砂土地基变形及承载力试验研究论文英文版.docx
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砂土地基变形及承载力试验研究论文英文版.docx
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砂土地基变形及承载力试验研究论文英文版
DeflectionsandBearingCapacitiesofSandGrounds
UnderTheModelShallowFoundation
1Preface
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% 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
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- 关 键 词:
- 砂土 地基 变形 承载力 试验 研究 论文 英文
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