浅基础承载能力学习教案.pptx

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1、会计学1浅基础浅基础(jch)承载能力承载能力第一页，共58页。2Bearing Capacity FailureGeneral shear failureLocal shear failurePunching shear failure第1页/共58页第二页，共58页。3Soil Conditions and BearingCapacity Failure第2页/共58页第三页，共58页。4 Load Displacement Curves(after Vesic(after Vesic (1973)(1973)a)General Shear Failureb)Local Shear Fai

2、lurec)Punching Shear Failure第3页/共58页第四页，共58页。5Comments on Shear Failuren nUsually only necessary to analyze general shear failure.Usually only necessary to analyze general shear failure.n nLocal and punching shear failure can usually be Local and punching shear failure can usually be anticipated by

3、settlement analysis.anticipated by settlement analysis.n nFailure in shallow foundations is generally settlement Failure in shallow foundations is generally settlement failure;bearing capacity failure must be analyzed,but in failure;bearing capacity failure must be analyzed,but in practical terms is

4、 usually secondary to settlement practical terms is usually secondary to settlement analysis.analysis.第4页/共58页第五页，共58页。6Development of Bearing Capacity Theoryn nApplication of limit equilibrium methods first done by Prandtl on the Application of limit equilibrium methods first done by Prandtl on the

5、 punching of thick massespunching of thick masses of metal.of metal.n nPrandtls methods adapted by Terzaghi to bearing capacity failure Prandtls methods adapted by Terzaghi to bearing capacity failure of shallow foundations.of shallow foundations.n nVesicVesic and others improved on Terzaghis origin

6、al and others improved on Terzaghis original theory and added other factors for a more complete analysis theory and added other factors for a more complete analysis第5页/共58页第六页，共58页。7Assumptions for Terzaghis Methodn nDepth of foundation is less than or equal to its widthDepth of foundation is less t

7、han or equal to its widthn nNo sliding occurs between foundation and soilNo sliding occurs between foundation and soil (rough foundation)(rough foundation)n nSoil beneath foundation is homogeneous semi infinite massSoil beneath foundation is homogeneous semi infinite massn nMohr-Coulomb model for so

8、ilMohr-Coulomb model for soiln nGeneral shear failure mode is the governingGeneral shear failure mode is the governing mode(but not the only mode)mode(but not the only mode)第6页/共58页第七页，共58页。8Assumptions for Terzaghis MethodAssumptions for Terzaghis Methodn nNo soil consolidation occursNo soil consol

9、idation occursn nFoundation is very rigid relative to the soilFoundation is very rigid relative to the soiln nSoil above bottom of foundation has no shearSoil above bottom of foundation has no shear strength;is only a surcharge load against the strength;is only a surcharge load against the overturni

10、ng load overturning loadn nApplied load is compressive and applied vertically to the Applied load is compressive and applied vertically to the centroid of the foundationcentroid of the foundationn nNo applied moments presentNo applied moments present第7页/共58页第八页，共58页。9Failure Geometry for Terzaghis M

11、ethodFailure Geometry for Terzaghis Method第8页/共58页第九页，共58页。10Notes on Terzaghis Methodn nSince soil cohesion can be difficult to quantify,conservative values of c(cohesion)Since soil cohesion can be difficult to quantify,conservative values of c(cohesion)should be used.should be used.n nFrictional s

12、trength is more reliable and does not need to be as conservative as cohesion.Frictional strength is more reliable and does not need to be as conservative as cohesion.n nTerzaghis method is simple and familiar to many geotechnical engineers;however,it does not Terzaghis method is simple and familiar

13、to many geotechnical engineers;however,it does not take into account many factors,nor does it consider cases such as take into account many factors,nor does it consider cases such as rectangular foundationsrectangular foundations.第9页/共58页第十页，共58页。11The General Bearing Capacity Equation.The General B

14、earing Capacity Equation.第10页/共58页第十一页，共58页。12The General Bearing Capacity Equation.The General Bearing Capacity Equation.第11页/共58页第十二页，共58页。13The General Bearing Capacity Equation.The General Bearing Capacity Equation.第12页/共58页第十三页，共58页。14Other FactorsOther Factors第13页/共58页第十四页，共58页。15Other Factors

15、Other Factors For continuous footing,s=1 For perpendicular load,i=1 For level foundation,b=1 For level ground,g=1 Need to compute factors -Bearing Capacity Factor N,-Depth Factor d第14页/共58页第十五页，共58页。16Groundwater Effects第15页/共58页第十六页，共58页。17Groundwater EffectsShallow groundwater affects shear streng

16、th in two ways:Shallow groundwater affects shear strength in two ways:n nReduces apparent cohesion that takes place when soilsReduces apparent cohesion that takes place when soils are not saturated;may necessitate reducing the cohesion measured in are not saturated;may necessitate reducing the cohes

17、ion measured in the laboratorythe laboratoryn nPore water pressure increases;reduces both effective stress and Pore water pressure increases;reduces both effective stress and shear strength in the soil(same problem as is experienced with shear strength in the soil(same problem as is experienced with

18、 unsupported slopes)unsupported slopes)第16页/共58页第十七页，共58页。18Groundwater EffectsGroundwater Effects第17页/共58页第十八页，共58页。19FOOTINGS WITH ECCENTRICOR INCLINED LOADINGSINCLINED LOADINGSEccentricityInclination第18页/共58页第十九页，共58页。20FOOTINGS WITH FOOTINGS WITH One WayOne Way EccentricityEccentricity In most i

19、nstances,foundations are subjected to moments in addition to the vertical load as In most instances,foundations are subjected to moments in addition to the vertical load as shown below.In such cases the distribution of pressure by the foundation upon the soil shown below.In such cases the distributi

20、on of pressure by the foundation upon the soil is not uniform.is not uniform.第19页/共58页第二十页，共58页。21第20页/共58页第二十一页，共58页。22第21页/共58页第二十二页，共58页。23FOOTINGS WITH FOOTINGS WITH One WayOne Way EccentricityEccentricityn nNote that in these equations,when the eccentricity Note that in these equations,when the

21、 eccentricity e e becomes becomes B/6B/6,q qminmin is is zerozero.n nFor For e e B/6 B/6,q qminmin will be will be negativenegative,which means that tension will,which means that tension will develop.develop.n nBecause soils can sustain very little tension,there will be a separation Because soils ca

22、n sustain very little tension,there will be a separation between the footing and the soil under it.between the footing and the soil under it.n nAlso note that the eccentricity tends to decrease the load bearing Also note that the eccentricity tends to decrease the load bearing capacity of a foundati

23、on.capacity of a foundation.n nIn such cases,placing foundation column off-center,as shown in In such cases,placing foundation column off-center,as shown in Figure is probably advantageous.Figure is probably advantageous.n nDoing so in effect,produces a centrally loaded foundation with a Doing so in

24、 effect,produces a centrally loaded foundation with a uniformly distributed pressure.uniformly distributed pressure.第22页/共58页第二十三页，共58页。24FOOTINGS WITH FOOTINGS WITH One WayOne Way EccentricityEccentricity第23页/共58页第二十四页，共58页。25Footing with Footing with Two-way EccentricitiesTwo-way Eccentricitiesn n

25、Consider a footing subject to a vertical ultimate load Consider a footing subject to a vertical ultimate load QQultult and a moment and a moment MM as shown in Figures a and b.as shown in Figures a and b.For this case,the components of the moment For this case,the components of the moment M M about

26、the x and y axis are about the x and y axis are MMx x and and MMy y respectively.This condition is equivalent to a load respectively.This condition is equivalent to a load QQ placed eccentrically on the footing with placed eccentrically on the footing with x=x=e eB B and and y=y=e eL L as shown in F

27、igure d.as shown in Figure d.第24页/共58页第二十五页，共58页。26Footing with Footing with Two-way EccentricitiesTwo-way Eccentricities第25页/共58页第二十六页，共58页。27Example 1第26页/共58页第二十七页，共58页。28Example 1第27页/共58页第二十八页，共58页。29Example 2第28页/共58页第二十九页，共58页。30Example 2第29页/共58页第三十页，共58页。31Footings with Inclined Loads第30页/共

28、58页第三十一页，共58页。32Footings with Inclined Loads1.1.Compute the inclination factors using the equations given below:Compute the inclination factors using the equations given below:inclination of load with respect to vertical inclination of load with respect to vertical2.2.Use the inclination factors jus

29、t computed to compute Hansen shape factors asUse the inclination factors just computed to compute Hansen shape factors as 第31页/共58页第三十二页，共58页。33Footings with Inclined Loads3.3.These are used in the following modifications of the edited“These are used in the following modifications of the edited“Hans

30、en bearing capacity equation:Hansen bearing capacity equation:Use the smaller value of qutut computed by either of Equations.第32页/共58页第三十三页，共58页。34The Bearing Capacity of Multi-Layered Soils第33页/共58页第三十四页，共58页。35The Bearing Capacity of Layered SoilsThe Bearing Capacity of Layered Soils第34页/共58页第三十五页

31、，共58页。36The Bearing Capacity of Layered SoilsThe Bearing Capacity of Layered Soilsn nIn layered soil profiles,the unit weight of the soil,the angle of friction and In layered soil profiles,the unit weight of the soil,the angle of friction and the cohesion are not constant throughout the depth.The ul

32、timate surface the cohesion are not constant throughout the depth.The ultimate surface failure may extend through two or more of the soil layers.failure may extend through two or more of the soil layers.n nConsider the case when the stronger soil is underlain by a weaker soil.If Consider the case wh

33、en the stronger soil is underlain by a weaker soil.If HH,the,the thickness of the layer of soil below the footing,is relatively large then the failure thickness of the layer of soil below the footing,is relatively large then the failure surface will be completely located in the top soil layer,which

34、is the upper limit surface will be completely located in the top soil layer,which is the upper limit for the ultimate bearing capacity.for the ultimate bearing capacity.n nIf the thickness If the thickness HH is small compared to the foundation width is small compared to the foundation width B B,a p

35、unching,a punching shear failure will occur at the top soil stratum,followed by a general shear failure in the bottom shear failure will occur at the top soil stratum,followed by a general shear failure in the bottom soil layer.soil layer.n nIf If HH is relatively deep,then the shear failure will oc

36、cur only on the top soil layer.is relatively deep,then the shear failure will occur only on the top soil layer.第35页/共58页第三十六页，共58页。37The Bearing Capacity of Layered SoilsThe Bearing Capacity of Layered Soilsn nMeyerhof and Hanna(1978)and Meyerhof(1974)Meyerhof and Hanna(1978)and Meyerhof(1974)第36页/共

37、58页第三十七页，共58页。38第37页/共58页第三十八页，共58页。39第38页/共58页第三十九页，共58页。40第39页/共58页第四十页，共58页。41第40页/共58页第四十一页，共58页。42第41页/共58页第四十二页，共58页。43The Bearing Capacity of Layered SoilsThe Bearing Capacity of Layered Soilsn nMeyerhof and Hannas punching shear coefficient Ks第42页/共58页第四十三页，共58页。44The Bearing Capacity of Lay

38、ered SoilsThe Bearing Capacity of Layered Soilsn nVariation Variation of of c ca a/c c1 1 with with q q2 2/q q1 1based on the based on the theory of theory of Meyerhof and Meyerhof and Hanna(1978)Hanna(1978)第43页/共58页第四十四页，共58页。45Example on layered soils第44页/共58页第四十五页，共58页。46Example on layered soils第

39、45页/共58页第四十六页，共58页。47Example on layered soils第46页/共58页第四十七页，共58页。48Ground Factors第47页/共58页第四十八页，共58页。49Base Factor n nFor footings with angled foundation basesFor footings with angled foundation basesn n When footing is level,b=1 When footing is level,b=1第48页/共58页第四十九页，共58页。50RigidityRigidityFactors

40、Factors第49页/共58页第五十页，共58页。51Bearing Capacity from Field Tests第50页/共58页第五十一页，共58页。52Bearing Capacity from SPT第51页/共58页第五十二页，共58页。53Bearing Capacity from SPT第52页/共58页第五十三页，共58页。54Bearing Capacity from SPT第53页/共58页第五十四页，共58页。55Bearing Capacity using CPT第54页/共58页第五十五页，共58页。56Bearing Capacity for Field B

41、earing Capacity for Field Load Tests Load Tests PLTPLT第55页/共58页第五十六页，共58页。57Bearing Capacity for Field Load Tests Bearing Capacity for Field Load Tests PLTPLTn nFor Granular Soils:For Granular Soils:n nFor Cohesive Soils:For Cohesive Soils:第56页/共58页第五十七页，共58页。58Correction of Standard penetration num

42、berCorrection of Standard penetration numbern nIt has been suggested that the SPT be standardized to some energy ratio It has been suggested that the SPT be standardized to some energy ratio E Er r which which should be computed asshould be computed asn nNote that larger values of Note that larger v

43、alues of E Er r decrease decrease the blow count the blow count N N nearly linearly,that is,nearly linearly,that is,E Er45 r45 gives gives N=20 N=20 and and E Er90 r90 gives N=10;gives N=10;n nExample of Example of N N for for E Er45r45=20 20 we obtain for the arbitrarily chosen we obtain for the arbitrarily chosen E Er r =70,(70,(E Er70r70):):N for Er70=13第57页/共58页第五十八页，共58页。