Effects of shear stress applied to surfaces in stationary contact on rock friction

In this paper we show how the applied shear stress affects the frictional strength of granite sliding surfaces initially held in stationary contact. We develop an experimental scheme to perform a slide and hold friction test whereby the shear stress is servocontrolled during the hold period of the test. A series of experiments conducted at a single normal stress of 5 MPa, show the following effects. First, unloading the shear stress during the hold period increases frictional resistance by up to 10–20%; a decrease in shear stress linearly increases the frictional resistance of the interface. Secondly, the static friction coefficient increases with the logarithm of contact time. The friction coefficient increases by 0.01 per decade of contact time at near zero shear stress levels, and by more than 0.02 at shear stress levels that are greater than half the sliding friction.