基础工程英文复习课件

第一章3/14/20231基础工程基础工程研究下部结构物与岩土相互作用共同承担上部结构物所产生各种强度、变形与稳定问题。桩、墩基、箱基、支挡结构、围护结构、水泥搅拌体，砂石桩体，锚固，土钉、砂井等等，都可视为结构物与岩土相互作用问题。FactorsofSafetyDependsonRequiredreliabilityConsequencesofafailureUncertaintiesinsoilpropertiesandappliedloadsConstructiontolerancesIgnoranceofthetruebehavioroffoundationsCost-benefitratioofadditionalconservatisminthedesign.0.75[]a0.75[]aAllowablestressdesign(useinthisbook)Thedesignloadisthemostcriticalcombinationofthevariousloadsources,asdefinedbycodes.ANSI/ASCE， D (2.1) D+L+F+H+T+(Lr

orSorR) (2.2) D+L+(LrorSorR)+(WorE) (2.3) D+(WorE) (2.4)3/14/20235（1）由可变荷载控制时：组合值系数c3/14/20236（2）由永久荷载控制时：组合值系数c3/14/20237ServiceabilityrequirementsIntendedtoproducefoundationsthatperformwellwhensubjectedtotheserviceloads.The

Requirementsinclude:SettlementHeave抬升TiltLateralmovementVibrationDurability3/14/20239MeasuresforalargesettlementAdjustthefoundationdesignUseamoreelaboratefoundationImprovethepropertiesofthesoilRedesignthestructuresoitismoretolerantofsettlements3/14/202310DifferentialsettlementsDefinitionThedifferentialsettlementisthedifferenceintotalsettlementbetweentwofoundationorbetweentwopointsonasinglefoundation.ReasonsforDifferentialsettlements:ThesoilpronotbeuniformacrossthesiteTheratiobetweentheactualloadandthedesignloadmaybedifferentforeachcolumn.Theratioofdeadloadtoliveloadmaybedifferentforeachcolumn.Theas-builtfoundationdimensionsmaydifferfromtheplandimensions.3/14/202311Whatisshallowfoundations?d/b<=2.5ShallowfoundationsThosetransmitstructuralloadstothenear-surfacesoil.Types:Spreadfootingfoundations(扩展基础)Matfoundations（筏板基础）dbdb3/14/202313Foundationswitheccentricormomentloads

One-wayloading

summary1.Shallowfoundationsarethosethattransmitstructuralloadstothenear-surfacesoils.Therearetwokinds:spreadfootingfoundationsandmatfoundations.2.Althoughothermaterialshavebeenusedinthepast,todayvirtuallyallshallowfoundationsaremadeofreinforcedconcrete.3.Spreadfootingsaremostoftenusedinsmall-tomedium-sizestructuresonsiteswithmoderatetogoodsoilconditions.Matsaremostoftenusedonlargerstructures,especiallythosewithdifferentialsettlementproblemsandthosewithfoundationsbelowthegroundwatertable.4.Thebearingpressureisthecontactpressurebetweenthebottomofashallowfoundationandtheunderlyingsoils.3/14/202317summary5.Afloatingfoundationisonewheretheweightofthefoundationissubstantiallylessthantheweightoftheexcavatedsoils.Thisoccursinbuildingswithbasementsandothersimilarstructures.6.Iftheloadsappliedtoafoundationareeccentric,orifmomentloadsareapplied,theresultingbearingpressuredistributionalsowillbeeccentric.Insuchcases,thefoundationneedstobedesignedsotheresultantofthebearingpressureiswithinthemiddlethirdofthefoundation(forone-wayeccentricity)orinadiamond-shapedkern(fortwo-wayeccentricity).Thisrequirementensurestheentirebaseofthefoundationhascompressivebearingpressures,andthusavoidsproblemswithuplift.3/14/202318第六章3/14/202319BearingcapacityfailurePunchingfailurecaseVeryloosesandAthincrustofstrongsoilunderlainbyaveryweaksoilWeakclaysunderdrainedconditionsFailuredevelopsgradually.3/14/202321BearingcapacityfailureLocalfailurecase-theintermediatecaseLoosetomediumsandsShearsurfacearewelldefinedunderthefoundation,andthenbecomevaguenearthegroundsurface.Failuredevelopsgradually.3/14/202322Example6.3A30-mby50-mmatfoundationistobebuiltasshowninFigure6.10.Computetheultimatebearingcapacity.3/14/202323Terzaghi’sbearingcapacityformulasAssumptions:D≤BNoslidingbetweenthefoundationandthesoil.Semi-infinitemassanduniformMCstrengthlawGeneralshearcaseNoconsolidationRigidfoundationSoilabovethefoundationbottomisNoshearstrengthCentralloading3/14/202325Terzaghi’sbearingcapacityformulas

3/14/202326

3/14/202329BearingcapacityonlayersoilsMethods:Evaluatethebearingcapacityusingthelowestvaluesofc',f'andg'inthezonebetweenthebottomofthefoundationandadepthBbelowthebottom.Evaluatethebearingcapacityusingtheweightedaveragevaluesofc',f'andg'inthezonebetweenthebottomofthefoundationandadepthBbelowthebottom.Rigorousanalysisuselimitmethod,similartoslopestabilityanalysis.3/14/202330第7章3/14/202331ThecausesofsettlementTheapplicationofstructuralloadsTheweightofarecentlyplacedfillAfallinggroundwatertableUndergroundminingortunnelingTheformationofsinkholesSecondarycompressionoftheunderlyingsoilsLateralmovementsresultingfromnearbyexcavations……Consolidationsettlement(NC)

Consolidationsettlement(OC-I)

Consolidationsettlement(OC-II)

EXAMPLE7.4TheallowablesettlementfortheproposedcontinuousfootinginFigure7.9is25mm.Uputeitssettlementanddetermineifitsatisfiesthiscriterion.

见课本224Settlementanalysesbasedonin-situtests

Schmertmann’sMethod

EsfromStandardPenetrationTest(SPT)resultsSoilTypeβ0(kPa)β1(kPa)Cleansands5,0001,200Siltysandsandclayeysands2,500600第八章3/14/202341Question:

canDbe0?MostoftheBearingCapacitywillbelost.easytobeundercutbyscour(冲刷),

especiallyonbridgepiers(桥墩)Thetopsoilusuallyispoorinstrength.Unabletoresisthorizontalload,orverypoorifany.Easytobeimpactedbytheweather.Frostheave（冻胀）

3/14/202342MakeDshallower,ifpossible

(bothbearingcapacityandsettlementaresatisfied.).Savetime,moneyandmaterial宽基浅埋MakeDabovethegroundwaterlevelinconstructionstage,ifpossible.OthergeneralprinciplestodetermineD3/14/202343TheinfluenceoftheadjacentbuildingsWhenanewfootingiscarriedoutnearaoldoneswhatwillhappen??

挖墙脚3/14/202344TheproceduretodevelopqABearingcapacityDthesmallestoneDw

TheshallowestoneFSafetyfactoragainstabearingfailure(PerFig.6.11)qaThesmallestappliednormalload.Bearing.xls,orqult_tzgorqult_vscSettlementdaanddDaAllowablesettlementandallowabledifferentialsettlementCh2dD/dTable7.5orlocalexperienceda=Min(da,dDa/(dD/d))由differentialsettlement算出来的允许沉降和由允许沉降的最小值。SettlementanalysisThelargestappliednormalloadSettlement.xlsorS4chmertmann.xls.keepd<da→q

3/14/202345Equivalentbearingpressuremethod

3/14/202346Example8.3GivenB=L=5ftD=2ftP=80kM=60ft-kqA=3500lb/ft2Dw=greatdepth.CheckSatisfactory?Solution3/14/202347试算法步骤：（1）进行深度修正，初步确定修正后的地基承载力特征值fa。（2）根据荷载偏心情况，将按轴心荷载作用计算得到的基底面积增大10%~40%，即取（3）选取基底长边l与短边b的比值n（一般取n≤2），于是有国标：2.偏心荷载作用要求：pk≤fapkmax≤1.2fae≤l/6（或pkmin≥0）基底最大压力Designforshearloads

3/14/202350Example8.4GivenB=L=6ftD=2.5ftDw=greatdepthP=112kV=20kf'=29o(siltyfine-to-mediumsand)CheckOK?Solution3/14/202351Conditionsforfrostheave容易出现冻状的土层ThereisanearbysourceofwaterThesoilisfrost-susceptible.Cleansandandgravel?×Clays?×Siltandfinesand?√Frostline冻深线3/14/202352第九章Spreadfootings

structuraldesign3/14/202353DesignloadsLRFDmethodFactoredloads3/14/202354Loadfactor,g

AmericanConcreteInstitute(ACI)CodeU=1.4D+1.7L (2.7)U=0.75(1.4D+1.4T+1.7L)U=0.9D+1.4FU=1.4D+1.7L+1.4FU=1.4D+1.7L+1.7H……U=1.4(D+T) (2.17)ANSI/ASCEandAISECodeU=1.4D (2.18)U=1.2(D+F+T)+1.6(L+H)+0.5(LrorSorR)U=1.2D+1.6(LrorSorR)+(0.5Lor0.8W)……U=0.9D+(1.3Wor1.0E) (2.23)3/14/202355summaryTheplandimensionsandminimumembedmentdepthofaspreadfootingaregovernedbygeotechnicalconcerns,andaredeterminedusingtheunfactoredloads.Thethicknessandreinforcementofaspreadfootingaregovernedbystructuralconcerns.StructuraldesignisgovernedbytheACIcode,whichmeanstheseanalysesarebasedonthefactoredload.Thestructuraldesignofspreadfootingsmustconsiderbothshearandflexural

failuremodes.Ashearfailureconsistsofthecolumnorwallpunchingthroughthefooting,whileaflexuralfailureoccurswhenthefootinghasinsufficientcantileverstrength.3/14/202356Sincewedonotwishtousestirrups(shearreinforcement),weconducttheshearanalysisfirstandselectaneffectivedepth,d,sothefootingthatprovidesenoughshearresistanceintheconcretetoresisttheshearforceinducedbytheappliedload.Thisanalysisignorestheshearstrengthoftheflexuralsteel.5.Oncetheshearanalysisiscompleted,weconductaflexuralanalysistodeterminetheamountofsteelrequiredtoprovidetheneededflexuralstrength.Sincedislarge,therequiredsteelareawillbesmall,anditisoftengovernedbyrmin.6.Forsquarefootings,usethesameflexuralsteelinbothdirections.Thus,thefootingisreinforcedtwice.3/14/2023577.Forcontinuousfootings,thelateralsteel,ifneeded,isbasedonaflexuralanalysis.Usenominallongitudinalsteeltoresistnonuniformitiesintheloadandtoaccommodateinconsistenciesinthesoilbearingpressure.8.Designrectangularfootingssimilartosquarefootings,butgroupagreaterportionoftheshortsteelnearthecenter.9.Practicalminimumdimensionswilloftengovernthedesignoflightlyloadedfootings.3/14/202358Ch10Mats(筏板基础)3/14/202359DesignmethodsRigidmethodstheconventionalmethod,theconventionalmethodofstaticequilibriumAssumption:ThematismuchmorerigidthantheunderlyingsoilsNoflexuraldeflectionBearingpressuredistributionislinear倒梁法DrawbacksoftheRigidmethodWidth-to-thicknessislargerformatfoundations.Notconsiderredistributionofbearingpressure.Cannotproducereliableestimatesofshears,momentsanddeformationsinthematNonrigidmethods

consideringsoil-structureinteractionWinklermethodCoupledmethodPseudo-coupledmethodMultiple-ParametermethodFiniteelementmethodTreatsoilsas“springs”Treatsoilsas“soil”WinklermethodLinearspringsNointeractionbetweensprings.Pseudo-coupledmethodSimilartowinklemodel,butHavedifferentksvaluesdependingontheirlocationks_outermost=2ks_innermost

summary1.Matfoundationsareessentiallylargespreadfootingsthatusuallyencompasstheentirefootprintofastructure.Theyareoftenanappropriatechoiceforstructuresthataretooheavyforspreadfootings.2.Theanalysisanddesignofmatsmustincludeanevaluationoftheflexuralstressesandmustprovidesufficientflexuralstrengthtoresistthesestresses.3.Theoldestandsimplestmethodofanalyzingmatsistherigidmethod.Itassumesthatthematismuchmorerigidthantheunderlyingsoil.whichmeansthemagnitudeanddistributionofbearingpressureiseasytodetermine.Thismeanstheshears,moment,anddeformationsinthematareeasilydetermined.However,thismethodisnotanaccuraterepresentationbecausetheassumptionofrigidityisnotcorrect.4.Nonrigidanalysesaresuperiorbecausetheyconsidertheflexuraldeflectionsinthematandthecorrespondingredistribution

ofthesoilbearingpressure.5.Nonrigidmethodsmustincludeadefinitionofsoil-structureinteraction.Thisisusuallydoneusinga"bedofsprings"analogy,witheachspringhavingalinearforce-displacementfunctionasdefinedbythecoefficientofsubgradereaction,ks.6.ThesimplestandoldestnonrigidmethodistheWinklermethod,whichusesindependentsprings,allofwhichhavethesameks.Thismethodisanimprovementoverrigidanalyses,butstilldoesnotaccuratelymodelsoil-structureinteraction,primarilybecauseitdoesnotconsidercouplingeffects.7.ThecoupledmethodisanextensionoftheWinklermethodthatconsiderscouplingbetweenthesprings.8.Thepseudo-coupledmethodusesindependentsprings,butadjuststheks

valuestoimplicitlyaccountforcouplingeffects.9.Themultipleparameterandfiniteelementmethodsaremoreadvancedwaysofdescribingsoil-structureinteraction.10.Theks

isdifficult

to

determine.Fortunately,thematdesignisoftennotoverlysensitivetoglobalchangesinks.Parametricstudiesareoftenappropriate.11.IftheWinkler

methodisusedtodescribesoil-structureinteraction,andthematgeometryisnottoocomplex,thestructuralanalysismaybeperformedusingclosed-formsolutions.However,thesemethodsaregenerallyconsideredobsolete.12.Moststructuralanalysesareperformedusingnumericalmethods,especiallythefiniteelementmethod.Thismethodusesfiniteelementstomodelthemat,anddefinessoil-structureinteractionusingtheWinkerorpseudo-coupledmodels.Inprinciple,italsocouldusethemultipleparametermodel.13.Adesigncouldbebasedentirelyonathree-dimensionalfiniteelementanalysisthatincludesthesoil,matandsuperstructure.However,suchanalysesarebeyondcurrentpractices,mostlybecausetheyaredifficulttosetupandrequireespeciallypowerfulcomputers.14.ThetotalsettlementisbestdeterminedusingthemethodsdescribedinChapter7.Donotusethecoefficientofsubgradereactiontodeterminetotalsettlement.15.Bearingcapacityisnotaproblemwithsandsandgravels,butcanbeimportantinsiltsandclays.ItshouldbecheckedusingthemethodsdescribedinChapter6.Ch11deepfoundationsG.Y.LUO3/14/202368definitiondeepfoundationisonethattransmitssomeoralloftheappliedloadtosoilswellbelowthegroundsurface.3/14/202369ApplicableconditionsTheuppersoilsaresoweakand/orthestructuralloadssohighthatspreadfootingswouldbetoolarge.Theuppersoilsaresubjecttoscourorundermining.Thefoundationmustpenetratethroughwater.suchasthoseforapier.Alargeupliftcapacityisrequired.Alargelateralloadcapacityisrequired.Therewillbeafutureexcavationadjacenttothefoundation.3/14/202370LoadtransferPiles(预制桩)DefinitionPilesareconstructedbyprefabricatingslenderprefabricatedmembersanddrivingorotherwiseforcingthemintotheground.TypeTimberpilesConcretepilesSteelpilesCompositepiles3/14/202372Drilledshafts(灌注桩)Drilledshaftsareanothercommontypeofdeepfoundation.Thefundamentaldifferencebetweenpilesanddrilledshaftsisthatpilesareprefabricatedmembersdrivenintotheground,whereasdrilledshaftsarecast-in-place.Type沉管灌注桩钻（冲、磨）孔灌注桩挖孔桩旋挖桩3/14/202373AdvantageCompactionincreasesthestrengthandbearingcapacitySandyandgravellysoilsImprovingthesidefrictionresistanceLargebasesDisadvantageGeneratelargegroundvibrationEquipmentisbulkyCompactedshaftscannotincludelargeamountsofreinforcingsteel.ExpensiveEconomicalonlywhenthelengthislessthanabout9mPressure-injectedfootings(夯扩桩)Ch13deepfoundations

axialloadcapacitybasedonstaticloadtestsFactorofsafetypiles≤Davisson’smethodP:kND:mmB:mmA:m2E:kPaExample13.3

Ch14deepfoundations

axialloadcapacitybasedonanalyticmethodsG.Y.LUO3/14/202379ChangesinthesoilduringconstructionPilesChangesinclaysDistortionCompressionandexcessporewaterpressureLossofcontactbetweenpileandsoilChangesinsandParticlerearrangement,crushing,anddensification.DrilledshaftsSide-frictionandtoe-bearingReductionNotethatforpiles,itisincrease(permanentally).DrillingmudSmearandremoldsImpactonAnalyticdesignmethodsMostoftheanalyticmethodsdescribedinthischapterareassociatedwithaparticulartypeofdeepfoundation.3/14/202380Toebearinginsands

effectivestressanalysis

3/14/202381

3/14/202382Clays

undrainedanalysis

3/14/202383Sidefriction

3/14/202384sands

3/14/202385

LateralearthpressurecoefficientSoil-foundationinterfacefrictionangle

SoilfrictionangleSandDifficulttodetermine.ClayEasytodetermine.3/14/202386sands

3/14/202387Gravels

3/14/202388Siltsandclays

3/14/202389Ch14deepfoundations

axialloadcapacitybasedonanalyticmethodsG.Y.LUO3/14/202390Increasedinloadcapacitywithtime孔隙水压力的减少将导致承载能力的增加3/14/202391

3/14/202392Clays

undrainedanalysis

3/14/202393Sidefriction

3/14/202394

LateralearthpressurecoefficientSoil-foundationinterfacefrictionangle

SoilfrictionangleSandDifficulttodetermine.ClayEasytodetermine.3/14/202395sands

3/14/202396Gravels

3/14/202397Siltsandclays

3/14/202398

Piles

3/14/202399

Upwardloadcapacity

forfoundationwithenlargedbase3/14/2023100Upwardloadcapacity

3/14/2023101Load-SettlementResponse

3/14/2023102ImaginaryfootingmethodApplicableconditions:deepfoundationsunderlainbycompressiblesoilssuchthatthecompressionofthesesoilsismoresignificantthanthesettlementsrequiredtomobilizethesidefrictionandtoebearing.zi=0.67D:forfrictionpileszi=D:forend-bearingpilesLinearinterpolationforothers

3/14/2023103CH16Deepfoundation-lateralloadcapacityG.Y.LUO3/14/2023104Shortvs.longfoundationsShortfoundation:Thetoeisabletorotate.ThefailureiscontrolledprimarilybythesoilsLongfoundationThetoeisfixedThefailureiscontrolledprimarilybythefoundationFlexuralfailureFortimberpilesD/B>20Forsteelandconcretepile,D/B>353/14/2023105RigidanalysisAssumeEIisinfinite.NotaccurateasthenonrigidmethodUsedprimarilyforlightweightshortlateralloadedfoundationStreetlightsSmallhighwaysigns3/14/20231061.Alateralloadisanyloadthatactsperpendiculartothefoundationaxis.Thus,shearmomentarelateralloads,butaxialcompressionortensionortorsionalloadsarenot.2.Untilthemiddleofthetwentiethcentury,engineersassumedthatdeepfoundationswereonlyabletoresistaxialloads,sotheyusedbatterpilestoresisthorizontalloads.Morerecently,wehavereconsideredthatassumptionandnowrelyonbothaxialandlateralcapacities.3.Theutilizationoflateralcapacitiesindesignoftenproducesfoundationsthataremoreeconomicaltobuild