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

Abstract

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 (IR) is derived considering both effects of contact sliding and rolling. The IR = 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 (ψe0) derived from the e-p′ CSL and the micro state parameter (ψMCN0) determined from the MCN-p′ CSL. The specimens with the same ψe0 may have distinct cyclic behaviors, while cyclic behaviors of specimens with the same ψMCN0 are close to each other, indicating that ψMCN0 is a more plausible state variable for characterizing the behavior of sand than ψe0.