Influences of grain comminution on the frictional properties of simulated fault gouge

Abstract

Previous DEM (Distinct Element Method) simulations of granular shear have qualitatively reproduced experimental observations of shear zone deformation and also provided insight into the frictional behavior of fault gouge. Gouge deformation, however, was accommodated by grain rolling and sliding alone, with no grain comminution. We examine the influences of grain comminution on frictional behavior of simulated fault gouge using DEM including breakable bonds between adjacent particles. In this way, arbitrarily shaped grains can be generated to reproduce more realistic fault gouge, and grain size and shape can evolve by grain fracture during shear. Two types of grains, rounded grains composed of 7 close-packed spherical particles and triangular grains composed of 6 close-packed spherical particles were designed to generate quartz gouge; Four different grain sizes were generated using four different particle sizes. Both the rounded and triangular grain have four breakable bonds and can break down into two irregular-shaped subgrains due to tensile and shear forces during shear, allowing for a wide range of grain sizes and shapes. DEM experiments were conducted by shearing identical granular assemblages composed of either the rounded or triangular grains under identical boundary conditions (i.e., wall surface roughness), over a range of normal stresses from 5 MPa to 100 MPa. The results show that the intensity of grain comminution is not only a function of normal stress, but also strongly dependent on grain shape. The triangular grains are much easier to break down at certain normal stresses than the rounded grains, a result of the smaller number of intergrain contacts and higher contact forces. The probability of breakage of the smallest grains is much higher than for the larger grains. The results support the constrained comminution mechanism in which the probability of particle fracture is strongly dependent on the relative size of nearest neighbors. As a less expensive deformation mechanism than rolling and sliding at high stress, our results demonstrate that comminution itself weakens the deformed granular assemblage. On the other hand, because comminution also changes grain shape and size, it increases the frictional strength of granular assemblage by increasing grain angularity. Grain shape becomes the most important factor that affects the frictional strength of fault gouge.