DEM simulations of thermally activated creep in soils

Discrete element modelling (DEM) has been used to simulate creep in assemblies of spherical grains possessing an interfacial coefficient of friction that varies with sliding velocity according to rate process theory. Soil stiffness is represented by a pair of values of linear spring stiffness normal and tangential to each inter-granular contact, and the limiting coefficient of contact friction is described as varying linearly with the logarithm of sliding velocity. DEM simulations of an assembly of 3451 spheres reproduce a number of significant phenomena including: creep rate as a function of the mobilisation of deviatoric stress; initially linear decay of creep strain rate with time plotted on log-log axes and with a slope m in the range -0.8 to -1; and ultimate creep failure in triaxial simulations at high deviatoric stress ratios. Creep-induced failure is shown to occur at a unique axial strain for a given state of initial packing, and to be linked with dilatancy. The numerical results are compared quantitatively with the test data of soils from the literature. The effects of activation energy are considered in relation to the different magnitudes of creep encountered in sands and clays.