Influence of normal stress and grain shape on granular friction: Results of discrete element simulations

Laboratory experiments of granular shear deformation demonstrate that loading conditions and grain characteristics can significantly affect the macroscopic friction of a granular material under shear. We have examined the variation of maximum sliding friction with normal stress and grain shape using a version of the distinct element method (DEM) that includes bonds between adjacent particles. In this way, arbitrarily shaped grains can be generated to reproduce more realistic fault gouge with a range of grain sizes and shapes. Two types of grains were designed to represent quartz gouge: rounded grains composed of seven close-packed particles and triangular grains composed of six close-packed particles. DEM experiments were conducted by shearing granular assemblages with different grain shape distributions using the identical boundary configurations (i.e., wall surface roughness) over a range of normal stresses from 5 to 100 MPa and were compared to equivalent experiments using reference circular particle assemblages. The results show an inverse power law relationship between normal stress and maximum sliding friction in all cases, where both its coefficient and exponent are dependent on gouge angularity. Under normal stress over 20 MPa, triangular grain assemblages exhibited the highest frictional strength and also the highest abundance of rotating grains, demonstrating that enhanced grain rolling alone does not explain the low frictional strength of simulated granular assemblages.