Micromechanics-based multimechanism bounding surface model for sands

Within the multimechanism framework, a micromechanics-based sand model is presented based upon the bounding surface and critical state theories. This model follows the assumption that the macroscopic responses of sands can be determined by summing the contributions from a macroscopic volumetric mechanism and an infinite number of virtual microscopic shear mechanisms in various orientations. Each virtual shear mechanism characterizes the microscopic shear deformations and the dilatancy-induced volumetric deformations in three mutually perpendicular directions. The deformations in each direction are described by using a microscopic shear stress-strain relationship founded upon the bounding surface theory and a microscopic stress-dilatancy relationship, respectively. The shear strength with the SMP yield criterion and the stress-dilatancy relationship introduce a state variable for compatibility with the critical state theory. The correlations between the microscopic and macroscopic model parameters are established, and most of them are defined by soil parameters with a clear physical meaning. With a spatial distribution of microscopic shear mechanisms, the model can intrinsically consider the stress-induced anisotropy, the non-coaxial behaviour of stress and strain increment in their principal directions, and the effect induced by principal stress rotations without requiring additional model parameters. The comparison of the simulated and experimental results indicates its excellent capability in predicting sand responses in stress-strain curve as well as stress path under different drainage and loading conditions.