Three-Dimensional Discrete Element Simulations of Direct Shear Tests

Using discrete element simulations, one can monitor the micro-mechanisms driving the macroresponse of granular materials and quantify the evolution of local stress and strain values. However, it is important to couple the se simulations with carefully controlled physical tests for validation and insight. Only then can findings about the micro- mechanics of the material response be made with confidence. Moreover, the sensitivity of the observed response to the test boundary conditions can be analyzed in some detail. The results of three-dimensional discrete element simulations of direct shear tests and as well as complementary physical tests on specimens of steel balls are presented in this paper. Previous discrete element analyses of the direct shear test have been restricted to two-dimensional simulations. For the simulations presented here, an analysis of the internal stresses and contact forces illustrates the three-dimensional nature of the material response. The distribution of contact forces in the specimen at larger strain values, however, was found to be qualitatively similar to the two-dimensional results of Zhang and Thornton (2002). Similarities were also observed between the distrib ution of local strain values and the distribution of strains obtained by Potts et al (1987) in a finite element analysis of the direct shear test. The simulation results indicated that the material response is the stress dependent. However, the response observed in the simulations was found to be significantly stiffer than that observed in the physical tests. The angle of internal friction for the simulations was also about 3o lower than that measured in the laboratory tests. Further laboratory tests and simulations are required to establish the source of the observed discrepancies.