SANISAND-FM: a sand plasticity model with fabric anisotropy and memory surface with emphasis on cyclic liquefaction

A new constitutive model based on bounding surface plasticity, developed within anisotropic critical state theory (ACST) and incorporating a memory surface, is proposed to simulate sand response with emphasis on cyclic liquefaction. The formulation introduces key innovations over prior SANISAND-type models: it eliminates auxiliary constructs such as the dilatancy fabric tensor and semifluidised range, replacing them with functions of the evolving fabric anisotropy tensor, an inherent entity of ACST. A scalar-valued internal variable regulates post-dilation shear stiffness, and two additional fabric-related factors address volumetric and shear stiffness and bedding plane orientation effects. These advancements enable consistent control over stiffness evolution, allowing simulation of progressive softening in saturated sand under constant-volume cyclic loading. The model reproduces pore pressure build-up and shear strain accumulation, capturing a range of post-dilation responses, from rapid patterns in looser sands to slower patterns in very dense sands. It also simulates shear accumulation under asymmetric cyclic shearing and the influence of fabric, reflecting bedding plane orientation, on undrained monotonic and cyclic response. In addition, a limited number of drained tests under monotonic and cyclic loadings are successfully simulated. Validation uses experimental data on Ottawa F-65 and Toyoura sands, including monotonic and cyclic tests under simple shear, torsional shear and triaxial loading. In conclusion, the model’s unified framework achieves data simulations under very diverse and varying stress and fabric conditions.