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Multi-scale modeling of cyclic shearing and liquefaction response of granular materials Barrero, Andres Ricardo

Abstract

Cyclic shearing of granular materials under undrained conditions can induce a reduction of mean effective stress and increase of the pore water pressure. In extreme cases, the mean effective stress can temporarily vanish and lead to a “semifluidized state", in which large shear strains are developed and accumulated. Predicting the level of deformations developed during liquefaction and especially in the post-liquefaction stage using constitutive models is a changeling task, and yet important to evaluate the safety of geotechnical structures. A sand plasticity model, which is the precursor of the SANISAND family of models, was considered as the reference model in this study. The model has proven success in the simulation of monotonic and cyclic response of sand in the pre-liquefaction state. A series of modifications were introduced out to improve the predictability of the model for the post-liquefaction cyclic shear strain. The modifications were motivated by carrying out a number of constant-volume cyclic shear triaxial simulations using the discrete element method (DEM). The DEM simulation results revealed that a high number of floating particles with zero contact in a semifluidized state, which explained the vanishing of load-bearing structures and large shear strain accumulations. Thus, linking discrete and continuum modelings via the semifluidized state, inspired introducing a new state internal variable named strain liquefaction factor (SLF) to model the degradation of stiffness. The SLF evolves within the semifluidized range; its constitutive role is to reduce the values of parameters controlling the plastic modulus and dilatancy, maintaining the same plastic volumetric strain rate, in the semifluidized range. The evolution rate equation of the SLF includes a back-to-zero recovery term under drained loading. The extended model was validated against a series of undrained cyclic simple shear tests at the element level. Then this model was implemented in a finite difference platform and used in the benchmark study LEAP for simulating centrifuge experiments of a submerged slope subjected to dynamic excitations. Comparisons between experiments and simulations were satisfactory, and especially the simulated horizontal displacement was improved using the SLF. This work is expected to extensively benefit the numerical modeling of liquefaction-related problems in the future.

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