DEM Simulations of Undrained Triaxial Behavior of Granular Material

The paper presents results of three-dimensional discrete element method (DEM) simulations of axisymmetric undrained tests on loose assemblies of polydisperse spheres using a periodic cell. In the work reported, undrained tests were modeled by deforming the samples under constant volume conditions. The undrained (effective) stress paths are shown to be qualitatively similar to published physical experimental results. The onset of liquefaction (or temporary liquefaction) is identified by a redundancy index equal to unity, which defines the transition from solidlike to liquidlike behavior. This corresponds to a critical mechanical coordination number slightly in excess of 4. The results of the simulations also suggest that a reversal in the direction of the undrained stress path does not necessarily indicate temporary liquefaction. The undrained behavior obtained by the DEM simulations is found to be dependent on strain rate and the so-called temporary liquefaction phenomenon is only observed if the strain rate is sufficiently high.