Effects of particle properties and boundary conditions on soil shear behaviour : 3-D numerical simulations

The mechanical response of cohesionless granular materials under general engineering stress conditions is examined by simulation of direct shear box and biaxial tests using a three dimensional distinct element method computer code PFC-3D. When shearing a random assembly of single sized spheres, the effective angle of shearing resistance at constant volume ø’cυ does not increase with interparticle friction angle øμ. The strength and volumetric dilation of such an assembly are low. The strength of an assemblage increases when different sized spheres are used or when their rotations are restrained. For non-spherical particles, an increase in øμ results in an noticeable increase in ø’peak, ø’cυ and volumetric dilation. The less spherical the particles, the higher the strength and the dilation rate of the assembly, probably as a result of particle interlocking. Loading conditions such as the initial porosity, confining stress and friction on the loading platen, also significantly affect a sample’s stress-dilation behaviour. Shear bands can be characterised by a defined zone of intensively rotated particles, associated with large voids and concentrated shear strain. The width of a shear band is related to particle size and shape and the direction of a shear band is related to the applied boundary conditions. Samples with low initial porosity and smooth loading platens or comprising spherical particles are less likely to develop shear bands. The present study suggests that, under specific conditions and to a certain extent, a DEM model consisting of tens of thousands particles can be a feasible approach to investigate both the macro and micro behaviour of granular material. However, to study the micro behaviour in greater detail or to simulate a more realistic and sophisticated test, further research will be needed.