Granular geomaterials generally are regarded as rate-insensitive under the conventional loading conditions. However, recent studies have observed that crushable geomaterials may exhibit rate-dependent behavior when subjected to high confining stresses. As a consequence, the evolution of the critical state with breakage in the void ratio–mean pressure space also varies with the applied strain rate. Thus, the concept of critical state soil mechanics (CSSM) may not be a suitable choice for mimicking the behavior of crushable sands because CSSM needs prior information about the unique critical state line (CSL). Hence, this work aims to enhance a constitutive model based on continuum breakage mechanics (CBM) that can not only consider the influence of crushing on CSL but also estimate the extent of breakage. Specifically, this study had the following two primary objectives: (1) to upgrade an existing CBM-based model suitable for granular geomaterials at high confining stresses by incorporating viscoplasticity; and (2) to overcome the presumption, often inherited by the CBM-based models, that the stress state transitions to the elastoplastic regime only when breakage initiates. In order to avoid any kind of predefined overstress function, the consistent viscoplasticity framework is utilized. Additionally, to achieve the second aim, the notion of an unconventional plasticity known as the subloading surface model is employed. Furthermore, adopting a semi-implicit method called the cutting plane algorithm, equations are derived for the numerical stress integration and are presented comprehensively. The calibration results demonstrate that the upgraded model is capable of providing more-realistic responses than those provided by the classical base formulation. The improved plasticity model explicitly takes into account the effect of the applied strain rate on the evolution of CSL with grain fragmentation.
Sinha et al. (Thu,) studied this question.