DEM simulations of critical state behaviour of granular materials under various drained triaxial stress path tests

The present study investigates the critical state behaviour of granular assemblies composed of clumped particles under four different drained axisymmetric triaxial stress paths, using the discrete element method (DEM). A series of numerical samples were prepared at initial states with different density indexes (I_D) and different initial confining pressures (p_o^′). These samples were sheared to large strains, at which constant stresses and volumes were maintained to reach the critical state. The evolution of stress ratio under the same loading mode (for the same intermediate principal stress ratio, b) is shown to yield an almost identical behaviour independent of stress paths, whereas the stress-strain response depends on the stress paths. Four different axisymmetric stress paths all share the same unique friction angle at critical state, indicating the Mohr-Coulomb failure criterion is the appropriate critical state strength criterion, which is at least true for the axisymmetric stress conditions. A unique coordination number (CN) is achieved at the critical state for a given p_o^′, which is independent of the stress path. The critical state CN is found to increase with the increase in p_o^′, which could be attributed to the decrease in the critical state void ratio (e_c) as mean effective stress (p^′) increases. Interestingly, a unique linear functional relationship is found between the critical state values of CN and e_c, and a unique polynomial functional relationship is found between the critical state values of CN and p^′. These functional relationships indicate no dependency on the stress paths or loading modes, thus characterizing unique features at critical states at both macroscopic and microscopic levels for a given type of granular material.