Elasto-plastic model for sand including time effect

Time effects on granular soils have been observed in the laboratory and by in-situ tests, but these cannot be reproduced by classical elasto-plastic models. To address these concerns, existing specific modelling approaches were based on the theory of viscoplasticity formulated by Perzyna or on a viscous evanescent relationship. This work explores an alternative elasto-plastic modelling framework formulated in a multiaxial structure space. The proposed elasto-plastic model is associated with a thixotropic-type framework through the use of a structure parameter, the evolution of which illustrates the competition between two effects: the time-dependent tendency of the granular system to reach its stable configuration - restructuration - and its destructuration under external perturbations. The structure parameter is linked to the existence of a stress-dependent target structure towards which the current granular material structure evolves. The timescale is explicitly introduced by postulating a rate for this structure evolution. The modelling of the material behaviour has shown good similarities with the response of granular soils observed in monotonic loading, as well as during creep and variable strain rate loading experiments.