The effect of particle dimensionality on Granular friction in laboratory shear zones

To match the boundary conditions of numerical models and to examine the effect of particle dimensionality on granular friction, we conducted laboratory experiments on rods sheared in 1-D and 2-D configurations, glass beads (3-D), and angular quartz sand (rough 3-D). The average coefficient of friction during stable sliding for 1-D, 2-D, smooth 3-D, and rough 3-D particles is 0.15, 0.3, 0.45, and 0.6, respectively. Frictional strength of 2-D layers exceeds 1-D friction by an amount associated with dilatancy and the additional contact plane in 2-D. We show that 3-D granular friction exceeds 2-D friction by the amount of interparticle friction on the out-of-plane particle contacts that do not exist in 2-D. Data from our 2-D experiments are remarkably similar to numerical results based on 2-D particle dynamic simulations. Our data indicate that application of numerical models of granular friction to tectonic faults will require computations involving rough, 3-D particles.