Experimental study on the velocity-dependent frictional resistance of a rough rock fracture exposed to normal load vibrations

Changes in shear velocity can strengthen or weaken the frictional resistance of joints/faults in natural systems, but the mechanism remains unclear. We investigated the shear behavior of a rough basalt fracture in well-controlled, repeatable shear tests under constant and dynamic normal load conditions at different shear velocities. Normal load vibrations, simulating a dynamic normal load, were applied to the upper block of a fractured basalt sample. Simultaneously, a shear load was applied to the bottom block, providing a constant shear velocity. The peak shear strength increased with increasing shear velocity under constant normal load conditions. The peak shear strength decreased at a lower shear velocity under normal load vibrations. When the shear velocity exceeded the critical value, v(c), the peak shear strength increased. The apparent coefficient of friction reduced under normal load vibrations. The reduction in the dynamic coefficient of friction increased with increasing shear velocity. We identified a phase shift between the peak normal load and peak shear load with peak shear load delay (D1) and a phase shift between peak normal load and the peak coefficient of friction with the peak coefficient of friction delay (D2). D1 and D2 were dependent on the quasi-static coefficient of friction and shear velocity, and both decreased with increasing shear velocity. D1 decreased with the increasing quasi-static coefficient of friction, while D2 was almost constant with changes in the quasi-static coefficient of friction. A new shear strength criterion was proposed for a rough joint under a constant shear velocity and normal load vibrations.