Micropile and Low-Mobility Grouting Foundation …

1 Micropile and Low-Mobility Grouting FoundationRemediation for a Coastal Development inBarbadosJ. , Carter, CarvalhoGolder Associates Ltd.,Mississauga,Ontario, BruceGeosystems ,Venetia, Pennsylvania, USAABSTRACTThis case study describes the investigation, design and implementation of Foundation improvements comprisingmicropiles and Grouting to remediate the settlement distress of an existing building founded on very weak, vuggy,uncemented coralline limestone located on the coast of the Caribbean Sea in Barbados. The Foundation remediationultimately included the construction of an 88 m long sub-surface sea-wall , the installation of 174 micropiles (providingdirect and indirect support to the building) and the Grouting of voids and interconnected fissures/fractures in thesubsurface below the building.

2 An in-depth study of the causes of the settlement, a flexible design and close monitoringof the drilling and Grouting during construction were all essential to the success of the project. In addition, contractorprocurement and operation based on the Alliance concept resulted in an excellent consultant-contractor teamrelationship throughout and was the key to the completion of the work withinthetightschedule required by the SUM Cette tude de cas d crit l investigation, la conception et l impl mentation des am liorations apport es aux fondationsd une b tisse existante fond e sur des sols tr s faible, vacuolaires, non-ciment s d origine calcaire sur la cote descara bes au Barbade. Les am liorations apport es pour contrer au tassement comprennent; l installation de microspieux et le fon age des puits par la m thode de r habilitation des fondations inclue la construction d unouvrage longitudinal souterrain de 88 m tres de longueur, l installation de 174 micros pieux (qui permettaient le supportdirect et indirect de la b tisse) et l injection de ciment dans le but de combler les vides, fractures et fissuresinterconnect s situ s sous la b tisse.

3 La r ussite d un tel projet d coule de l tude approfondie des causes dutassement, d un design flexible et d une surveillance en continu des travaux de construction. De plus, l laboration des quipes de travail c est bas e sur un concept de partenariat entre le ma tre d uvre et les sous-traitants, ce qui apermis de r aliser l ouvrage dans un court laps de Sandy Covedevelopment is located on the westcoast of Barbados between Bridgetown and Holetown inthe Parish of St. James. Phase 1of the project includesa six-storey luxury condominium complex, including aonelevelbasement (on the northern half of the building only)and five levels ofabove groundunits, set-backapproximately 15m to 20 m from theedge of a 3 m to5m high coralline cliff bordering the Caribbean Sea to thewest of the approximately 4 m deepgully/drainage channel exists immediately adjacent tothenorth side of the iscomprised of reinforced concrete and concrete block-wallconstruction designed to besupported onshallowstripfootingsfounded on engineered fill and/or the nativecorallinelimestone rockmass.

4 This paperbrieflydescribes theoriginal buildingconstructionandinitialdistress,t hesubsequentinvestigationandmechanismass essment,theremediation design,thecontractor selectionprocess, thefoundation improvements and subsequent performanceofthe Phase 1 Sandy Cove ,involving site grading and excavation for thebasement and foundations,commencedon the northernhalfof the siteinMarch 2005. Between March and May2005, several small caverns and fractures(some clay in-filled)were encountered. As a result offinding thesefeatures, a local geotechnical consultant wasretainedwho performed geophysical surveys at the (GPR)andElectricalResistivity (ER)surveyshad a survey penetration depthlimited to about 5 m, but theinterpreteddatasuggestedapattern of deep linear features, mostly oriented NW-SE,crossing the southern half of the site.

5 In the northern halfof the site, severalanomalies/zones of disturbed groundwere identified and noted to bepotentiallyeither voids,clay filled fractures or very loose pockets of coral of the majority of the anomalies identified bythe geophysical surveys and/or encountered duringexcavationmainlyinvolvedsub-excava tionandreplacement with a well-graded, limestone or marl fill,placed in lifts not exceeding 200 mm thickness andcompacted to at least 98% of the Modified Proctormaximum dry density. This type of ground treatment isconsidered to becommon practise in the northern side of the structure, in the area ofone anomaly considered too deep for sub-excavation andreplacement,six(6)augeredpiles,measur ingapproximately m in diameter and reportedly m in length were installed.

6 No logs of the strataencountered during drilling were maintained and thefounding conditions at the base of the piles is a few locations, thefoundation excavationencounteredvertical fissures and a sub-horizontal voidwithin the coralline rock that extended below the footprintof the fissure,on the western sideof the building,reportedly appeared to be connected to acrevice on theocean-sidecliff face as a constant streamof air was observed to be coming up through the remediation of the horizontal void and vertical fissureinvolved filling with a high slump concrete (by pouringfrom surface, not tremied) and reportedly requiredvolumes on the order of 11 cubic metres and 16 cubicmetres for the horizontal and vertical void, addition to the localized ground treatmentdescribed above, the Foundation design on the northernhalf of the building(in the basement area)was modifiedfrom strip footings to a reinforced mat/raft nominally mthick, locally thickened at load bearing wall/columnlocations to up to m Shell Completion andInitial DistressThe building structureand exteriorshell was substantiallycompleted in April 2006 without incident.

7 Between Apriland August 2006, the building performed as designedwhile interior and exterior finishes were in August 2006, following several days of heavy seas,it is reported that cracking appeared on several walls inthe northwest corner of thebuilding, near the confluenceof the ocean-side, cliff face(to the west)and drainagegully(to the north).Observation of these initial cracks, mostly viacrackplatesandmarkings,suggested little change overthenext fewmonths and accordingly thecracks were patchedand interior finishing was continued. No new cracking orany other observable signs of building movement werenoted from this time until early February 2007, againfollowing violent sea conditions, when these originalcracks re-opened and additional sets of cracks April and May 2007, six (6) boreholes wereadvancedat the siteto investigate the Foundation conditionsbeneath and adjacent to the Phase 1 building and to helpunderstand the cause of the crackingpatterns observedon the walls of the boreholes werevertically oriented while the other three were drilled tocross the two prevailing joint sets.

8 All of the boreholeswere advanced using rotary coring techniques using atriple-tube core barrel system (HQ3)combined withvarious flush methods aimed at improving recovery fromthe very weak substrata. Upon completion, all boreholeswere examined using a downhole video this same time period, qualitative crackmapping surveys were initiated and crack gauges werepositioned on various key cracks to quantitatively assessrates of movement across the existing points were installed around and within thebuilding and regular precision surveys were carried out tomonitor vertical building addition,off-shore wave height and local rainfall data were sourced forthe period of time since building ConditionsThe results of the borehole drilling andcoringrevealedthatthe engineered marl fillimmediately below thebuilding foundations overlies a variably vuggy andheterogeneous.

9 Weak coralline limestone rockmass containing numerous voids and subhorizontal andsubvertical fissures and joints. The coralline limestonestratum containszones of marly/friable limestonethatareinterbedded with morecrystalline limestone technically a rock, the term rock is a bit of amisnomer for much of the Foundation zone, as in manyzones the rockmass strength is so low that the materialhas properties approaching that of a soil, with relict rockfabric and incipient FillThe engineered fill below the footings and floor slab isdescribed asdense to very dense, coralline sand andgravel (marl) Penetration Testing (SPT) wasnot carried out in the boreholes, however, duringsubsequent test pitting into the marl filltoexpose the topof the exterior stripfootingsit wasfound that the marl fillhad a very dense relative density and required ajackhammer on the conditions encountered in some of theboreholes and fromgeologicalmapping exercisescarriedout at and adjacent to the site, thereis evidence that theweak coralline limestone rockmass,hasa locallyindurated (or hardened) cap present along areas of theshoreline in the crest zones of the from the remnant coral sea stacksimmediately in front of the west side of the building ( the shore side)

10 , and fromanecdotal and photographicinformation, suggests that notching along thehardenedcoral cliff face locally occursnearsea level and alongprominent sub-horizontal weaknesses. In addition, thereis evidencethat a set of sub-vertical major fissures existsextending landward from the seathrough the cap and intothe underlying coralline cap is described as moderately weathered,medium bedded,amorphous to reefal,weak(R2), porous,fineto very fine grained, cream to white,corallinelimestonewith some more competentmaterial was likely present (up to 3 m in thickness)overparts of the top, if not all, of the rockmasswithin thebuilding footprint prior to ,theexcavation for construction ofthe basementof thebuilding likely removed most of this cap zone in the northpart of the building StratumBelow the cap,the coralline stratum is described asmoderately to slightly weathered, medium bedded,amorphousto reefal, weak to veryweak(R1 to R2)