friction

The smaller of elther the actual wall fncuon or adhes~on measured by test or 75 % of desgn sod strength (see 1.3.4) See 2.2.8 and 3.2.6

1.3.8 disturbing force

The force exerted by retained soil on a r e t m m g wall, tendmg to cause the wall to move It mcludes the surcharge loads, external loads and water pressure The mnnmum value is the fully actwe earth pressure (see 1.3.11)

1.3.9 earth pressure coefficients

Ratlo of honzontal effective stress to vert~cal effective stress K, IS the fully active earth pressure

(see 1 3.11) coefficient, K p is the fully passive earth resistance (see 1.3.12) coeffic~ent Both are based on the des~gn sod strength (see 1.3.4) Des~gn values are determined from design values of soil parameters Graphs are provlded in annex A for values of honzontal component of K, and KP The values a v e n m the varlous graphs m annex A are for vanous ratlos of 9' and wall f n c t ~ o n

6

1.3 10 embedded walls

Formerly known as sheet plle walls, this term embraces walls of s m ~ l a r structural behav~our whether constructed of steel sheet piles, concrete piles, concrete diaphragms or t~mber They are supported, at least m part, by passwe earth resistance (see 1.3.15)

1.3.11 fully active earth pressure

The minimum value of the actwe earth pressure (see 1.3.1), which occurs after suffment movement or deflect~on of the retailung wall, the necessary movement 1s usually w~thin the semlceability h m ~ t state (see 1.3.18) of the wall

1.3.12 fully passive earth resistance

The maxlmum value of the passwe earth reslstance (see 1.3.15), wluch occurs after suffmeut

movement or deflect~on of the r e t m m g wall The necessary movement 1s often outs~de the

semceabihty h i t state (see 1.3.18) of the wall 1.3.13 limit state

Any state of stabhty beyond which the retainmg wall no longer sat~sfies the design performance

requirements A h ~state is not assoc~ated t wlth any part~cular method of structural des~gn See ultmate h i t state (1.3.19) and serviceablhty hmit state (1 3.18)

1.3.14 mobilizat~on factor

A factor M of 1 2 or 1 5 (or more, see 3.2.4 and 3.2.5) appl~ed to the representatwe so11 shear strength to produce the desgn sod strength (see 1.3.4) M deternunes the proportion of the representative strength w h c h may be mob~hzed at a lmut state (see 1.3.13)

1.3.15 passive earth resistance

The earth pressure generated by the sod when it reslsts movement of a retanmg wall

1.3.16 rapid shearing

In the context of total stress a n a l y s ~ , the sheanng of a sod at a rate suffic~ent to prevent or l n h ~ b ~ t any s~gnlficant pore water pressure dffis~pat~on so that c, is the operatwe shear strength

1.3.17 representative soil streugth

Consematwe est~mate of the mass strength of the so11 The value e determmed from reliable site mvestigation and so11 test data In the absence of such data, see tables 1, 2, 3 and 4

1.3.18 serviceabll~ty limit state

State of deformation of a retamng wall such that ~ t s use is affected, its durab~hty is mpaued, its maintenance requirements are substant~ally mcreased or damage 1s caused to non-structural elements Alternatively such movement of the earth retaming structure which may affect adjacent structures or semces in a hke manner

1.3.19 ultimate limit state

State of collapse, instability or forms of falure that may endanger property or people or cause major econormc loss

1.3.20 unplanned excavation

The m i m u m depth, below the nommal finished surface in front of the wall, wluch it is assumed, for design purposes, w~ll be excavated at some tlme durmg the Me of the r e t m m g wall See 3.2.2

1.4

Major

symbols

effectwe cohes~on base adhesion

u n d m e d shear strength undrruned wall adhes~on effective gram slze effective gram sue Young's modulus moment of mertia

flow-net parameter (see figure 9) fully actwe earth pressure coeffment actwe pressure coefficient for cohesion ratlo of horizontal to vert~cal effective stress for sol1 at rest (no stram) coeffic~ent of earth pressure a t rest

(Ko

-

KJ

fully passwe earth reslstance coeffment mobiluation factor

result of standard penetration test modlf~ed value of N (see figure 2) beanng capacity factor

beanng capacity factor beartng capaclty factor

total active thrust normal to the wall total passwe thrust normal to the wall pore water pressure

rad~us

surcharge pressure water pressure load

depth

depth to water table inchnation of the wall

mclinat~on of the surface of the retained so11

u ~ u t welght of so11 (kNIm3) unlt welght of water angle of wall fnct~on angle of base fnctlon

active pressure normal to the wall passwe pressure normal to the wall total vert~cal pressure

effectwe vertical pressure

effectwe angle of shearmg reslstance c r ~ t ~ c a l state angle of shearmg reslstance maxlmum value of 9' determmed from conventional tr~axlal test

res~dual f n c t ~ o n angle base reslstance

1.5 Selection

and

types of structure

1.5.1 General

There is a w d e vanety of different forms of earth retainmg structure Many structures mclude a combinat~on of wall and support system

1.5.2 Selection of type

The select~on of a particular form of earth retanlng structure w ~ l l depend on

a) the locat~on of the wall, its posltlon relat~ve to other structures and the amount of space

awlable, lncludmg the necessity or othennse to confine the support system witlun the slte boundanes,

b) the proposed helght of the wall and the topography of the ground, both before and after construcbon,

c) the ground conditions,

d) the ground water and t ~ d a l cond~t~ons, e) the extent of ground movement acceptable dunng construction and in s e m c e and the effect of movement of the earth retmlng structure on e x ~ s t ~ n g or supported structures and services,

f) external h e loadmg,

g) the avallabhty of materials, h) appearance,

I) reqwed Me and mamtenance,

Where several alternatwes are su~table then an economc companson should be made

2.1

Site and geotechnical

data

2.1 1 General

The deslgn of an earth r e t m m g structure requlres mformation on the physlcal condlt~ons in the v~cmlty of the structure, mcluding the topography and layout of the slte, detalls of adjacent

foundat~ons and services, the nature of the ground and the ground water condit~ons mcludmg, where apphcable, the t ~ d a l and seasonal vanations An adequate site investigation should be carned out to prov~de the necessary informat~on When the site investlgatlon has been carried out and so11 test results are obtamed, these are processed to prov~de values for the representatwe soil parameters

(see 2.2.2) Once representatwe values have been established, dewgn values should be denved for use m the equihbrlum and structural design

calculat~ons

NOTE The denvatmn of deslgn values is explaned In 3 1 8 2.1.2 Site investigations

Sufflclent informat~on should be obtamed on the ground and ground water conditions and the strength and deformation propertles of the so~ls wh~ch w ~ l l be retamed and the soils whlch will support the earth retam~ng structure Major earth retamlng structures require an extenslve Site

lnvestlgation Mmor earth retammg structures requlre sufficlent mformat~on about the site together w ~ t h so11 data to permit the selection of representative values and des~gn values of the sod parameters to permlt a sat~sfactory d e s w to be prepared Geolo@cal maps, memoirs and

handbooks should be consulted together wlth any other source of local knowledge

The code of practlce for site lnvestlgatlon BS 5930

describes the general cons~derat~ons to be taken into account and deta~ls the methods of slte investlgatlon avdable Informat~on on methods of m situ and laboratory testlng 1s glven In BS 1377 P a r t l t o P a r t 9

The number of boreholes, or other form of mvestigatlon, should be adequate to establ~sh the ground cond~t~ons along the length of the wall and to ascertam the var~ability in those conditions The centres between boreholes wlll v a q from site to slte but should generally be at Intervals of 10 m to 50 m along the length of the wall The depth of mvestlgation w ~ l l be related to the geology of the slte and to the type of wall

a) for a backfilled gravlty or reinforced stem wall the borehole depth below foundmg level should be at least t w c e the proposed retamed helght, b) where excavation will be carried out in front of the waU the borehole depth, below excavatlon level, should be at least three tlmes the proposed retamed he~ght,

c) where the type of wall or method of constructlon 1s uncertain at the t m e of mvestigatlon the borehole depth, below

excavatlon level, should be at least three tlmes the proposed retamed helght

If ground anchorages are proposed the mvestigatlon should be of sufficlent extent and depth to provide data for the strata m wh~ch the anchorages will attam their bond length

The essent~d propertles of the so~ls, in the lrnmed~ate vlcmlty of the retammg structure, should be ascertained together w ~ t h the details of foundations of any adjacent structures The relat~onsh~p of the slte to the overall geology should be establ~shed mcludmg the exlstence of any specla1 condit~ons such as geologcal faults, movement jomts, areas of Iandshp or any tendency of the slte to shift, creep or settle, as for example in areas of mmng subsidence The poss~bhty of externally generated v~brat~ons and their effect upon earth pressures should be ascertained The process of slte mvestlgation contmues during constructlon Inspections should be carr~ed out from tlme to tlrne, durmg constructlon, to determme that the conditions revealed are m accordance wrth the design assumptions If the cond~t~ons differ then the desgn should be checked agamst the changed cond~tions

2.1.3 Ground water

An adequate design requres knowledge of the ground water levels and seepage pressures at the site, together with ~nformat~on as to the exlstence of any hydrostatic uphft pressures Information on ground water cond~tions may be avalable from records of the slte, geologcal maps or memoirs, or from knowledge of other simllar sltes in the local~ty Ground water conditions may be

pred~ctable from a knowledge of the local geology The poss~b~My of flooding should be ascertamed together w ~ t h its effect on the ground water conditions

Standp~pes or piezometers should be mstalled where necessary to determine the ground water cond~tions, they should be mstalled in accordance w ~ t h BS 5930 Where layers or strata of markedly different permeabhty emst, then the hydrostat~c levels w t h m each stratum should be obtamed NOTE Water levels encountered dunng bonng operations are unrelrable, they seldom -present equlhbrlum condmons Poss~ble changes in ground water levels due to the presence of the retammg wall and seasonal or other causes, ~nclnd~ng future trends and acc~dent circumstances, should be mvestigated Future works, in the vicm~ty of the wall, may @ve nse to changes in the long-term ground water cond~t~ons, where such future works can be reasonably

anticipated, the potentlal changes in ground condlt~ons should be assessed

Section

2

BS 8002

:

1994

.

-

The presence of deletenous chem~cals m the ground water and sod should be estabhshed m

accordance w ~ t h BS 1377 Part 3 and the effect of such deletenous chemcals upon the corrosion of

the proposed structure should be assessed in accordance w ~ t h BS 8110 Part 1 and Part 2 and BS 5493

2.1.4 Flood tides and waves

Ground water cond~t~ons, both for waterfront structures and also for structures a short distance mland, may be mfluenced by tidal cond~t~ons The maxlmum t ~ d a l range to waterfront structure should be established includmg potentlal or poss~ble surge t ~ d e s and flood cond~t~ons The he~ght, length and angle of approach of waves and the resultmg forces on the structure should be determmed

2.1.5 Climate

The clunat~c vanatlons and their effect on the structure should be determmed, including

a) diurnal and seasonal temperature changes and the effect on earth pressures of temperature changes, particularly ground freenng,

b) short-term and long-term rainfall vanations and the effect on earth pressures of the resulting molsture content changes,

c) artifmally mduced clunat~c changes such as those produced in boiler houses or cold stores and their effect on earth pressures and stabhty

2.1 6 Trees

Retammg walls budt adjacent to exlstlng trees may suffer deletenous effects from the penetration of root-systems Dunug the course of the site investlgatlou, the presence of trees and large shrubs should be noted so that dec~s~ons can be taken at the desgn stage concermng the retention or removal of such trees or shrubs

Trees and large shrubs In general, should not be p e n t t e d nor planted wrthm a distance from the retammg wall equal to half of thew expected mature heght and dec~duous forest trees such as alder, beech, oak, poplar and d o w should not be p e m t t e d w~thln a d~stance equal to the mature helght of the tree BS 5837 prov~des useful adv~ce Where ~t is requwed to plant or retam trees or large shrubs close to the retamng wall after its

construction, the locat~on and choice of the tree or shrub specles should be such as to munmlze or elmunate the adverse effects of root penetratlon and the changes m the molsture content of the sod and any assoc~ated desiccation and shnnkage of the sod

The adverse effects of trees and root penetratlon mcludes mcreased loading on the structure and penetration of roots Into joints or dramage systems

2.2

Soil properties

2.2.1 General

The desgn of earth retalnmg structures usually involves an effective stress analysis, although m

some circumstances a total stress des~gn may be appropnate, accordingly data on the so11 propertles m respect of both strength (see 1.3.17) and

stiffness under both dramed and undramed conditions should be obtamed Sod propertles are determmed as part of the slte lnvestlgatlon process but may be amphfied by data from back analysis of comparable retainmg structures In smular ground conditions

The u n ~ t we~ghts of materials, m table 1, prov~de reasonable values for umt we~ghts of sods m the absence of reliable test results

A

-

Granular

Gravel

1

16 0

a b l e 1. Unit weights of soils (and similar materials)

Well graded sand

and gravel 1 9 0 Materm1 Peat (very vanable) Coarse or medium sand

Well graded sand F ~ n e or sdty sand Rock fill Bnck hardcore Slag fill Ash fill Orgamc clay Soft clay F ~ r m clay St~ff clay Hard clay Stlff or hard glac~al clay y m morst bulk welght (kN/m3) 16 5 18 0 17 0 15 0 13 0 12 0 6 5 Loose y, saturated bulk wezght ( k ~ l m ~ ) B - Cohesive

comervatwe estnnates (see 1.3.2) of the propertles of the sod

as

it exists m situ Care should be taken that the representatlve value is properly applicable to the part of the des~gn for whlch it IS intended

The second stage, the denvat~on of sat~sfactory des~gn parameters from representatwe so11 parameters, 1s considered m 3.1.8

The first step in obtaming representative values of the measured so11 parameters, is to make a cntical

examination of the raw data assisted by estabhshed calibration factors between different types of so11 tests Cons~stency md~ces, denved from molsture content and hquid and plastic limlt tests, provlde a useful correlat~on w t h sol1 strength and stiffness ind~ces Data from d~fferent samples and different locations wlLl spread over a range of values

Isolated low or high values should be scrutin~zed to determ~ne thew accuracy, where such values are attnbutable to errors they should be rejected, where they are due to extreme local varlatlons t h e ~ r relevance requlres further cons~derat~on For soil parameters, such as dens~ty, for which field values can be determmed with confidence from test results w h ~ c h show httle vanation, the representatwe value should be the mean value of the test results Where greater vanatlons occur or where values cannot be fixed with confidence then the representative value should be a cautious assessment of the lower h m ~ t (or the upper h m ~ t if that IS the relevant bound) of the acceptable data

In the absence of detaled test information, representatwe values should be selected by the apphcation of conservative bounds to generally avalable parameters

The selection of representative values of soil parameters should take the followng matters into account

a) geologxal and other background information, b) differences between the in situ condit~ons and the propertles measured by field or laboratory tests,

c) the effect of construction activities on the properties of the ground,

d) changes w k c h may occur in the field due to vanations in the environment or weather, e) relevant data from prevlous projects and the performance of exlstmg faclllties

Careful assessment of the so11 parameter values 1s necessary to ensure select~on of those values which are pertment to the behav~our of retaming

matenal A range of values should be considered part~cularly,lf the sol1 parameter values are hkely to change dunng the hfetime of the retammg structure

Under serv~ceab~l~ty condlt~ons, where

deformations are comparatively small, the so11 wll operate at below peak strength conditions The appropriate strength and stiffness values may be obtamed by examlnmg the stress-stram hehamour of the sod, as gwen for example by laboratory

tnaxlal tests Under ultimate hmlt state cond~t~ons where deformations are comparatwely large, the so11 will operate at beyond peak strength condlt~ons and may d~late to approach the cnt~cal state values conslstent with the strength envelope for loose or normally consohdated solls

Tables 2, 3 and 4 promde gu~dance on the empmcal relat~onshp between class~ficat~on and mdex tests and representatwe values of the angle of sheanng

resistance and the denslty of various materials

2.2.3 Clay soils

The construction of a reta~nmg wall may result in changes in the strength of the ground m the viclnlty of the wall Where the mass permeab~lrty of the ground 1s low the changes of strength take place over some tlme and therefore

a

m necessary to determine parameter values apphcable to both short-term and to long-term cond~tlons, 1 e

undmned and drained conditions

The undrained shear strength of a clay so11 is not a fundamental so11 property D~fferent values may be recorded m tnaxlal compression and extension, in

dlrect shear and m pressuremeter tests in situ Although convent~onal practlce has been based on tnaxlal compression tests, w h c h are conslstent with active so11 conditions, extenslon tests may be required

d

the behav~our of a passive zone IS of

particular concern

The undmned strength of a soft clay with a small overconsohdated ratio (less than 3) Increases when the posltlve pore pressures d~ss~pate, but the negative pore pressures Induced by sheanng a stlff clay, w t h a high overconsolidation ratio, cause it to swell and soften m the long-term

Lf

the undramed strength of a shff clay is to be rehed upon dunng temporary works construction then care IS

necessary to ensure that there are no sand or sdt partings contaming free ground water wh~ch would affect the undrained shear strength, such

In assessing the strength of clay so~ls, part~cularly

from undmned tests in accordance w t h BS 1377

Part

7, the procedures used for sampl~ng and testing should be taken Into account For example, UlOO sampllng of stlff clays leads to partla1

remouldmg and the creatlon of excess negatlve

pore pressures, these m turn cause excesslve ~ n ~ t ~ a l effectlve stresses whlch can lead to unconsolldated tests reglstenng erroneously lugh undramed

strengths, even when the water content has been preserved T h ~ s is due to the mode of fa~lure of heavlly overconsohdated clays, whlch, by stram softenmg, lead to shear rupture Such fa~lures occur at strengths lower than those apphcable at the same water content but lower

overconsohdat~on ratlo More consistent results are obtalned if samples are consohdated to a best estimate of in s ~ t u effect~ve stresses pnor to shearlng Representatlve values for undralned strength parameters should be assessed for the peak strength and for the remoulded strength of the sod The values for the representatlve peak strength should make due allowance for the

Influence of samphng and the method of testmg, as well as for hkely softenmg on excavation

To determme the strength of clay sods, for an effective stress analysis, t n m a l tests may be carned out e ~ t h e r fully dramed or undramed w ~ t h pore pressure measurement, provlded the samples are fully saturated m accordance m t h BS 1377 Part 7 and Part 8 The tests are carned out

sufficiently slowly to ensure equahzatlon of pore pressures The Mohr-Coulomb falure envelope for overconsohdated clays, of lnltlally Identical samples, IS generally curved, see flgure 1 At

effect~ve pressures close to the preconsolldatlon pressure, the sod mobll~zes rts cnt~cal state angle of sheanng , p',, At lower ~ n ~ t ~ a l effect~ve stresses,

I e at hgher overconsohdated rat~os, the so11

e x h ~ b ~ t s a ddatant peak at failure before a s

strength drops to, and poss~bly beyond, the cnt~cal state value Representatlve values should be

assessed separately for the peak strength and for the cnt~cal state strength of the sod The

representatlve peak strength should be appropnate to the antmpated stress state of the so11 in the ground Where the stress-strm curve never reaches a peak, dunng the maximum stram range

ach~evable dunng test, the peak strength should be assumed to be the largest strength mobhzed dunng the test It may be represented by values of c' and

p' or by secant values of p' The representatlve crltlcal state strength 1s represented by the cntlcal state angle of sheanng resistance, p'c,lt Cohes~ve sods w ~ t h hlgh clay contents e x h b ~ t the greatest fall from peak to resldual strength, formmg a pohshed rupture surface Prev~ous shear surfaces, m plast~c clays, may be reactwated a t low res~dual

fnctlon angles p', F m t tlme shdes due to new constructlon have been found to moblllze mass strengths no lower than pic,,

Two approaches may be adopted for the

convent~onal hneanzat~on of the peak so11 envelope over some deslred range of stress, see figure 1 A secant p' value can be selected as a funct~on of stress level If a smgle value 1s chosen, the resultmg envelope is h e a r to the on@n and falls safely ms~de the envelope of tests caxned out from ~ d e n t ~ c a l m ~ t ~ a l condit~ons Slnce sod samples from the field are not ident~cal, the method should normally be apphed by select~ng the lowest secant

p' for any sample tested wlthm the target range of stress Alternatwely, the tangent parameters (c', p') may be used, where each IS a funct~on of

stress level for ident~cal samples Sample vaxlatlon causes scatter m the tangent parameter values and conservatwe values are best selected by fitting a lower bound to the relevant data, tak~ng care to conslder the range of effect~ve stress requxed In the absence of rehable laboratory test data, the conservatwe values of p f o t gwen m table 2 may be used, m t h c' = O

If samples of clay contmmg vems or seams of sand or silt are remoulded for the plastic~ty Index tests the test results gwe lower plastlc~ty md~ces than the clay Itself Care should be taken to carry out the tests on the clay alone if there are doubts as to the mclus~on of sand or sdt then, in table 2 use the next value of the plast~clty mdex h~gher than recorded m the tests

'hble 2. pIcnt for clay soils

I

In all tests a n o n - h e a r so11 response should be ant~c~pated, so that stress-strm curves form hysteres~s loops on load-unload-reload cycles In assessmg the deformat~on properties of so~ls, the stlffness measured m convent~onal laboratory tests

m accordance wlth BS 1377 Part 5 and Part 6 generally underestimates m s ~ t u values denved from back analys~s of instrumented field structures Appropnate stiffness values can be measured in the laboratory by laboratones expenenced in thls spec~ahst work provlded part~cular care IS taken m

sample preparation and local s t m n measurement Sample d~sturbance IS corrected by flrst t a h g the

sample through its most recent effectwe stress cycle so that its m situ state IS properly recreated

a1 Peak and u l t ~ m a t e strength most sods Stress range

A

0' c Secant parameters f ( 0 ' 1 C L 0' 0'

bl Peak and ultimate strength s o i s with > 50% t h y minerals _{d ) Tangent parameters s,,,=r'+g' tan 0'}

Figure 1. Strength envelopes for a gwen pre-consolidation

St~ffness parameters can be determmed from certam field tests which cause httle d~sturbance m accordance with BS 1377

Part

9 and CIRIG Ground engmeenng report, 1987

2.2.4 Cohesionless soils

The strength and st~ffness of cohes~onless so~ls are determmed md~rectly by m situ statlc or dynanuc penetration tests Detals of three types of

penetration tests as well as plate loadmg tests are Sven m BS 1377 Part 9 The peak and crit~cal state angles of sheanng resistance for siliceous sands and gravels may be estimated from the following equations

The est~mated peak effectwe angle of sheanng reslstance 1s gwen by

p' , ,

,

-

30

+

A + B

+

C (1)

The estmated cnt~cal state angle of sheanng reslstance is gwen by

pfCnt = 30 + A

+

B

The values of

A = angularity of the part~cles

B

-

gradlng of the sandlgravel

C

-

results of standard penetratlon tests are gwen in table 3

The standard penetratlon test (SPT) values should be corrected for the effect of overburden pressure

m accordance with figure 2 (see Thorburn, 1963), other correction effects may be necessary See ClRIA Report FRICP17Ll Bolton (1986) has mtroduced emp~ncal relations between ,,,,p' p',,

effect~ve stress at fa~lure to reflect the change in the secant value of peak angle of sheanng

resistance wlth the change m the mean effective stress m the ground

2.2.5 Silts

It 1s d~fficult and often mpract~cable to obtam und~sturbed samples of s ~ l t s and fine sands, even employmg speclal samphng techn~ques Loose slits are read~ly hqu~fled by vlbrat~on, both dunng prob~ng and dunng the life of the retaining wall, accordmgly excess pore pressures should be taken into account lnorgan~c s~hceous SINS can generate

as much dllatancy as sands, at the same relative denslty, but they more easlly soften to cntlcal states in thm rupture bands In the absence of other data and where disturbed samples have shown the s ~ l t 1s a rock flour wlth neghgble organlc or clay mineral content, the representatwe effectwe angle of sheanng may be conservatwely taken as q',,,, m table 3

a b l e 3. q' for siliceous sands and gravels A - A ~ u l a r i t v l )

1

A Rounded

/

(degrees) 10 Sub-angular 12 Angular B - Grading of soil2) Moderate gradmg Well graded C - h"3) (blows 300 mm)

<

10 10 2 2 6 Rock

The engneermg properties of rock relevant in deslgn are controlled by the extent and onentatlon of the beddlng planes andjomts wlthm the rock mass together with the water pressures on the dlscont~nu~ty planes The slte mvestgation should estabhsh the strength and onentatlon of the discontinuity planes Weak rocks, part~cnlarly weakly cemented sandstones, f~ssured shales and chalk, are often difficult matenals to sample and test

Some correlat~on has been obtamed between the standard penetration test in accordance w ~ t h BS 1377 Part 9 and the strength and stiffness properties for c e r t m weak rock masses In addition the mass rock properties may be derived from compression wave and shear wave veloc~ty

measurements

The f o l l o m g lud~cative values of the effect~ve angle of fr~ctlon in table 4 relate to rocks which can conservatively be treated as composed of granular fragments, i e they are closely and

randomly jolnted or othenv~se fractured, havmg an

RQD (rock quahty des~gnation) value close to zero

20 40 60 2 6 9 Weak sandstone 42 Weak s~lstone 3 5 a b l e 4. 9' for rock

'I Angularity ie emmated fwm vrsunl descnptlon of soil " ~ r a d m g can be determmed from grading CUNe by use of Unlfonnzty coefficient = D,,lD1,

where Dlo and D,,, are part~cle slzes such that m the sample. 10 X of the matenal is finer than D,o and 60 ih is finer than

Dhii

Gradmg Unlformlty e o e f f m e n t Uniform < 2

Moderate grading 2 to 6 Well graded > 6

A step-graded sol1 should be treated as umform or moderately graded so11 according to the gradmg of the fmer fractmn

i V

' from results of standard penetration test modified where necessary by figure 2

lntermed~ate vaules of A. B and C bv mterwolahon

Weak mudstone

NCTE 1 The presence of a prefemd onentatmn of jomts, bedding or cleavage in a dlrectlon near that of a poss~hle failure plane may requre a reduction m the above values, especially d the dlscontinuitles are filled w t h weaker materlals

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

Chalk Clayey marl Sandy marl

2 2 7 Fill

A wide range of matenals may be used as fill behind retamng walls Selected cohesionless granular fill placed m a controlled manner such as well graded small rockfds, gravels and sands, are smtable as fill Cohes~ve matenals. sub~ect to the

B' (degrees)

35 28 33

further recommendat~ons below, may 6e

suitable

but other materlals such as mdustnal, chem~cal and domestic wastes should not be used All fill

matenals should be properly invest~gated and class~fied

The use of coheswe soil as f ~ l l may involve

problems durmg deslgn and construction additional

to those w h c h occur with granular fill, but the use of coheswe soil may result m sigmf~cant economies

Figure 2. Derivation of

N'

from SPT value N

The cohesive sod should be w t h m a range su~table Chalk wlth a saturation moisture content of 20 %

for adequate compactmn, for guidance on the selection of such fill see the Transport Research Laboratory pubhcations LR406, LR750, SR522 and RR90, the proceedmgs of the conference on clay fills, ICE 1979, the DOT Specification for h~ghway works, 1991 and DOT Standard BD30187 The placement moisture content of cohewve fill should be close to the final equdbnum value to prevent elther the swelling of clays placed too dry or the consohdat~on of clays placed too wet Volume changes in clay soils wdl affect the

pressure d~stnbut~on on the wall in the medlum to long-term Compaction pressures should also be taken into account, see 3.3.3.6 Problems

associated w t h swelling and consolidation wdl be m~nimized d clay fill IS limited to clays with a hquid

l~rnit not exceeding 45 % and a plast~city index not exceeding 25 !& (DOT Spec~ficat~on for highway

works, 1991)

or less IS acceptable

as

fill and may be compacted

as a well graded granular soil The saturation molsture content of chalk is evaluated from the dry denslty of individual lumps, determined m

accordance with 7.3 of BS 1377 Part 2 1990

Saturation moisture content =

where

yd = dry density in mg/m3

Exceptionally, some granltes are found whch detenorate by weathering of the feldspars If it is proposed to use such granltlc rocks, due allowance should be made for deterioration in estlmatmg the angle of fnctlon

Conditioned pulverized fuel ash (PFA) from a single source may be used as fill it should be supphed at a moisture content of 80

X

to 100 % of the optlrnum molsture content

Section

2

BS

8002

:

1994

.

.

Shale, mudstone and steel slag swell when they absorb water These matenals should not be used

as

fdl, except at some distance from the retaining wall Peaty or highly organic sol1 should not be used as fill

2.2.8 Wall friction, base friction and undrained wall adhesion

Representatwe values of the strength of the soil shdmg as a mass against the wall can be

determined from appropriate drained and

undmned shear box tests The wall matenal should be placed m the bottom half of the box w ~ t h its ~nterface on the plane of shdmg The so11 is then placed in the upper part of the box m the requ~red state With large scale surface roughnesses (1 e concrete formed on or agalnst coarse granular sods) side and end effects of the small shear box

(60 mm x 60 mm) w~ll affect the laboratory test results and large shear boxes should be used Tests should be carned out over the range of normal stresses hkely to exlst on the wall dunng its hfe Testing should be contmued to determine any reduction in strength \nth continued sllding

In the absence of large shear box test results the representatwe strength, m terms of effect~ve stress, should not exceed values calculated usmg

a) 6 = P',,,~ for the sod, for rough surfaces \nth a texture coarser than that of the medlan particle size,

b) 6

-

20°, for smooth surfaces w t h a texture finer than that of the median part~cle sue

No effect~ve adhesmn

c'

should be taken for walls or bases in contact with sod

The effects of wall construction on the mterface fnction between the soil and the wall should be taken in account The undramed shear strength mobillzed on a wall surface may be ~rrelevant due to the presence of drainage mater~al creating effectwe fnct~on conditlons on the boundary Craclung and arr entry against the wall also tend to produce fnctlon conditlons \nth zero (atrnosphenc) pore pressures against the wall, m contrast to the possibly negatlve pore pressures mobfized

temporanly w~thin the clay mass Under these circumstances the representative coefficient of effective fnction on the boundary 1s tan 6 and the normal effective stress at the boundary IS equal to

the normal total stress a, in the soil so that the representatwe wall fnction is a, tan 6 Where the undramed sod strength agamst a surface 1s

relevant, and in the absence of appropnate tests, the representatwe value should not exceed the remoulded undramed strength of the soil

2.3 Externally applied loads

All necessary details should be obtamed of statlc, translent and dynam~c loads that may be applied externally to the earth retainmg structure

3.1 Design philosophy

3 1 1 General

The design of earth retammg structures requlres cons~derat~on of the mteractlon between the ground and the structure It requlres the performance of two sets of calculatlons

1) a set of equ~hbnum calculat~ons to determme the overall proportions and the geometry of the structure necessary to acheve equ~hbnum under the relevant earth pressures and forces,

2) structural design calculat~ons to determme the sue and properties of the structural sectlons necessary to reslst the bendmg

moments and shear forces determmed from the equ~hbnum calculat~ons

Both sets of calculat~ons are camed out for spec~fic design s~tuations (see 3.2.2) m accordance wlth the pnnc~ples of hmrt state deslgn The selected des~gn s ~ t u a t ~ o n s should be suffic~ently severe and vaned so as to encompass

all

reasonable cond~t~ons w h c h can be foreseen dunng the penod of construction

and the l ~ f e of the retalnmg wall 3.1.2 Limit state design

This code of practlce adopts the phlosophy of h ~ t state des~gn This ph~losophy does not Impose upon the des~gner any specla1 requirements as to the manner in whch the safety and stab~hty of the retammg wall may be ach~eved, whether by overall factors of safety, or partial factors of safety, or by other measures Llm~t states (see 1.3.13) are class~fied Into

a) ult~mate h i t states (see 3.1 31, b) servlceabhty hmit states (see 3.1.4)

Typ~cal ult~mate h m ~ t states are dep~cted in figure 3 Rupture states whch are reached before collapse occurs are, for s~mphc~ty, also class~f~ed and treated as ult~mate 11m1t states Ultlmate hmlt states mclude

a) mstab~hty of the structure or any part of it, mcludmq supports and foundat~ons, cons~dered as a n@d body,

b) fa~lure by rupture of the structure or any part of it, mclud~ng supports and foundat~ons 3.1.3 Ultimate limit states

3.1.3.1 General

The followmg ultimate 11m1t states should be considered b l u r e of a r e t a n ~ n g wall as a result of

a) mstability of the earth mass, e g a shp fadure, overturnmg or a rotat~onal fafiure where the disturbmg moments on the structure exceed the restonng moments, a translat~onal fa~lure where the d~sturbulg forces (see 1.3.8) exceed the restonng forces and a beanng fadure Instability of the earth mass mvolving a shp fa~lure may occur where

1) the wall IS bulk on sloping ground wh~ch

~tself 1s close to l m t m g equil~bnum, or 2) the structure IS underlam by a slgmficant

depth of clay whose undramed strength mcreases only gradually wlth depth, or

3) the structure 1s founded on a relatively

strong stratum underlam by weaker strata, or 4) the structure 1s underlain by strata w~bhin wh~ch high pore water pressures may develop from natural or artificial sources

b) fallure of structural members lncludmg the wall ~tself m bendmg or shear,

c) excesswe deformat~on of the wall or ground such that adjacent structures or servlces reach theu ultimate lumt state

Section

3

BS

8002

:

1994

.

.

F a ~ l u r e of gravlty wall due t o slldmg

G r a v ~ t y hall w i t h shear key Fallure may be on A B or along

B C sloplng plane

Bear~ng capacity fallure of gravity wall on r e s t r ~ c t e d depth of s o f t SOIL

Embedded wall cantiever fallurc by forward rotahon

Bend~ng moment fa~lure of embedded wall

Figure 3. Limit states for earth retaining structures (concluded)

-

fivwx

1 3 -

Fa~lure of embedded wall by rotahon Fa~lure by y ~ e l d of anchor or t ~ e (or prop 1

about anchor ( o r prop

I

I

3.1.3.2 Analysis method

Where the mode of fadure mvolves a shp fa~lure the methods of analysis, for s t a b ~ l ~ t y of slopes, are descnbed in BS 6031 and in BS 8081 Where the mode of fadure mvolves a beanng capaclty fa~lure, the calculat~ons should establ~sh an effectwe w ~ d t h of foundat~on The beanng pressures as determmed from 4.2.2 should not exceed the ult~mate beanng capaclty In accordance w ~ t h BS 8004

Where the mode of fa~lure 1s by translat~onal movement, w t h passwe resistance excluded, stable e q u ~ l ~ b n u m should be ach~eved usmg the desgn shear strength of the sod m contact w ~ t h the base of the earth retammg structure

Where the mode of fadure mvolves a rotat~onal or translat~onal movement, the stable equ~hbr~um of the earth r e t a l m g structure depends on the mob~l~zat~on of shear stresses w t h m the sol1 The full mob~hzatlon of the so11 shear strength gwes nse to hm~tmg actwe and passive thrusts These

hm~ting thrusts act m concert on the structure only at the pomt of collapse, I e ultrmate l ~ r n ~ t state

3.1.4 ServiceabiLity limit states

The f o l l o m g semceablbty h m ~ t states should be cons~dered

a) substantla1 deformat~on of the structure, b) substantial movement of the ground

The so11 deformat~ons, w h c h accompany the full mob~hzat~on of shear strength in the surroundmg sod, are large m companson w t h the normally acceptable strams in s e m c e Accordmgly, for most earth reta~nmg structures the semceab~hty h m ~ t state of d~splacement wlll be the govenung

cntenon for a sat~sfactoly equ~hbnum and not the ultmate h m ~ t state of overall stabd~ty However, although it IS generally nnposs~ble or ~mpract~cal to

calculate dls~lacements dlrectlv. serwceab~htv can be s u f f i c ~ e n t i ~ assured by hm&g the propor& of avadable strength actually mobll~zed m semce, by the method gwen in 3.2.4 and 3.2.5

The des~gn earth pressures used for serv~ceab~l~ty h m t state calculat~ons wlll d ~ f f e r from those used for ult~mate h m ~ t state calculat~ons only where structures are to be subjected to drffenng des~gn values of external loads (generally surcharge and hve loads) for the ult~mate h m ~ t state and for the semceablhty h m ~ t state

3.1.5 Limit states and compatibility of

deformations

The deformatlon of an earth retalnlng structure 1s

important because it has a du-ect effect upon the forces on the structure, the forces from the retamed sod and the forces whlch result when the structure moves agalnst the sol1 The structural forces and bendmg moments due to earth pressures reduce as deformat~on of the structure increases

sufficient deformat~on of the structure, the active earth pressure (see 1.3.1) m the retamed so11 reduces to the fully actwe pressure and the passwe resistance (see 1.3.15) tends to mcrease to the full available passive remtance (see 1.3 12)

The compatibil~ty of deformation of the structure and the correspondmg earth pressures 1s Important where the form of structure, for example a

propped cantilever wall, prevents the occurrence of fully active pressure at the prop it is also

particularly mportant where the structure behaves as a bnttle matenal and loses strength as

deformat~on increases, such as an unremforced mass gravity structure or where the soil is hable to stram softenmg as deformat~on increases

3 1.6 Design values of parameters

These are applicable at the specified lirmt states in the speclf~ed deslgn situations All elements of safety and uncertainty should be incorporated into the deslgn values

The selection of design values for soil parameters should take account of

a) the poss~b~hty of unfavourable vanations in the values of the parameters,

b) the independence or mterdependence of the var~ous parameters mvolved in the calculat~on, c) the quality of workmanshlp and level of control s p e c ~ f ~ e d for the constructlon 3.1.7 Applied loads

The des~gn value for the dens~ty of fill materials, should be a pessmistic or unfavourable assessment of actual density

For surcharges and live loadmgs different values may be appropriate for the differing conditions of serviceabihty and ult~mate limlt states and for different load combmations The mtention of thls code of pract~ce IS to determine those earth

pressures w h ~ c h w~ll not be exceeded in a hmit state, if external loads are correctly predicted

External loads, such as structural dead loads or veh~cle surcharge loads may be spec~fied in other codes as nominal or charactenstic values Some of the structural codes, m t h wluch this code

interfaces, spec~fy different load factors to be apphed for semceab~hty or ult~mate hmlt state checks and for different load combmat~ons, see 3.2.7 Design values of loads, denved by factonng or otherw~se, are intended, here, to be the most pesslmlstlc or unfavourable loads whlch should he used in the calculat~ons for the

structure Similarly, when external loads act on the

account should be taken also of the short-term conditions which apply dunng and unmed~ately followng the penod of constructlon Single des~gn values of so11 strength should be obtamed from a considerat~on of the representatwe values for peak and ultmate strength The value so selected w l l satisfy, simultaneously, the cons~derat~ons of

ultimate and semceabhty h m ~ t states The des~gn value should be the lower of

a) that value of sod strength, on the stress-strain relat~on leadmg to peak strength, which is mob~l~zed at so11 strains acceptable for

semceabhty Tlus can be expressed a s the peak strength reduced by a mob~hzat~on factor M as glven in 3.2.4 or 3.2.5, or

b) that value which would be mobihzed a t collapse, after sign~ficant ground movements Tlus can generally be taken to be the cntical state strength

Des~gn values selected m t h way should be checked to ensure that they conform to 3.1.6 Design values should not exceed representatwe values of the fully softened cnt~cal state so11 strength

3.1.9 Design earth pressures

The deslgn values of lateral earth pressure are mtended to gwe an overestunate of the earth pressure on the actwe or retained s ~ d e and an

underestunate of the earth remtance on the passive slde for small deformations of the structure as a whole, in the worlung state Earth pressures reduce as fully active conditions are mobhzed a t peak soil strength in the retamed sod, under deformations larger than can be tolerated for serv~ceabihty As collapse threatens, the retamed soil approaches a cnt~cal state, m which its strength reduces to that of loose matenal and the earth pressures consequently tend to mcrease once more to active values based on cntlcal state strength

The ~ n ~ t ~ a l presumption should be that the des~gn earth pressure wdl correspond to that ansmg from the des~gn sod strength, see 3.1.8 But the

mobhzed earth pressure in semce, for some walls, w~ll exceed these values m s enhanced earth pressure wdl control the design, for example

a) Where clays may swell in the retained sod zone, or be subject to the effects of compaction m layers, larger earth pressures may occur m that zone, causmg correspondmg res~stance from the ground, proppmg forces, or anchor tenslons to mcrease so as to mamntam overall equihbnum

Section

3

_BS

₈₀₀₂

_:

₁₉₉₄

b) Where clays may have lateral earth pressures In excess of the assessed values takmg account of earth pressures pnor to construction and the effects of wall ~nstallat~on and sol1 excavation or fillmg, the earth pressure in retamed so11 zones wlll be Increased to maintam overall equrl~bnum C) Where both the wall and backfill are placed on

compressible solls, d~fferentlal settlement due to consolidation may lead to rotatlon of the wall Into the backfill Thls Increases the earth pressures m the retamed zone

d) Where the structure 1s part~cularly stlff, for example fully plled box-shaped bndge

abutments, h~gher earth pressures, caused, for example by compaction, may be preserved, notw~thstandlng that the degree of wall

displacement or f l e x ~ b ~ l ~ t y requred to reduce retamed earth pressures to them fully actwe values m cohes~onless matenals is only of the order of a rotatlon of 10-3 radians

In each of these cases, mobilized so11 strengths w~ll mcrease as deformat~ons continue, so the

unfavourable earth pressure cond~t~ons WIU not perslst

as

collapse approaches

The des~gn earth pressures are denved from des~gn soil strengths usmg the usual methods of plast~c analys~s, w ~ t h earth pressure coeffments

(see 1 3.9) p e n in ttus code of practlce bemg based on Kensel & Absi (1990) The same des~gn earth pressures are used in the default c o n d ~ t ~ o n for the deslgn of structural sections, see 3.2.7

3.2

Design

method

3.2.1 Equilibrium calculations

In order to determme the geometry of the retammg wall, for example the depth of penetration of an embedded wall (see 1.3.10), equil~bnum calculat~ons should be earned out for carefully formulated design sltuatlons The des~gn

calculatlons relate to a free-body diagram of forces and stresses for the whole r e t a l m g wall The des~gn calculat~ons should demonstrate that there 1s global equ~hbr~um of vert~cal and honzontal forces, and of moments Separate calculat~ons should be made for d~fferent des~gn sltuat~ons The structural geometry of the retamlng wall and the equllibnum calculations should be determ~ned from the design earth pressures denved from the design sol1 strength using the appropnate earth pressure coefficients

Des~gn earth pressures wdl lead to actwe and passwe pressure d~agrams of the type shown in figure 4 The earth pressure distnbut~on should be checked for global equ~hbnum of the structure Honzontal forces equhbnum and moment

e q h b n u m WIU @ve the prop force in figure 4a and the locat~on of the point of reversed stress

cond~tlons near the toe in figure 4b Vertlcal forces e q u ~ l ~ b n u n should also be checked

3.2.2 Design situations

3.2.2.1 General

The spec~ficat~on of des~gn situations should mclude the d ~ s p o s ~ t ~ o n and class~ficat~on of the var~ous zones of so11 and rock and the elements of constructlon w h ~ c h could be mvolved m a h m ~ t state event The spec~ficat~on of des~gn s ~ t n a t ~ o n s should follow a cons~derat~on of all uncertamtles and the r ~ s k factors mvolved, mcludmg the followng

a) the loads and them combmat~ons, e g

surcharge and/or external loads on the actlve or retamed s ~ d e of the wall,

b) the geometry of the structure, and the ne~ghbounng sod bod~es, representmg the worst credlble cond~tlons, for example over-excavation

durmg or after constructlon,

c ) the material charactenstlcs of the structure, e g followmg corroslon,

d) effects due to the envlronment w ~ t h m whlch the des~gn IS set, such as

- ground water levels, mcludlng thelr vanatlons due to the effects of dewatenng, possible floodmg or falure of any dramage system,

-

scour, eroslon and excavatlon, leading to changes m the geometry of the ground surface, - chemlcal corroslon,

- weathenng, - freezmg,

- the presence of gases emergmg from the ground,

- other effects of tlme and envlronment on the strength and other properties of matenals, e) earthquakes,

f) subs~dence due to mlnlng or other causes, g) the tolerance of the structure to deformations, h) the effect of the new structure on exlstmg structures or serwces and the effect of exlstlng structures or semces on the new structure,

I) for structures restrng on or near rock, the

considerat~on of

- mterbedded hard and soft strata, - faults, jolnts and fissures,

- solutlon cavltles such a s swallow holes or fissures, filled w ~ t h soft matenal, and contlnumg solutlon processes

3.2 2.2 Minimum surcharge and minimum unplanned excauataon

In checkmg the stable equ~l~bnum and sod deformat~on all walls should be des~gned for a min~mum des~gn surcharge loadmg of 10 kN/m2 and a mlmmum depth of add~t~onal unplanned excavatlon in front of the wall, wh~ch should be

a) not less than O 5 m, and

b) not less than 10 % of the total heght retamed for cantilever walls, or of the he~ght retamed below the lowest support level for propped or anchored walls

These mmmum values should be rewewed for each deslgn and more adverse values adopted in

particularly cntlcal or uncertain clrcumstances The requirement for an addit~onal or unplanned

excavatlon as a des~gn crltenon IS to provlde for

unforeseen and accidental events Foreseeable

excavations such as servlce or dramage trenches m front of a retalnlng wall, w h ~ c h may be requ~red at some stage in the hfe of the structure, should be treated as a planned excavatlon Actual excavatlon beyond the planned depth IS outs~de the des~gn

cons~derat~ons of t h ~ s code

3.2.2.3 Water pressure regsme

The water pressure regme used m the design should be the most onerous that IS cons~dered to be

reasonably possible

3.2 3 Calculations based on total and effectwe stress parameters

The changes In loaduig assoc~ated w t h the constructlon of a retammg wall may result m changes m the strength of the ground m the vlcmty of the wall Where the mass permeabhty of the ground is low these changes of strength take place over some tlme and therefore the des~gn should conslder cond~t~ons

m

both the short- and long-term Whch cond~t~on w ~ l l be cntical depends on whether the changes m load apphed to the sod mass cause an mcrease or decrease m sod strength The long-term cond~tlon 1s ltkely to be cntical where the so11 mass undergoes a net reduct~on ~n load as a result of excavatlon, such as adjacent to a cant~lever wall Conversely where the so11 mass IS

subject to a net Increase m loadmg, such as beneath the foundat~on of a gravity or remforced stem wall at ground level, the short-term c o n d ~ t ~ o n 1s l~kely to be cnt~cal for s t a b ~ l ~ t y When

cons~denng long-term earth pressures and

equhbnum, allowance should be made for changes In ground water cond~t~ons and pore water pressure reame w h ~ c h may result from the construction of the works or from other agencies

Calculat~ons for long-term cond~tlons requlre shear strength parameters to be m terms of effectwe stress and should take account of a range of water pressures based on cons~derat~ons of poss~ble seepage flow cond~t~ons w t h the earth mass Effectwe stress methods can also be used to assess the short-term cond~t~ons prov~ded the pore water pressures developed dunng construction are known A total stress method of analys~s may be used to assess the short-term condlt~ons m clays and sods of low permeab~lity, but an Inherent

assumption of t h ~ s method IS that there w ~ l l be no

change m the sod strength as a result of the changes in load caused by the construction For granular materials and solls of hlgh permeab~llty all excess pore water pressure w~ll d~sslpate rapidly so that the relevant strength IS always the dramed

strength and the earth pressures and equhbnum calculat~ons are always m terms of effectlve stresses