Influence of Grain Shape and Wall Roughness on Rock Friction in DEM Simulations

Experimental data on rock friction for normal stresses up to 200 MPa are characterized by large scatter in the data, a result thought to be due to poorly constrained variations in the roughness of shear zone walls. Recent laboratory experiments have demonstrated that grain characteristics can also significantly affect the macroscopic friction of a granular material under shear. We have examined the effects of wall roughness on friction, and the variation of friction with normal stress and grain characteristics using a modified version of the Distinct Element Method (DEM), in which bonds are added between adjacent particles. In this way, grains of arbitrary shape and size can be generated to reproduce more realistic fault gouge, which can evolve by grain fracture during shear. Two types of grains, rounded grains composed of 7 close-packed particles and triangular grains composed of 6 close-packed particles were designed to simulate quartz gouge. DEM experiments were conducted by shearing identical granular assemblages, subject to the same wall surface roughness, over a range of normal stresses from 1 MPa to 100 MPa. Similar experiments were conducted on both types of gouge. The results show an inverse power law relationship between normal stress and friction. For the rounded grain assemblages, this relationship can be represented by a best fit equation: μ = 0.8569σ _n^-0.2627. Base friction of the rounded-grain assemblages is around 0.4, similar to results of DEM simulations using unbonded, circular particles. Gouge composed of triangular grains is much stronger, with base friction of about 0.65, close to the results of laboratory experiments. At lower normal stresses, grain interlocking is a dominant deformation mechanism, particularly favored by triangular grains. Grain rolling becomes more significant, and effects of grain shape on friction becomes less important, with increase of normal stress. Our study confirms that friction of simulated granular assemblages with identical wall roughness obeys the power law friction relationship derived from Hertz's contact law, and that grain shape is a key factor that affects the magnitude of friction.