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Behaviour during construction of a stabilisedbase embedded wall at Coventry
Prepared for Quality Services (Civil Engineering),
Highways Agency
T Hayward (University of Southampton), D R Carder (TRL), W Powrieand D J Richards (University of Southampton), and K J Barker (TRL)
TRL REPORT 446
TRANSPORT RESEARCH LABORATORY
First Published 2000ISSN 0968-4107Copyright Transport Research Laboratory 2000.
This report has been produced by the Transport ResearchLaboratory, under/as part of a Contract placed by theHighways Agency. Any views expressed are not necessarilythose of the Agency.
TRL is committed to optimising energy efficiency, reducingwaste and promoting recycling and re-use. In support of theseenvironmental goals, this report has been printed on recycledpaper, comprising 100% post-consumer waste, manufacturedusing a TCF (totally chlorine free) process.
The Transport Research Laboratory and TRL are trading names of TRL Limited,a member of the Transport Research Foundation Group of CompaniesTRL Limited. Registered in England, Number 3142272Registered Offices: Old Wokingham Road, Crowthorne, Berkshire, RG45 6AU.
CONTENTS
Page
Executive Summary 1
1 Introduction 3
2 Site location 3
3 Ground conditions 3
3.1 Geology 3
3.2 Ground water 3
3.3 Geotechnical parameters 4
4 Construction sequence 4
5 Instrumentation 4
5.1 Wall movements 4
5.2 Temporary prop loads 9
5.3 Wall and base bending moments 9
5.4 Vertical stresses beneath the stabilising base 9
6 Discussion of results 9
6.1 Wall movements 9
6.2 Temporary prop loads 11
6.3 Wall and base bending moments 13
6.4 Vertical stresses beneath the stabilising base 14
7 Simple assessment of wall and base bending moments 18
8 Finite element analysis 19
8.1 Parameters used in the analysis 19
8.2 Comparison of measurements with predictions 19
9 Conclusions 21
10 Acknowledgements 24
11 References 24
Abstract 26
Related publications 26
iii
iv
1
Executive Summary
For roads located below existing ground level increasinguse is being made of embedded retaining walls with astabilising base. This form of construction is generallymore economical than either an unpropped wall of deeperembedment or, in the case of a wide road cutting, theinstallation of a continuous prop at formation levelbetween the opposing walls. The stabilising base isessentially a stub prop at formation level that extends onlya short distance from the wall. Minimal design guidance isavailable for this particular class of wall: thus high qualitycase studies have an essential role to play in enhancing ourunderstanding of the way in which these structures behaveand hence in improving the methods of analysis andassumptions used in design.
Field monitoring has been carried out to establish thebehaviour of a contiguous bored pile retaining wall with astabilising base founded in Bromsgrove Sandstone duringand after its construction as part of the Coventry North-South Road scheme in Warwickshire. Wall movements,wall and base bending moments, loads in the temporaryprops, and vertical pressures beneath the permanentstabilising base were monitored. In this report, the groundconditions, construction sequence and instrumentation aredescribed and the results of monitoring over a two yearperiod in service are discussed.
A finite element back-analysis of performance is alsoincluded together with outline design recommendations forfuture structures of the same type.
2
3
1 Introduction
For roads located below existing ground level increasinguse is being made of embedded retaining walls with astabilising base. This form of construction is generallymore economical than either an unpropped wall of deeperembedment or, in the case of a wide road cutting, theinstallation of a continuous prop at formation levelbetween the opposing walls. The stabilising base isessentially a stub prop at formation level that extends onlya short distance from the wall. Minimal design guidance isavailable for this particular class of wall: thus high qualitycase studies have an essential role to play in enhancing ourunderstanding of the way in which these structures behaveand hence in improving the methods of analysis andassumptions used in design.
A contiguous bored pile embedded retaining wall, with astabilising base has been constructed as part of Phase II ofthe Coventry North-South Road in Warwickshire. Phase IIof the Coventry North-South Road is the second of threeschemes to link the M6 to the A45. The ground conditionsat the site comprise the Bromsgrove Sandstone formationoverlain by made ground and glacial till.
Extensive field instrumentation was used to monitor thebehaviour of a section of the bored pile retaining wallduring and subsequent to construction. At the instrumentedsection, the wall was formed from 14m deep by 1mdiameter contiguous bored piles, spaced at 1.1m centres,with a retained height of 7.8m. The centre of the stabilisingbase was located 8.3m below the top of the retaining walland the base extended 5m from the pile face under the newcarriageway. This report describes the field observationsmade during construction of the retaining wall and overthe first two years in service.
The case study forms part of a wider research programmeinto the behaviour of embedded retaining walls beingcarried out by the Transport Research Laboratory on behalfof the Highways Agency. This study is a sequel to an earlierstudy of the long term performance of a bored pile wall witha stabilising base founded in stiff overconsolidated clay(Carder et al, 1999; Powrie et al, 1999).
2 Site location
Phase II of the Coventry North-South Road projectcomprised the construction of approximately 2km of dualcarriageway east of Coventry City Centre. The routepasses through a residential area and generally follows theline of a disused railway cutting. The existing cutting waswidened to construct the new carriageway. This wasaccomplished by installation of contiguous bored pileretaining walls having a total length of approximately1.8km. The instrumented section of wall, bay W69, lies onthe west side of the cutting just north of the Caludon Roadfootbridge (Figure 1).
N
Bay E71MonitoredBay W69
RetainingWalls
Caludon Road
Stepney RoadSwan Lane
North-SouthRoad
No5
Figure 1 Location of instrumented section
3 Ground conditions
3.1 Geology
Exploration Associates carried out site investigations onbehalf of the Coventry City Council in 1989, 1993 and1994. The geology at the instrumented section was obtainedfrom borehole records during these site investigations,together with field observations during the construction ofthe road (Figure 2). The top 1.5m of the cutting at theinstrumented section was made ground, below this theBromsgrove Sandstone formation was encountered.
The Bromsgrove Sandstone formation consists of crossbedded sandstone units interbedded with mudstone units.The sandstone units are grey to buff in colour, with agypsum or calcite cement. The mudstone units are generally2m to 3m thick, and of a dark red-brown colour. Theformation exhibits cyclic sedimentation, with the sandstoneoften passing upward into mudstone. Weathering in thecutting occurs mainly along the discontinuities, with thediscontinuities typically infilled with around 10mm of sandor clay. The strata at the top of the cutting are frequentlyhighly weathered. The lower strata are typically slightly tomoderately weathered. Davies and Barton (1998) givefurther details of the site geology.
3.2 Ground water
The uppermost sandstone unit that exists at theinstrumented section of the retaining wall contains alocalised perched water table, the level of which variesseasonally. Below final carriageway level there is someevidence of a water table in the lower aquifer. Owing to alack of piezometric data its exact level is unclear, butprobably lies between 84.5 and 82mAOD.
4
3.3 Geotechnical parameters
The results from weak rock self-boring pressuremeter andhigh pressure dilatometer tests undertaken at the site as partof the 1993 site investigation are given in Figure 3. Theshear stiffness parameters presented in Figure 3 weredetermined from the data given in the site investigationreport, using the methods of Gibson and Anderson (1961).Typically, the unload/reload shear moduli, G
r, were
measured over increments of 0.05% to 0.8% cavity strain,with the total cavity strains not exceeding 3%. Figure 3bindicates a trend of increasing stiffness with depth althoughthere is a wide scatter that is possibly due to the valueshaving been obtained over different cavity strain increments.
The in situ horizontal stresses indicated by thepressuremeter lift-off pressures are shown in Figure 3c:these indicate that the in situ earth pressures coefficients,K
o, within the Bromsgrove formation are generally in the
range 1 to 2. A range of values is usual for weatheredweak rock profiles, as the stiffer materials in the profiletend to attract higher stresses.
The geotechnical properties determined from the mainsite investigation are summarised in Table 1. Furtherdetails are given by Hope et al (1998).
4 Construction sequence
Prior to the installation of the piled retaining wall, therailway cutting was backfilled to provide a platform for thepiling rig. Piles were then installed in a sequence in whichalternate piles were constructed shortly after the concrete ofthe piles on either side had cured. During boring, the top 2mof each pile shaft was temporarily supported by a casing. Asteel reinforcing cage was then lowered into each pile shaftand the concrete poured. Following pile installation, aconcrete capping beam was cast on top of the wall.
Bulk excavation of the backfill was then carried out instages to about 6.3m depth in front of the wall along its
entire length, leaving behind temporary earth berms ofdecreasing cross section (Table 2). Temporary tubular steelprops were then installed at approximately 5m centres inthe instrumented section and pre-loaded to 1300kN. Theremaining earth berm was then excavated. Final trimmingof the excavation to base formation level took placeimmediately before stabilising base construction. Prior tocasting the base, starter bars in the bored piles wereconnected to the steel reinforcement in the base to providea moment connection between the retaining wall and thestabilising base. Following construction of the permanentstabilising base, the temporary steel props were removed.This was followed by the construction of the non-structuralbrickwork facing to the wall and the road pavement (Plate 1).The road was opened to traffic in December 1997.
The principal stages of retaining wall construction at theinstrumented section (bay W69) are summarised in Table 2.
5 Instrumentation
Instrumentation was installed to monitor wall movements,wall and base bending moments, temporary prop loads andvertical pressures beneath the stabilising base. Figures 4and 5 show a cross section and plan of the instrumentlocations while Table 3 gives the dates of installation ofthe various instruments.
5.1 Wall movements
Wall movements were monitored by three independentsystems.
l A tape extensometer system was used to measure thecrest level movements at the instrumented bay W69.Measurement lines were extended from an eyebolt (S2)fixed into the capping beam to survey stations S3, S4and S5 (Figure 5). Each of the survey stations consistedof a concrete block founded about 0.3m below ground
Level(mAOD)
Description
94.7 93.2 Made ground
93.2 90.5 Highly weatheredBromsgrove sandstone
90.5 87.4 Highly moderatelyweathered Bromsgrovemudstone
87.4 84.2 Moderately weatheredBromsgrove sandstone
84.2 Moderately slightlyweathered Bromsgrovemudstone
94.77 m.AOD
86.93 m.AOD
80.93 m.AOD
Perchedwater table
Water tableof loweraquifer
Figure 2 Soil profile at the instrumented section (Bay W69)
5
Table 1 Summary of geotechnical properties
Moisture Bulkcontent density c
uc' φ'
Description Weathering (%) (Mg/m3) (kPa) (kPa) (degrees)
Red brown clay with occasional Highly to 25 1.91 37 na namudstone lithorelicts. completely weathered
Red brown clay, interbedded Moderately to 15 2.12 209 na nabands of mudstone. highly weathered
Red brown very weak to moderately Slightly to 9.7 2.11 na 5 34strong mudstone. moderately weathered
Buff fine to medium and Highly to completelycoarse sand. weathered na na na 8 34
Buff sand, weakly cemented Moderately toin zones with bands of very highly weatheredweak sandstone. na 2.01 na na na
Buff very weak to strong Slightly tosandstone. moderately weathered na na na na na
0
2
4
6
8
10
12
14
0 50 100 150 200
Initial shear modulus
Gi (MPa)
Dep
th (
m)
0
2
4
6
8
10
12
14
0 200 400 600
Unload/reload shear modulus Gr
(MPa)
Dep
th (
m)
Sandstone(loop 1)
Sandstone(loop 2)
Sandstone(loop 3)
Mudstone(loop 1)
Mudstone(loop 2)
Mudstone(loop 3)
0
2
4
6
8
10
12
14
0 200 400 600
In situ horizontal stress
(kPa)
Dep
th (
m)
Ko=1 Ko=2
Sandstone
Mudstone
Figure 3 Soil properties determined from weak rock self-boring pressuremeter and high pressure dilatometer tests
6
Table 2 Main stages of construction at the instrumented section (Bay W69)
Stage Description Schematic Date Day
I Installation of contiguous pile wall 03/02/96 —
II First stage excavation 14/05/96 0
III Second stage excavation 24/09/96 133
IV Third stage excavation 16/10/96 155
V Temporary props installed and pre-loaded to 1300 kN 22/11/96 192
VI Earth berm excavated 25/11/96 195
VII Excavation to stabilising base blinding level/start of 11/12/96 211stabilising base construction
VIII Temporary props removed 28/02/97 290
IX Road opened to traffic 12/12/97 577
1m5m
4m1m
5m4m
1m4.5m
5m
1.5m2m
4.8m
7
Plate 1 View showing the completed structure
Capping beam
Survey station (S)
Inclinometer (I)
Strain gauge (VS, VE, VT)
Pressure cell (VP)
Stabilising base
Bored pile
VP1VP2VP3
VE20
VE8
VE6
VE4
VE18
Temporary prop
I3S1
VT2VT1
I1
VS1
VS3
VS2,4 Temporary prop P1
S2S3
80.93 mAOD
VE13
VE15
Bay W69 (pile no 3)
94.77 mAOD
86.93 mAOD
Bay E71
VE9
VE7
VE5
VE3
VE11
VE1
VE19
VE17
Figure 4 Cross section through field instrumentation
8
S4
S3
Bay W69Bay W70 Bay W68
Pile 3
Pile 5
S2
I2 I1
NP1
NP3
NP2
VP1
VP3
VP2
Temporary prop P2 Temporary prop P1
S5
Survey station (S)
Inclinometer (I)
Pressure cell (VP, NP)
Figure 5 Plan view of field instrumentation
Table 3 Installation of instruments
Vibrating wire Surface movementstrain gauges Inclinometers stations Pressure cells
Dates Nos. Location Nos. Location Nos. Location Nos. Location
Feb 1996 VE1-40 Bored pile I1-3 Bored – – – –pile
May 1996 – – – – S1-5 Retained – –ground
Nov 1996 VS1-8 Temporary – – – – – –prop
Dec 1996 – – – – – – VP1-3 Beneath& NP1-3 stabilising
baseFeb 1997 VT1-12 Stabilising – – – – – –
base
9
level, fitted with a stainless steel socket designed toaccept pillars to which the tensioned tape was attached.Readings were corrected for the effects of temperatureon the steel tape. The accuracy of the measured wallcrest movements was estimated to be within ±0.5mm.
l Three 13m long inclinometer tubes were installed in theretaining walls, two in the instrumented bay W69 andone in the opposite bay E71. Horizontal displacementprofiles of the walls were then determined from theresults of inclinometer surveys carried out at suitabletime intervals. The reproducibility of the measurementswas found to be better than ±1mm.
l A Geomensor electronic distance measuring system wasused to monitor the change in span at crest levelbetween the instrumented bay W69 and the opposite bayE71. A tribrach to accept the Geomensor system wasfixed onto the capping beam of bay W69 and amachined socket for a target reflector into the capping ofbay E71. The accuracy of the span measurements atcrest level was considered to be ±0.2mm.
5.2 Temporary prop loads
Axial loads and temperatures in the two temporary propsin the instrumented bay were measured using surfacemounted vibrating wire strain gauges with thermistors,located at quarter points around the circumference and 3mfrom the end of the steel prop. A data logger was used toobtain a continuous record of the strain gauge output. Theaxial load (P) was calculated using the following equation.
P = A.Eave .
where εav is the average strain from all four gauges, A is
the net cross-sectional area of the prop which is equal to0.0192m2, and E is the Young’s Modulus of steel.
5.3 Wall and base bending moments
Two profiles of twenty vibrating wire embedment straingauges, incorporating thermistors, were cast into theretaining wall. Two separate profiles of six gauges wereinstalled in the stabilising base. Gauges were placed inpairs at intervals of depth as shown in Figure 4 to enablebending moments to be determined. The data logger wasused to obtain a continuous record of the strain gaugeoutput. Standard engineering beam theory was used toconvert the longitudinal strains å
1 and å
2 measured by the
vibrating wire strain gauges near the back and front of thewall and at the same depth into bending moment (M).
M =EI
y
e e1 2
2
-b g
where E is the Young’s Modulus of concrete, I is the secondmoment of area and y is the distance from the gauge to theneutral axis. Flexural rigidities (EI) of 1.4×106 kNm2 per pileand 2.2×106 kNm2 per metre run of base were calculatedusing an E of 26×106 kN/m2 for concrete which assumes thatno cracking occurs at the small strain levels involved.
5.4 Vertical stresses beneath the stabilising base
Six pressure cells were installed beneath the stabilisingbase, in two rows of three as shown in Figure 5. All cellswere fluid filled with three of the cells having a pneumaticpressure transducer (NP) and three having a vibrating wiretransducer (VP). Prior to casting the blinding concrete forthe stabilising base, each cell was placed in a carefullytrimmed pocket in the ground and surrounded with mortarbedding. The vibrating wire cells were monitoredcontinuously using the data logger, while readings for thepneumatic cells were taken manually at suitable intervals.
6 Discussion of results
In this section the results of the measurements arepresented, both for the main stages of construction and anineteen month period after the road was opened.
6.1 Wall movements
Figure 6 shows the change in span at crest level betweenthe monitored bay W69 and the opposite bay E71 asdetermined from the Geomensor measurements. Figure 6also includes data on the span calculated from theinclinometer surveys assuming base fixity of the wall.During the main construction period close agreement wasobtained between the two techniques indicating that theassumption of base fixity was realistic. The possibleexception to this is the reading on day 379 where a smalldiscrepancy was apparent between the Geomensor andinclinometer measurements. However readings could notbe repeated at a later date because the line of sight for theGeomensor was lost after day 379 due to the constructionof a boundary wall. The construction of this wall and otheroperations on the capping beam, particularly in the regionof inclinometer tube I3, may also have accounted for thevariations between inclinometer results after day 379.These operations eventually led to tube I3 being damagedbeyond repair on day 633.
Lateral movements between the crest of the retainingwall and the tape extensometer stations are shown inFigure 7, together with data from inclinometer I1(assuming base fixity) and the semi-span of the underpassmeasured by Geomensor. Movements determined bytaping have been calculated assuming no absolutemovement at stations S4 and S5, approximately 12m awayfrom the wall, as taping was not possible beyond thisdistance. During the initial period of monitoring, theextensometer eyebolt in the capping beam was obstructedand had to be re-installed at a new location. A new datumreading on the replacement eyebolt was established on day155 and corrected to the Geomensor reading. A similarprocedure was carried out on day 469 when the eyebolthad to be moved from the capping to the parapet whentopsoil was placed over the retained ground.
Figure 7 generally shows good agreement between themovements of the crest of the wall measured using thevarious techniques. However very small differences areapparent between the measurements using the inclinometer
10
-20
-10
0
10
20
0 50 100 150 200 250 300 350 400 450 500 550 600 650
Days
Cha
nge
in s
pan
(mm
)
Geomensor Inclinometer (I1+I3) Inclinometer (I2+I3)
Props pre-loaded+ berm excavated
Excavation to formation level
Stabilising base constructed
Props in Bays 70and 71 removed
Props in Bay 69 removed
Note: Inclinometer readings assume base fixity
InclinometerI3 damaged
Construction operations inprogress on capping beam
Figure 6 Change in span measured between Bays W69 and E71
-20
-10
0
10
20
200 3000 100 400 500 600 700 800 900 1000 1100 1200
Days
Mov
emen
t tow
ards
exc
avat
ion
(mm
)
Geomensor (readings halved) Wall to STN4 (extensometer)
Wall to STN5 (extensometer)Inclinometer I1
Props pre-loaded+ berm excavated
Berms excavatedto blinding level
Stabilising baseconstructed
Props in Bays 70and 71 removed
Props in Bay 69 removed
Note: Inclinometer readings assume base fixity
Access not available forextensometer readings
Figure 7 Change in wall crest movements (Bay W69)
11
and those using the other two techniques when thetemporary props in bay 69 were released on day 290. Thisindicated that an outward movement of the wall toe of nomore than 2mm may have occurred at this time. Theinclinometer results and those available from the tapeextensometer up to day 710 showed no evidence of anyfurther wall toe movement. Inclinometer surveys on tubeI1 also confirm that over nearly two years in service, thereis no indication of any seasonal or long term movements ofthe structure.
The horizontal movement profiles measured using thethree inclinometer tubes are shown in Figure 8 for severalstages during and after construction of the underpass.These movements have been calculated assuming fixity atthe base of the inclinometer tubes and, as discussedpreviously, this assumption is certainly valid up to the timeof temporary prop release. After this stage, it is possiblethat a small outward movement of the toe of the wall of nomore than 2mm occurred. These toe movements have notbeen taken into account in Figure 8. Little or no movementoccurred during the excavation in front of the wall up today 155 (Table 2). More significant movements areapparent by day 196, after the props had been pre-loaded,
with the crest of the wall at both bays W69 and E71 beingpushed about 5mm into the retained ground. By day 279the movement profiles indicate that the temporary propshad restricted the crest movements but berm excavation,for stabilising base construction, had induced a 2mmoutward movement of the wall at about 6m depth. Onremoval of the props, the wall cantilevered towards theexcavation. By day 632 the crest of the wall was about2mm beyond its original position and most of the shortterm movement due to the construction work had probablytaken place. The results in Figure 8a show that little furtherchange occurred over the following 15 months.
6.2 Temporary prop loads
Continuous records of the axial load and temperatureagainst time are plotted in Figures 9 and 10 for temporaryprops P1 and P2 respectively. Figures 9a and 10ademonstrate the characteristic effects of thermal expansionon the prop loads. These effects are typical of thoseidentified at other sites where steel props have been used(Twine and Roscoe, 1999). Figures 9b and 10b show theaxial loads corrected for the effect of temperature using themethod proposed by Batten et al (1999), ie. so as to show
0
2
4
6
8
10
12
14
-10 -5 0 5
Lateral movementtowards excavation (mm)
Dep
th (
m)
day 155
day 196
day 279
day 315
day 632
day 1091
Bay W69
0
2
4
6
8
10
12
14
-10 -5 0 5
Lateral movementtowards excavation (mm)
Dep
th (
m)
Bay W69
0
2
4
6
8
10
12
14
-10 -5 0 5
Lateral movementtowards excavation (mm)
Dep
th (
m)
Bay E71
Note: Inclinometer readingsassume base fixity
b) Inclinometer I2a) Inclinometer I1 c) Inclinometer I3
Figure 8 Development of lateral movements of the wall
12
0
400
800
1200
185 195 205 215 225 235 245 255 265 275 285 295
Days
Axi
al lo
ad (
kN)
-10
10
4030
Tem
pera
ture
(C
)
Average load
Average temperature
Pre-load in pressure jacks of 1294kN
Prop pre-loaded Prop removed
0
400
800
1200
185 195 205 215 225 235 245 255 265 275 285 295
Days
Axi
al lo
ad (
kN)
-100
10
4030
Tem
pera
ture
(C
)
P2 pre-loaded
Props in Bay 70 pre-loaded
Earth berms excavated
Prop pre-loaded Prop removed
Prop re-loaded prior to removal
a) Axial load
b) Axial load after temperature correction
0
20
20
a) Axial load
b) Axial load after temperature correction
0
400
800
1200
185 195 205 215 225 235 245 255 265 275 285 295
Days
Axi
al lo
ad (
kN)
-10
10
30Average load
Average temperature
Pre-load in pressure jacks of 1216kN
Prop pre-loaded Prop removed
0
400
800
1200
185 195 205 215 225 235 245 255 265 275 285 295
Days
Axi
al lo
ad (
kN)
-10
10
30
Props in Bay 70pre-loaded
Earth berms excavated
Prop re-loadedprior to removal
Prop pre-loaded Prop removed
Tem
pera
ture
(C
)T
empe
ratu
re (
C)
20
40
0
40
20
0
Figure 9 Change in axial load measured in temporary prop P1
Figure 10 Change in axial load measured in temporary prop P2
13
the loading due to construction activities only. Thismethod quantifies the magnitude of the temperaturecorrections by evaluating the relationship between propload and temperature over a period when no constructionactivity was taking place.
Figures 9 and 10 show the changes in prop loads in themonitored bay as the construction sequence progressed.Prop P1 was the first prop in the monitored bay to be pre-loaded. When adjoining prop P2 was pre-loaded, the axialload in prop P1 fell as some of its load was redistributedonto prop P2. Similarly changes in the loads on the props inthe monitored bay also occurred when the props in theadjoining bay were pre-loaded. On excavation of the soilberms in front of the retaining wall, a very slight increase inaxial loads in the props was observed (Figures 9b and 10b).Subsequent construction events did not cause any noticeablechange in the magnitude of the measured prop loads,although as expected loads dropped to zero on prop release.
6.3 Wall and base bending moments
Continuous records of bending strain, and hence bendingmoment, were obtained from the vibrating wire gauges inthe wall. Typical time plots for gauges at about 3m intervalsof depth are shown in Figure 11. In all cases a sharp change
in values occurred when the temporary props were pre-loaded and subsequently released. These changes areparticularly noticeable in Figures 11a and 11b at depthsabove base formation level. Below formation level (Figure11c), the changes are not so pronounced. Little change inbending moment was recorded after the end of constructionup until monitoring was terminated on day 1157.
The profiles of wall bending moment with depthdetermined at key stages of the construction sequence areshown in Figure 12. Prior to the pre-loading of thetemporary props on day 192, monitored bending momentsin the wall were very small. After pre-loading the temporaryprops, the bending moment profile was typical of a proppedcantilever, with a maximum bending moment of around215kNm/m. Excavation of the berm induced an outwardwall movement at about 6m depth, increasing the maximumbending moment at about this level to 280kNm/m. Onreleasing the temporary props, the rotation of the walltowards the excavation generated a bearing pressure on theunderside of the stabilising base, which imparted a restoringmoment to the retaining wall with a bending momentdeveloping in the slab. The results in Figure 12 demonstratethat the maximum bending moment over the retained part ofthe wall for design purposes is likely to occur afterexcavation to full depth beneath the temporary props.
a) At 3.73m below top of wall
b) At 6.33m below top of wall
Ben
ding
mom
ent
(kN
m/m
)B
endi
ng m
omen
t(k
Nm
/m)
c) At 9.43m below top of wall
Ben
ding
mom
ent
(kN
m/m
)
-400
-200
0
200
100 200 300 400 500 600 700 800 900 1000 1100 1200Days
Propspre-loaded
Props in Bay 69 removed
-500
-300
-100
100
100 200 300 400 500 600 700 800 900 1000 1100 1200Days
Propspre-loaded
Props in Bay 69 removed
-100
100
300
500
0 100 200 300 400 500 600 700 800 900 1000 1100 1200Days
Props in Bay 69 removedPropspre-loaded
pile 3 pile 5
0
0
Figure 11 Development of wall bending moments at different depths
14
Figure 13 shows continuous records with time of thebending moments developed in the stabilising base at twolocations, ie. in front of piles 3 and 5. These confirm that onreleasing the temporary props, bending moments wereimmediately developed in the stabilising base as the outwardrotation of the wall was resisted. As would be anticipated thelargest bending moment of about 100kNm/m was measuredusing the strain gauges closest to the wall (Figure 13c) withthe moment reducing to about 40kNm/m at 4.5m away(Figure 13a). A reduction in the bending moment in the basewith time was initially recorded although, by about day 500,readings had begun to stabilise.
6.4 Vertical stresses beneath the stabilising base
Figure 14 shows the development of vertical pressure asmeasured by the pressure cells beneath the stabilising base.Figure 14a gives the readings which were taken manuallyon cells with pneumatic transducers beneath the base infront of pile 5. Figure 14b shows the continuously loggeddata from the cells with vibrating wire transducers in frontof pile 3. In both cases, a significant increase in thepressures was recorded when the temporary props wereremoved. Also shown in the figures are the average
temperature variations at both locations. The pressuresshow some thermal fluctuation as the cells are fluid-filledand therefore are temperature susceptible. As nosignificant thermal variations in bending moments wererecorded in the stabilising base, it was concluded that thethermal dependence of the cells was the sole cause of thefluctuations shown in Figure 14. General trends in pressurechanges can be assessed however by comparing values attimes when the temperature was the same.
The results in Figure 15 show the variation of verticalstress with distance from the wall for three dates when thecell temperatures were near identical. These dates wereshortly after temporary prop removal (day 335), about oneyear later (day 714) and a further year later (day 1091).Generally the vertical stresses beneath the base increasewith distance from the wall in front of pile 5, although thistrend is less evident in front of pile 3. A comparison of thevarious stress distributions given in Figure 15 indicates aslight reduction in vertical stress with time.
0
2
4
6
8
10
12
14
-200 0 200 400
Bending moment (kNm/m)
Dep
th (
m)
day 155
day 196
day 212
day 279
day 315
day 632
day 1157
Props pre-loaded
Props removed
Stabilising base level
0
2
4
6
8
10
12
14
-200 0 200 400
Bending moment (kNm/m)
Dep
th (
m)
Props pre-loaded
Stabilising base level
Props removed
a) Pile 3 b) Pile 5
Readings between6 and 8m depthunavailable forday 279
+ ve moment iscompression of theexcavated faceof the wall
Figure 12 Development of wall bending moments (Bay W69)
15
a) At 4.5m from wall face
b) At 3.2m from wall face
Ben
ding
mom
ent (
kNm
/m)
Ben
ding
mom
ent (
kNm
/m)
c) At 1.8m from wall face
Ben
ding
mom
ent (
kNm
/m)
Adjacent to pile 3 Adjacent to pile 5
-50
0
50
100
150
275 475 675 875 1075 1275
Days
-100
-50
0
50
100
150
275 475 675 875 1075 1275
Days
Temporary props removed
-100
-50
0
50
100
150
275 475 675 875 1075 1275
Days
Temporary props removed
Temporary props removed
Figure 13 Change in bending moment in the stabilising base
16
Tem
pera
ture
(C
)
Ver
tical
str
ess
(kP
a)
Tem
pera
ture
(C
)
a) Stresses beneath the base adjacent to pile 5
b) Stresses beneath the base adjacent to pile 3
0
50
100
150
200
250
300
275 475 675 875 1075 1275Time (days)
0
5
10
15
20
4.5m from wall face (NP1)3.2m from wall face (NP2)
1.8m from wall face (NP3)average temp
Propsremoved
Roadopened
Ver
tical
str
ess
(kP
a)
0
50
100
150
200
250
300
275 475 675 875 1075 1275
Time (days)
0
5
10
15
20
1.8m from wall face (VP3)3.2m from wall face (VP2)
4.5m from wall face (VP1)average temp
Propsremoved
Roadopened
Figure 14 Vertical stresses beneath the stabilising base
17
Distance from wall (m)32
a) Shortly after temporary prop removal (day 335)
3.50
50
150
100
3.52.5 4 4.5
Adjacent to pile 5 (NP)
Adjacent to pile 3 (VP)
Ver
tical
str
ess
(kP
a)
Distance from wall (m)32
b) About 1 year later (day 714)
3.50
50
150
100
3.52.5 4 4.5
Ver
tical
str
ess
(kP
a)
Distance from wall (m)32
c) About 2 years later (day 1091)
3.50
50
150
100
3.52.5 4 4.5
Ver
tical
str
ess
(kP
a)
Figure 15 Variation of vertical stress beneath the stabilising base with distance from the wall
18
7 Simple assessment of wall and basebending moments
A simple assessment of wall and base bending momentswas carried out to give a benchmark on which to assesswhether the measured values were of the right order ofmagnitude. In a clay soil this assessment would normallybe performed using a limit equilibrium analysis andfactored soil strength parameters: at this site, where thewall is founded in and retains weak rock, this approachwas considered inappropriate.
In situ horizontal pressures measured by pressuremeter(Section 3.3) indicated that the earth pressure coefficient,K
o, within the Bromsgrove formation was generally in the
range 1 to 2. Given that some stress relief will occurduring piling and excavation in front of the wall, an upperbound K-value of unity might be anticipated on theretained side of the wall after these operations. On thisbasis, and using the mean measured temporary prop loadof 160kN per metre run of the wall, bending momentswere calculated over the retained height of the wall andthese are shown in Figure 16a. Also shown in Figure 16a isthe measured profile of wall bending moment at this stagereproduced from Figure 12. The maximum values of the
measured and calculated moments were similar, althoughthe depth to the maximum measured moment wasnoticeably more.
A similar prediction of bending moment was attemptedafter the temporary props were removed and this is shownin Figure 16b. At this stage, little correlation between thecalculated and measured moments was obtained using thissimplistic approach. This was because the calculationstook no account of the stress history in so far as thestabilising base was cast with the temporary props in placeand this influenced the development of bending momentthereafter. For this reason it was considered necessary tomore accurately model the construction sequence andstress history using finite elements and these results arereported in Section 8.
A crude evaluation of design values for the verticalstresses beneath and hence the bending moments in thestabilising base can also be carried out following a similarprocedure. For this purpose the benefit of the embedded partof the wall was ignored so that the remaining structure, thatis the exposed part of the wall face and the base, can beregarded as L-shaped. Assuming the perturbing moment isgenerated by a K-value of unity in the retained ground, therestoring moment developed by the stabilising base can then
Distance from wall (m)
-400 4002000-2008
6
4
2
0
Ben
ding
mom
ent (
kNm
/m)
Distance from wall (m)
-400 4002000-2008
6
4
2
0
Ben
ding
mom
ent (
kNm
/m)
a) Temporary prop in place a) Temporary prop removed
Measured (pile 3)
Predicted
800600
Figure 16 Measured and predicted wall bending moments after bulk excavation
19
be determined. On the basis of this idealised situation, threepossible distributions of stress on the underside of thestabilising base,all of which provide the required restoringmoment are indicated in Table 4.
The plane strain finite element analysis was carried outusing the CRISP package to model the weak rock as anelastic perfectly plastic material obeying the Mohr-Coulomb failure criteria. The mesh used in the analysiscomprised 272 linear strain quadrilateral elements with thetemporary prop being modelled using a bar element. Themesh was constructed to have smaller elementsconcentrated around the wall, where the most significantchanges in stress in the ground due to construction wereexpected to occur. The effectiveness of the mesh designwas investigated by doing one analysis with double thenumber of elements and no significant differences in theresults were observed.
8.1 Parameters used in the analysis
Although the sensitivity of the analyses to the groundparameters was investigated in more depth by Hayward(2000), for the purpose of this report the parameters usedin the design (Mott MacDonald, 1994) were adopted.These parameters are summarised in Table 5.
Table 4 Estimated vertical stress distributions beneaththe stabilising base
Bearing pressure beneath stabilising base (kPa)
Stress distribution Close to wall Remote from wall
Triangular 0 233Rectangular 156 156Trapezoidal 93 187
It must be noted that because a K-value of unity in theretained ground has been assumed, the bearing pressuresgiven in Table 4 are likely to be an upper bound for designpurposes provided that the mechanism of failure is that offorward rotation about a point near to the wall toe. Othermechanisms of failure, that is forward or backwardrotation about the stabilising base, are technically possible:however, Carder et al (1999) established that based onlimit equilibrium calculations, these are unlikely. Thepressures estimated in Table 4 are well below theallowable bearing pressure for sandstones given in Table 1of BS8004. Although the Bromsgrove Sandstoneformation beneath the stabilising base was moderatelyweathered, the effect of this and of discontinuities wasconsidered unlikely to result in any significant reduction inthe engineering performance of the strata.
A comparison of the estimated stresses from Table 4with the measured values is given in Figure 17a. Thebending moments in the stabilising base derived fromthese estimated stresses are compared with the measuredvalues in Figure 17b. The calculated distributions ofbending moment in the base proved to be relativelyinsensitive to the distribution of vertical stress beneath it.As anticipated, the measured bending moments at day 350were much less than those calculated because the latterassume a K-value of unity in the retained ground andignore the influence of the embedded part of the wall.
8 Finite element analysis
At the test section, the Bromsgrove Sandstone was bothweathered and discontinuous, and it was important that thepotential influence of the discontinuities (primarily jointsand bedding planes) was considered when developing anumerical model for the retaining wall. Details ofdiscontinuity spacing and orientation at a location just northof the instrumented area were obtained, which indicated thatthe dip direction of the bedding planes was such that thesediscontinuities were unlikely to dominate the behaviour ofthe wall. It was therefore concluded that a continuumapproach using the finite element method would besatisfactory for the purpose of back-analysis. Discreteelement analyses were separately carried out by Hayward(2000) which verified that the continuum approach wasrealistic for the engineering geology at this site.
Table 5 Design parameters for the weak rock
Weathered rock
ModeratelyMade Highly to slightly
Parameter ground weathered weathered
Strength c'=0, φ'=30o c'=0, φ'=35o c'=0, φ'=35o
Elastic modulus E'=10 E'=30 at top of E'=100 at top of(MN/m2) stratum + 5 per stratum + 20 per
metre depth metre depth
The results given in Section 3.3 indicate that the in situearth pressures coefficients, K
o, within the Bromsgrove
formation are generally in the range 1 to 2. Given thatsome stress relief will occur during pile installation andthat the wall is ‘wished in place’ for the purpose of theanalysis, in situ lateral stresses consistent with a K-value ofunity were adopted as appropriate.
The retaining wall was modelled as a linear elasticmaterial 1m thick with an elastic modulus of 15×106kN/m2
which gave the same flexural stiffness as the contiguouspiles forming the wall. The stiffness of the stabilising basewas taken as 26×106kN/m2 and a Poisson’s ratio of 0.15was used for both the stabilising base and the wall.
The construction sequence employed on site and givenin Table 2 was carefully modelled, although the averagemeasured pre-load in the temporary props of 160kN/m wasused in the analysis.
8.2 Comparison of measurements with predictions
The calculated and measured lateral movements of the wallare compared in Figures 18a and 18b. With the possibleexception that the analysis indicated a small outwardmovement of the wall during third stage excavation (stageIV in Table 2), generally the correlation between measuredand calculated movements was remarkably good. Thistended to indicate that the soil stiffnesses derived at the
20
Trapezoidal
Rectangular Mean measured (day 335)
50
21 54 5
Distance from wall (m)
Ver
tical
str
ess
(kP
a)
30
100
150
200
250
Triangular
a) Vertical stress beneath stabilising base
500
21 54 5
Distance from wall (m)
Ben
ding
mom
ent (
kNm
/m)
30
1000
1500
2000
b) Bending moments in stabilising base
Figure 17 Measured and predicted performance of the stabilising base
21
a) Finite element b) Inclinometer measurements I1
0
2
4
6
8
10
12
14
-10 -5 0 5
Lateral movement towards excavation (mm)
Dep
th (
m)
Third stageexcavation(Stage IV)
Earth bermexcavated(Stage VI)
Excavationat stabilisingbase level(Stage VII)Propsremoved(Stage VIII)
0
2
4
6
8
10
12
14
-10 -5 0 5
Lateral movement towards excavation (mm)
Dep
th (
m)
day 155
day 196
day 279
day 632
Bay W69
Figure 18 Comparison of measured and predicted lateral movements of the wall
design stage and given in Table 5 were a reasonableassessment. It is also worth noting that the analysis showeda lateral movement of the wall toe of about 1mm. Thevalues shown in Figure 18b were measured by inclinometerand assume base fixity of the tube, although results fromtape extensometer and Geomensor measurements hadtentatively suggested that a toe movement of no more than2mm may have actually occurred.
Agreement was also good between the calculated andmeasured bending moment profiles which are shown inFigure 19. Generally calculated moments (restoring) wereslightly higher than those measured whilst the temporaryprops were in place. On removal of the temporary propsthe overall shape and magnitude of the calculated momentswere very close to those measured. These resultsdemonstrate the importance of predicting behaviour usinga numerical technique (such as finite elements) where theconstruction sequence is accurately modelled, simpleassessment procedures such as those given in Section 7 areof value but need to be treated with caution.
The measured and calculated performance of thestabilising base are shown in Figure 20. Figure 20acompares the calculated vertical stress distribution beneaththe base with the mean of the measured values at locationsadjacent to pile 3 and pile 5. The calculated and measuredvalues were similar in magnitude although there was somescatter in the measured values. The calculated bendingmoments developed in the base are compared with thosemeasured in Figure 20b: generally the latter values werebelow those obtained from the analysis.
9 Conclusions
Field monitoring has been carried out to establish thebehaviour of a contiguous bored pile retaining wall with astabilising base founded in weak rock. Measurements duringconstruction of the wall and its first 2 years in service as partof the Coventry North-South Road in Warwickshire arereported. The following conclusions were reached.
22
a) Finite element b) Pile 3 measurements
0
2
4
6
8
10
12
14
-400 -200 0 200
Bending moment (kNm/m)
Dep
th (
m)
Third stageexcavation(Stage IV)
Earth bermexcavated(Stage VI)
Excavationat base level(Stage VII)
Propsremoved
(Stage VIII)
0
2
4
6
8
10
12
14
-400 -200 0 200
Bending moment (kNm/m)
Dep
th (
m)
day 155
day 196
day 279
day 632
Figure 19 Comparison of measured and predicted wall bending moments
23
100
2.5 4.53.5
Mean measured (day 632)
Mean measured (day 335)
40
1.5 2 54 5
Ver
tical
str
ess
(kP
a)
3
160
80
100
120
Finite element (Stage VIII)
a) Vertical stress beneath stabilising base
-10021
5
4 5
Distance from wall (m)
Ben
ding
mom
ent (
kNm
/m)
3
0
200
300
400
b) Bending moments in stabilising base
20
Figure 20 Measured and predicted performance of the stabilising base
24
i Pre-loading of the temporary props, used for supportduring excavation of the soil berm in front of the wall,pushed the top of the wall about 5mm into the retainedBromsgrove Sandstone formation. After finalexcavation and construction of the stabilising base thetemporary props were removed and the wallcantilevered towards the excavation until its top wasabout 2mm beyond its original position. During the laterstages of construction, it is possible that a small outwardmovement of the toe, and hence a translation of the wall,of no more than 2mm occurred. Little further movementoccurred during the first two years in service.
ii The axial loads in the temporary steel props varied withtemperature. Correction of the loads for temperaturedependence showed that only small changes in loadfrom the original pre-load values occurred over theconstruction period. During excavation of the soil bermbeneath the temporary props, the props were effective inrestraining lateral movement of the top of the wall.
iii Prior to the pre-loading of the temporary props,measured wall bending moments were very small. Afterpre-loading, the bending moment profile was typical ofa propped cantilever, with a maximum moment ofaround 215kN/m. Excavation of the berm induced anoutward wall movement at about 6m depth, increasingthe maximum bending moment at about this level to280kNm/m. On releasing the temporary props, a bearingpressure was developed on the underside of thestabilising base which imparted a restoring moment tothe wall. On this basis, the maximum bending momentover the retained part of the wall for design purposes islikely to occur after excavation to full depth beneath thetemporary props.
iv Measurements of vertical pressures beneath and bendingmoments in the stabilising base confirmed theirimmediate development on release of the temporaryprops. As would be anticipated the largest bendingmoment of about 100kNm/m was measured closest tothe wall with the moment reducing to 40kNm/m at 4.5maway. Generally the vertical stresses beneath the baseincreased with distance from the wall in front of pile 5,although this trend was less evident in front of pile 3. Aslight reduction in stress beneath the stabilising baseoccurred over the first 2 years in service.
v A crude assessment of the bending moments developedin the structural members demonstrated that they couldonly be realistically determined at all stages of theconstruction using numerical methods which accuratelymodelled the construction sequence and stress history.At this site, it was decided from details of discontinuityspacing and their orientation in the weatheredBromsgrove Sandstone, that a continuum approachusing the finite element method would be satisfactoryfor the purpose of back-analysis. Reasonable agreementbetween the analytical results and measurements wasobtained. In some instances and for other weak rocks, adiscrete element analysis may be a more appropriatedesign tool.
10 Acknowledgements
The work described in this study forms part of the researchprogramme of the Structures Department of the TRL. Thestudy was carried out in collaboration with the Universityof Southampton. The project received funding fromQuality Services (Civil Engineering) of the HighwaysAgency and the Engineering and Physical SciencesResearch Council. The authors gratefully acknowledge theassistance of Mr P Darley, Mr M D Ryley, Mr G H Alderman(TRL), Dr M Barton, Mr H Skinner and Miss T Davies(Southampton University).
Thanks are due to the City of Coventry Council forpermission to undertake this study. The advice of Mr NVincent and the site staff of Babtie Group (formerlyBroadgate Consultants) is gratefully acknowledged. Thecooperation of Mowlem Construction in providing on siteassistance and Mott MacDonald in providing siteinformation was also much appreciated.
11 References
Batten M, Powrie W, Boorman R Yu H-T and Lieper Q(1999). Use of vibrating wire strain gauges to measureloads in tubular steel props supporting deep retainingwalls. Proc Instn Civ Engrs, Geotechnical Engineering,Vol 137, pp3-13.
British Standards Institution (1986). BS8004: Code ofpractice for foundations. British Standards Institution,London.
Carder D R, Watson G V R, Chandler R J and Powrie W(1999). Long-term performance of an embedded retainingwall with a stabilizing base slab. Proc Instn Civ Engrs,Geotechnical Engineering, Vol 137, pp63-74.
Davies T J and Barton M E (1998). Precise geologicalcharacterisation for the wider application of high qualitysite data. Proc 1st Int Conf on Site Characterisation,Atlanta, USA.
Exploration Associaties LTD (1989). Coventry North-South Road, Phase II. Factual report on preliminary andmain ground investigations. (For City of CoventryCouncil).
Exploration Associates LTD (1993). Coventry North-South Road, Phase II. Supplementary site investigation.(For City of Coventry Council).
Exploration Associates LTD (1994). Coventry North-South Road, Phase II. Interpretative report on groundinvestigations. (For City of Coventry Council).
Gibson R E and Anderson W F (1961). In situmeasurement of soil properties with the pressuremeter.Civ Engng Works Review, Vol 56, pp615-618.
25
Hayward T (2000). The behaviour of retaining wallsembedded in weak rock. PhD Thesis, University ofSouthampton.
Hope V S, Clayton C R I and Sutton J A (1998). The useof seismic geophysics in the characterisation of a weakrock site. Proc 1st Int Conf on Site Characterisation,Atlanta, USA.
Mott Macdonald (1994). Coventry North-South RoadPhase II. Geotechnical investigations interpretive reportfor highway structures. Document No 24318/30/A. (ForCity of Coventry Council).
Powrie W, Chandler R J, Carder D R and Watson G V R(1999). Back-analysis of an embedded retaining wall witha stabilizing base. Proc Instn Civ Engrs, GeotechnicalEngineering, Vol 137, pp75-86.
Sutton J A and Clayton C R I (1998). The use of seismicgeophysics in the characterisation of a weak rock site.Proc 1st Int Conf on Site Characterisation, Atlanta, USA.
Twine D and Roscoe H (1999). Temporary propping ofdeep excavations - guidance on design. CIRIA PublicationC517. Construction Industry Research and InformationAssociation, London.
26
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For further details of these and all other TRL publications, telephone Publication Sales on 01344 770783 or 770784,or visit TRL on the Internet at http://www.trl.co.uk.
Abstract
Instrumentation has been installed to monitor the behaviour of a bored pile retaining wall with a stabilising baseduring and after its construction as part of the Coventry North-South Road scheme in Warwickshire. Wallmovements, wall and base bending moments, loads in the temporary props, and vertical pressures beneath thepermanent stabilising base were monitored. In this report, the ground conditions, construction sequence andinstrumentation are described and the field measurements during construction and its first two years in service arediscussed. The results are compared with a finite element back-analysis and outline recommendations made for thedesign of similar structures in the future.
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