Cyclic loading and fabric evolution in sand: A constitutive investigation

An anisotropic plasticity model is proposed to describe the effect of fabric and fabric evolution on the cyclic behaviour of sand within the framework of anisotropic critical state theory. The model employs a cone-shaped bounding surface in the deviatoric stress space and a yield cap perpendicular to the mean stress axis to describe sand behaviour in constant-mean-stress shear and constant-stressratio compression, respectively. The model considers a fabric tensor characterizing the internal structure of sand associated with the void space system which evolves with plastic deformation. The fabric evolution law is assumed to render the fabric tensor to become codirectional with the loading direction tensor and to reach a constant magnitude of unit at the critical state. In constant-stress-ratio compression, the final degree of anisotropy is proportional to a normalized stress ratio. An anisotropic variable defined by a joint invariant of the fabric tensor and the loading direction tensor is employed to describe the fabric effect on sand behaviour in constant-mean-stress monotonic and cyclic shear. Good comparison is found between the model simulations and test results on Toyoura sand in both monotonic and cyclic loadings with a single set of parameters.