Modelling of a direct shear test in sand using the 3D Discrete Element Method

Abstract

In the present article, a direct shear test was modelled using the 3D Discrete Element Method. This approach describes the soil as an assembly of particles, reproducing the macroscopic soil behaviour from micro-mechanics interactions between individual particles. About 70000 particles were used in a 3D model, developed to reproduce a direct shear test. The numerical model results will be compared with experimental tests of a coarse sand from previous stages of this research. The real material was modeled as perfect individual spheres, with sizes consistent with the real grain size distribution and with a rolling friction approach to include the sand's grain shape. Several methodologies were followed in order to reproduce an initial void ratio similar to those obtained experimentally. The goal is to accurately reproduce the macro-scale response of the laboratory tests with the DEM model, and to study the effect of the micro-mechanical parameters in the stress path and the material deformation. Two specimens with different initial void ratio were modelled to study the effect of the micro-mechanical parameters and the initial fabric compactness on the model results. Afterwards, a model calibrated against experimental data is presented with a brief analysis of the particle migration and the contact orientation. The results show the development of a clear shear zone in the middle portion of the specimens, with an evident out of plane migration of particles. It is shown that, including a rolling resistant model, the stress path can be appropriately reproduced, but the dilatancy characteristic of a dense sand was very difficult to replicate.