A New Sand Constitutive Model for Pre- and Post-liquefaction Stages

A new model, named SANISAND-MSf, is presented that is an extension of the two-surface constitutive model within the framework of bounding surface plasticity. It addresses the modeling of sand behavior under undrained cyclic loading by successful simulation of stress-strain curves, undrained stress paths and strength curves, for various cyclic stress ratios and loading conditions, at both pre- and post-liquefaction stages separately, as opposed to overall simulations of past efforts. The model incorporates a new form of a memory surface for the pre-liquefaction stage and the concept of semifluidized state for the post-liquefaction stage; both address the proper modification of stiffness and dilatancy. The formulation avoids common shortcomings of previous models related to stiffness degradation and singular analytical eventualities. The SANISAND-MSf maintains the well-known ability of the two-surface bounding surface model to simulate successfully monotonic loading at various pressures and densities with one set of model constants and be critical state compatible. This paper addresses the novel capabilities of the proposed model in capturing the challenging aspects of modeling seismic site response analysis of a soil deposit in both pre- and post-liquefaction state of response. The developed modeling framework will contribute to future applications in a realistic and thorough seismic site response analysis.