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Behavior of raft on settlement reducing piles: Experimental model study
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   JournalofRockMechanicsandGeotechnicalEngineering5(2013)389–399 JournalofRockMechanicsandGeotechnicalEngineering  Journal   of    Rock   Mechanics   and   GeotechnicalEngineering  journalhomepage:www.rockgeotech.org Behavior   of    raft   on   settlement   reducing   piles:   Experimental   model   study Basuony   El-Garhy ∗ ,   Ahmed   Abdel   Galil,   Abdel-Fattah   Youssef,   Mohamed   Abo   Raia CivilEngineeringDepartment,FacultyofEngineering,MinufiyaUniversity,ShebinEl-Kom,Egypt  a   r   t   i   c   l   e   i   n   f   o  Articlehistory: Received5December2012Receivedinrevisedform18February2013Accepted11March2013 Keywords: RaftSettlementreducingpilesPiledraftModeltestsSandsoil a   b   s   t   r   a   c   t An   experimental   program   is   conducted   onmodel   piled   rafts   in   sand   soil.   The   experimental   program   isaimedto   investigate   the   behavior   of    raftonsettlement   reducing   piles.   The   testing   program   includestests   onmodels   of    single   pile,   unpiled   rafts   andrafts   on1,   4,   9,   or   16   piles.   The   model   piles   beneaththerafts   are   closed   ended   displacement   piles   installed   bydriving.   Three   lengths   of    piles   are   used   in   theexperiments   to   represent   slenderness   ratio,   L/D ,   of    20,   30and   50,   respectively.   The   dimensions   of    themodel   rafts   are   30cm ×   30   cm   with   different   thickness   of    0.5   cm,   1.0   cmor   1.5   cm.   The   raft-soil   stiffnessratios   of    the   model   rafts   ranging   from   0.39   to   10.56   cover   flexible   to   very   stiff    rafts.   The   improvement   in   theultimate   bearing   capacity   isrepresented   bythe   load   improvement   ratio,   LIR ,   and   the   reductions   in   averagesettlement   anddifferential   settlement   are   represented   bythe   settlement   ratio,   SR ,   and   the   differentialsettlement   ratio,   DSR ,   respectively.   The   effects   of    the   number   of    settlement   reducing   piles,   raft   relativestiffness,   and   the   slenderness   ratio   of    piles   on   the   load   improvement   ratio,   settlement   ratio   anddifferentialsettlement   ratio   are   presented   and   discussed.   The   results   of    the   tests   show   the   effectiveness   of    using   pilesas   settlement   reduction   measure   with   the   rafts.   As   the   number   of    settlement   reducing   piles   increases,   theloadimprovement   ratio   increases   and   the   differential   settlement   ratio   decreases.©   2013   Institute   of    Rock   and   Soil   Mechanics,   Chinese   Academy   of    Sciences.   Production   and   hosting   byElsevier   B.V.All   rights   reserved. 1.Introduction Pilescanbeusedwitharaftfoundationinordertoprovideade-quatebearingcapacityortoreducesettlementstoanacceptablelevel.Thecommondesignofpiledraftisbasedontheassumptionthatthetotalloadofthesuperstructureissupportedbythepiles,ignoringthebearingcontributionoftheraft.Thisresultsinacon-servativeestimateofthefoundationperformance,andthereforeanoverdesignofthefoundation.Adifferentapproach,involvingtheuseofpilesassettlementreducers,hasbeenreportedbyRandolph(1994),Burland(1995),Sanctisetal.(2002),andFioravanteetal. (2008).Thebasicconceptofthisapproachisthatthefoundationcomprisesonlyanumberofpilesthatarenecessarytoreducesettlementstoatolerableamountandtheloadsfromthestructure ∗ Correspondingauthor.Tel.:+966548188134. E-mailaddress: belgarhy@hotmail.com(B.El-Garhy).PeerreviewunderresponsibilityofInstituteofRockandSoilMechanics,ChineseAcademyofSciences.1674-7755©2013InstituteofRockandSoilMechanics,ChineseAcademyof Sciences.ProductionandhostingbyElsevierB.V.Allrightsreserved.http://dx.doi.org/10.1016/j.jrmge.2013.07.005 aretransmitted,viaaraft,inparttothepilesandinparttothefoundationsoil(loadsharedbetweentheraftandpiles).Thisapproachallowsthepiledraftdesigntobeoptimizedandthenum-berofpilestobesignificantlyreduced.Fig.1showsschematicallytheprinciplesbehindthedesignof pilestoreducedifferentialsettlement.Assumingthatthestructuralloadisrelativelyuniformlydistributedovertheareaoftheraft,andthentherewillbeatendencyforunpiledrafttodishinthecenter.Afewpiles,addedbeneaththecentralareaoftheraftandprob-ablyloadedtoabouttheirultimatecapacity,willreducecentralsettlement,andthusminimizedifferentialsettlement.However,arelativelysmallnumberofpilescouldraisetheproblemsofhighbendingmomentsandcrackingintheraftandaconcentrationof axialstressesinthepileheads(Wongetal.,2000).Manyresearchershaveconductednumericalanalysisofpiledrafts(e.g.RussoandViggiani,1998;HorikoshiandRandolph,1999;Poulos,2001;Viggiani,2001;Mandolini,2003;Randolph,2003;Randolphetal.,2004;Badelowetal.,2006;SanctisandMandolini,2006;SanctisandRusso,2008).Butonlylimitedinformationisavailableintheopenliteratureontheexperimentaldataofpiledrafts(e.g.Horikoshietal.,2003;LeeandChung,2005;BajadandSahu,2008;Fioravanteetal.,2008;Phung,2010).Theexperimen-taldataarehelpfulinverifyingtheresultsofnumericalanalysisof piledrafts.Horikoshietal.(2003)investigatedtheload-settlementbehav-iorandtheloadsharingbetweenthepilesandtheraftinthe  390 B.El-Garhyetal./JournalofRockMechanicsandGeotechnicalEngineering5(2013)389–399 Fig.1. Centralpilestoreducedifferentialsettlement. piled-raftsystemthroughaseriesofstaticloadingtests(verticallyandhorizontally)onpiledraftmodelsinsandbyusingageotechni-calcentrifuge.LeeandChung(2005)pointedoutthatforaproper pilegroupdesign,factorssuchastheinteractionamongpiles,theinteractionbetweencapandpiles,andtheinfluenceofpileinstal-lationmethodallneedtobeconsidered.LeeandChung(2005)studiedtheeffectofthesefactorsontheperformanceofpilegroupsinsandsoilthroughmodeltestsonsinglepile,single-loadedcen-terpilesingroups,unpiledfooting,freestandingpilegroups,andpiledfootings.BajadandSahu(2008)investigatedtheeffectofpile lengthandnumberofpilesonloadsharingandsettlementreduc-tionbehaviorofpiledraftsrestingonsoftclaythrough1gmodeltestsonpiledrafts(i.e.10cm × 10cmraftwithdifferentthicknessonfour(2 × 2),nine(3 × 3),andsixteen(4 × 4)piles).Fioravanteetal.(2008)investigatedthebehaviorofraftsonsettlementreduc-ingpilesthroughacentrifugemodeltestonrigidcircularpiledraftsrestingonabedoflooseandveryfinesilicasand.Thetest-ingprogramincludedanunpiledraft,raftson1,3,7or13piles.Phung(2010)presentedthedataofthreeextensiveseriesoflarge-scalefieldmodeltestsperformedonpiledfootingsinnon-cohesivesoilinordertoclarifytheoverallcap-soil-pileinteractionandtheloadsettlementbehaviorofpiledfooting.Allthepilegroupsweresquareandconsistedoffivepiles(i.e.onecenterandfourcornerpiles).Inthispaper,thebehaviorofpiledraft(i.e.raftwithalimitednumberofpilesbeneaththecentralraftareacalledsettlementreducingpiles)isinvestigatedthroughmodeltestsonpiledraftinloosesand.Modeltestsonsinglepileandunpiledraftarealsocarriedoutforthepurposeofcomparison. 2.Experimentalprogram Aseriesoflaboratorytestswereperformedonmodelsof singlepile,unpiledraftandcentralpiledraft(i.e.raftonset-tlementreducingpiles).Theexperimentalprogramconsistsof fortytests.Onetestwascarriedoutonsinglepile,threetestswerecarriedoutonunpiledraftsandthirtysixtestswerecar-riedoutoncentralpiledrafts.TestsonunpiledraftandcentralpiledraftarepresentedinTable1.Thepilesconfigurationsand modelraftsdimensionsofthestudiedcasesofcentralpiledraftsareshowninFig.2.Thedimensionsofthetestmoldandmodel raftswereselectedtoensurenoeffectoftheboundarywallsonthestressesinthesoil,andtheheightofthesoilwasselected2timesgreaterthanthemaximumpilelengthtoensureinsignif-icanteffectofarigidbaseonthebehaviorofpiles(HorikoshiandRandolph,1999). 30         3        0 1215   1530303013.513.53         3        0        3        0        3        0 123310.510.5333 Fig.2. Studiedcasesofcentralpiledrafts(unit:cm).  2.1.Testedsoil Drysandwas   usedasfoundationsoilinthisstudy.Sieveanalysistestswerecarriedoutonthreerandomsamplestodeterminethegrainsizedistributioncurveofthetestedsoil.Thegrainsizedistributioncurveparametersare: D 10 =0.30mm, D 30 =0.45mm,   D 60 =0.60mm,   C  u  (coefficientofuniformity)=2.0,and C  c  (coefficientofcurvature)=1.125.AccordingtotheUnifiedSoilClassificationSystem(USCS),thetestedsoilisclassifiedaspoorlygradedsand,SP.Thedirectsheartestswerecarriedoutonfoursamplestodeterminetheangleofinternalfrictionofthetestedsand.Thesandispouredinthedirectsheartestmoldonlayerstogiveaunitweightof15.5kN/m 3 .Theangleofinternalfrictionisdeterminedtobe33 ◦ .  2.2.Modelofraftsandpiles Threesquaresteelplates,withdifferentthickness,servedasmodelrafts.Thedimensionsoftheraftswere30cm × 30cm × 0.5cm,   30cm × 30cm × 1.0cm,   and30cm × 30cm × 1.5cm,   respectively.Themodulusofelastic-ityandPoisson’sratioofthesteelplateswere2.1 × 10 8 kPaand0.20,respectively.Themodelpilesusedintheexperimentsweresteelhollowpipesof10mminoutsidediameterand1.5mminwallthickness.ThemodulusofelasticityandPoisson’sratioofthesteelpipewere2.1 × 10 8 kPaand0.20,respectively,asdeterminedfromthedatasheetofthetechnicaldepartmentofthemanufacturedcompany.Theembeddedpilelengthsof200mm,   300mm,   and500mmwereusedintheexperiments.Theselengthsrepresent L/D ratiosof20,30,and50,respectively.Topheadofeachpilewasprovidedwithaboltof10mmindiameterand40mmlongtoconnectthepiletothecapthroughtwo   nutstoensureacompletefixationbetweenthepileandthecap.Inaddition,thepiletipwasprovidedwithasteelconicalshoetofacilitatethepiledriving,asshowninFig.3.  B.El-Garhyetal./JournalofRockMechanicsandGeotechnicalEngineering5(2013)389–399 391  Table   1 Summaryofthemodeltestsonunpiledandpiledrafts.StudiedcasesRaftmodeldim.(cm × cm × cm)   No.ofpiles L / DS  / D No.oftestsUnpiledraft 30 × 30 × 0.5––– 330   × 30 × 1.0–––30   × 30 × 1.5–––Raft   +1centralpile 30 × 30 × 0.5 150– 3130–1   20–30   × 30 × 1.0 150– 3130–1   20–30   × 30 × 1.5 150– 3130–1   20–Raft   +4centralpiles 30 × 30 × 0.5 4503 343034   20330   × 30 × 1.0 4503 343034   20330   × 30 × 1.5 4503 343034   203Raft   +9centralpiles 30 × 30 × 0.5 9503 393039   20330   × 30 × 1.0 9503 393039   20330   × 30 × 1.5 9503 393039   203Raft   +16centralpiles 30 × 30 × 0.5 16503 31630316   20330   × 30 × 1.0 16503 31630316   20330   × 30 × 1.5 16503 31630316   203 3.Testingsetupcomponents  3.1.Steeltankandmainframe Thetestmoldconsistsofasteeltankandamainframe.Thesteeltankrestsonamovablerollingframebase.Thetankwas   1.0mlong,1.0mwide,and1.0mhigh.Thetankwasprovidedbyfourhorizontalstiffeners(L40 × 40)at0,20cm,   50cm,   and85cmlevelsfromthebottombaseofthetankasshowninFig.4.Themainframe was150cminclearwidth,215cminclearheight,andconsistedof twoverticalcolumnsandonehorizontalbeamasshowninFig.4.  3.2.Measuringdevices Threedialgaugesof0.01mmaccuracywereusedtomeasuretheverticalsettlements.Onedialgaugewaslocatednearthecenterandtwowerelocatedatthemiddlesidesoftheraft.Thedialgaugeswerefixedtotheraftbymeansofsteelrods.Thesteelrodconsistedofaverticalrodconnectedtothehorizontalbeamofthemainframeandahorizontalrodwhichcarriedthedialgauge.Thetwo   rodswereconnectedtoeachotherbyhollowtubeswhichhadtwoscrewgroovesasshowninFig.5.Thisrodsystemhadtheabilitytosupport thedialgaugeatanyhorizontalplane.LoadswereappliedbyahydraulicjackfixedatthemiddleofthehorizontalbeamofthemainframeasshowninFigs.4and5.The hydraulicjackwasusedmanuallytoproducetheincrementalload.Calibratedprovingringswithdifferentcapacitieswereattachedtothejacktomeasuretheloads.Duringtestsonsinglepileaverticalloadingbarwasattachedtotheprovingringtoproducepointcentralverticalload.Duringtestsonunpiledraftandcentralpiledraft,theverticalloadingbartransmittedthejackloadtothetestedraftmodelthroughaspe-cialloadingcap.Theloadingcapwascomposedofasquaresteelplate,ofdimensions30cm × 30cm × 2cm,   supportedbyninesteelcolumns.Eachcolumnwas2.54cmindiameterand26cminheight.Thecentralspacingbetweencolumnswas   10cmasshowninFig.5. 4.Testprocedures (1)Eachexperimentstartedwithplacingthesandsoilinthesteeltankinlayers.Themaximumlayerthicknesswas10cm.   Thetotalheightofthetankwasdividedintointervalsfromtheinnersidebymakingsignsevery10cmheighttohelptoputaspecifiedweightinaspecifiedvolumetogettherequiredsanddensitybycompaction.Apre-weightedquantityofsandwascompactedbymeansofaspecifiedcompactiontoolinthesteeltank.Thecompactioncontinueduntilthesoilwascompactedtofillthefirst10cmlayer.Asteelarmwithcircularplateof   392 B.El-Garhyetal./JournalofRockMechanicsandGeotechnicalEngineering5(2013)389–399 10 mmTop nutSteel cap(1.5 cm thickness)Bottom nutHollow steel pipe tubewith outer diameter 10 mmand inner diameter 7 mmSteel conicalshoeDiameter=10 mmCopper weld8 mm15 mm    P   i   l  e   l  e  n  g   t   h   1   0  m  m Fig.3. Connectionbetweenthepileandthecap. 15cmindiameterand0.8cminthicknesswasusedforcom-paction.Theprocesswasrepeateduntilreachingtheheightof thesteeltank(i.e.95cm).Thefinalsoillayerwas5cmthicktoavoidsoiloverflowingduringthecompactionprocess.(2)Forthecasesofcentralpiledraft,woodentemplateswereusedtolocatethepilesinthecorrectpositions,andtheneachpilewasinsertedverticallyintothesandbydrivingwithasteadysuccessionofbellowsonthetopofthepileusingasteelham-mer   weighting2kg.Theinclinationsofthepileswerecheckedcarefullybyalevelduringdriving.Thesequenceofpilesinstal-lationstartedwiththeinnerpiles,thencornerpiles,andfinallytheedgespiles.(3)Aftertheinstallationofpilestotherequireddepth,thewoodentemplateswereremoved.Then,theraftmodelwasplacedonthesandsurfaceandthehorizontalityoftheraftmodelwasadjustedbyalevelandeachpilewasconnectedtotheraftmodelbytwonuts.(4)Theloadingcapwasplacedontheraft.Then,threedialgaugeswerelocated(onedialgaugenearthecenterandtwo   atthemiddlesidesoftheraft).(5)Averticalloadingbarandacalibratedprovingring,of50kNmaximumcapacity,wereconnectedtothehydraulicjack.The jackarmwasloweredslowlytowardtheloadingcap,untilthedialgaugeoftheprovingringstartedtorespond.Theraftmodelwasthenloadedincrementallybyusingthehydraulic jack.Theverticalsettlementswererecordedattheendofeachloadincrement.Therateofloadingwas0.1kN/min.Theloadingwascontinuedtillthesettlementreachedabout25mm. SIB 1600.23 m0.23 m0.23 m0.23 mHydraulic jack     2 .   1   5  m Steel tank 1.0 m   1.0 m   1.0 mWalling frames    1  m Breaker rodBase connectionRolling base20 cm concrete baseSteel plateSteel anchores1.5 m    0 .   2  m   V  e  r   t   i  c  a   l  c  o   l  u  m  n   C   1   4   0 Fig.4. Verticalcrosssectioninthesteeltankandmainframe. 5.Raft-soilstiffnessratio Theshearmodulusofthetestedsandsoilwasdeterminedfrombackanalysisofthemeasuredload-settlementcurvesforsinglepilewith L/D ratioof50.Theshearmodulusofsandsoilwas   assumedtochangelinearlywiththedepthfrom300kPaatthegroundsur-face(i.e.beneaththemodelraft)to G l  attheendofthepilelength.ThecomputerprogramPGROUPdevelopedbyEl-Garhy(2002)was usedtopredicttheelasticload-settlementcurveofsinglepileatdifferentvaluesof  G l .Thebestmatchbetweenmeasuredandpre-dictedvalueswas   obtainedatthevalueof  G l  equalto500kPa.Therefore,thevalueofshearmodulus, G ,inkPaatanydepth,  z  , Fig.5. Photographshowingloadingcapandmeasuringdevices.

Portlets and Tiles

Jul 23, 2017
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