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Micro- and macromechanical modeling of granular material under constant volume cyclic shearing Yang, Ming


Cyclic liquefaction of granular soils during earthquakes often results in catastrophic damages to civil infrastructure. Understanding and modeling of this complex phenomenon are of crucial importance in geotechnical engineering. Motivated by its practical importance, this study focuses on modeling the granular materials response under constant volume cyclic shearing from both micromechanics and continuum mechanics. At the micromechanical level, discrete element method was used to carry out an extensive set of uni- and multidirectional cyclic shear simulations on idealized granular assemblies. Unidirectional simulations were analyzed to explore the microstructural evolution concerning particle connectivity, force transmission, and anisotropies. Liquefaction state was marked by a significant drop in coordination number, where the granular system became fluid-like, and deformed significantly to rebuild the contact network. Stress-force-fabric relationship was verified, revealing increasing and decreasing patterns, respectively, for the proportions of fabric and force anisotropies. The multidirectional analysis explored the effects of shear paths on the cyclic response of granular assembly. Multidirectional simulations presented lower cyclic liquefaction resistance than unidirectional ones. Microscopically, particle connectivity, particle-void fabric, and anisotropies were investigated to shed light on the stability, deformation, and load-bearing network of the granular assembly, respectively. At the continuum level, the study focused on constitutive modeling of sand response in both pre- and post-liquefaction stages. A new constitutive model is formulated by incorporating two new constitutive ingredients into the platform of a reference critical state compatible bounding surface plasticity model with kinematic hardening. The first ingredient is a memory surface for more precisely controlling stiffness affecting the plastic deviatoric and volumetric strains and ensuing pore pressure development in the pre-liquefaction stage. The second ingredient is the concept of semifluidized state and the related formulation of stiffness and dilatancy degradation, aiming at modeling large shear strain development in the post-liquefaction stage. The new model successfully simulates undrained cyclic torsional and triaxial tests with different CSRs, separately for the pre- and post-liquefaction stages, as well as liquefaction strength curves. The new model was also assessed in the simulation of several multidirectional cyclic shear tests. The development of this constitutive model contributes to future applications in seismic site response analysis.

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