geotechnical engineering 13外语英文电子书

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1、CHAPTER13SHALLOWFOUNDATIONII:SAFEBEARINGPRESSUREANDSETTLEMENTCALCULATION13.1INTRODUCTIONAllowableandSafeBearingPressuresThemethodsofcalculatingtheultimatebearingcapacityofsoilhavebeendiscussedatlengthinChapter12.Thetheoriesusedinthatchapterarebasedonshearfailurecriteri

2、a.Theydonotindicatethesettlementthatafootingmayundergoundertheultimateloadingconditions.Fromtheknownultimatebearingcapacityobtainedfromanyoneofthetheories,theallowablebearingpressurecanbeobtainedbyapplyingasuitablefactorofsafetytotheultimatevalue.Whenwedesignafoundatio

3、n,wemustseethatthestructureissafeontwocounts.Theyare,1.Thesupportingsoilshouldbesafefromshearfailureduetotheloadsimposedonitbythesuperstructure,2.Thesettlementofthefoundationshouldbewithinpermissiblelimits.Hence,wehavetodealwithtwotypesofbearingpressures.Theyare,1.Apre

4、ssurethatissafefromshearfailurecriteria,2.Apressurethatissafefromsettlementcriteria.Forthesakeofconvenience,letuscallthefirsttheallowablebearingpressureandthesecondthesafebearingpressure.Inallourdesign,weuseonlythenetbearingpressureandassuchwecallqnathenetallowablebear

5、ingpressureandqsthenetsafebearingpressure.Indesigningafoundation,weuse545546Chapter13theleastofthetwobearingpressures.InChapter12welearntthatqnaisobtainedbyapplyingasuitablefactorofsafety(normally3)tothenetultimatebearingcapacityofsoil.Inthischapterwewilllearnhowtoobta

6、inqs.Evenwithoutknowingthevaluesofqnaandqs,itispossibletosayfromexperiencewhichofthetwovaluesshouldbeusedindesignbaseduponthecompositionanddensityofsoilandthesizeofthefooting.Thecompositionanddensityofthesoilandthesizeofthefootingdecidetherelativevaluesofqnaandqs.Theul

7、timatebearingcapacityoffootingsonsandincreaseswithanincreaseinthewidth,andinthesamewaythesettlementofthefootingincreaseswithincreasesinthewidth.Inotherwordsforagivensettlement5pthecorrespondingunitsoilpressuredecreaseswithanincreaseinthewidthofthefooting.Itistherefore,

8、essentialtoconsiderthatsettlementwillbethecriterionforthedesignoffootingsinsandbeyondaparticularsize.Experimentalevid