Performance of a reinforced earth bridgeabutment at Carmarthen

by K C Brady

w

,

(\

The tibrawTranspoti and Road Research Laborato~i)epartment of Transpoti

Cr:~l~:A; Berks

Digest RR111 1987

PERFORMANCE OF A RE~ORCED EARTH B~GE ABUTMENT AT CARMARTHEN

by

K C Brady

The route of the A48 Carmarthen Southern Bypass required that two bridges be constructed over the alluvial floodplain of theAfon Tywi. In this area the alluvial deposits are approximately 10 metres thick, are highly compressible and have low bearingcapacities. It was therefore decided to construct the bridge abutments in reinforced earth using the Websol system. This systemincorporated polyester reinforcing straps (Paraweb) and sheets of a melt-bonded geotextile (Terrain). A view of the westabutment of the rail bridge during construction is shown in Plate 5.

Plate 5: View of west abutment during construtiion (CR 571/81/19)

The Report describes the geology of the site, the instrumentation of the west abutment of the rail bridge and the sequence ofconstruction. Data obtained from laboratory tests and from the instruments installed on site are analysed and discussed in thecontext of the behaviour of the west abutment.

During construction of the abutment, piles were driven to form the foundation of the adjacent bridge pier. Although thesettlement of the abutment was influenced by the piling operations, no significant changes in earth pressures or strains wererecorded within the backfill. Some outward movements of the facing panels were recorded during construction, but nosignificant movements have been detected since the abutment was completed.

The recorded distributions of pressure within the structure were not entirely as anticipated. The data obtained from the pressurecells were inconsistent. In general, the data obtained from load cells attached to the Paraweb straps indicated that the lateralthrust acting on the back of the wall varied between active and at-rest earth pressure conditions.

Measurements taken up to July 1986indicate that no significant changes have occurred to the reinforced earth structure since itwas opened to traffic in July 1983.

The work described in this Digest was earned out in the Ground Engineering Division of the Structures Group of TRRL.

I

If thti information is imufficient for your needs a copy of the full Report RRII1 (price at publication U price code B) may beobtained on written request to the Cashier, Transport and Road Research Laboratory, Old Wokingham Road, Crowthorne,Berkshire RGI 16A U. Cheque made payable to TRRL.

Crown Copyright. The views expressed in this Digest are not necessarily those of the Department of Transport. Extracts fromthe text may be reproduced, except for commercial purposes, provided the source is acknowledged.

TRANSPORT AND ROAD RESEARCH LABORATORY

Depatiment of Transpoti

RESEARCH REPORT Ill

CONTENTS

Abstract

1. Introduction

2. Geology

2.1 Superficial deposits

2.1.1 Alluvia

2.1.2 Tills

3. Bridge and Embankment design andconstruction

3.1 Reinforced Earth

3.2 Construction sequence

4, Properties of backfill

4.1 Particle size distribution

4.2 Shearbox tests

4.3 In situ properties

5. Instrumentation and site data

5.1 Settlement of wall

5.2 Vertical alignment of wall

5.3 Strains within backfill

5.4 Earth pressures

5.4.1

5.4.2

5.4.3

5.4.4

5.4.5

Installation

Calibration

Variation of vertical pressure

Page

1

1

1

1

1

1

1

1

4

6

6

6

7

7

7

10

10

11

11

11

12

Variation of horizontal pressure withinbackfill 12

Variation of lateral pressure acting onthe back of the wall 12

5.5 Strap tensions 15

5.6 Measurement of temperature 18

5.7 Data acquisition system 18

6. Discussion 18

6.1 Settlement of wall 18

7.

8.

9.

6.2 Vertical alignment of wall 18

6.3 Strains within backfill 18

6.4 Earth pressures 19

6.4.1 Performance of the pressure cell 19

6.4.2 Vertical pressure 19

6,4.3 Relation between horizontal andvertical pressures 20

6.5 Strap tensions 21

6.6 Lateral pressures acting on the back of thewall 23

6.7 Temperatures within backfill 23

Conclusions 23

Acknowledgements 24

References 24

@CROWN COPYRIGHT 1987Extracts from the text may be reproduced,

except for commercial purposes,provided the source is acknowledged

PERFORMANCE OF A REINFORCED EARTH BRIDGEABUTMENT AT CARMARTHEN

ABSTRACT

Four bridge abutments were constructed overthe alluvial floodplain of the Afon Tywi nearCarmarthen using the Websol reinforced earthsystem. This system incorporated polyesterreinforcing straps (Paraweb) and sheets of melt-bonded geotextile (Terrain). The constructionand instrumentation of one of the abutments isdescribed and an assessment of the structureduring construction and up to the end of its thirdyear in service is also given. Measurementstaken in the three years after the opening of thebypass to traffic in 1983 indicate that no signifi-cant changes have occurred to the reinforcedearth structure during this period.

1 INTRODUCTION

As part of the programme of works to improvethe London to Fishguard trunk road (A48),construction of a bypass to the town ofCarmarthen commenced in 1981. The route ofthe bypass, shown in Figures 1 and 2, requiredtwo bridges to be built over the alluvial flood-plain of the Afon Tywi. Because of the lowbearing capacity and high compressibility of thealluvial deposits, the abutments of these bridgeswere constructed in reinforced earth using theWebsol system (Agrement Board, 1979). Asthese were the first reinforced earth bridgeabutments on a major trunk road scheme in theUK to incorporate plastic reinforcing elements,the Transport and Road Research Laboratory(TRRL) took the opportunity to assess theconstruction and performance of the abutmenton the western side of the rail bridge.

In addition to providing details of the geology ofthe site and the instrumentation, this RepoRcontains a description of the method of con-struction and an assessment of the performanceof the structure during the three years followingthe opening of the bypass to traffic.

2 GEOLOGY

The bedrock of the area consists of shales andmudstones of the Ordovician period, the uppertwo metres or so of which was weathered to ashaley clay. The superficial deposits consist of

tills (sand/gravels and ‘boulder clay’) and recentalluvia (silts, clays, peats). The geological sectionof the part of the bypass that crosses the Tywifloodplain is shown in Figure 3.

2.1 SUPERFICIAL DEPOSITS

Information on these deposits was obtainedfrom site investigation reports by Threadgoldand Daley (1971), Stott and Rodger (1975),Newbery (1975) and Foster and Coomber (1979).

2.1.1 Alluvia.These can be divided, according to age, into twogroups. The older deposits consist of mixtures ofsilts and clays: following a change in sea level,the Afon Tywi cut into these, and the youngerbeds of gravel and mixtures of organic silts andclays were then deposited. A summary of theproperties of the alluvial deposits is given inTable 1.

2.1.2 Tills.These deposits can again be convenientlydivided into two groups (i) sands and gravelsand (ii) blue ‘boulder clay’; but the boundarybetween them was not always clearly defined.The coarse grained deposits are a mixture ofsands and gravels with pockets of clay and silt.The blue clay is essentially a mixture of sand andgravel with a firm to stiff clay matrix.

3 BRIDGE AND EMBANKMENTDESIGN AND CONSTRUCTION

The low shear strength and high compressibilityof the upper alluvial deposits were the majorfactors that influenced the design and construc-tion of the bridge and earthworks.

The proximity of the in-service railway linesdictated that the piers to the rail bridge befounded on piles. Reinforced earth was chosenfor the bridge abutments because of the abilityof this medium to tolerate large differentialsettlements. Stability considerations demandedthe provision of a berm to the embankment andalso some control on the rate of construction.

3.1 REINFORCED EARTH

The Websol system of soil reinforcement,described in Agrement Board Certificate No

1

10

5

0

E –5o<

:

c –loo,-;~w —15

–20

–25

–30

ROAD PROFILERAILBRIDGE

TRIVERBRIDGE

Position of b

4

H.W.S,T. 5.790m (Not all indicated)n. “--

)T ~~= ‘=s<:–

3 +.?-7—————————__ ----- --—- /-——— L

.—— —1

~ Fill

~ Organic clay

~ Peat

~ Sandy gravels

~ :;:,::~s

~ Boulder clay’

G Iacial sands= and gravels

H M.dsto”esand shales

Inferred base ofyounger alluvium

Inferred base ofolder alluvium

Highest recordedZ piezometric

reading and date

o Lo 5m

Fig.3 Geological section across Tywi Floodplain

TABLE 1

Geotechnical properties of Alluvium

In situ

Moisture Consistence Compressibility data In situ

Description Content from Iaboratoy

Alluvium (per cent) Gradin~Limits

permeability

of Soil ‘Strength’ oedometer tests K, Ki/KL

Upper

Organic silts PL: 15t045 ~- 0.7m2/Vr 3.3 x 10-9

25 to 80and clavs LL: 35 to 75 “

-20 kN/m2 mc; - 1.0m2/MN to -20

KL - 2.2 x 10-’0 m/see 6 x 10-11 m/see

I Peat inclusions-200

withi’n abovec“ - 10 kN/m2 m. - 2 m2/MN

D50 - 10mm

ISandV gravels Coefficient of N-25

uniformity - 10

Lower

I SiltV and PL -20c.: 20 C. >15 m2/Vr 3 x 10-7

-25 tosandv clavs LL -30

m. <0.1m2/MN to >1000

400 kN/m2 KL variable 4 x 10+ mlsec

PL:

LL:

c“:

N:

D50:

Plastic limit c“ : Coefficient of consolidationLiquid Limit per cent

m.: Coefficient of volume changeUndrained cohesion K,: Coefficient of permeability from laboratory testsSPT blow count Ki : Coeticient of permeability from in situ testsmean panicle size

79/18, was used for the bridge abutments. In this of polyolefine. Straps, 90 mm wide and 3.5 mmsystem the main reinforcement was Paraweb* thick, having a short term breaking load of 50 kNstraps; these are continuous, aligned, high- were used on this scheme; a maximum designtenacity polyester fibres enclosed within a casing load of 12.5 kN was adopted.

3

Neg. no. R59018417

Plate 1 Installation of Paraweb straps

The system also incorporated sheets of Terrain*1000 fabric; it was not possible to quantify howthese sheets would affect the performance of thestructure and their reinforcing effect was nottaken into account during design.

3.2 CONSTRUCTION SEQUENCE

Construction of the west abutment of the railbridge commenced in July 1981 with theremoval of the soft subsoil and its replacementsby rock fill. The plain concrete strip footing of theabutment was then cast and the first row ofprecast facing units were erected upon it;crushed limestone fill was placed behind theunits up to the level of the first layer of Paraweb.The Paraweb straps were looped aroundanchorage pins on the facing units and theanchor bar at the back of the reinforced earthzone, see Plates 1 and 2. The straps were thentightened and held in position by steel pinssecuring the anchor bar. The 100 metre lengthsof Paraweb were overlapped by 2 metres and, asshown in Plate 3 joined together with a plywoodclamp. To prevent ingress of water, the ends ofthe Paraweb were sealed with a proprietarycompound. As shown in Plate 4, each layer ofParaweb was overlain by a sheet of Terrainbefore the next layer of fill was placed. Thenorthern end of the west abutment was built upfirst to enable a temporary haul road to beconstructed; the remainder of the abutment wasbuilt up uniformly along its length, Constructionof the wall was affected by the late delivery of a

* (Paraweb and Terrain are registeredtrademarks of ICI.)

Neg. no. R590184114

Plate 2 Installation of Paraweb straps

few facing units for the uppermost row and wasfinally completed in November 1981.

The embankment across the floodplain wasconstructed in stages; approximately 5 metresof mudstone/shale fill was placed during thesummer of 1981 and the bulk of the remainderwas placed in the spring and summer of thefollowing year.

The foundations of the west pier of the railbridge were supported by sixty six H-sectionsteel piles; these can be seen on the left hand

4

Neg. no. CR417I81I1O

Plate 3 Installation of clamp joining together lengthsof Paraweb

r

Neg. no. CR46718115

Plate 4 Construction of West Abutment

I .. .. .. “

Neg. no. CR571/81/19

Plate 5 View of West Abutment during construction

side of Plate 5. The first lengths of these piles backfill at the back of the bankseat was placed inwere driven in June 1981 and subsequent July and August of that year. Plate 6 shows asections were added and driven between view of the abutment shortly after the installationOctober 1981 and March 1982. of the bridge beams.

Construction of the bankseat of the west The fill directly in front of the wall was removedabutment commenced in February 1982. The prior to the installation of a rising main inbridge beams were placed in June 1982 and the September 1982, see plate 7. After reinstatement

5

Neg. no. CR3921a2111

Plate 6 View of West Abutment after installationof bridge beams

Plate7 Installation ofrising main in frontofWest Abutment

the level of fill was approximately one metre ‘above the level of the footing.

The bypass was opened to traffic on the 22ndJuly 1983.

4 PROPERTIES OF BACKFILL

4.1 PARTICLE SIZE DISTRIBUTION

The results of sieve analyses showed that thegrading of the backfill to the abutment wasreasonably consistent and the mean of theresults is shown in Figure 4.

100

aomc.-

60 ;Rmg

40 :u&n

20

0

0.1 1 10 100

Particle size (mm)

IFINE I MEDIUM I COARSE I FINE I ME.,.. I cOARsE

SAND GRAVEL

Fig 4 Mean particle size distribution curve for backfill

4.2 SHEARBOX TESTS

Tests were performed using a 300 mm wideshearbox to determine, (i) the frictional proper-ties of the backfill and (ii) the coefficient ofinterface friction between the backfill and re-inforcement; the results are shown in Figure 5.The values of the angle of friction, 0’, weredetermined using the following simple relation;

()O)=tan-l FL .......................... (1)

where SF is the shear force,and NL is the applied normal load.

Previous work by Bishop and Skempton (1950),Rowe (1969) and Brady et al (lg84) has shownthat the shearing resistance of a soil can bedivided into two components; one associatedwith shearing at constant volume, 0’C., and theother derived from particle interlock, 0’d. Thus,

O’=O’~v +O’d . . . . . . . . . . . . . . . . . . . . . . . . . . . . (2)

6

Furthermore, the particle interlock componentcan be related to the change in volume of thespecimen during shear,

O’~=C.tan-l (dv/dh) . . . . . . . . . . . . . . . . . . . . . . . (3)

where dv is the change in height of the specimenthat occurs during a horizontal movement of dh,and C is a constant.

The data obtained from the tests on the backfillindicated that the magnitude of the constantvolume component was approximately 41degrees, and O’d was equal to 0.64. tan-1 (dv/dh).

In the second series of tests, the reinforcementswere attached to the leading edge of the bottomhalf of the she,arbox and laid along the mid-height, ie on the assumed shear plane. As shownin Figure 5 the data from these tests werescattered but they did show that; (i) little dilationoccurred during shearing and (ii) the shearingresistance along the interface was less than thatof the backfill alone.

4.3 IN SITU PROPERTIES

The results of sand replacement tests (Test 15B,BS1377:1975) indicated that the mean in situ drydensity and moisture content of the backfill were2080 kg/m3 and 2.6 per cent respectively. Thedata given in Figure 5 indicate that at that

density the peak angle of friction of the backfillwas approximately 61°.

5 INSTRUMENTATIONAND SITE DATA

The congested nature of the site dictated that theinstrumentation cabin had to be situated to theeast of the railway. This meant that all the cablesto the instruments in the west abutment had tobe installed from manholes through pipe ductsunder the railway and temporary haul road;some of the cables were over 100 m long. Inaddition, the rate of construction was relativelyrapid and not all the instruments were placed intheir originally intended positions. The finalposition of the instruments are shown in Figures6(a) and 6(b).

The measurement of earth pressures, strains,temperatures and tensions was terminated twoyears afier the opening of the bypass; measure-ment of settlement and vetiical alignment iscontinuing.

5.1 SE~LEMENT OF WALL

The settlement was initially assessed by opticallyIevelling the footing and the tops of 1 metrehigh, 20 mm, diameter mild steel rods that were

Normal stress (kN/m2)

80 160

Backfill onlv 00Interface with Paraweb A A

Interface with Paraweb and Terrain ❑ ■

[~ I 1 1 1A

35I I

65

60

55

■

50

45 –P~

@ :“❑

40 -A

A35 1 1 1

1500 1600 1700 1800 1900 2000 2100 2200 0 0.2 0.4 0.6 0.8

Initial drv densitv (kg/m3) (dv/dh)

Fig.5 Data obtained from shearbox tests

7

Limit of bankseatc

1Facing panel number Pressure cell attached to

1’

I 142rear of facing panel

o1’

I133 124

1’113 101

I 10 0

87 73 88

1’ 1’057 42

1’

I I \I

0’‘Centre profile’

1m high, 20mm diameter

u Ievelling rod fixed in footing

(a) Position of pressure cells on back of wall and Ievelling rods in footing

‘CENTRE’ PROFILEBANK SEAT

I IRein for~e:e:nt

Facing [ 1 ———— ——-— ———2 ,11

No.n

2B -O-J o —’ o

.:-g; @132 I

2 , ;Horizontaldistance ❑

{ 5beweenreinforce- i-merits 112

t~

——

(0.67m) 7

~ Tension cell attached to Paraweb

I Pressure cell recording horizontal earth pressures

Pressure cell recording verlical earth pressures

Thermocouple

&Set 01 strain cOIls

I“srr”me”ts “ot to scale)

19m FROM CENTRELINE OF ABUTMENT

ReinforcementFacing Iaval

ti:e~ ;1

I1A

117

- —--: z ~ o, L-— — I

-~_~ L-11- -- —- ~1 L1 —1 5A

2 1 1 1- -- --—

56— —

4 . - —- -—_ =_ -- 115 61+ - -T i

(0.5m) ;: +1 1 1

— —ri —— -- I (0.5m)~ 1

———_ ——

Number of cells al parlic~a;posit,on1- -1

F.G. L. ~ Rei”forceme”tsi 29i I

24 ! 8A] Footing level

——— ——— ——— J

Footing 4 w 5.04 D 6,54 * 8.04 10.0 0 lm

4 - I I

(b) Position of instrumen~ within reinford earth zone

Fig.6 Layout of instrumentation

fixed into the footing. These rods were protected the construction of the abutment and adjacentwith 100 mm diameter plastic sleeves to prevent pier, settlements during the later stages ofearth pressures from acting on them when the construction and subsequently were determinedground in front of the wall was brought up to from the levels of secondary points on the facelevel. Because the rods were displaced during of the wall.

8

Relations between settlement and time for structure was approximately 155 mm, thisvarious positions along the abutment are given ~ increased by a further 5 mm in the followingin Figure 7. At the opening of the bypass to three years; Figure 8 shows the settlementtraffic the settlement at the centreline of the profile along the structure in July 1986.

z—-E?- .-Og

10 -;0~+ ----- -

“~ Y1: 0

a

% 40&

Distance from centreline of abutment (m)E

80 -fa=~ 120 -= —*___ .—. —.—. — ._. —.—. ~

N“mb@r

160

I~\~\

~\

Date

Fig.7 Relationsbetweensettlement and time

Centreline of abutment

IExtent of bankseat

Berm

1/

I

8 .5m I

I

I

32m

d~

dl

1 10

30 20 10

II

Maximum differential settlement 14—

Half width of structure . 200

me/

Vertical exaggeration 200:1

●

●O=0.48 degs

D I-o

10 20 30

Distance from centreline (m)

Fig.8 Settlement profile of west abutment on 20 July 1986

9

5.2 VERTICAL ALIGNMENT OF WALL

The layout of the site and the requirement toplace’datum points well away from the zone ofinfluence of the construction works precludedthe use of optical methods to assess the absoluteposition of the facing panels. Therefore, thechange in vertical alignment of a column ofpanels was assessed using a plumb line. Initially,these lines were established using coneddepressions provided in the tops of the 1 metrehigh mild steel rods. However because the rodswere disturbed, secondary points on the facingpanels were used in the later stages of construc-tion and subsequently. Offset readings betweenthe face of the wall and the plumb line weretaken at a number of points during and after theconstruction of the abutment. The reproducibilityof the readings was approximately * 1.5 mm instill air conditions.

Examples of the alignment of columns of facingunits in December 1981, just after the wall hadbeen completed, are shown in Figure 9. Nofurther significant changes were recorded in theperiod up to July 1986, three years after thebypass had been opened to traffic.

5.3 STRAINS WITHIN BACKFILL

Three sets of inductive sensors were used tomeasure vertical and horizontal movements

within the backfill. The sensors were 100 mm by5 mm thick disc-shaped coils of wire encapsu-lated within a waterproof plastic coating (BisonInstruments, undated). The sensors werecalibrated in the laboratory using an inductancebridge and micrometer.

The coils were installed in both co-planar and co-axial alignments. Initially they were placed ontop of successive layers of backfill. However, thelower coils were displaced laterally to such anextent during the placing of the next layer of fillthat it proved impossible to obtain reliable zeroreadings. Moreover, the time required to positionthe upper set of coils was excessive. Conse-quently, the coils were installed in trenches duginto the compacted backfill.

The relations between vetiical strain and depthof cover for the coils installed 3 metres above thelevel of the footing are shown-in Figure 10.Taking into account the in situ unit weight of thebackfill, the data given in Figure 10 indicates thatthe modulus of the backfill was approximately3.3 MN/m2, at least an order of magnitude lowerthan anticipated. The data obtained from theother coils was less consistent, but indicatedmore plausible modulus values of between 15MN/m2 and 70 MN/m2.

The variation in strain with time is shown inFigure 11. In the two years following the openingof the bVDaSS all the coils recorded smallincrease; in compressive vertical strain.

;1-\-//

‘/Footing level

Top of abutment

Reinforced

“t

Outwardearth inclination

zoneof facingpanel

1- 1Omm Scale exaggeration 50:1

/

/

III

142

124

101

73

42

10 r

\

\

/

I

155

115

59

27

L L

Fig.9 Vertical alignment of wall in December 1981

1/\\/

Although these changes may have been inducedby traffic, it is pertinent to note that no signifi-cant changes were recorded during pilingoperations. Figure 11 shows that the horizontalstrains recorded by the uppermost set of coilswere approximately double the vertical strainswhereas those recorded by the other two setswere significantly less.

/

Number of strain coils (See Fig.6)

= H = horizontal strain, V = vertical strain

l/2:HI

-[ [

Data obtained fromcoils installedapproximately 3mabove footing

‘o 1 2 3 4 5

Compressive vertical strain (per cent)

Fig.10 Relation btween vertical strain

and depth of cover

L

5.4 EARTH PRESSURES

Pneumatic pressure cells were used to monitorthe earth pressures acting within the backfill andon the back of the facing panels. Details of theoperation and performance of this design of cellwere given by Carder and Krawczyk (1975). Theexpansion of the diaphragm creates passivethrusts within the soil and hence with this type ofcell the recorded pressures are usually greaterthan the stresses in the backfill acting normal tothe diaphragm.

5.4.1 Installation.The cells installed to record the lateral earthpressures were attached to the back of the facingpanels with an epoxy resin compound. Theve~ically-aligned cells in the backfill wereinstalled in slots excavated in a completed layerof backfill. To reduce the effects of point loadingfrom large particles all the cells were bedded-inand covered with a thin layer of fines obtainedfrom the backfill using a 1 mm sieve.

5.4.2 Calibration.Data obtained from laboratory calibration trials,in which a number of cells were buried in thebackfill used on site, are presented in Figure 12.The relations between gauge reading and appliednormal stress were reasonably linear. Theenvelope to the data points in the figurerepresents gauge factors, defined as the ratio ofgauge reading to applied normal stress, ofbetween 1.9 and 3.3.

The relations between gauge reading and depthof cover for cells installed some distance fromthe back of the wall, where it was to be expectedthat the vertical stress would be approximatelyequal to the overburden pressure, are shown in

~

Number of strain coils (See Fig.6)

etlo v 3/4. H

ev

—— ————— ___ -— —__ _==-

/{ 9/10 v t~

=>-.b T I ----

II 1/2’ H

1/3, V 9/11. H 5/6 v

2/4 V ----

----

Fig.11

Date

Variation of strain with time

11

.

.

v●

●

●.a8

1

--” / EnveloDe to

1

0 20 40 60

Applied normal stress (kN/m2)

Fig.12 Laboratory calibration data for pressure cells

Figure 13. All the relations obtained by thismethod were non-linear but lay within theenvelope given in Figure 12. All ‘corrected’pressures reported hereafter are gauge press- ‘ures adjusted with the mean calibration curvegiven in Figure 13.

5.4.3 Variation of vetiical pressure.The data given in Figure 14 indicate that thevertical pressure during construction varied froman equivalent height of overburden at the backof the reinforced earth zone to much smallervalues immediately behind the facing panels.However, the pattern was not so consistent inthe later stages of construction where as shown

12

Envelope tosite data

calibrationcurve

o 2 4 6 8

Oepth of cover (m)

Fig. 13 Relations be~een gauge pressure and

depth of cover for ceils installed at

rear of reinforced earth zone

in Figure 15 significant variations in pressurewere recorded by a number of cells. Nearly allthe cells installed within 4.5 metres of the back ofthe wall recorded significant reductions in press-ure towards the end of the construction period.Large variations in pressure were also recordedby the majority of cells installed in the lower halfof the backfill during December 1981 and January1982.

Most of the cells recorded increases in pressurefollowing the construction of the bankseat, in-stallation of the bridge deck and placement ofbackfill behind the bankseat (see section 3.2).The recorded pressures were essentiallyconstant during the two years following theopening of the bypass.

5.4.4 Variation of horizontal pressure withinbackfill.Relations between corrected horizontal pressureand depth of cover are given in Figure 16; therecorded pressures were corrected using thecalibration shown in Figure 13 for cellsmeasuring vertical pressures.

The variation in horizontal pressures withdistance from the back of the wall is shown inFigure 17; to allow comparison with Figure 14,the horizontal pressures have been normalizedby the nominal overburden pressure. As with thevertical pressures, Figure 18(a) shows that in thelater stages of construction, more than half thecells recorded significant reductions in pressure.Large variations in pressure were also notedduring the winter of 1981/82.

Most of the cells recorded some increase inpressure following the construction of thebankseat and placing of the bridge beams. Againthe pressures were essentially constant inthe two years following the opening of thebypass.

5.4.5 Variation of lateral pressure acting on theback of the wall.Most of the cells attached to the back of the wall

Height of fillabove cells (m)

.,● ✍✍✍✍✍✍

▼

I

:’/

I/

(i) Cells placed approx. 3mabove footing

/A Centre profile

,LExtent of reinforcedearth zone ~

I I 1 10 2 4 6 8 10

Distance from rear of facing panel (m)

.2

t

.0

t

Height of fillabove cells (m)

0.8

0.6

0.4

0.2P0.4

,: ::;

,11 1.6

,@\ . :Ji,11.

/’ ‘bjf

/1 ‘h’;;?’ “ , ,. (ii) Cells placed appro

.A 4.6m above footir

Centre profile

Extent of reinforce(earth zone —

o I I 1 Io 2 4 6 8

Distance from rear of facing panel (m)

1.2

1.0

0.8

0.6

0.4

0.2

0

* 4Height of fill ● 0.4

- above cells (m) 1

II

()\

(iii) Cells installedapprox. 1.75and 3.Omabove footing,19m fromcentrelineof abutment

o 2 4

Distance from rear

of facing panel (m)

Fig. 14 Distribution of vertical pressure within reinforced earth zone

180

140

I . n Jw100

J 1.1,7.0,ce”trepro file 7.5m abovefooting,4.Om from rearof facingpanel,centreprofile ----

60

‘E~ 20

--w-/ 4.5, 4,0, centreprofflle

----

~ I I I I I I 1 I 1 I I I I I I I I———

; 160w

F A~”:

Cells installed 1m from rear of facing panel, centre profile 2k 1.1 above footing J/

j 120 ~.7.-EA /%; 80 4.6

~ .o~>

-----I 1 I 1 I I 1 1 I I I I 1 I 1 I I

———

50 r1

Approximate height of cell above footing in metres

- Cells installed close to rear of facing panel, centre profile 7%

30

10----

_—. - ---1 I I I I I I I

———%5

~\

‘1

\O~~\ %6 \b\ Q

~%

Date

Fig.15 Variation of vertical pressureWith time

13

recorded very low pressures and no clear relationcould be established between these readingsand the depth of cover.

1The variation of lateral pressure with time isshown in Figure 18(b). All the cells recordedsudden reductions in pressure at some time

30

25

20

15

10

5

0

Fig. 16

30

20

10

0

80

60

40

20

0

/“Cell installed 19m● from centre line,

1.75m above footing1.5m from rear of

●

facing panel

{

.

●

Centre profile; 3,0m above footing,

/

4m from rear of facing panel

0 ~-----~_,sG ~- Y.+-0--

-----

-*. LCell installed on centre profile,

.. ‘ 4.5m above footing,2.25m from rear of facing panel

1 1 1 1 1

0 1 2 3 4 5 6

Height of backfill above cell (m)

Relation between height of backfill above celland measured horizontal pressure

(a) Cells installed within backfill

k

Cell installed 19.Om from centreline, 1.75m above fooling,1.5m from rear of facing panel

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

Fig. 17

Mean height offill above cells (m)

0.4

/

0.8

~

G

1.6

3.2

Extent of reinforcedearth zone ~

Cells installed at centre profile, approx,3.Om above footing

o 2 4 6 8 10

Distance from rear of facing panel (m)

Variation of horizontal pressure with distancefrom back of wall

---L

~Centre profile, 4.5m, 2.25m

----

% Cenlre prof,le, 3.Om, 1.Om----

———1

Distance from centreline (Inetres)

r (b) (i) Cells installd on rear of facing panels//

Height of cell above footing lmetres)~Panel number

other than mntre profile ,7,0, 5,52 (114)

a.”, ..0” ,a, ,---————

1

/Height of cell above footing

40 r (b) (ii) Centra profile 5.95m (112)

5.02m (112) ----

20

0 ---—————

Date

Fig.18 Variation of horizontal pressure with time

14

during December 1981 or January 1982.Although a recovery of pressure was recorded bymost of the cells installed in the upper half of thewall, many in the lower half continued to recordvery low pressures. The cells installed on theupper two rows of panels recorded someincreases in pressure following the constructionof the bankseat and placement of the bridgedecking. The pressures remained relativelystable during the two years following the openingof the bypass.

The data in Figure 19 show that the tensionsgenerally increased with increasing depth ofcover: the data points represent time — averagedvalues of tension for a particular cell at a particu-lar depth of cover. Once a cell had been coveredby more than a metre of fill, the range of tensionsfor a particular depth of cover was usually lessthan t 0.2 kN. The tensions recorded by load cellsinstalled on different straps, but on the same levelof reinforcement and the same distance from theback of the wall were in reasonable agreement.

5.5 STRAP TENSIONS

Fig. 19 Relations between tension and depth of cover

The distributions of tension along the lowerlevels of reinforcement are shown in Figure 20;the tensions recorded on the upper levels wererelatively low. The data given in this figure.showthat maximum tension was not developed at theanchorage points on the facing panels. The vari-ation of tension with time is shown in Figure 21.

The working life of the load cells was shonerthan expected. Premature failures were causedby:

(i)

(ii)

(iii)

penetration of water through the junctionbetween the body of the cell and the protec-tive encapsulating compound,slow absorption of moisture through thecable, sheathing,damage to cables during installation.

Failure modes (i) and (ii) were characterised by aconstant drift in output with time, mode (iii) by

15

The form of the Paraweb strap precluded thedirect application of strain gauges and so a loadcell was designed to clamp over a fixed length ofstrap. To ensure that the polyethylene sheath wasnot damaged, the surface area of the clampingplates was made fairly large, see Plate 8. It is likelythat a load cell of this size will affect the localstress regime in the surrounding backfill andhence the distribution of tension along the straps.

Neg. no. CFf4271al/2

Plate 8 Installation of load sell

A range of tensions was recorded by the cellsimmediately following their installation, ie beforeany fill was placed over them, in general thesetensions were approximately 0.25 kN. The dataindicated that the pre-stress applied to the straps,using the technique shown in Plate 2, reducedwith time when the straps were not covered withbackfill.

-------------- Level 15, 1.5m from rear of facing panel

...........””.. Level 12,1 .5mfrom rear of facing panel

-. —.- ---- Level 14, 1.5m from rear of facing panel

Level 17, close to rear of facing panel

-------- Level 16, close to rear of facing panel

Position of load cell-see Fig. 6

7

6

5

4

3

2

1

0

Level 15....,.~.

..-.,..,.

,.., . . . . . . . Level 12, . .,.’. .~.. ..7

,’,’

,,.. ,,,’

. ,,

~

O 2 4 6 a 10 12 14 16

Number of levels of reinforcement above load cell

(1 level QO.4m)

6

4

2

0

6

4

2

0

6

4

2

Level 14 Number oflevels of

x reinforcementebove loadcell(1 level =0.4m)

+1: (23.9.81)

42 1

0 0.2 0.4 0,6 0.8 1.0

-A Level 15

~

(239.81) 14

● I 1 1

0 0.2 0.4 0.6 0.8 1.0

Level 16

/

15 (23.9.81)

6

4

2

0

0 0.2 0.4 0.6 0.8 1.0

Level 17

A

o 0.2 0.4 0.6 0.8 1.0

(Distance from facing panel/length of reinforcing strap)

(a) Strapsattached to Panel W

6

‘Al,

Level 10

4 9 (23981)

42

2

0

0 0.2 0.4 0.6 0.8 1.0

6 —Level 11

2

0

6

4

2

0

4

2

0

4

0 0.2 0.4 0.6 0.8- 1.0

0 0.2 0.4 0.6 0.8 1.0

Level 13

‘~ , ,

12 (23.9.81)

531

0 0.2 0.4 0,6 0.8 1.0

(Distance from facing panel/length of reinforcing strap)

(b) Straps atteched to Panel 86

Fig. 20 Distribution of tension along reinforcements

16

6

4

2

0

a

6

4

2

r Panel 61 and 915A,l,5m

,- . i .* *----1-

-- .+---- .-.. - A.

F“___ /-;--

I1 --- 4------ ti~

.1I

‘ 3A,l.5m

- --, 5A, F●

●

‘4,\

\

~ Panels 112 and 132

L.........................0.

. . . . .

.“O...*.“ -----

..”. -“----- . **--- --

.“ ----.* *---

. . ...0. --- 3,1.5m-------- .-- _._.. . . . . . . . . . . . *. ”,.@ ~-

. . .-- z-------- - “ ------ -- _<-- ---*

---e.d-~.4, F

.4.-_. —.- .—.

7,F

6,F

2,1.5m

“4, .

-+0

/+---- -

---- \\ -- 12, F

10 v Panel 56 Reinforcement level, see F ig.6

/\\

a

/

60 Distance of cell from rear

of facing panel(m)F - Immediately behind panel

..d#.*:’~***.*. . . . .

4.-

.-. . . . . . 17,1.5m\

● - 14,3.Om

. . . . . ...***.

-z =------ --------- -

15,5.Omo

.-c 20 ~o ,g:ZDmc 10

E;? o I 111 1.11,1. UIIIL I1111IiNote: Pile sections not equal length

> ,= .= ,z au I I I

Date’~\ \o\%’ ~\

\T\g\%%

\~\,o\~’ ,9\T\0T

%%

~~\ \\ ~\\~\

Fig. 21 Variation of tension with time

17

either zero output or a sudden cha”nge in output.As parts of the cables were buried below thewater table and other parts were in contact withthe hard, angular limestone backfill, somefailures were inevitable.

5.6 MEASUREMENT OFTEMPERATURE

Copper-constantan thermocouples were used torecord the temperature at various points withinthe backfill; their positions are shown in Figure6.

As expected, the diurnal and seasonal variationsof temperature within the reinforced earth zonereduced with distance from an exposed surface.The records of temperature taken at varioustimes indicated that the mean temperature of thebackfill was approximately 12°C.

5.7 DATA ACQUISITION SYSTEM

The output from the load cells andthermocouples was recorded and processedautomatically by a mini-computer system locatedin the instrumentation cabin.

The interval between sets of readings was variedaccording to the construction activity.

Fluctuations in the mains supply voltage causedthe system to fail at least once a week. Althoughthis was not a serious problem during construc-tion when the site was continuously manned, itresulted in gaps in the records after the wall wascompleted. To reduce the number and durationof the aaDs, a standbv batterv svstem was

installe-d in June 1982.

6 DISCUSSION

6.1 SETTLEMENT OF WALL

The data presented in Figure 7 show that the rateof settlement was significantly affected by thepiling operations, particularly by the driving ofthe raking piles in the row closest to the abut-ment on 15 October 1981. Some of the settlementmay have been due to the densification ofthe coarse-grained subsoils induced byvibrations generated during pile driving but asmentioned previously little movement appearedto have occurred within the backfill itself.

The data presented in Figure 8 show that theabutment was subjected to significant differentialsettlement. The mean angular distortion (~),defined as the ratio of the differential settlement(5) between two points at a distance (1) apart,

18

was approximately 1/200 in July 1986: the maxi-mum angular distortion at this time wasapproximately 1/1 10. This maximum value iswell in excess of the limits quoted by Polshinand Tokar (1957), Sowers (1962), Skempton andMacDonald (1956) to avoid structural damage totraditional brick and reinforced concrete walls.Smith (1986), however, reported a reinforcedearth bridge abutment at Burton-on-Trent hadsustained a maximum distortion of 1/120 withoutshowing any signs of structural distress.

Data collected in the three years following theopening of the bypass indicated that the settle-ment of the subsoils was virtually completed byJuly 1986.

6.2 VERTICAL ALIGNMENT OF WALL

Because the datum rods were disturbed duringconstruction, it was difficult to assess the effectsof the various construction activities on thealignment of the wall. However the alignment ofthe panels was affected by the positioning oftemporary props. The variation in attitude of thelower panels, as shown in Figure 9, probablyreflects the variation in the distribution andefficiency of the props, see Plate 5. The maxi-mum deviation from the vertical at any oneprofile was approximately 45 mm; however thishas not unduly affected the appearance of theabutment, (Plate 9). In situations where the finalappearance is of patiicular concern the wallcould be provided with a stone facing. Plate 10shows such a solution at the bridge over theAfon Tywi. Cheaper alternatives, such asbattering back the face or providing an exposedaggregate finish to the panels could also beconsidered.

Although some changes in the width of the gapsbetween adjacent panels was noted duringpiling operations, the relative alignment of thepanels was not affected. No significant changesin alignment of the panels were noted betweenDecember 1981, just after the wall was com-pleted, and July 1986, three years after theopening of the bypass to traffic.

6.3 STRAINS WITHIN BACKFILL

The strains measured by the inductive coils wereto a large extent determined by the techniqueused to install them and little confidence can beplaced in the data obtained from this site.

Some of the problems of installation were due,in part, to the requirement to obtain the maxi-mum amount of data from the least number ofinstruments. In hindsight it would have beeneasier though more costly to install pairs of coilsto record either vertical or horizontal move-

Neg. no. CR453/a414

Plate 9 View of mmpleted West Abutment of rail bridge

ments, rather than combinations of coils ar-ranged to petiorm both functions.

6.4 EARTH PRESSURES

6.4.1 Performance of the pressure cell.As noted in Section 5.4, and shown in Figures 15and 18, many cells recorded sharp reductions inpressure towards the end of construction andalso during the winter of 1981/2: a number ofthese cells did not show any significant recoveryof pressure.

The pressures recorded by the cells were affectedby the manner of operation; excessive or pro-longed inflation of the diaphragm significantlyaffected the gauge factor. Carder and Krawczyk(1975), using a medium grained sand, found thatthe gauge factor reduced from 1.33 for thenormal method of discontinuous inflation, to0.66 when the cell was continuously inflatedduring loading. This effect was confirmed in aseries of laboratory trials using the backfill fromthe Carmatihen site. In these trials the cells werepressurised to provide five times their normalrate of flow for one minute whilst they weresubjected to relatively small (5 kN/m2) appliednormal stresses, the cells were then calibrated inthe normal manner. The mean gauge factor offour cells after over-inflation was approximately1.0; this can be compared to a value of 2.35obtained using the normal method of inflationthroughout calibration (see Figure 12).

.. ,— .#— 1

.. -., ..4

. ,.4, -. >, . -., .*. -

4 m - ‘-. ~“,---

. ... = ~c

~

,, .> ;;(y;;. ,’ .. *.<:. - ,*’ -- ? ;::’”., - ~ *:%*—_________ ~_a. _.._ -__.. ,_ ———-–-— , ~-. -—-A _“

Neg.no. CR4531a4/1 o

Plate 10 View of abutment of bridge overthe Afon Tywi

Excessive or repeated deflection of the dia-phragm may enable the backfill to arch over thebody of the cell or provide a preferential path forthe gas to return at lower pressures than themean. Arching of the soil around the body of thecell may have been induced by ground move-ments within the backfill, eg settlement or move-ment of the facing panels. It is pertinent to notethat the reductions in pressure occurred beforethe start of piling operations and that most of thecells installed towards the top of the abutmentdid not record any sharp reductions in pressure.Given these reservations, it is quite possible thatthe relatively low earth pressures recorded bythe cells may not have accurately reflected theactual stresses within the backfill.

6.4.2 Vertical pressure.The trapezoidal distribution of pressure shownin Figure 14 is consistent with a backwardrotation of the reinforced eatih block. In view ofthe difference in the depth of soft subsoilbeneath the block and the adjacent embankment,this rotation would not be unexpected.

However, the data given in Figure 14 indicatethat the mean ratio of measured vertical pressureto overburden pressure across the reinforcedearth zone was less than unity. Therefore either(i) the calibration used to correct the gaugepressure readings was incorrectly assessed, or(ii) a significant propotiion of the dead-weight ofthe backfill was transferred through friction tothe facing panels. It was noted in section 5.4.2

19

that the laboratory and in situ calibration datawere in reasonable agreement. Frictional forcesacting on the back of the facing panels mayreduce the vertical stress acting in a zoneimmediately behind the wall. However themagnitude of the reduction and extent of thezone of influence suggested by the data shownin Figure 14 are both greater than could bereasonably expected. For example, the datagiven in Figure 14(i) suggest that at a depth of fillof 0.4 metres the zone of influence was 4 metreswide. Moreover, the vertical frictional forcewhich would need to be transferred to the backof the wall at this depth demands a coefficient ofinterface friction some 60 per cent greater thanthe equivalent value for the backfill alone.

The distribution of vertical pressure given by theuppermost set of cells is shown in Figure 22. Thedata presented in Figure 22(a) indicate that themean corrected pressure was only some 60 to 70per cent of the nominal overburden pressure;the pattern of distribution shown in this figure isdifferent to that shown in Figure 14. Figure 22(b)shows that the recorded increase in verticalpressure due to the placing of the bankseat andbridge decking was approximately 30 kN/m2.However, calculations based upon the dead-weight of the components indicate that theincrease should have been approximately 90kN/m2. Even with an allowance of 35 per cent forovercorrection of the gauge readings, the differ-ence between corrected and estimated verticalpressures was significant. This suggests thatsome of the bankseat load was transferred byfrictional forces to the facing panels and also tothe backfill at the rear of the bankseat. Itindicates that the pattern of loading beneathbankseats may not be as simple as assumed indesign.

Clearly, interpretation of the data from thepressure cells is not straightforward. In additionto the doubts surrounding the gauge factor ofthe cells, the presence of the sheets of Terrainmay have significantly affected the distributionof internal stresses within the backfill.

6.4.3 Relation between horizontal and verticalpressures.The stress conditions within the backfill can beassessed by comparing the measured horizontaland vertical pressures, but the limitations of thepressure cell measurements just discussed mustbe borne in mind. The magnitude of the ratio ofthe measured horizontal to measured verticalpressures, designated kpc where pc stands forpressure cell, varied widely for each pair of cellsduring the construction period. Values of kpCaregiven in Table 2.

Although the data were scattered, the sharpchanges in horizontal and vertical pressures,

c.~1.2,? a

I ,=

i: :/u—.- —

$8 0.4>$Uogn 0.2 Approx. 1 m of backfill over cellsUm

Distance from rear of facing panel (m)

(a) Before construction of bankseat

40

30

20

L\ Before opening of bypass

tv\mm~; 1 year after opening30%2 of bvpass to traffic

;~10

- !.

=E o 1 2 3 4 5

Distance from rear of facing panel (m)

(b) Following mmplation of bridge

Fig. 22 Distribution of vertical stress beneath bankseat

shown in Figures 14 and 17, occurred at thesame time and the distribution of pressurewithin the backfill showed similar trends.

The values of kpc given in Table 2 can becompared to the values of the coefficients of at-rest pressure, k~, and active pressure, k., qivenin Table 3 where

kO= (1 –sin O’) .k,= (1 –sin O’)/(1

where 0’ is againbackfill

--

(4)+sin O’’)::::::::::::::::(5)

the angle of friction of the

Less than half of the nineteen pairs of cells gavevalues for kpcthat consistently fell within therange of k. and k, values for angles of friction ofbetween 45° and 60°.

The coefficient of earth pressure at the back ofthe wall can similarly be assessed by consideringthe data obtained from the pressure cellsattached to the wall. But here interpretation is

20

TABLE 2

Values of kpc for the backfill

Position of cells Ratio of measured horizontal to measured vertical pressures

Height Distanceabove from rear

footing of wall Nov July Nov July Nov JulProfile (m) (m) at9t81 I 2t9/a~ I 7t9t81 I 9al 19a2 1982 I 9a3 I 9a3 1984

2.03.03.03.03.03.0

Centre4.64.64.64.67.57.57.57.5

19mfrom

1.75

centre1.75

line 3.0

of3.0

wail 3.0

0.10.11.02.254.0a.o0.11.02.254.00.11.02.254.0

0.11.50.11.53.0

0.430.240.330.560.420.34

—

—

0.141.00.620.600.61

0.540.530.290.25o.2a0.190.590.680.900.77

0.171.70.550.240.09

0.650.240.310.110.540.93o.al0.350.740.26

—

0.203.30.570.150.30

TABLE 3

Values of kOand k.

0’ (degrees) 40 45 50 55 60

kO 0.36 0.29 0.23 0.18 0.13k. 0.22 0.17 0.13 0.10 0.07

further complicated by the uncertainty of thedistribution of vertical pressure within thereinforced earth zone (see section 5.4.3). There-fore values of the mean lateral pressure havebeen normalized with respect to the nom;na/overburden pressure and these values are givenin Table 4.

However, values of the coefficient of earthpressure implied by the distributions ofmeasured vertical pressure shown in Figures 14and 22(a) are larger than the values given inTable 4. These distributions suggest that thevalues in Table 4 should be multiplied by factorsof approximately 5 for the lower panels and byapproximately 1.35 for the upper panels. As theactual coefficient of earth pressure would beexpected to lie between these factored valuesand the unfactored values the data indicate that;

(i) on completion of the structure, pressurescorresponding to k. or below acted on thelower rows of panels,

o.ao0.250.140.170.151.90.660.421.20.170.240.420.910.40

1.01.80.460.040.15

0.320.070.240.230.172.1o.a70.231.30.160.170.220.360.25

0.550.720.56o.oao.2a

0.220.210.300.190.211.60.690.161.10.110.170.240.340.07

0.160.690.560.110.30

0.460.240.280.190.201.50.930.071.10.070.120.23o.3a0.05

0.320.57o.3a0.100.25

0.32 0.42o.la 0.190.21 0.140.16 0.120.20 0.191.6 l.ao.aa 0.810.05 0.041.0 1.00.08 0.070.12 0.100.21 0.210.33 0.360.05 0.05

0.29 0.310.69 0.660.42 0.540.10 0.050.24 0.24

(ii) pressures corresponding to kOor aboveacted at various stages on the upper rows ofpanels.

6.5 STRAP TENSIONS

Estimates of the coefficient of earth pressurehave also been derived from the tensionsmeasured by the load cells installed immediatelybehind the wall, viz,

Fv.klc= T~. N . . . . . . . . . . . . . . . . . . . . . . . . . . . ...(6)

where F. is the normal vertical thrust, calculatedfrom the area of the panel and nom;na/, ieunfactored, overburden pressure acting at themid-height of the panel.

k“ is the estimated coefficient of earth pressure,where Ic stands for load cell

T~ is the mean measured tension, (where poss-ible, the mean has been determined from themean values of tension for each level ofreinforcement)

and N is the number of reinforcements connectedto the panel.

Values of k“ are given in Table 5. The data forpanels 56, 61 and 86 suggest that the lateral

21

TABLE 4

Values of kpc at the back of the wall

Panel No ofNo cells Mean lateral pressure/nominal overburden pressure

on panel(see Fia 6)

Date 819181 12/9/81 15/9/81 1/10/81 Ill 1/81 19/1 1/81 18/12/81 1711182 1714182 2519182 July 1983–July 1984

56 5 0.06 0.03 0.02 0.0257 1 0.09 0.05 0.02 0.0159 1 0.08 0.05 0.02 0.0361 3 0.13 0.03 ‘ 0.04* 0.06*

86 4 0.18 0.10 0.05 0.05 0.0187 1 0.16 0.15 0.03 0.02 0.0188 1 0.09 0.04 0.01 0.0189 1 0.90 >1 0.02 0.70 0.2290 1 0.08 0.03 0.03 0.0191 3 0.08 0.12 0.18 0.18* 0.15

112 3 0.16 0.21 0.20 0.2’5 0.16 0.32114 1 0.36 0.30 0.32 0.42 0.08 0.36117 1 >1 0.74 0.66 0.31 0.19 0.67

132 4 0.14 0.16 0.06 0.06 0.06 0.22 0.09133 1 0.15 0.10 0.18 0.16 0.16 0.05134 1 0.23 0.17 0.06 0.04 0.14 0.04135 1 0.25 0.19 0.05 0.04 0.21 0.08136 1 0.19 0.16 0.11 0.12 0.31 0.24

0.010.010.010.05*

0.020.020.01>1’0.050.26*

0.210.42

●

0.140.050.050.090.30

*Variable data

TABLE 5

Values of k’c

PanelNumber Coefficient of earth pressure on back of wall, derived from load cell data

(see April JulyFig 6) 8/9/81 9/9/81 10/9/81 11/9/81 12/9/81 12/9/81 15/9/81 23/9/81 1/10/81 1/1 1/81 19/1 1/81 19/1 2/81 17/1/82 19/2/82 1982 1982

560.30 0.28 0.27 0.24 0.15 0.12 0.17(8) (8) (8) (8) (8) (8) (8)

610.16 0.19 0.21 0,19 0.19 0.11 0.22(1) (1) (1) (1) (1) (1) (1)

860.25 0.24 0.18(6) (6) (6)

112

132

0.21 0.26 0.30 0.27 0.27 0.28 0.28(7) (5) (3) (3) (3) (3) (3)

0.22 0.22 0.22 0.22 0.18 0.18 0.21(1) (1) (1) (1) (1) (1) (1)

0,15 0,18 0.18 0.18 0.33 0.36 0.38(6) (5) (5) (5) (4) (3) (3)

0.17 0,19 0.38 0.46 0.48 0.52 0.47(3) (3) (3) (3) (2) (2) (2)

0.12 0,08(6) (5)

(n) = Number of load cells used to calculate values’of k“

pressure behind these panels varied from activeto at-rest conditions with an angle of friction ofapproximately 46°. The data obtained from panel112 suggest that the pressures were greater thanthe at-rest condition. The limited amount of dataavailable for panel 132 could be taken to indicatethat either active pressures for an angle offriction of approximately 53°, or at-rest pressuresfor an angle of friction of greater than 60°, actedon the back of this panel.

As in previous sections, the values given inTable 5 could be factored to take account of thedistribution of measured vertical pressureshown in Figures 14 and 22(a). However, very

few of these factored values would fall within therange of values of k given in Table 3.

It is important to note that the tensions measuredby the cells installed immediately behind thewall did not vary uniformly with the depth ofburial nor were they uniform on one particularpanel. In addition the ratio of the number ofinstrumented straps to the total number ofstraps attached to one panel was low: forexample, only one-quarter of the 32 strapsattached to panel 56 carried load cells.

As shown in Figure 20 the maximum tensions inthe straps were recorded at positions between

22

0.15 and 0.4 times their length from the back ofthe facing panels. The maximum tensions wereup to six times the value recorded by the cellsinstalled immediately behind the facing panels.The maximum tension recorded by any of theload cells was 9 kN, which was comfotiablybelow the maximum allowable design value forthe Paraweb straps of 12.5 kN. Generally, asshown in Figure 20, the maximum tensionrecorded in the straps was about 5 kN.

6.6 LATERAL PRESSURES ACTING ONTHE BACK OF THE WALL

It is difficult to reconcile the difference betweenthe data derived from the pressure cells and theload cells given in Tables 4 and 5 respectively.However, both sets of data show some similarvariations of pressure with time. For example,significant reductions in lateral pressure wererecorded on 12 September 1981 by the pressurecells and the load cells installed behind facingpanel 56. Around this time the temporary propsto the panel were being rearranged. Althoughthe load cells recorded a recovery in tension,only the upper pair of pressure cells recorded asimilar increase in pressure. Moreover, althoughthe pressures recorded by this pair of cellsreduced significantly in December 1981 the loadcells only recorded a slight reduction in tension.It is possible that the pressure cells may not haveaccurately recorded the actual earth pressureswhen ground movements occurred. Thus, thedistribution of pressures shown in Figure 14 mayreflect movements within the backfill rather thanvariations in earth pressure.

The data suggest that conditions close to kOprevailed on the lower panels until the temporaryprops were removed or rearranged. Someoutward movement of the panel was accom-panied by a reduction in pressure, to approx-imately k. conditions. Subsequently, the lateralpressures increased again — perhaps toconditions close to kO. The data obtained fromthe upper rows of panels indicate pressuresequal to or greater than kOconditions.

6.7 TEMPERATURES WITHINBACKFILL

The damping effect of the backfill and facingpanel on diurnal and seasonal changes intemperature will vary from structure to structureas will the micro-climate. However, it wouldseem adequate to determine the properties of‘plastic’ reinforcements at around 2WC, ie the

standard temperature adopted for mostlaboratory tests.

7 CONCLUSIONS

1) In July 1986, some three Vears after the

2)

3)

4)

5)

6)

7)

opening of the bypass the”differential settle-ment between the centre and ends of theabutment was approximately 160 mm andthe maximum angular distoflion wasapproximately 1/1 10. It is almost certain thatunder these conditions a conventionalreinforced concrete wall would have sufferedstructural damage.There is evidence to suggest that the totalsettlement and the rate of settlement of theabutment were increased bv piling oper-ations. The settlement of the subsoils wasvirtually completed within three years of theopening of the bypass to traffic.The alignment of the facing panels wasaffected by the provision of temporary propsand the installation of adjacent panels. Nosignificant changes in vertical alignmenthave been noted since the wall was com-pleted, ie the alignment of the panels wasnot significantly affected by the installationof the bankseat and bridge decking or byopening the bypass to traffic.The data obtained from the pressure cellswere not entirely consistent. The measuredvertical pressure in the backfill showed atrapezoidal distribution with a maximum atthe back of the reinforced earth zone.However, onlv a few of the values of theratio of the measured horizontal to themeasured vertical pressures fell within theanticipated range of earth pressure coef-ficients. The accuracy of the pressurereadings may have been adversely affectedby movements within the backfill that per-mitted arching to occur.The data obtained from the load cells on theParaweb straps indicated that the lateralpressures acting on the back of the lower

rows of facing panels varied between at-rest

and active conditions. Some of the data

obtained from the upper rows of panels

indicated that the lateral pressures were

greater than those corresponding to at-rest

conditions.

The maximum tensions in the straps wererecorded at positions between 0.15 and 0.4times their length from the back of the facingpanels. The maximum tension recorded wascomfortably below the maximum value of12.5 kN allowed in design for the Parawebstraps.Measurements made in the three year periodfollowing the opening of the bypass indicatethat no significant changes had occurred tothe reinforced earth structure over thatperiod.

23

8 ACKNOWLEDGEMENTS

The work described in this repoti forms part ofthe research programme of the Ground Engin-eering Division (Division Head: Dr M P O’Reilly)of the Structures Group, TRRL.

The TRRL research team consisted of Dr R TMurray, the late Mr M J Irwin, Mr P Darley andthe author. A large proportion of the site workwas undertaken by Mr K. Drysdale and Mr TDenton of Messrs Sandberg Ltd.

The experiment could not have been undertakenwithout the help of the staff of the ConsultingEngineers — Howard Humphreys and Partners— and thanks are due to Mr J Clunas, Mr MChampion and Mr P Bloomfield.

Assistance on site was also given by the staff ofSoil Structures Ltd, Dyfed County council, BritishRail and Sir Alfred McAlpine Ltd — the maincontractor for the scheme. Thanks are also dueto Michael Barclay Partnership and the WelshOffice — Transport and Highways Group.

9 REFERENCES

AGREMENT BOARD (1979). Websol System ofSoil Reinforcement: Roads and Bridges Certifi-cate No 79/18. The Agrement Board, Garston.

BISHOP, A W and SKEMPTON, A W (1950).Measurement of shear strength of soil. Geo-technique, London, 1950, 4, No 2.

BISON INSTRUMENTS (undated). Instructionmanual for soil strain gauge model 4101A. BisonInstruments Incorporated, Minneapolis.

BRADY, K C, AWCOCK, I and WIGHTMAN, N R(1984). A comparison of shear strength measure-ments using two sizes of shearbox. Departmentof the Environment, Depaflment of TranspotiTRRL Laboratory Report 1105, Crowthorne.

CARDER, D R and KRAWCZYK, J V (1975).Performance of cells designed to measure soilpressure on earth retaining structures. Depati-ment of the Environment, TRRL LaboratoryReport 689, Crowthorne.

DEPARTMENT OF TRANSPORT (1978). TechnicalMemorandum (Bridges) BE 3f18, Reinforced EarthRetaining Walls and Bridge Abutments for Embank-ments. Department of Transport.

FOSTER, S A and COOMBER, D B (1979). Siteinvestigation at Carmarthen. GKN Ke//er Foun-dations Report No S4039.

NEWBERY, J (1975). Carmarthen Bypass siteinvestigation; interpretative repoti. HowardHumphreys & Sons. Ref 2.038.0.

POLSHIN, D E and TOKAR, R A (1957). Maximumallowable non-uniform settlement of structures.Proc. 4th Int. Conf. Soil. Mech. & Found. Engng.,Vol. 1, 402.

ROWE, P W(1969). The relation betweentheshearstrength of sand in triaxial compression, planestrain and direct shear. Geotechnique, London,1969, 19, No. 1.

SKEMPTON, A W and MacDONALD, D H (1956).Allowable settlement of buildings, Proc. Instn. Civ.Engrs., Part Ill, Vol 5, 727–768.

SMITH, R J H (1986). Reinforced and anchored earth.Ground Engineering, Vol 19 Number 7. London.

SOWERS, G F (1962). Shallow Foundations.Foundation Engineering, G A Leonards (Ed),McGraw-Hill, New York.

STO~, M Fand RODGER, I R (1975). Preliminary siteinvestigation Carmarthen By-Pass. Hoist SoilEngineering Ltd, Ref S1 1716/F 2956.

THREADGOLD, Land DALEY, J (1971 ). Report on thesite investigation for the London-Fishguard TrunkRoad, Carmarthen Southern By-Pass. CementationReport No. 7999/L T.

BRITISH STANDARDS INSTITUTION (1975).Methcds of test for soils for Civil Engineeringpurposes. British Standard 6S1377: 1975. BritishStandards Institution, London.

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