DEM analysis of monotonic and cyclic behaviors of sand based on critical state soil mechanics framework

A series of monotonic and cyclic triaxial shearing tests on Toyoura sand are simulated by discrete element method (DEM) incorporating rolling resistance. The input parameters are calibrated against laboratory triaxial test data considering different initial states and monotonic loading paths. The calibrated DEM model successfully captures the density and stress dependency of sand behavior under both monotonic and cyclic conditions. It is shown that when approaching liquefaction, contact yielding alters from sliding-dominant to rolling-dominant. The expression of mechanical redundancy index (I-R) is derived considering both effects of contact sliding and rolling. The I-R = 1.0 condition is shown to be a unified criterion distinguishing between solid state and liquid state upon both monotonic and cyclic liquefactions. A unique micro critical state line (CSL) is obtained in MCN-p' plane. The cyclic resistance ratio is exponentially correlated with both the macro state parameter (Psi(e0)) derived from the e-p' CSL and the micro state parameter (Psi(MCN0)) determined from the MCN-p' CSL. The specimens with the same Psi(e0) may have distinct cyclic behaviors, while cyclic behaviors of specimens with the same Psi(MCN0) are close to each other, indicating that Psi(MCN0) is a more plausible state variable for characterizing the behavior of sand than Psi(e0).