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1、2.6 Seepage pressure and critical hydraulic gradientSeepage pressure Seepage pressureWhen seepage occurring in groundwater,the flowingof water in the voids of soil exerts a force on particleskeleton,which is called seepage pressure or dynamicwater pressure,expressed by the symbol Gd(KN/m3).The seepa
2、ge water will also subjected to the resistanceT(KN/m3)from the soil particle skeleton.T=-Gd土力学土力学Soil MechanicsSoil MechanicsProf.Prof.H.J.LiaoH.J.LiaoThe stress analysis of water column BA with length L,sectional area F and water head difference H=H1-H2iscarried out.Note:Neglecting the inertia forc
3、e of seepage water.Derivation of formula T(Gd)wLFBase levelPiezometer 土力学土力学Soil MechanicsSoil MechanicsProf.Prof.H.J.LiaoH.J.Liao Total water pressure on section B:f1=wh1 F Total water pressure on section A:f2=wh2 F Weight of water column BA:f3=wL F Total resistance of soil particle skeleton to see
4、page water:f4=TLF Component of the gravity of water column BA in the flow line direction:f5=wL F cosDerivation of formula T(Gd)wLFPiezometer Base level土力学土力学Soil MechanicsSoil MechanicsProf.Prof.H.J.LiaoH.J.LiaoAlong the flow line direction of BA,the equilibrium relation of force iswh1F+wLFcos-wh2F-
5、TFL=0 （a）Remove F and substitute cos=(z1-z2)/L into it,thenwh1+w(z1-z2)-wh2-TL=0w(z1-z2)+(h1-h2)TL=0（b）Since z1h1=H1，z2h2=H2i=(H1-H2)/L=(z1-z2)+(h1-h2)/L（c）It can be concluded that T wiNamely the seepage pressure is Gd wiThe minus sign means that the two directions are opposite and the direction of
6、Gd is the same as seepage water.wLFPiezometer Base level土力学土力学Soil MechanicsSoil MechanicsProf.Prof.H.J.LiaoH.J.LiaoDownwardseepage：Thedirectionofseepage pressure is the same as the gravitydirection of soil.The effective stress in the soillayer increases,while the pore water pressuredecreases,which
7、makes the soil particle morecompacted and is beneficial to engineering.Seepage direction 土力学土力学Soil MechanicsSoil MechanicsProf.Prof.H.J.LiaoH.J.LiaoUpward seepage：The direction of seepagepressure is opposite to the direction of gravity.The contact force between the particles in the soillayer decrea
8、ses,while the pore water pressureincreases,which reduces the weight of the soiland causes the quicksand phenomenon.This isharmful to engineering.Seepage direction 土力学土力学Soil MechanicsSoil MechanicsProf.Prof.H.J.LiaoH.J.LiaoSeepage in foundation pitSeepage in preloading sand drainLoadSoft soil layerS
9、and drainImpervious bed Drainagesand layer土力学土力学Soil MechanicsSoil MechanicsProf.Prof.H.J.LiaoH.J.LiaoEngineering cases dwGiSeepage PressureGdWs sW 渗流方向Vs sGd+W-Ws=0Gd=Ws-W=(satw)Vs=Vs Gd+W Ws Gd Vs Gd (unit volume)Gd=（critical state）That is,wicr=Critical hydraulic gradient：icr=/wCritical hydraulic
10、gradientSeepage direction(The direction is consistent with seepage)土力学土力学Soil MechanicsSoil MechanicsProf.Prof.H.J.LiaoH.J.LiaoDamage effect：Affecting seepage stabilityanddeformation.Forexample,thescouring effect produced in the process ofseepage,alsoknownasmechanicalsuffosion,is a kind of destructive effect.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.