Discussion on sand liquefaction and its static approachDongdong Chang 2014/3/8 1 Introduction Liquefaction of saturate sands Static approach—steady state approach Some divergent opinions Difficulties in this approach Suggestions 2014/3/8 2 What’s liquefaction When soil loses its strength and stiffness and behaves like a fluid For a loose, saturate sand Under earthquake or quick loading Soil particles loses contact with each other 2014/3/8 3 bays. Japan. commonly near rivers. lakes.Where and how Occurs in saturate sands. 1995 4 . structures damages 2014/3/8 Kobe. and oceans Foundation failures. Why liquefaction Rising pore water pressure reduced effective stress reduced shear strength In extreme case effective stress turns zero loses shear strength soil acts like fluid 5 2014/3/8 . Related terms and concepts 2014/3/8 6 . 1996) 2014/3/8 7 .Critical Void Ratio Behavior of dense and loose soils in monotonic strain controlled triaxial tests (after Kramer. CVR line in e-logp’ space Critical void ratio varies with effective confining pressure A critical void ratio (CVR) line in e-logp’ space constitute the boundary between dilative and contractive behavior in drained triaxial compression 2014/3/8 8 . Steady State A steady state line (SSL) in e-logp’ space (at large strains) Boundary of flow liquefaction The difference between CVR and SSL is the existence a "flow structure". in which the grains orient themselves so the least amount of energy is lost by frictional resistance during flow 2-D Projection of SSL in e-logp’ space 2014/3/8 9 . 3-D Location of SSL 2014/3/8 10 . Critical state CSL = tan ult e Straight line e = e0 . ln e CSL ln d d 2014/3/8 d d v = = = 0 d d 11 . can be used interchangeably 12 2014/3/8 .Critical state Steady state Steady state is developed in empirical manner Critical state is on theoretical basis Essentially the same. and pile driving are all example of dynamic loads that could trigger flow liquefaction 2014/3/8 13 . blasting.Flow liquefaction A phenomenon when the static equilibrium is destroyed by static or dynamic loads with low residual strength Residual strength is the strength of a liquefied soil Earthquakes. the Kawagishi-cho apartment in Niigata earthquake 1964 2014/3/8 14 . like a remarkable bearing capacity failure.Failures mode of flow liquefaction Large and rapid movements Disastrous effects. Flow liquefaction surface(FLS) 2014/3/8 15 . Cyclic Mobility A liquefaction phenomenon Triggered by cyclic loading Occurring with static shear stresses lower than soil strength Deformations due to cyclic mobility develop incrementally Lateral spreading is a common result of cyclic mobility 1976 Guatemala earthquake caused lateral spreading 2014/3/8 16 . Phase transformation line(PTL) A key to understand cyclic mobility is PTL PTL:The stress path points at which the dense or medium sands transform from contractive to dilative behavior 2014/3/8 17 . A stress path example Before PTL: contraction. u increases. u decreases. p' increases 2014/3/8 Undrainded stress path 18 . p' decreases On PTL:no contraction or dilation. p' constant After PTL: dilation. Figure showing zones of flow liquefaction and cyclic mobility susceptibility 2014/3/8 19 . Evaluation of Liquefaction Potential By comparing earthquake loading (CSR: cyclic stress ratio) and liquefaction resistance (CRR: cyclic resistance ratio ) FS = CRR / CSR FS: factor of safety against liquefaction 2014/3/8 20 . Steady-State approach to sand-liquefaction 2014/3/8 21 . medium. and dense) (Castro 1966) 2014/3/8 22 .Steady-State deformation Static triaxial test stress paths for three specimens of different densities (very loose. Limited liquefaction The stiffness of the soil depends on p'. the stiffness decreases (stress path below the PTL) but then increases (stress path above the PTL) This change in stiffness produces the "limited liquefaction” 23 2014/3/8 . Steady-State approach Use the unique relationship between shear strength and void ratio at high shear strains Behavior of soil is dominated by its initial state relative to SSL Used for both loose and dense sands 2014/3/8 24 . undrained strength is higher than drained strength 2014/3/8 25 .The lowest safety margin for loss of limiting equilibrium Loose sand: contractive. drainage increase shear strength. drained strength is higher than undrained strength Dense sand: dilative. 3 sin -0.4 0.Stress ratio (sinm) versus Rate of dilation (sin) sinm 0. after Stroud 1971 2014/3/8 .6 0.6 Whatever their density or state.2 0 0.8 0.4 -0.2 0.5 Rate of dilation is function of stress ratio (Wood 1990) 0. sands are contractive at m < crit dilatant at m > crit 26 Adapted from Wood 1990.4 0.7 0. A divergent opinion Is the SSL or CSL unique? 2014/3/8 27 . Sample preparation procedures. Stress path.Uniqueness of SSL/CSL Specially designed simple shear tests show that even dilatant sands reach a unique critical state in the failure zone regardless of their initial density (Cole 1967. Consolidation stress prior to shear) do not influence true steady state (Mcroberts 1992) True steady state or critical state is unique 2014/3/8 28 . Stroud 1971) From series of tests. all theoretical influencing factors of SSL (Strain rate. Problems of this approach Difficulties in determining the steady-state line and the in-situ void ratio (very flat SSL for sands is highly sensitive to its parameters) Theoretical limitations in the validity of the concept and the potential influence of factors that are not considered 2014/3/8 29 . Limitations No claims to following aspects: the potential for progressive failure the magnitude of the disturbing force required to trigger liquefaction the influence of in-situ stress state on liquefaction potential effects of redistribution of void ratio in cyclic loading 30 2014/3/8 . ..So. not invalidate the steady-state approach but makes it inappropriate for the back analysis of actual liquefaction failures 2014/3/8 31 . Conclusions Useful in understanding the basic mechanics of true liquefaction Difficult to apply in practice highly sensitive to its parameters for sands extremely difficult to measure required parameter with sufficient accuracy might lack theoretical basis might ignore factors that may be important 32 2014/3/8 . consider several methods when evaluating liquefaction Consider case records: useful particular at a yes-no level.Suggestions Do not rely on only one method. study and rework previous case records Rely on traditional approaches 33 2014/3/8 . Thanks 2014/3/8 34 .