Influence of grain characteristics on the friction of granular shear zones

Numerical models of granular shear show lower friction and a greater tendency for stick slip than laboratory studies designed to investigate fault mechanics. Here we report on laboratory experiments designed to reproduce the conditions of numerical models and to test the role that grain characteristics play in controlling frictional behavior. Friction and microstructural data are compared for direct shear experiments on thin layers (2-3 mm) of angular quartz sand and spherical glass beads. We study the effect of grain shape, roughness, size distribution, and comminution. In a nonfracture loading regime, sliding friction for smooth spherical particles (μ ∼ 0.45) is measurably lower than for angular particles (μ ∼ 0.6). A narrow particle size distribution (PSD) of spherical beads (105-149 μm) exhibits unstable stick-slip behavior, whereas a wide PSD of spheres (1-800 μm) and the angular gouge display stable sliding. At higher stress, where grain fracture is promoted, initially spherical particles become stable with accumulated slip, and friction increases to the level observed for angular gouge. We find that frictional strength and stability of a granular shear zone are sensitive to grain shape, PSD, and their evolution. We suggest that a low friction translation mechanism, such as grain rolling, operates in gouge composed of smooth particles. Our results show that the first-order disparities between laboratory and numerical studies of granular shear can be explained by differences in grain characteristics and loading conditions. Since natural faults predominantly contain angular gouge, we find no evidence for a fault-weakening mechanism associated with the presence of gouge.