The HySand hyperplasticity constitutive model for sand: theory

Advanced finite-element analyses are increasingly being used in geotechnical design, and central to these analyses are reliable and accurate constitutive models to represent soil behaviour. These models need to address response across a wide range of conditions, including for instance cyclic loading for offshore applications. Existing advanced constitutive models for sand have limited capability for modelling cyclic loading, are often complex and rarely respect the laws of thermodynamics. HySand_base, a new constitutive model for sand under cyclic loading, is presented. Its concise, rigorous and simple formulation within the hyperplasticity framework is given. Each element of the formulation is explained through the progressive construction of the model, and the improvements brought by each additional feature are illustrated for drained monotonic, undrained monotonic and undrained cyclic tests. HySand_base uses 14 material parameters. It is a density- and pressure-dependent multi-surface plasticity model, rooted in critical state theory, with yield surfaces based on the fusion of Matsuoka–Nakai type surfaces with consolidation surfaces. Non-associated plasticity is adopted, with volumetric plastic strains arising from two mechanisms: anisotropic dilation and consolidation.