Research On Physical Fundamentals And Constitutive Model Of Large Post-Liquefaction Deformation Of Sand

The basic laws of post-liquefaction deformation of saturated sand subjected tocyclic loading have been summarized. Based on it, a rational approach was exploredsystematically for quantitative evaluation of large post-liquefaction deformation ofsaturated sand from aspects of physical fundamentals, constitutive model, numericalimplementation and practical application. The main achievements obtained in thethesis are as follows:1. A new mechanism of large post-liquefaction deformation in saturated sandwas established, based on the fact that the volumetric strain of sand due to dilatancyis composed of a reversible dilatancy component and an irreversible dilatancycomponent, and through the analysis of three probable physical states of soil particlesafter initial liquefaction and corresponding constraint requirements on the threevolumetric strain components. The intrinsic relationship between thepost-liquefaction shear deformation and the two dilatancy componets was revealed. Itwas found that the post-liquefaction shear strain is governed by the evolution laws ofthe three volumetric strain components, i.e., a component due to the change in meaneffective confining stress and the two shear dilatancy components.2. A new cyclic constitutive model for saturated sand, based on the abovephysical fundamentals, was developed within the theoretical frame of boundingsurface plasticity. Only through the description of the two dilatancy components, themodel can be used well, not only to simulate the development of shear strain from asmall to large range during an entire pre-and post-liquefaction process, but also toevaluate the accumulation process of large volumetric strain induced duringpost-liquefaction reconsolidation, while other redundant assumptions and modelparameters were not needed. Another prominent strongpoint of the present model isthat it can reflect the interaction of residual volumetric strain and residual shearstrain.3. A pragmatic stress integration algorithm of the model was developed andimplemented in FEM softwares or codes. The algorithm was confirmed to beapplicable to both drained and undrained conditions and can effectively be used tocalculate the large shear strain development in zero effective stress state duringliquefaction process.4. Numerical simulation was performed on several dynamic centrifuge modeltests of VELACS. The macroscopic phenomena observed in the tests were wellexplained through the stress-strain response analysis using the present model. Thegood agreement was shown to exist between the tested and calculated results,confirming the effectiveness of the model and related algorithm for the analysis ofboundary-value problems involving seismically induced liquefaction of sand.5. As a typical example of engineering application, the seismic response ofDaikai subway station during the 1995 Hyogoken-Nambu Earthquake wasinvestigated by the analysis procedure based on the presented model with specialattention to the liquefiable saturated sandy stratum around the subway station. It wasshown that large relative displacements can occur when liquefaction is induced in thesaturated sandy stratum. It is the main reason causing serious damage to the stationduring the earthquake.