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  • 1IVE EngineeringYour Partner in Professional DevelopmentIVE EngineeringYour Partner in Professional Development

    CivilEngineeringConstructionI(CBE5031)

    Foundations

  • 2IVE EngineeringYour Partner in Professional Development

    TheBritishStandardCodeofPracticeBS8004definesafoundation asthatpartofthestructuredesignedandco9nstructedtobeindirectcontactwithandtransmittingloadstotheground

    DefinitionofFoundation

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    Shallowfoundationsarefoundatadepthoflessthan3mbelowthefinishedgroundlevel.

    1.ShallowFoundation

    1.1Padfooting

    Apadfootingisanisolatedfoundationtospreadaconcentratedload.

    Padfootingsaregenerallyusedtotransfertheloadfromacolumn,pierorheavymachinerytotheground.

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    1.1Padfooting

    Theplanshapeofapadfootingisusuallysquare,butifthecolumnisclosetothesiteboundaryitmaybenecessarytousearectangularplanshapeofequivalentarea.

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    1.1Padfooting

    Blindingconcreteisalayerofnonstructuralconcreteofabout50mmthicklaidontheearth.

    Itprovidesacleanandflatplatformforsteelfixingandformworking,andtopreventcontaminationofthefreshconcretebysoil.

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    1.1Padfooting

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    1.1Padfooting

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    1.2StripFoundation

    Astripfoundationisafoundationprovidingacontinuouslongitudinalgroundbearing.

  • 9IVE EngineeringYour Partner in Professional Development

    1.2StripFoundation Stripfoundationsareusedtotransfertheloadfromawall,or

    fromasuccessionofcloselyspacedpiersorcolumns,totheground.

    Theyconsistofacontinuousribbonshapedstripformedofreinforcedconcrete.

    Mainbarsareplacedtransverselytoresistbendingwhilelongitudinalbarsareusedforthecontinuityofthestripfoundationandtobridgesoftspotsinthesoil.

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    1.3RaftFoundation Araftfoundationisafoundationcontinuousintwo

    directions,usuallycoveringanareaequaltoorgreaterthanthebaseareaofthestructure.

    Thestructuremovestogetherwiththeraftfoundationwhengroundmovementsoccurssuchthatcrackingordamagecanbeprevented.

    Araftfoundationisalsocalledafloatingfoundation.

  • 11IVE EngineeringYour Partner in Professional Development

    1.3RaftFoundation Raftfoundationsareusefulinthefollowingcases:

    forstructureswherethecolumnloadsand/orsoilconditionsaresuchthattheresultingfootingsoccupymostofthesite;

    wherebuildingshavetobeerectedonsoilssusceptibletoexcessiveshrinking,swellingorfrostheave;

    wheredifferentialsettlementsarelikelytobesignificant.

    Raftfoundationscanbesubdividedintothefollowinggroups:

  • 12IVE EngineeringYour Partner in Professional Development

    1.3.1SolidSlabRaft Solidslabraftsaresuitableforlightlyloadedstructuressuch

    assmallhouses. Asolidslabraftconsistsofareinforcedconcreteslab,usually

    slightlylargerthantheareaofthebuilding. Reinforcementintheformofameshfabricisprovidedon

    boththetopandbottomfacesoftheslab.

    SolidslabRaft (Source:R.Chudley)

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    1.3.1SolidSlabRaft Theslabissometimesstiffenedundertheperipheralwall

    withedgethickeningoranedgebeam.

    SolidslabRaft (Source:R.Chudley)

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    1.3.2BeamandSlabRaft Beamandslabraftfoundationsareusedwherepoorsoil

    conditionsareencountered. Theslabandbeamsprovidestiffnessandpreventthe

    distortionofthebuilding. Groundbeamsareusuallydesignedasdownstandbeam. Themainadvantageofthedownstandbeamraftfoundation

    isthatitprovidesalevelsurfaceslabwhichcanformthegroundfloorofthebuilding.

    Groundbeamscanalsobedesignedasupstandbeamswithaprecastconcretesuspendedflooratgroundlevelthuscreatingavoidspacebetweenraftandgroundfloor.

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    1.3.2BeamandSlabRaft

    Beam and slab raft with downstand beams (Source: R. Chudley)

    Beamandslabraftwithupstand beams(Source:R.Chudley)

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    1.3.3CellularRaft Cellularraftfoundationissuitablewherepoorsoilis

    encounteredatashallowdepth,andwhereitwouldbeuneconomicaltouseslabandbeamraftfoundations.

    Thistypeoffoundationconsistsoftworeinforcedconcreteslabslinkedbyinternalwallswhichdividethevoidintocells.Thewallshelptospreadtheloadovertheraft.

    Openingscanbeformedinthecellwallsallowingthevoidstobeutilisedforthehousingofservices,asstoreroomsorforgeneralaccommodation.

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    1.3.3CellularRaft Withcellularraftfoundations,thecolumnsshallbe

    positionedattheintersectionoftheinternalwalls.

    Celleular Raftfoundation (Source:R.Chudley)

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    1.4Combinedfoundation Rectangularfooting isusedforcloselyspacedcolumns.

    Balancedbasefooting isusedforeccentricallyloadcolumn.

    Thisoftenhappeninperimetercolumnwherethefootingislimitedbysiteboundary.

    Balancedbasefooting(Source:R.Chudley)

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    1.4Shallowfoundation

    Shallowfoundations(Source:R.Chudley)

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    2.PiledFoundation classifications Pilescanbeclassifiedbythewayinwhichtheytransmittheir

    loadstothegroundandbythewayinwhichtheyareinstalled.

    Endbearingpiles (Source:Jufri &Wellman)

    2.1.1ClassifiedbyMethodofLoadTransmissionEndbearingpiles theyactascolumnscarryingtheloadsthroughtheoverlayingweaksubsoils toafirmstratumonwhichthepiletoesrest.

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    2.1.1ClassifiedbyMethodofLoadTransmission

    Frictionpiles theytransfertheloadsbytheadhesionorfrictionactionofthesoilaroundtheperimeterofthepileshaft.

    Frictionpile(Source:Jufri &Wellman)

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    2.1.1ClassifiedbyMethodofLoadTransmission

    Besidessupportingadownwardload,africtionpilemayalsobeusedtosupportanupwardload,whichiscalledananchoragepile

    Inmostsituationspilesworkonacombinationofthetwoprinciplesoutlinedabovebutthemajordesignconsiderationidentifiesthepileclass.

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    2.1.2ClassificationbyMethodofInstallationDisplacementPiles thepilesareforcedintotheground,thesoilis

    displaceddownwardsandsideways,butmaterialisnotactuallyremoved.

    alsocalleddrivenpiles

    Displacementpile (Source:Jufri &Wellman)

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    2.1.2ClassificationbyMethodofInstallationReplacementPiles ashaft(orhole)isexcavatedandthesoilreplaced

    withconcretetoformapile.

    Replacementpile (Source:Jufri &Wellman)

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    2.2UsesofPiledFoundation asanalternativetoothertechniqueswhenthefollowing

    conditionsareencountered:1. Whentheloadofastructure,cannotbespreadsufficientlyoverthe

    availablegroundareawithoutexceedingtheallowablebearingcapacityofthesoil.

    2. Whensettlementduetoweakunderlyingstrataisunpredictable(i.e.overmudandpoorfilledground).

    3. Whenwindforcetendstooverturnatallbuilding,anchorpilesareneededonthewindwardside.

    4. Whenthereisneedtoresistupliftduetothefloatationofgroundwater.

    5. Toresistlateralloads e.g.rakingpilescanusedresisttheearthpressurebehindaretainingwallorthesideimpactfromshipsalongsideajetty.

    6. Tounderpinexistingfoundations.

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    3.DisplacementPiles Displacementpilesarepreformedpilesmadeinafactory,

    transportedtothesite,andthendrivenintotheground.

    Alsocalledpreformedpilesordrivenpiles.

    Preformedpilesmaybemadeofreinforcedconcreteorsteel.

    (drivencastinsitupile bydrivingashelloracasingwithaclosedendintothegroundandthencastingtheannularspacewithconcrete.Thistypeofpileisnowobsolete)

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    3.1.Pilingequipmentfordisplacementpiles

    Displacementpilesaregenerallydrivenintothegroundbyoneofthefollowingmethods:1.Percussion2.Vibration

  • 28IVE EngineeringYour Partner in Professional Development

    3.1.1Percussivedriving

    Pilesaredrivenintothegroundbytheimpactofheavypilehammers.

    Thereareseveraltypesofpilehammers:a.Drophammerb.Dieselhammer(Thesetwotypesofpilesarenowobsolete.)

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    3.1.1Percussivedriving

  • 30IVE EngineeringYour Partner in Professional Development

    c.HydraulicHammer mountedonthepilewhileoperating. similartoadrophammerbuttheweightisraisedby

    hydraulicjacks. Theweightthenfallsfreelyundergravityontothepilehead. sopowerfulthattheweightcanbeupto18tonnes. Thestrokeheightisabout12m. Theblowratedependsonthestrokeheightandrangesfrom

    40to100blowsperminutes. Mosthydraulichammersareequippedwithsilencers

    nowadays.Theyproducelessnoisethandieselhammersandtheydonotemitexhaustfumes.

    Theyaresuitableforharddrive

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    c.HydraulicHammer

    Hydraulichammer (Source:Junttan)

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    3.1.2Vibratorydriving vibrationhammer

    vibrationhammeriscomparativelysilent. usedindrivingsmalldisplacementpilessuchasH

    pilesandsheetpiles. consistsofavibratingunitmountedonthepilehead

    transmittingvibrationsdownthelengthofthepileshaft.

    vibrationsareinturntransmittedtothesurroundingsoil,reducingitsshearstrengthenablingthepiletosinkintothesubsoilunderitsownweightandalsothatofthevibrationhammer.

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    3.1.3Pilingrigs

    Apilingrighasthefunctionofguidingthepileatitscorrectalignmentfromthestageoffirstpitchinginpositiontoitsfinalpenetration.

    Italsocarriesthehammerandmaintainsitinpositioncoaxiallywiththepile.

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    3.1.3Pilingrigs

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    3.1.4Helmets Theheadofdisplacementpilesmustbeprotectedfrom

    damageduringdriving. Theprotectionconsistsofresilientpackingwhichisheldin

    positionbyasteelhelmet. Theresilientpackingdistributestheimpactloadfromthe

    pilinghammerevenlyontothepilehead. Thehelmetshouldfitlooselyaroundthepile,sothatthe

    pilemayrotateslightlywithoutinducingtorsiononthepile.

    Pile helmet (Source: R. Chudely) (left)Pile helmet (Source: Jufri & Wellman)

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    3.2Precastprestressedconcretetubularpile Precastprestressedconcretetubularpiles,e.g.

    DaidoSSpileandNiponHimeSSPile,hasbeenwildlyusedinHongKongbuttheyarenotobsolete.

    ProductionofPrecastPrestressedConcretePIle

    DrivingofPrecastPrestressedConcretePIle

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    3.2Precastprestressedconcretetubularpile

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    3.3SteelHpile

    SteelHpileisstrongandtough.

    Thedisplacementofthesoilislittlecomparedwithothertypesofdisplacementpiles.

    ItiswidelyusedinHK

    Drivingsteelpilesgenerallydonotrequirepileshoes.

    Inhardconditions,thetoeofthepilemaybestrengthened.

  • 39IVE EngineeringYour Partner in Professional Development

    3.3SteelHpile

    Strengtheningmaybecarriedoutbyweldingplatesoranglestothetoeofthepile,ora'rockpoint'isfittedtothetoewhenthepilehastopenetraterock.

    PreformedpileshoesforHpilesarealsoavailable.

    Hpilespoint(Source:HousingDepart)

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    3.3SteelHpile

    LengtheningofsteelHpileisbybuttwelding

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    3.4Procedureofdrivingadisplacementpile

    1. Prepareapilingrigandapilinghammer.2. Putmarksonthepilelengthat500mmintervaltoindicate

    thelengthbeingdriven.3. Pitchthepileinthepilingrig;checkitsalignment,

    verticalityordesiredinclinedangle.4. Placeasuitablepilehelmetonthepileheadthenmount

    thepilinghammerontopofitandstartdriving.5. Continuedriving.Ifthewholelengthofthepilehasbeen

    drivenintotheground,spliceanewlengthonitbybuttwelding.

    6. Continuedrivinguntilitrenderssufficientresistance,thenmeasuretheset.Finishthedrivingifthepredeterminedsetisattained.Otherwiserepeatfromstep5untilitdoes.

  • 42IVE EngineeringYour Partner in Professional Development

    3.4MeasurementofSet

    Setrelatedtotheultimatebearingcapacityofthepilecanbedeterminedbypiledrivingformulaebyconsideringtheparametersofthepile,thesoilandthegroundcondition

    Therearemanypiledrivingformulae,amongwhichHileyformulaismostcommonlyused.

    Thefinalsetshallbemeasuredas: 1.penetrationper10blows(forendbearingpiles),or 2.thenumberofblowsrequiredtoproduce25mmpenetration(for

    frictionpiles).

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    3.5Advantagesofdisplacementpiles:

    1. Wherelargenumbersofpilesaretobeinstalledineasydrivingconditions,itcanbemoreeconomic.

    2. Projectionabovegroundleveladvantageoustomarinestructures.

    3. Thepilecanbeinspectedforqualityandsoundnessbeforedriving.

    4. Constructionoperationsnotaffectedbygroundwater.5. Notliabletosqueezingornecking.

  • 44IVE EngineeringYour Partner in Professional Development

    3.6Disadvantagesofdisplacementpiles

    1. Transportingthecompletelengthofpilethroughnarrowand/orcongestedstreetsmaybedifficult.

    2. Thedrivingprocess,whichisgenerallypercussion,cancauseunacceptablenoiseandvibrations.

    3. Piledrivinghourislimitedbyenvironmentalregulations.4. Maybreakduringdriving,causingdelaysandreplacement

    charges.5. Maysufferunseendamagewhichreducecarrying

    capacity.6. Displacementofsoilduringdrivingmaydamageadjacent

    structureorcauseliftingofadjacentpiles.7. Cannotbedriveninverylargediameters.8. Cannotbedriveninconditionsoflowheadroom.

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    DrivenpilesoverwaterdisplacementpileNeckingofdisplacing pile

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    4.ReplacementPile

    Replacementpileisalsocalled'nondisplacementpile'. Itisformedbyboringapileshaftandreplacingthesoilwith

    insituconcrete. itismostlycalledboredpile.

    4.1Methodsandequipmentforboringpileshafts

    4.1.1.Rotaryboring Rotaryboringinvolvesadrillingrigwhichmaybemounted

    onamobilecraneoratruck. Thedrillingrigconsistsofatelescopicorextendablekelly

    baron

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    4.1MethodsandequipmentforboringpileshaftsFlight auger (Source: Tomlinson) Cheshire auger (Source: R. Chudley)

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    BoringTools

    1. TheCheshireauger whichhasonly1to2helixturns.Thesoiliscutbytheauger,raisedtothesurfaceandspunoffthehelixtothesideoftheboreholefromwhereitisremovedfromsite.

    2. Thecontinuousauger orflightauger bringsthespoiltothesurfacewithspiralmotion.Thedrillingrigmustprovidesufficienttorquetoovercomethegreatfriction.

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    BoringTools

    4. Thedrillingbucket isdesignedtowithstandthehightorqueforcesdevelopedduringpenetrationofdenseandcompetentstrata.Thebucketbaseisfittedwithflapdoors.Spoilisextractedbydrillingwiththebucket,raisingtothesurface,openingtheflapsandclearingofthepilebore.Adaptationsofthestandardbucketareavailableforcoringandforusewithbentonite.

    5. Thecoringbucket isusedtoraiseasolidcoreofrock.6. Thechiselisusedwhenboulderorrockisencountered.

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    CheshireaugerBentonitebucket&CoringbucketChisel

    Cheshire auger Bentonite bucket Coring bucket Chisel (Source:Tomlinson)

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    4.1.2Grabboring

    Grabsconsistofclamshellbucketswhichareopenedandclosedbymeansofwireropes.

    Theyaresuspendedfrommobilecranes. Thegrabisdroppedontothesoilinanopenpositionand

    isthenclosed. Itisthenraisedtothesurfaceandemptiedatthesideof

    theshaft

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    4.1.2Grabboring

    Whenboulderisencountered,itcanbebrokenbyfreefallingachiselontoit.

    Therockfragmentsarethenremovedbythegrab. Chiselandgrabareoftenusedalternativelytobreak

    throughobstructions.

    Chisels

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    4.1.3Downthehole(DTH)drilling

    DTHisalargesizehammerdrillequippedwithabuttonbitthatcandrillholesintorockwithdiametersupto750mmandtodepthsofseveralhundredmeters.

    Aconventionalhammerdrillbecomeslessefficientasthelengthofboreholeincreasesbecauseofthelossofenergyindrillstem.

    Toovercomethesedifficultiesthedowntheholedrillwasdeveloped.

    Itsrotarymotorremainsabovegroundlevelwhilethebitisfolloweddowntheholeactuatedbyitscompressedairdrivenpneumatichammer

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    4.1.3Downthehole(DTH)drilling

    Thedrillstemrotatesinslowmotionwhilethedrillbitstrikesrapidly.

    Thedrilldebrisisblownoutbycompressedair.

    Downtheholedrill

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    4.1.4Reversecirculardrilling

    reversecirculationdrillingrigcandrillatafastrateinawiderangeofgroundconditionsincludingrocks.

    InHongKong,itiscommonlyusetodrillthebedrocksocketsforboredpiles.

    Forboringinrock,rockrollercuttersareused.

    TheReversecirculationdrillsitsonthetemporarysteelcasingwhileoperating.

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    4.1.4Reversecirculardrilling

    Itconsistsofadoublewalldrillingpipeandoperatesontheprincipleofairlifting.

    Compressedairisinjectedthroughtheannularspaceofthedrillingpipeanddischargenearthebase.

    Therisingcolumnofairandwateratthecentreofthedrillingpipeliftsthesoilorrockfragmentswhichhasbeenloosenedbyrotatingcutters.

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    4.1.4Reversecirculardrilling

    (Airliftingisalsocommonlyusetocleanthebottomofaboredpilebeforeconcreting.)

    Rock roller Cutter Reverse Circulation Drill

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  • 59IVE EngineeringYour Partner in Professional Development

    4.1.6Underreaming

    Thebaseortoeofthepilecanbeenlargedtothreetimestheshaftdiametertoincreasethebearingcapacityofthepile.

    Themethodofenlargingthebaseisknownasunderreamingorbelling.

    Underreamingisachievedbyabellingbucket,whichisrotatedbythekellybarofthedrillingrig.

    Thebellingbucketsareequippedwitharmshingedatthetoporbottom.

    Thearmsofthebucketarejackedouttoexcavatethesidesoftheshaftandhenceformthe'bell'.

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    4.1.6Underreaming

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    4.2Methodsofsupportingpileshafts

    4.2.1Selfsupported(unsupported)

    Forsmalldiameterboredpilesandinstablesoils,theboredpileshaftmayremainunsupportedwithoutcollapsing.

    aboredpileshaftwithoutsupportisnotrecommendedinHongKong.

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    4.2.2Supportedwithtemporarycasing

    Inunstableorwaterbearingground,theboredpileshaftcanbesupportedbyasteelcasing.

    Topreventthecollapseoftheboredshaft,thecasingshallbedrivenaheadofboring.

    Thecasingcanbedrivenintothegroundbyavibrationhammeroracasingoscillator.

    CasingoscillatorVibrationhammer

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    4.2.2Supportedwithtemporarycasing

    Theoscillatorconsistsofaclampandapairofhydraulicjack.

    Theoscillatorclampsitselftothecasingwithitstwohydraulicjacksextendandretractinoppositephrase.

    Thisoscillationmotionhelpstocutthesoilbythecasingshoeandsinksthecasingintotoground.

    Thesteelcasingisusuallythickandexpansiveandwillberetrievedafterconcreting.

    Vibratorsandthecasingoscillatorscanbeusedtowithdrawnthecasing.

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    4.2.3Lightgaugepermanentcasing

    Sometimesextractionofthethicktemporarycasingmaybedifficultafterconcretingespeciallyinlongpiles.

    Inthesecases,corrugatedlightgaugepermanentcasingsmaybeaddedtoovercometheproblem.

    Thethinpermanentcasingischeaperandwillbeleftinplace.

    Itpreventsthethicktemporarycasingfromstickingbythefreshconcrete.

    itpreventsneckingandcontaminationofthefreshconcretebythesoilandgroundwater.

    Corrugatedsteelcasingsareoftenusedinlongpilesandindifficultgroundssuchasreclaimedland.

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    4.2.3Lightgaugepermanentcasing

    Light gauge corrugated steel casing

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    4.2.4Supportedwithbentoniteslurry

    Bentonite isakindofclay.Whenmixedwiththecorrectamountofwaterbentoniteslurryproducesthixotropicproperties,itgivesaliquidbehaviourwhenagitatedandagelstructurewhenundisturbed.

    Duringboring,theshaftisfilledwithbentoniteslurry. Theboringactionstirstheslurrysoitremainsliquidstate. Buttheslurrypenetratesslightlyintothesubsoilandforms

    asoftgelorsocalled'filtercake'attheinterfaceoftheexcavationsides.

    Hydrostaticpressurecausedbytheslurrythrustingonthe'filtercake'holdsbackthesubsoilandgroundwater.

    Thisalleviatestheneedforcasingsupport.

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    4.2.4Supportedwithbentoniteslurry

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    4.3ExamplesofReplacementPiles

    4.3.1Boredpile

    BoredpileisthemostpopulartypeofpileinHongKong Themaximumdiametercanexceed3mandthepilelength

    canbeover100m. Itcanbedesignedtocarryloadsupto30000kN. Therearevarioustechniquesforboringpileshaftsand

    supportthem. Thechoicedependsonthegroundconditionandthepile

    design. Combinationofthesetechniquesgeneratesvariouskindsof

    piles. Sometypicalexamplesareasfollows:

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    4.3.1.1Boredpilesupportedwithtemporaryandpermanentcasing

    Procedure1. Pitchatemporarysteelcasinginthecorrectpositionand

    driveitintothegroundwithacasingoscillator.2. Borewithinthecasingwithahammergrab.Boringshallnot

    exceedthebottomofthecasingtopreventcollapseoftheborehole.

    3. Extendthesteelcasinganddriveitintothegroundifnecessary.

    4. Ifrocksorbouldersareencountered,useachiselandthegrabalternativelytobreaktherocksandremovethefragments.

    5. Repeatsteps2to4untilthefoundingrockisreached.

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    4.3.1.1Boredpilesupportedwithtemporaryandpermanentcasing

    6. Placeareversecirculationdrillontopofthesteelcasing.Formthesocketofdesireddepthintothebedrockwitharollercutter.Thedebrisisremovedbyairlifting.

    7. RemovetheRCDandlowerapermanentcorrugatedsteelcasingintotheshaft.

    8. Lowerthereinforcementcagewithinthepermanentsteelcasing.

    9. Checktherateofingressofgroundwater.Ifitdoesnotexceed0.3L/s,placehighslumpconcreteintotheshaftimmediatelyafteritispumpeddry.Otherwise,filltheshaftwithtremieconcrete.

    10. Removethetemporarysteelcasing.Thepileprocessiscompleted.

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    4.3.1.3Boredpilesupportedwithbentoniteslurry

    Procedure1. Pitchasteelcollarcasinginthecorrectpositionanddriveit

    intothegroundwithavibrationhammer.2. Borewithinthecasingwithahammergrab,andthe

    boreholeisfilledwithbentoniteslurry.3. Continueboringthroughtheslurry,maintaintheslurrylevel

    towithin500mmbelowthetopofthecasingduringthewholepilingprocess.

    4. Ifrocksorbouldersareencountered,useachiselandthegrabalternativelytobreaktherocksandremovethefragments.

    5. Repeatsteps2to4untilthefoundingrockisreached.

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    4.3.1.3Boredpilesupportedwithbentoniteslurry

    6. Usethechiselandthegrabalternativelytoformasocketintherock.

    7. Lowerthereinforcementcageintotheboreholethroughtheslurry.

    8. Insertanairliftpipetothebottomoftheborehole.Cleanthebottomoftheborewithairlifting.Slurryloadedwithsoilparticlesshallbepumpedtoasettlingtank,stainedandrecycled.

    9. Fillthepileshaftwithtremieconcrete.Displacedslurryshallberecycled.

    10. Removethetemporarycasing.Thepilingprocessiscompleted.

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  • 74IVE EngineeringYour Partner in Professional Development

  • 75IVE EngineeringYour Partner in Professional Development

    4.3.2Barrette

    Bydefinition,abarretteisexactlythesameasaboredpilesupportedwithbentoniteslurry.However,abarretteisoftenreferredasapileinrectangularcrosssection.

    Itsformationisthesameasaboredpileexceptthatfourconcreteguidewallsareusedinsteadofthecollarcasing.

    Theguidewallsareabout150mmthick,arrangedasatoplessandbottomlessboxwithinternaldimensionthesameasthebarrettesection.

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    4.3.3SockettedsteelHpiles

    pilesformedbyinsertingsteelHpilesinpreboredholessunkintosoundrock,andsubsequentlyfillingtheholeswithcementgrout.

    IntheconstructionofsockettedsteelHpiles,thefollowingshouldbeconsidered:1. Thepreboredholesshouldbelargeenoughtoenablethe

    installationofsteelHpilesandtoallowaminimumcoverof40mmtothesteelHpiles.

    2. Atemporarycasingshouldbeprovidedinthepreboringprocessdowntorockheadleveltopreventsoilfromfallingintothepreboredhole;

    3. BeforethesteelHpileisinsertedinthepreboredhole,theholeshouldbecleanedtoensurethatitisfreefromdebrisandsoil;and

    4. Thegroutshouldbenonshrinkandhaveaminimumcharacteristicstrengthof30MPaat28days.

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    4.3.4Minipile(Micropile)

    pileshavingadiameteroflessthan300mm. rangeinshaftdiameterfrom50to250mm,withworking

    loadsintherangeof50to500kN. Therearemanywaysofformingminipiles. AtypicalmethodcurrentlyusedinHongKongis:

    Drillaboreholeofabout150mmdiameterwitharotarydrillingrigandlinetheboreholewithasteelcasing.

    Grouttheboreholewithcementorcementsandgroutfromthebottomofthehole.

    Insertthepilereinforcementintothecasing.Normallyitconsistsof24numbersofT4050bars.(Thesteelcasingisleftinplacetoenhancecorrosionprotection.)

    Theprincipaluseofminipilesisforunderpinningworkorinsteepslopewherelargepilingmachinecannotaccess

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    4.4.1Advantagesofreplacementpiles:

    1. Soilorrockremovedinboringcanbeinspectedforcomparisonwithsiteinvestigationdata.

    2. Canbeinstalledinverylargediameters.3. Canbeinstalledinverylonglength.4. Endenlargementsuptothreediametersarepossible5. Materialformingpileisnotgovernedbyhandlingor

    drivingstresses.6. Canbeinstalledwithoutappreciablenoiseorvibration.7. Canbeinstalledinconditionsofverylowheadroom.8. Noriskofgroundheave.

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    4.4.2Disadvantagereplacementpiles:

    1. Squeezingorneckingmayoccurinsoftgroundwhereconventionaltypesareused.

    2. Waterunderartesianpressuremaypipeuppileshaftwashingoutcement,specialtechniquesneededforconcretinginwaterbearingsoil.

    3. Concretecannotbeinspectedafterinstallation.4. Drillinganumberofpilesingroupcancauselossofground

    andsettlementofadjacentstructures.

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    4.5PlacingConcreteinPiles

    4.5.1Concretingbytrunking

    Inboreholeswheretherateofingressofwaterdoesnotexceed0.3L/s,thepilesshallbedriedimmediatelybeforeconcreteisplaced.

    Thentheconcretecanbeplacedusingareadilyworkablemix(slump100mm,whichisselfcompactingbutdoesnotsegregate)throughatrunking.

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    4.5.2Concretingunderwater(tremiemethod)

    Iftheexcavationsforpilesaresupportedbybentoniteslurryoriftherateofingressofwaterexceeds0.3L/s,concreteshallbeplacedbyatremie.

    Atremieisasteeltubesuspendedinthewaterbyacrane,withahopperfixedtothetopendtoreceivetheconcrete.Thetubemustbewatertight,smoothbored,diametersof150 200mm.

    Thetremieconcretemustbeofhighworkability(minimum150mmslump).

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    4.5.2Concretingunderwater(tremiemethod)

    Beforeconcreting,thetremieiserectedverticallyinthepileshaftwiththelowerendrestingonthebottom.

    Atravellingplug,formedfromplasticbags,foamedplasticorsimilarmaterial,isplacedinthepipeasabarrierbetweentheconcreteandwater.

    Thewaterinthepipeisdisplacedastheweightoftheconcreteforcestheplugtothebottom.

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    4.5.2Concretingunderwater(tremiemethod)

    Afterthepipehasbeenfilledwithconcreteitisraisedoffthebottomtoallowtheconcretetoflow.

    concreteshouldbecontinuallyfedintotheinterioroftheinitialmass.

    Thetremietubemayberaisedprogressivelytomaintaintherateofflowofconcrete,butkeepingatleast3membenmentintheconcrete.

    Concretingiscontinueduntiltheconcretelevelreaches750mmabovethecutofflevel.

    Concretingiscompletedandthetremiepipeisremoved. Thesurplusconcreteistrimmedbeforethepilecapis

    formed.

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    Reference:

    1. Piledesignandconstructionpractice,fourthedition(1994),M.J.Tomlinson,E&FNSpon.

    2. Introductiontocivilengineeringconstruction,thirdedition(1995),RoyHolmes,Thecollegeofestatemanagement.

    3. ConstructionTechnology,secondedition(1987),R.Chudley,LongmanScientific&Technical.

    4. CivilEngineeringConstructionIV(1991),S.A.R.Jufri&R.J.Wellman,HongKongPolytechnic.

    5. GeneralSpecificationforCivilEngineeringWorks(1992),HongKongGovernment.