DEM simulation of the undrained shear behavior of sand containing dissociated gas hydrate

A numerical simulation technique is proposed using discrete element method (DEM) to study the undrained shear behavior of sand containing dissociated gas hydrate. Dissociation of gas hydrate in sand samples under undrained conditions is simulated first, followed by undrained bi-axial compression of these samples, which consist of three phases, i.e., sand particles, water, and methane gas. The simulations demonstrate that hydrate dissociation under undrained conditions generates significant excess pore pressure and volumetric dilation, the magnitudes of which are found to be comparable with those predicted by theoretical analysis. During the undrained bi-axial compression, samples with different initial degrees of hydrate saturation exhibit markedly different shear behavior. Complete strain softening behavior, i.e., static liquefaction, is observed for the samples with relatively high initial degrees of hydrate saturation, while other samples reach quasi-steady state followed by strain hardening at large strains. The observed static liquefaction is believed to be a combined result of the looser sand structure induced by hydrate dissociation, and the essentially stiff modulus of fluid under high pore pressure, despite the existence of gas in sand pores. The influence of initial sample porosity and particle shape is also investigated.