土壤力学土壤力学英文 (11).ppt

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1、Outline of the lectureSelf-weight stress in soilIncrease of stress in soil due to external loading Stress state and Mohr circle Effective stress concept1.Stress state and Mohr circleSign conventionsSign conventionsMaterial mechanicsMaterial mechanics+-+-Soil mechanicsSoil mechanicsNormal Normal stre

2、ssesstressesCompression positiveShear stressesShear stressesCounterclockwise positiveIn soil mechanics1.Stress state and Mohr circleMohrs Circle for Stress State Mohrs Circle for Stress State 1.Stress state and Mohr circlePole Pole method&2method&2 method methodDetermine the stress component on EF p

3、lane2.Self-weight stress in soilz=zHomogeneousHydrostatic/Geostatic condition-vertical direction Hydrostatic/Geostatic condition-vertical direction Homogeneous soilHomogeneous soilLayered soilLayered soilNote:below water level use Note:below water level use above water table use (1)(2)2.Self-weight

4、stress in soilStress distribution Stress distribution in soilin soilHomogeneous soilHomogeneous soilLayered soilLayered soil3.Effective stress conceptaa Cross section a-a passes through the contact points of soil particlesA:cross section area of soil elementAs:cross section area occupied by solid-to

5、-solid contactsAw:cross section area occupied by pore waterEffective stress Assume that the As is extremely small and can be neglected.Stresses in a saturated soil without seepage(1)(2)(3)(4)3.Effective stress conceptStresses in a saturated soil without seepageThe effective stress represents the ave

6、rage stress carried by the soil solids and is the The effective stress represents the average stress carried by the soil solids and is the difference between the total stress and the pore water pressure.difference between the total stress and the pore water pressure.Deformations of soils Deformation

7、s of soils is is a function of effective stresses,not total stresses or pore water a function of effective stresses,not total stresses or pore water pressure.pressure.The principle of effective stresses applies only to normal stresses and not to shear The principle of effective stresses applies only

8、 to normal stresses and not to shear stresses.(The pore water cannot sustain shear stresses,and therefore the soil solids stresses.(The pore water cannot sustain shear stresses,and therefore the soil solids must resist the shear forces)must resist the shear forces)3.Effective stress conceptStresses

9、in a saturated soil with upward seepagePoint A:Point B:3.Effective stress conceptStresses in a saturated soil with upward seepagePoint C:hydraulic gradient i caused by the flow(1)(2)(3)(4)3.Effective stress conceptStresses in a saturated soil with upward seepage3.Effective stress conceptStresses in

10、a saturated soil with downward seepage3.Effective stress conceptSeepage forceUpward seepageDownward seepageAs water flows through soil,it exerts a frictional drag on the soil particles,resulting in head losses.The frictional drag is called seepage force in soil mechanics,which is denoted by jsDownwa

11、rd seepage increases the resultant effective stress;upward seepage decreases the resultant effective stress3.Effective stress conceptCritical hydraulic gradientFor most soils,the value of icr varies from 0.9 to 1.1.Since Gs is constant,the critical hydraulic gradient is solely a function of the void

12、 ratio of the soil.Under such a situation,soil stability is lost.This situation generally is referred to quick condition.(1)(2)4.Stress increase due to external loadingStress caused by a point loadStress caused by a point loadConsider a homogeneous,elastic and isotropic semi-infinite half spaceyzxoP

13、MxyzrRMBoussinesqs solution for normal stress increases at point M due to the point load P.4.Stress increase due to external loadingyzxoPMxyzrRM(1)(2)(3)Stress caused by a point loadStress caused by a point load4.Stress increase due to external loadingVertical Vertical stress caused stress caused by

14、 a by a line loadline load(1)4.Stress increase due to external loadingVertical stress caused by a vertical strip load Vertical stress caused by a vertical strip load(1)(2)4.Stress increase due to external loadingVertical stress caused by a Vertical stress caused by a uniform loaded circular areauniform loaded circular area(1)(2)(3)4.Stress increase due to external loadingVertical stress caused by a rectangular loaded areaVertical stress caused by a rectangular loaded area(1)(2)(3)=