土的工程绪论土的工程 (183).pdf

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1、6.7 Determining bearing capacity according to code for design of building foundationAccording to a large number of engineeringpractice experiences,the Chinese national standard,i.e.,“code for design of building foundation”,hassummarizedtherichengineeringpracticeexperiencesinChinabase on thetheoretic

2、alresearch on subsoil bearing capacity.Based onbasic physical indexes of soil and in-situ test results,such as:field loading test,standard penetration test,portable penetration test and indoor geotechnicaltest and their statistical data,and considering acertain degree of safety,the national standard

3、 givesthe determination method of bearing capacity.Determine the bearing capacity according to code for design of building foundation土力学土力学Soil MechanicsSoil MechanicsProf.Prof.H.J.LiaoH.J.Liao ffbd(3.0)(0.5)aakbd0Calculation formulafaThe modified characteristic value of bearing capacity;fakThe char

4、acteristic value of bearing capacity,which canbeobtained from field load test and other in-situ tests,and can alsobe calculated by the formula and determined in combinationwith engineering experience;（a）Base widthBuried depthCalculation formula of bearing capacity determined by the code土力学土力学Soil Me

5、chanicsSoil MechanicsProf.Prof.H.J.LiaoH.J.Liaob、dThe correction coefficients of baring capacity for thewidth and embedment depth of foundation;The unit weight of soil below the foundation base,thebuoyant unit weight is adopted below the groundwater level;0The weighted average unit weight of soil ab

6、ove thefoundation base,the effective unit weight of soil is adoptedbelow the groundwater level.Calculation formula of bearing capacity determined by the code土力学土力学Soil MechanicsSoil MechanicsProf.Prof.H.J.LiaoH.J.LiaoWhen the base width b is greater than 3m or the buried depth isgreater than 0.5m,th

7、e bearing capacity is calculated accordingto the above formula(a).when the base width b is less than 3m,its value is regarded as 3m;when b is greater than 6m,the valueis b=6m.ffbd(3.0)(0.5)aakbd0（a）Buried depthBase width土力学土力学Soil MechanicsSoil MechanicsProf.Prof.H.J.LiaoH.J.LiaoFor the buried depth

8、 d of the foundation,it shall be carried outaccording to the requirements of the code;it is generally calculatedfrom the elevation of the outdoor surface.In the filling and levelingarea,the depth can be calculated from the filling ground elevation.Ifthe filling is completed after the superstructure

9、construction,it shallbe calculated from the natural ground elevation.For the basement,when the box foundation or raft foundation is used,the buried depthof the foundation shall be calculated from the elevation of outdoorground.When the independent foundation or strip foundation isused,it shall be ca

10、lculated from the elevation of the indoor ground.土力学土力学Soil MechanicsSoil MechanicsProf.Prof.H.J.LiaoH.J.LiaoWhen e0.033b，“the code for design of building foundation”gives the formula(b)to determine the characteristic value ofsubgrade bearing capacity according to the shear strength indexof soil(b)f

11、aThecharacteristicvalueofsubgradebearingcapacitydetermined by the shear strength index of soil；Mb、Md、McThe bearing capacity coefficients which aredetermined according to the internal friction angle standardvalue kwithin the depth of one time the short width of base；fMbMd M c+abd0c k土力学土力学Soil Mechan

12、icsSoil MechanicsProf.Prof.H.J.LiaoH.J.Liao土力学土力学Soil MechanicsSoil MechanicsProf.Prof.H.J.LiaoH.J.LiaockThe standard value of cohesion of the soil withinthe depth of one time the short width of the base；b The width of the foundation base;when b isgreater than 6m,b=6m is taken;when b is less than3m

13、for sand,b=3m.Other symbols have the samemeaning as before.土力学土力学Soil MechanicsSoil MechanicsProf.Prof.H.J.LiaoH.J.LiaoThe national standard“code for design of building foundation”specifies that the bearing capacity characteristic value of theintact,relatively intact and relatively broken rock subgr

14、ade canbe calculated using formula(c)according to indoor saturateduniaxial compressive strength.frkthe standard value of saturated uniaxial compressivestrength of rock,which can be determined according to the testof saturated uniaxial compressive strength of rock in the code;(c)ffarrk土力学土力学Soil Mech

15、anicsSoil MechanicsProf.Prof.H.J.LiaoH.J.Liaorthe reduction coefficient,determined by regionalexperiences according to the integrity of rock massand the spacing,width,occurrence and combinationof structural planes.When there is no experiencesupports,r=0.5 for the intact rock mass,r=0.20.5for the rel

16、atively intact rock mass,and 0.10.2 for themore broken rock mass.土力学土力学Soil MechanicsSoil MechanicsProf.Prof.H.J.LiaoH.J.LiaoThe characteristic value of bearing capacity of brokenand extremely broken rock can be determined accordingto the plate loading test in the code.For the bearing capacity chara

17、cteristic value of the intact,relatively intact and relatively broken rock,it can also bedetermined according the loading test method of rocksubgrade in the code.土力学土力学Soil MechanicsSoil MechanicsProf.Prof.H.J.LiaoH.J.LiaoThe bearing capacity formula(d)given in Chinesenational standard“standard for

18、building constructionin collapsible loess regions”ffbd(3.0)(1.5)aakbd0(d)The symbolic meaning of the formula is the same asformula(a),and the relevant correction coefficients ofbearing capacity can be found by the code.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.