THE MICROMECHANICS OF FRICTION IN A GRANULAR LAYER

A grain bridge model is used to provide a physical interpretation of t he rate- and state-dependent friction parameters for the simple shear of a granular layer. This model differs from the simpler asperity mode l in that it recognizes the difference between the fracture of a grain and the fracture of an adhesion between grains, and it explicitly acc ounts for dilation in the granular layer. The model provides an explan ation for the observed differences in the friction of granular layers deformed between rough surfaces and those deformed between smooth surf aces and for the evolution of the friction parameters with displacemen t. The observed evolution from velocity strengthening to velocity weak ening with displacement is interpreted as being due to the change in t he micromechanics of strain accommodation from grain crushing to slip between adjacent grains; this change is associated with the observed e volution of a fractal grain structure.