baker et al 2010 on central_saint_giles.pdf

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CENTRAL SAINT GILES – BASEMENT AND FOUNDATIONS DESIGNED FORFUTURE CROSSRAIL TUNNELLINGChris Barker, Marek Niewiarowski and Dinesh Patel, Arup, London, United Kingdom

In London, it is more common to design the foundations and substructure of new

developments adjacent and over existing tunnels. However, designing economicsubstructures for future tunnelling presents a set of unique design challengesresulting in bespoke solutions tailored to the structure, ground and severity of theimposed tunnelling constraints. This paper presents several geotechnicalchallenges in the design of a basement structure that cantilevers out over thesafeguarded Crossrail tunnel corridor and the foundations which need toaccommodate ground movements induced by this future tunnelling. The design andconstruction outcomes for the basement foundation scheme are discussed andcomments are made on construction observations, pile testing and monitoring.

INTRODUCTION

The Central Saint Giles development is located inthe West End of London where future Crossrailtunnels will be constructed. These tunnels will besufficiently close as to impact on the basement andfoundations for this new development.

Crossrail is a major proposed east-west railwayacross London. Since 1990, the route alignmenthas been safeguarded through the local authorityplanning application process to ensure futureconstruction of Crossrail is not prejudiced by anyother development. Crossrail construction isplanned to commence in 2010 with servicesstarting in 2017.

Through central London, fifteen kilometres ofseven metre diameter twin bored tunnel will weaveits way between existing railway and other tunnels,sewers and building foundations. At Central SaintGiles, the eastbound tunnel alignment passesbeneath the southern boundary of the site. Thetunnel crown is about 16.5m below ground levelwith a 4m exclusion zone above and 3.5m eitherside of the tunnel to allow Crossrail flexibility tomove the proposed tunnel during design.

It is mandatory that all new projects within the safeguarded zone must consult Crossrail. At CentralSaint Giles consultation with Crossrail commencedduring concept design stage where tunnel location,geometry, loading and settlement constraints wereadvised by Crossrail for which the design of theCentral Saint Giles development had to address indesign. These constraints had a defining influenceover the foundation and substructure solution asdiscussed below.

CENTRAL SAINT GILES DEVELOPMENT

Central Saint Giles is a striking new landmark inLondon’s West End just east of Centre Point Tower

by architect Renzo Piano. The mixed usedevelopment comprises a fifteen storeyreinforced concrete frame residential building andan eleven storey steel frame office building setaround a central courtyard. The superstructure isclad by dramatic facades of vivid colours red,orange, green and yellow glazed ceramic whichsit on the perimeter basement wall, Figure 1.

Figure 1: Impression of development (Legal &General)

This eye-catching development is supported on asite wide seven metre deep single levelbasement substructure which, with the use of900mm thick full basement height transfer beamsalong the southern frontage, overhangs theproposed eastbound Crossrail tunnel exclusionzone by up to 8.5m, see Figure 2. A local threemetre deep second level basement is located inthe east of the site.

Demolition of the pre-existing building andbackfilling the site was completed by Keltbray Ltd

in late 2007 with Stent Foundations Limitedfollowing on immediately with preliminary pile



Figure 2: Foundation layout with 14m wideCrossrail tunnel exclusion zone

testing and piling which was completed in April2008. Building frames were completed in July2009 and the Stanhope Plc development is due forfinal completion in March 2010.

THE SITE AND GROUND CONDITIONS

The large city site is irregular in shape withmaximum dimensions 280m east-west and 190mnorth-south. Surrounding street levels are+25mOD in the north falling to +23mOD in thesouth east. The previous building on site was a

1950s seven to nine storey S shaped masonryMinistry of Defence (MoD) building with a singlelevel basement founded on shallow footings.

A ground investigation of twenty-one geotechnicaland archaeological trials pits, three cablepercussive with follow on rotary boreholes andtwelve concrete cores at the site encounteredvariable thickness of Made Ground up to 4.5mdeep which included numerous truncated andbackfilled historic basements and foundations pre-dating the MoD building. Underlying this MadeGround was about 2.5m of Terrace Gravels then

only 21m of London Clay. Cable percussiveboreholes where then continued by rotary coring toprove 17.5m of Lambeth Group clays and a thin

6m horizon of Thanet Sands overlying thebedrock Chalk proven to a couple of metresdepth.

Ground water comprises a shallow aquifer some4m below ground level (up to 1.8m above thesurface of London Clay) and a depressed deepaquifer 12m below the top of the chalk with theoverlying London Clay, Lambeth Group(comprising Upper Mottled Clay, LaminatedBeds, Shelly Clay, Lower Mottled Beds andUpnor Formation) and Thanet Sand strata being

underdrained.

SUBSTRUCTURE DESIGN

The proposed eastbound Crossrail tunnelbeneath the southern boundary had a defininginfluence over the foundation and substructuresolution. Crossrail imposed the following designconstraints on the engineers designing the newbuilding:

• outside tunnel diameter of 7m;

• allowance for the advised tunnel alignment tomove up to 3.5m horizontally and 4mvertically;



• loading imposed on the tunnels does notexceed existing overburden plus loading fromexisting development (or plus 50kPa at groundsurface);

• design of foundations for the development toallow for 1.7% face loss from tunnelling; and

• operational noise and vibration values werealso provided.

While a few substructure possibilities wereconsidered at concept design stage, the chosensolution was for a bottom up basement supportedon piles and cantilevering out over the tunnel. Thepiles being designed for future tunnelling inducedground movements and the excavation formed witha load bearing perimeter embedded retaining wallwith temporary propping.

To carry façade column loads up to 5.5MN, theload bearing perimeter embedded wall was1180mm diameter (reducing to 1050mm diameterbelow casing at +15.5mOD) hard-firm rotary boredsecant wall at 980mm centres with between 1.7%and 2.6% steel. At about 10m from the tunnelexclusion zone, it was continued as a proppedtemporary 600mm diameter CFA hard-firm secantwall to allow the cantilever sub-structure to be built.

Figure 3: Cross section through cantileverbasement

The permanent southern boundary basementwall was a 900mm thick cast in situ reinforcedconcrete beam and was cantilevered with eight900mm thick full basement height transfer beamstied back into pile caps, basement and groundfloor slabs as shown in Figure 3. The basement

slab through this zone including pile caps wasunderlain with compressible void former tomaximise the build out of basement settlementduring construction. Structural loads (excludingdowndrag allowance) on southern pile capstypically ranged between 20MN to 35MN with thewestern concrete framed residential building pilecap carrying 110MN.

A prediction of ground movements resulting fromfuture Crossrail tunnelling was made using theOasys computer program, TUNSET. Based onCrossrail’s anticipated face loss of 1.7% from

construction of running tunnels, groundmovements above the tunnel in front of thebasement were predicted to be in the order of30mm to 50mm between ground level and tunnelcrown. Also a 45 degree wedge extending upfrom the tunnel to basement formation leveldefined a zone of predicted tunnelling inducedground settlements greater than 5mm. Thisdemarcated a 6m wide zone from the edge of thetunnel exclusion zone for piles to be designed fordowndrag.

The substructure was founded on 900mm and

1500mm diameter rotary bored bearing pilesfounding in the London Clay and Lambeth Groupclays carrying loads between 1.6MN to 10.1MN.However, to reduce the additional effect ofdowndrag within the 6m wide ‘downdrag zone’,piles were sleeved with a 12mm thick 1560mmOD permanent steel casing coated with 6mm ofBitumen Compound SL and installed and groutedin an over size bore to one metre above tunnelinvert level. Piles within this zone were foundedabout 47m below ground level, a metre into theThanet Sand with the twenty piles under theresidential pile cap also base grouted.

TRIAL BORES

Of the 59 No. 1500mm diameter rotary boredpiles 35 were founded into the Thanet Sand and16 others at various levels within the LambethGroup. It was anticipated that due to the basalLondon Clay and Lambeth Group beinghistorically underdrained for a considerableperiod of time seepages and instability would beminimal. However, it was difficult to assess shaftdegradation in the relative small diameterboreholes during the ground investigation.

Consequently, bentonite support fluid had beenallowed for at tender due to the known instability



in the Thanet Sand and sandy horizons of theLambeth Group and to a lesser extent theremainder of the Lambeth Group and basal sandyLondon Clay horizon.

Also within the tender scope, two trial bores with

CCTV recording were also specified as part of thepreliminary pile testing programme to identify thespecific formations within the bores that exhibiteddegradation during a piling period of up to severalhours. This in turn allowed an assessment to bemade as to what stage in the boring process thesupport fluid would need to be introduced, and towhat level the fluid would need to be retainedwithin the bore.

The trial bores were constructed using a SoilmecR-625 rotary piling rig at 900mm and 1050mmdiameters, which allowed free movement and

rotation of the CCTV camera during subsequentmonitoring and was sufficiently large to observebore degradation in the large diameter piles beingadopted for the site. After installation of temporarycasing into the London Clay, boring withoutsupport fluid was carried out uninterrupted throughthe London Clay and Lambeth Group to the top ofthe Thanet Sand. A record of the materialencountered during excavation was made by theResident Engineer and CCTV monitoringcommenced immediately upon the completion ofboring.

CCTV monitoring of the type more commonly usedin drainage surveys, was carried out over acontinuous period of about 26 hours using a coloursurvey camera lowered into the bore. The camerawas mounted with strong lighting and had 360degree turn and tilt functions which were remotelycontrolled from a monitoring van at the surface.Images were continuously recorded onto DVD.

After completion of boring, the camera waslowered to the base of the excavation, recordingthe location and extent of any seepages ordegradation of the bore surface along the way. A

slow process of travelling up and down the borecontinued until the first major degradation wasobserved in sandy beds near the base of theLower Mottled Beds four hours into monitoringafter which the camera was then left to record thedevelopment of the major bore degradations or‘breakouts’ with ‘up and down’ surveys to observethe development of new breakouts undertakenapproximately every 30 minutes.

Figure 4 summarises the location and extent ofseepages and degradation recorded over themonitoring period for Trial Bore 1. Degradation of

the pile shaft continued steadily with smallfragments of soil ‘raining’ to the base of the pile.

Also significant spalling of the lower LambethGroup occurred over a basal 1.5m height. Moreseepages were evident in the London Clay thanLambeth Group.

Auger boring of the dry Thanet Sand stratum was

then attempted and it was found that collapsewas occurring immediately upon withdrawal ofthe auger with the result that after four augerloads the bore still dipped at the top of theThanet Sand. CCTV footage showedundercutting and slumping beneath the UpnorFormation. Re-drilling resulted in the same effectand it was considered that due to the drymoisture conditions, the low suction in the ThanetSand was overcome by a piston effect of a fullyloaded auger being withdrawn which resulted inbore collapse. While this could have beenovercome by using a vented digging bucket or

double flight auger, the significant spalling of thelower Lambeth Group was already a decidingfactor. This has also been observed by theauthor more recently on the Canary WharfCrossrail Station project, again during trial boringin the Thanet Sand stratum.

Figure 4: Degradation & seepage in trial bore



From the trial bore observations it was concludedthat bentonite was not required to be introducedduring boring through the basal London Clay orLambeth Group strata, but would be introducedinto the pile bore upon reaching the readilyidentifiable flint marker beds in the Upnor

Formation immediately above the Thanet Sand.This enabled considerable programme benefits tobe realised with the number of scheduled bentonitebored piles being reduced from 51 to 35. Thisreduction was achieved by being able to re-calculate a higher pile toe level, above theobserved trial bore pile instabilities, using a highernon-bentonite shaft adhesion alpha value.

TEST PILES

Two preliminary pile tests were carried out in August 2007, approximately one month in advance

of the main contract pile works. Preliminary testpile PTP1 was specified to confirm assumeddesign basis for piles founded entirely in LondonClay. Preliminary test pile PTP2 was specified todemonstrate settlement performance of pile endbearing in the Thanet Sand and to measurebitumen coated shaft adhesion to minimisecalculated downdrag loads. Two working test pileswere carried out in April 2008 during the mainpiling works, as required by the London DistrictSurveyors Association guidance, LDSA (2000) forthe factor of safety adopted.

PTP1 was a 900mm diameter straight shaftedrotary bored pile founded entirely in London Clay.The design philosophy followed the LDSAguidance notes for the design of straight shaftedpiles in London Clay. Table 1 of the LDSAguidance note suggests a FoS of 2.0 and shaftadhesion factor, α = 0.5, for a Maintained Load(ML) test. The pile test was chosen to be a ML testsince these better reflect pile-loading conditionsand are less susceptible to rate effects which instiff clay can lead to high excess pore pressureeffects. The location near to BH3 (see Figure 2)was selected taking into account the proposed

permanent works pile locations, location of groundinvestigation boreholes, basement demolitionworks and archaeological investigations beingundertaken on site. Double sleeved casing wasinstalled to minimise the maximum test load byisolating the pile shaft through the Made Groundand River Terrace Deposits and also to negate therisk of structural failure of the pile head under load.

Pile construction was independently witnessed bythe Resident Engineer. No notable difficulties wereencountered during construction. The pile boreand base were clean and dry, and free of

seepages. Boring and base cleaning of the pilewas uninterrupted and took approximately 1.5

hours. The duration from base cleaning to thestart of pile concreting was approximately 2.5hours. Concreting of the pile was uninterruptedand took approximately 1.25 hours. Concretecube test and sonic logging results weresatisfactory.

PTP1 was instrumented with 4 levels of straingauges, with extensometers fitted near to the pilebase and 0.5 m below the bottom of the doublesleeving. Electronic displacement transducersmeasured movement at the head of the pile.Precise levelling of the top of the pile head wasalso carried out as a check on pile headmovements. The sister bar type strain gaugeswere used to analyse the load shed down thelength of the pile, from which the designparameters could be back-calculated. OneGeokon retrievable multi-point extensometer was

installed in the pile for the duration of loadtesting. The extensometer was divided into twosections through the use of three pneumaticanchor points linked by displacement transducersand variable lengths of steel rod. All of the straingauges appeared to be recording throughout thetest.

PTP1 was tested by a maintained load test in25% increments to 100% DVL, followed by anextended proof load test to DVL + 50% SWL. Itwas then intended to follow this with amaintained load test to 200% DVL. However, the

maximum load achieved in the maintained loadtest was 168% DVL. In order to calculate the loadtransfer occurring along the length of the pile, arelationship between the pile secant modulus andmeasured strain was calculated using thetangent stiffness method described by Fellenius(2001).

The pile behaved in an elastic manner during thefirst loading up to 100% DVL (3100 kN).Settlement at this stage was 3.3mm which iscomfortably within the 4 mm maximum set in thespecification after Patel (1992). On loading up to

125% of DVL the pile continued to behaveelastically, the majority of the load being carriedby shaft friction with little transferred to the base.When 150% DVL was applied the settlement was0.5mm in the first hour. This load was maintainedfor 36 hours during which the settlement declinedslowly but erratically. The settlement rate was0.18mm per hour in the last half hour reaching amaximum settlement of 19.2mm. The pile wasthen unloaded and reloaded to 150% DVL. Thesettlement rate was over 1mm/hr in the first hour.The load was maintained for 14 hours duringwhich the rate diminished, again erratically, to

0.2mm/hr in the last 30 minutes. An attempt wasmade to raise the load to 175% DVL (5425 kN),



but the pile settled 50mm in 20 minutes and theload was not reached.

Back analysis on the basis of the design claystrength profile and an overall factor of safety FoS= 2.0 (as per the LDSA approach for ML tests)

showed that the mobilised shaft adhesion factorpeaked at α = 0.4 at 129% of DVL after this theshaft friction dropped and the load was transferredto the base. This alpha value is lower than thealpha of 0.5 suggested in the LDSA guidancenotes (LDSA, 2000). Possible causes investigatedincluded an error in the test process (calibrations,etc), problems during construction, and the claystrength profile. However, no unusual difficulties ordelays were experienced during construction, andthe clay strength profile was reviewed in detail andwas considered appropriate for the site. A reviewof previous London Clay pile tests showed that the

performance, measured by α, was within the rangeof previous London tests but below the 0.5 valuerecommended by the LDSA. It is noted that theLDSA guidance alpha values are not lower boundvalues and very occasionally alpha values below0.5 are back calculated from preliminary pile tests.For the detailed design of piles founded entirely inthe London Clay it was preferred to use the LDSAparameters for Maintained Load (ML) tests, that isFoS = 2.0, but to reduce the α value from 0.5 to 0.4to account for the low mobilisation of shaft frictionin the PTP1 and the existing design mean claystrength profile of cu = 90 + 8.0z kPa was retained.

Two 900mm diameter working test piles founded inthe London Clay were carried out to giveconfidence in the performance of the pilefoundations and revised design basis. Bothworking test piles were successfully tested to thespecified loads (i.e. up to 100%DVL + 50%SWL)with acceptable settlements of less than 4mm at100%DVL and less than 9mm at 100%DVL +50%SWL. It was concluded that the design methodadopted was sufficient to ensure the margin ofsafety for the piles.

Preliminary Test Pile 2 (PTP2) was specified todemonstrate the performance of a pile constructedthrough the London Clay and Lambeth Group,bearing in the Thanet Sand, constructed underbentonite support fluid. More specifically, the testwas intended to confirm:

• shaft adhesion through the Lambeth Group;

• a minimum end bearing factor in the ThanetSand; and

• back calculation of shaft friction for bitumencoated sleeving.

PTP2 was not base-grouted. A 1200mm diameter

test pile was chosen to provide both a practicalconstruction trial of this unusual pile for

programming and to keep the maximum test loadwithin limits of readily available load frames.

PTP2 was instrumented in a similar fashion toPTP1, in this case with 10 levels of strain gaugesand 3 levels of retrievable multi-point

extensometers, as shown in Figure 5.

Figure 5: Diagrammatic cross sectionarrangement, PTP2

The PTP2 construction sequence can be

summarised as follows:• From ground level install 1860mm diameter

temporary casing into top of London Clay

• Bore 1800mm diameter to the design levelfor friction reduction bitumen coated sleeving

• Install 1260mm diameter bitumen slip coatedpermanent sleeve to base of open bore,having checked pre-applied bitumen coatingfor consistency of coverage

• Check position, level and verticality ofsleeve, and grout annulus between openbore and permanent sleeve

• Bore 1200mm diameter to approx 1m abovetop of Thanet Sand



• Introduce bentonite and continue boring totarget toe depth using a bypass digging bucket

• Clean base of pile using bypass cleaningbucket. Check hardness of base usingweighted tape

• Exchange bentonite in the bore with freshbentonite (not undertaken in works piles)

• Test bentonite and base hardness and installreinforcement cage

• Check base hardness again using weightedtape, and concrete pile by tremmie.

Satisfactory base hardness checks demonstratedthe base to be Grade 2 ‘Firm’ prior to concreting,using the base hardness scale (of 1 to 5) forThanet Sand piles constructed under support fluid.Checks were also made on the minimum 6mmthickness of bitumen slip coating material (prior to

installation of the permanent sleeve); bentonitesupport fluid was sampled and tested for density,viscosity, sand content and pH; concrete cubeswere sampled and tested for strength; and soniclogging was carried out to confirm the integrity ofthe pile.

PTP2 was tested by a maintained load test in 25%increments to 100%DVL, followed by an extendedproof load test to DVL + 50% SWL, followed by amaintained load test to 225% DVL. The test wasceased at 23.7MN (225% of DVL) with a pile headsettlement of 79mm and a rate of settlement within

the specification limit of 0.24mm/hr. As with theback analysis of PTP1, the tangent stiffnessmethod described by Fellenius (2001) was used tocalculate the load transfer occurring along thelength of the pile. The measured load vs pile headsettlement is shown in Figure 6 and summarised inthe table below, along with some other similardeep bitumen sleeved preliminary test pilesundertaken in London.

SitePile

Diameter(mm)

PileLength

(m)

LengthSleeved *

(m)

BaseGrouted

Central SaintGiles 1200 47 7 / 16 No

Moorhouse900 56 26 / 22 Yes

Pinnacle900 64 11 / 40 Yes

King’s CrossCTRL C105

1200 37 0 / 25 Yes

* Pile length sleeved: air void (i.e. double sleeved) length /bitumen coated length.

These central London preliminary test piles wereall founded into the Thanet Sand. While pilediameter and amount of sleeving differs across thesample, Figure 6 illustrates pile performances

achievable with deep large diameter sleeved pilesin London.

0

5

10

15

20

25

0 25 50 75 100 125

Settlement at Pil e Head (mm)

L o a d a t P i l e H e a d ( M N )

Central Saint Giles

Moorhouse

Pinnacle

King's Cross CTRL C105

Figure 6: Test pile load settlementcomparison

As noted by Whitworth et al. (1993), the use ofbitumen applied as a slip coating to driven pileshas been investigated. According to Laybond

(1989), load transmission through BitumenCompound SL caused by moving soil transmittedthrough bitumen coating is a function of:

• Temperature of coating

• Rate of settlement or movement of soil

• Thickness of coating

• Flow characteristics of coating under load

However, published data from bored test pilecase histories of bitumen coated permanent steelcased piles are much less common. Whitworth etal. (1993) reported the results of comparison pile

testing on coated (6mm bitumen) and non-coated12m deep 750mm diameter pile tests at AngelSquare, London. At maximum test load of 325kNat 17mm head displacement (2% pile diameter) astill increasing average bitumen f s value of 11kPacan be determined.

From the Central Saint Giles back analysis ofthree sets of strain gauges the instrumentationinstalled for the bitumen coated sleeving, theresults shown in Figure 7 indicate a rapidlyincreasing initial average sleeve friction whichperhaps peaked at 15kPa then reduced withaverage shaft movement over the length ofsleeve.



0

2

4

6

8

10

12

1416

18

20

0 10 20 30 40 50 60

Displacement (mm)

B i t u m e n s l e e v e f r i c t i o n ( k P

a )

Central Saint Giles (average

pile shaft movement)

Angel Sq, Islington (pile head

displacement)

Figure 7: Bitumen coated sleeve adhesion (f s)

After consideration of the expected range oftunnelling induced ground settlement for thebitumen coating, a design skin friction, fs, of 20kPawas adopted in calculations of downdrag acting onbitumen coated piles.

During the programme of preliminary pile testing itwas observed that the black bitumen slip coatingcompound was found to melt and run during thewarm summer weather (~30°C). However, thiswas not found to be a problem during the main

piling works, which were carried out over the winterperiod. Methods of controlling bitumen run duringhot periods included shielding the bitumen coatingfrom direct sunlight with tarpaulins and sheets, andregular rotation of the sleeves, which were beingstored horizontally on timbers.

With a large proportion of the London Clay beingpotentially mobilised during tunnelling it wasimportant to maximum shaft capacity through theLambeth Group and confirm this by preliminary piletesting. Case history reporting of shaft frictionvalues in the Lambeth Group suffer from significant

variation reflecting the variable nature of thisstratum. Table A3.4 in CIRIA C583 reports fsvalues such as: 84kPa Canary Wharf; 66kPaEuston; 75kPa-113kPa British Library; 185kPaBlackwall Yard and at a recent project at King’sCross 146kPa was back calculated from test piling.

In the Lambeth Group at Central Saint Giles, ashaft adhesion factor of α = 0.32 was backcalculated and adopted for design. Although PTP2was constructed under bentonite support fluid, ashaft adhesion factor of α = 0.4 was again backcalculated for the short section of shaft in the

London Clay. However, it was decided to limit thedesign shaft adhesion in both the London Clay and

Lambeth Group to the maximum verified value of100kPa.

Similarly, in the Thanet Sand the end bearingfactor value of Nq was shown to be continuallyincreasing with load up to the maximum pile test

load, and therefore a limiting value for Nq of 13was considered appropriate for design purposes(no base grouting).

Individual 1500mm diameter Thanet Sand rotarybored working piles were successfullyconstructed over a period of four days. Thesequence of construction for a typical pile atCentral saint Giles is shown in the table below.

Day ActivityTypical

duration(hours):

1Install 1860mm diameter temporary

casing into top of London Clay0.75

Bore 1800mm diameter to the designlevel for friction reduction bitumencoated sleeving

0.75

Install 1560mm diameter bitumen slipcoated permanent sleeve to base ofopen bore

2.002

Grout annulus between open bore andpermanent sleeve (stage 1)

0.50

3Grout annulus between open bore andpermanent sleeve (stage 2)

0.50

Bore 1500mm diameter to bentoniteintroduction level

1.75

Introduce bentonite 1.00

Bore under bentonite and clean pile

base

0.75

Exchange bentoniteNot

required

Install reinforcement cage and concretetremmie

4.00

4

Concreting (approx. 78m3) 4.00

16

RETAINING WALLS

In recent times the demands on commercial andresidential basements at least in London, is forincreased space for mechanical and electricalplant necessary to also meet sustainability andplanning requirements as well as commerciallyoptimising roof space and appearance, thetraditional location for some plant. Hencebasement heights have been on the increase sothat the single level seven metre basementheight at Central Saint Giles to facilitate plantitems and their maintenance is now notuncommon.

With the complexity and associated cost of thecantilevered area of basement, the challengewas to also maximise savings for the remainingperimeter wall. With large basements likeCentral Saint Giles there is less pressure onminimising wall size as there is on small



basements where basement space is a premium.It was therefore more feasible to considerincreasing the secant wall pile diameter andcantilevering the basement wall during constructionin order to benefit from unrestricted access duringbasement and building core construction.

In the temporary condition, retained heights to B1basement formation along the permanent secantwall varied from 5.5m to 8.2m with some sectionsof wall close to pile cap and B2 basementexcavations giving effective retained heights up to8.8m. A review of central London case studiesshowed limited published case histories ofcantilever basements above 6m.

Nevertheless, CIRIA C580 Figure A2.1 indicatescantilever wall deflections in excess of 0.6% of theretained height. A damage assessment was

undertaken to consider surrounding buildings,buried utilities, footpaths and road pavements. Thenearest surrounding buildings were generallyabout 10m from the perimeter wall and with one ortwo levels of basement and therefore aconservative assessment classified them within anacceptable very slight to slight damage categoryaccording to Boscardin and Cording (1989).

A review of surrounding utility services indicatedfavourable conditions. Pressurised utilities whichwere considered more susceptible to damage suchas gas were modern relatively flexible low pressure

2.5” and 5” polyethylene butt welded constructionand the existing mains water pipes were scheduledto be replaced in 2007 with similar MDPE buttwelded pipes as part of the Thames Water mainsrenewal programme prior to construction. Also, the12” concrete and 48” oval masonry sewers wereset back beneath the middle of the surroundingstreets and relatively deep at between four to fivemetres below street level.

The assessment indicated that generally for areasof single level basement cantilever deflectionswould be acceptable because the significant

settlements would be expected to occur closebehind the wall and as part of this development thesurrounding footpaths and Dyott Street wherebeing reinstated.

Therefore the temporary propping scheme for thepermanent secant wall was economised to a singlelevel of horizontal props at capping beam level inthe north east and north west corners of the sitewhere deeper excavations for the B2 basementand residential pile cap approached the secantwall and effective retained heights increased.These props were removed after construction of

the basement slab (acting as base prop tocantilever) to allow less restricted ground floor slab

and building core construction. The southernboundary temporary CFA secant wall having anembedment less than its retained height due tothe Crossrail exclusion zone required berms andraking props with horizontal props set abovecapping beam level at the corners until

construction of both basement and ground floorslabs. The July 2008 progress photo in Figure 8shows an easterly view the basement underconstruction with temporary propping in place.

Figure 8: Site progress photo July 2008 (MikeO'Dwyer)

Instrumentation and monitoring of theconstruction comprised reflective survey targetson surrounding buildings, levelling studs onfootpaths, thirteen inclinometers in selected malesecant piles and reflective targets on the cappingbeam. In addition, precise levelling points forfuture monitoring were set in the basementtransfer beams at the request of Crossrail.

Monitoring of two cantilever inclinometers asshown in Figure 9 gave maximum deflectionsmuch less than case history data indicated.

Maximum total deflections of 16mm and 18mm,being 0.25% and 0.23% of their retained heights,of 6.5m and 7.8m respectively were measured.This includes a ‘creep rate’ of 1.5mm and 2.5mmper month respectively. The duration fromreaching maximum formation level toconstruction of basement slab was about sixmonths and ground floor slab followed aboutthree months later.



Figure 9: Cantilever wall deflection profi les

These cantilever deflections are plotted below withCIRIA C580 and St John et al (1992) data in Figure10 for comparison and to show that significantcantilever heights are possible in urbanenvironments.

0

10

20

30

40

50

60

70

0 2 4 6 8 10 12Maximum excavation depth below groun d level H (m)

M a x i m u m h

o r i z o n t a l w a l l d e f e c t i o n d e l t a ( m m )

Figure 10: Embedded cantilever retaining walldeflections after CIRIA C580 and St John (1992)

CONCLUSIONS

The Central Saint Giles basement has beendesigned and constructed to cantilever out overthe proposed Crossrail alignment and to

accommodate future tunnelling induced groundmovements.

Preliminary instrumented pile testing proved aLondon Clay alpha value of only 0.4 andconfirmed a maximum Lambeth Group adhesionof 100kPa and bitumen friction of 15 kPa.

The perimeter basement wall was alsoinstrumented and monitored during constructionand confirmed maximum cantilever deflection of0.25% H for retained heights up to 7.8m.

REFERENCES

BOSCARDIN, M.D., and CORDING, E.J., 1989.Building Response to excavation inducedsettlements, ASCE Journal of GeotechnicalEngineering, Vol 115, No.1, Jan 1989.

FELLENIUS, B.H., 2001. From Strain

Measurements to Load in an Instrumented Pile,Geotechnical News Magazine, Vol. 19, No. 1,pp.35-38.

GABA, A.R., SIMPSON, B., POWRIE, W.,BEADMAN, D., 2003. Embedded retaining walls – guidance for economic design, CIRIA C580.

HIGHT, D.W., ELLISON, R.A., PAGE, D.P.,2004. Engineering in the Lambeth Group. CIRIAC583, pp 206-210.

LDSA, 2000. Guidance Notes for the Design of

Straight Shafted Piles in London Clay, LondonDistrict Surveyors association, October 2000.

LAYBOND PRODUCTS LTD, 1989 BitumenCompound SL, Slip layer for bearing piles andburied structures, C1/SfB 18, June 1989.

O’RIORDAN, N.J., 1982. The mobilisation ofshaft adhesion down a bored, cast-in-situ pile inthe Woolwich and Reading Beds, GroundEngineering April 1982, pp 17-26.

Patel, D.C., 1992. Interpretation of results of pile

tests in London Clay, Piling Europe, Institution ofCivil Engineers, April 1992.

ST JOHN, H.D., POTTS, D.M., JARDINE, R.J.,HIGGINS, K.G., 1992. Prediction andperformance of ground response due toconstruction of a deep basement at 60 VictoriaEmbankment, Proc Wroth Mem Symp, PredictiveSoil Mechanics, Oxford pp 581-608.

WHITWORTH, L.J., TURNER, A.J., LEE, R.G.,1993. Bitumen slip coated trial piles andprototype undeream trial pile at Angel Square,

Islington, Ground Engineering, January/February1993, pp 28-33.

0.2%H

0.4%H

0.6%HKeyCentral Saint Giles data

+ C580 & St John et al (1992) data

baker et al 2010 on central_saint_giles.pdf

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