Refined SANISAND-F Model for Non-Proportional Loading: Development and Application

On-site sand usually suffers non-proportional loading that alters its fabric, strength and deformation characteristics. This study introduces a bounding surface plasticity model specifically designed for non-proportional loading conditions, by refining the SANISAND-F model that accounts for fabric effects. Key innovations include the development of a fabric-dependent elastic relation, a refined plastic loading tensor direction incorporating Lode's angle, and modified plastic flow and kinematic hardening rules tailored for non-proportional shearing. These advancements significantly enhance the accuracy and applicability of the model in capturing complex stress-strain behaviors under non-proportional loading condition. As a result, the stress-induced evolution of contact normals and the principal stress rotation-induced softening of sand can be considered. The return mapping with cutting plane algorithm is adopted to implement the developed model through UMAT subroutines in Abaqus. Then, the model is validated against a series of drained and undrained shear test results of sand under various loading paths, including the triaxial and torsional shear with fixed principal stress axes, as well as the simple shear paths with rotated principal stress axes, etc. It is found that the model simulates well the key stress-strain behaviors of sand, e.g., the strain softening with volumetric dilatancy, liquefaction, non-flow as well as the evolution of the intermediate stress ratio. Further application of the model to solve boundary value problems are provided. It is found that the model can provide a reasonable prediction of the bearing capacity of a strip footing on sand. Typical failure modes with general shear failure and local shear failure can be well reproduced.