DEM simulations of soil creep due to particle crushing

One mechanism that may contribute to creep in sand is that heavily loaded grains may progressively crush, following the growth of microcracks due to hydrolysis at crack tips. Weibull's statistical model of brittle failure can accommodate these time effects. Discrete-element modelling (DEM) simulations of the temporal deterioration of loaded grains were conducted using Robertson's bonded agglomerates of microspheres to represent the grains. A time-dependent failure equation that describes crack growth corresponding to increasing grain stress was developed. The equation was incorporated into a single DEM agglomerate to simulate the deterioration of loaded grains. It is found that a particular relationship can be derived between the rate of loss of bond strength at a contact and the tension carried at that contact, such that the Weibull modulus of this rate-dependent model matches that of short-term loading tests on individual sand grains. A strength-probability-time diagram was generated that enables estimation of delayed fracture. The predicted grain strength decreases in a nearly linear fashion with the logarithm of time, which is in good agreement with previous results for ceramics. This DEM technique was then applied to the simulation of creep in a triaxial test with 378 breakable grains. The DEM results show that crack growth can lead to softening of samples with volumetric contraction due to grain breakage accompanying dilation due to shearing. Creep-induced failure might be anticipated when the axial strain reaches about 5%.