土的工程绪论土的工程 (162).pdf
5.5.2Consolidated undrained and consolidated drained shear strengthFor the consolidated undrained shearstrength,becausetheconsolidatedundrained test is to fully drain andconsolidate first,there is no pore waterpressure in the sample,and then thedrain valve is closed and the sample issheared under the deviator stress untilthe sample is failure.At this time,thepore water pressure is generated in thesample.Drain and consolidateUndrainedshear failuresampleConsolidated undrained shear strength土力学土力学Soil MechanicsSoil MechanicsProf.Prof.H.J.LiaoH.J.LiaoThe consolidated undrained shearstrength of saturated cohesive soil isaffected by the stress history,whichrequires to distinguish whether thesample is normally consolidated oroverconsolidated.Drain and consolidateUndrainedshear failure土力学土力学Soil MechanicsSoil MechanicsProf.Prof.H.J.LiaoH.J.Liaosample13The consolidated undrained test results of the normal consolidated saturated cohesive soilo 31uEffective stress circleTotal stress circlecuIt can be seen that the characteristics of the consolidated undrainedshear strength of the normally consolidated sample is that the volumetends to decrease when shearing,and is called shear contraction.Normally consolidated sampleAccording to the principle of effective stress,theeffective stress circle is on the left of the total stress circle,and their diameters are equal.Both the total stressfailure envelope and the effective stress failure envelopepass through the origin,which means that the soil withzero consolidation pressure will not have shear strength.土力学土力学Soil MechanicsSoil MechanicsProf.Prof.H.J.LiaoH.J.LiaoThe consolidated undrained test result of overconsolidated saturated clayey soilEnvelope of shear strengthocuNormal consolidated conditionOverconsolidated conditionbcaccuConsolidated saturated cohesive soil 土力学土力学Soil MechanicsSoil MechanicsProf.Prof.H.J.LiaoH.J.LiaoThe consolidated undrained test result of overconsolidated saturated clayey soilocuccu1331u1u2cTheshearstrengthcharacteristicsoftheoverconsolidatedsampleis that the volume tends toincreaseduringshearfailure,and is called sheardilatancy.Thenegativeporewaterpressuregeneratedundertheundrained conditions,andthe effective stress circlesare on the right side of thetotal stress circles.Overconsolidated samplesNormal consolidated conditionOverconsolidated condition土力学土力学Soil MechanicsSoil MechanicsProf.Prof.H.J.LiaoH.J.LiaoInengineeringpractice,theeffectiveshearstrengthofconsolidatedundrainedsoilisusually used to analyze the long-term stability or long-term bearingcapacity of foundation,such as thelong-term stability of soil slope,thelong-term bearing capacity of softfoundation soil and so on.In thiscase,the effective stress methodshould be adopted for analysis.土力学土力学Soil MechanicsSoil MechanicsProf.Prof.H.J.LiaoH.J.LiaoFor the consolidated drained shear strength,because the drain valve ofconsolidated drainage test always remains open,the pore water pressurein the sample is always zero.13The consolidated drained test result of normally consolidated saturated cohesive soilod=Thecharacteristicsoftheconsolidated drained shearstrengthofnormallyconsolidated samples is thatthe total stress circle is theeffectivestresscircle,thetotal stress shear strengthenvelopeistheeffectivestressshearstrengthenvelope,and the envelopepasses through the origin.土力学土力学Soil MechanicsSoil MechanicsProf.Prof.H.J.LiaoH.J.LiaoThe consolidated drained test result of the overconsolidated saturated cohesive soilod=13cd=cThecharacteristicsofthe consolidation drainageshearstrengthoftheoverconsolidatedsamplesare:thevolumehasanincreasing trend when theshear failure,thereis awaterabsorptiontrendbecause of shear dilatancyunderthedrainageconditions,and the watercontent increases.Consolidation drainage shear strength of the overconsolidated samples土力学土力学Soil MechanicsSoil MechanicsProf.Prof.H.J.LiaoH.J.LiaoIn engineering practice,the consolidateddrained shear strength of soil is usually used toanalyzetheconstructionsitewithgooddrainage conditions and the construction withslow loading,e.g.,there are sand interlayers inclay stratum.土力学土力学Soil MechanicsSoil MechanicsProf.Prof.H.J.LiaoH.J.LiaoStatementStatement:Thanks for some pictures and materials fromthe Internet used in this courseware only for teaching.
