砂土地基变形及承载力试验研究论文英文版Word格式文档下载.docx

砂土地基变形及承载力试验研究论文英文版Word格式文档下载.docx

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2ExperimentalEquipment

3DeflectionandSettlement

3.1InducedStressesbeneathShallowFoundations

Thebearingpressurefromshallowfoundationsinducesaverticalcompressivestressintheunderlyingsoils.Wecallthisstress

becauseitisthechangeinstressthatissuperimposedontheinitialverticalstress:

Where:

=inducedverticalstressduetoloadfromfoundation；

=stressinfluencefactor

=bearingpressurealongbottomoffoundation

=verticaleffectivestressatadepthofDbelowthegroundsurface

3.2Boussinesq’sMethod

Boussinesq（1885）developedtheclassicsolutionforinducedstressinanelasticmaterialduetoappliedpointload.Newmark（1935）thenintegratedtheBoussinesqequationtoproduceasolutionfor

atadepth

beneaththecornerofarectangularfoundationofwidthBandlengthL.Thissolutionproducesthefollowingtwoeqations:

Squarefooting

Continuousfooting

4BearingCapacityofnatureground

4.1BearingCapacityFailures

Generalshearfailureisthemostcommonmode．Itoccursinsoilsthatarerelativelyincompressibleandreasonablystrong，inrock,andinsaturated，normallyconsolidatedclaysthatareloadedrapidlyenoughthattheundrainedconditionprevails.Thefailuresurfaceiswelldefinedandfailureoccursquitesuddenly.Aclearlyformedbulgeappearsonthegroundsurfaceadjacenttothefoundation.Althoughbulgesmayappearsonbothsideofthefoundation,ultimatefailureoccursononesideonly,anditisoftenaccompaniedbyrotationofthefoundation.

Theoppositeextremeisthepunchingshearfailure.Itoccursinveryloosesand,inathincrustofstrongsoilunderlainbyaveryweaksoil,orinweakclaysloadedunderslow,drainedcondition.Thecompressibilityofsuchsoilprofilescauseslargesettlementandpoorlydefinedverticalshearsurface.Littleornobulgingoccursatthegroundsurfaceandfailuredevelopsgradually.

Localshearfailureisanintermediatecase.Theshearsurfacesarewelldefinedunderthefoundation,andthenbecamevaguenearthegroundsurface.Abulgemayoccurs,butconsiderablesettlement,perhapsontheorderofhalfthefoundationwidth,isnecessarybeforeaclearshearsurfaceformsneartheground.Eventhen,asuddenfailuredoesnotoccur,asperhapsinthegeneralshearcase.Thefoundationjustcontinuestosinkintotheground.

Ves

（1973）investigatedthesethreemodesoffailurebyconductingloadtestsonmodelcircularfoundationsinsand.Thesetestsincludebothdeepfoundationsandshallowfoundations.Theresultsindicateshallowfoundations（D/Blessthanabout2）canfailinanyofthethreemodes,dependingontherelativedensity.However,deepfoundations（D/Bgreaterthanabout4）arealwaysgovernedbypunchingfailure.AlthoughthesetestresultsapplyonlytocircularfoundationsinVes

’ssandandcannotnecessarilygeneralizedtoothersoils,itdosegiveageneralrelationshipbetweenthemodeoffailure,relativedensity,andtheD/Bratio.

Completequantitativecriteriahaveyettobedevelopedtodeterminewhichofthesethreemodesoffailurewillgoverninanygivencircumstance,butthefollowingguidelinesarehelpful:

.Shallowfoundationsinrockandundrainedclaysaregovernedbygeneralshearfailure.

.Shallowfoundationsindensesandsaregovernedbythegeneralshearcase.Inthiscontext,adensesandisonewitharelativedensity,Dr,greaterthanabout67%.

Shallowfoundationsinloosetomediumdensesands（30%<

Dr<

67%）areprobablygovernedbylocalshearfailure.

Shallowfoundationsonveryloosesand（Dr<

30%）areprobablygovernedbypunchingshearfailure.

4.2MethodsofAnalyzingBearingCapacity

Full-scaleloadtests,whichconstructingrealspreadfootingsandloadingthemtofailure,arethemostprecisewaytoevaluatebearingcapacity.Howeversuchtestsareveryexpensive,andthusarerarely,ifever,performedasapartofroutinedesign.Afewsuchtestshavebeenperformedforresearchpurpose.

Modelfootingtestshavebeenusedquiteextensively,mostlybecausethecostofthesetestsisfarbelowthatforfull-scaletests.Unfortunately,modeltestshavetheirlimitations,especiallywhenconductedinsands,becauseofuncertaintiesinapplyingproperscalingfactors.However,theadventofcentrifugemodeltestshaspartiallyovercomethisproblem.

Limitequilibriumanalysesarethedominantwaytoassessbearingcapacityofshallowfoundations.Theseanalysesdefinetheshapeofthefailuresurface,thenevaluatethestressandstrengthsalongthissurface.ThesemethodsofanalysishavetheirrootsinPrandtl’sstudiesofthepunchingresistanceofmetals（Prandtl,1920）.Heconsideredofverythickmassesofmetal（i.e.,notsheetmetal