Shear Strength of Rock Fractures Under Dry, Surface Wet and Saturated Conditions

For rock fractures, the degradations in the strength of contacting asperities and the surface frictional resistance are responsible for the water-induced weakening in the shear strength. To quantitatively examine their independent roles, direct shear tests on sawtooth fracture samples of granite and sandstone under three moisture conditions: dry, surface wet and saturated, were conducted subject to three levels of normal stresses. The surface wet condition only resulted in the variation in the basic friction angle and the saturated samples underwent the degradation in both unconfined compressive strength (UCS) and basic friction angle, which were obtained via unconfined compression test and direct shear test, respectively. Two weakening coefficients that represent the reductions in UCS and basic friction angle, respectively, were proposed and incorporated into an analytical model, which quantifies the entire shear stress evolutions during shear based on a continuous yielding mechanism. The difference in the shear strength between dry and surface wet conditions originates from the lubricant effect of water represented by the change in basic friction angle with a weakening coefficient less than 10% for both rocks. Under the saturated condition, the weakening coefficient of the UCS varies significantly from 15.17% for granite to 50.39% for sandstone. A series of datasets that characterize the reductions in UCS and basic friction angle induced by water were collected from the literature, which were then incorporated into the analytical model to estimate the general weakening trend in the shear strength of the common rocks in practices. For crystalline rocks, the water-mediated lubrication seems to be the primary mechanism reducing the shear strength, while for sedimentary rocks, the remarkable degradation in UCS may dominate the weakening mechanism. The quantified weakening coefficients and the revealed weakening behavior of various rocks can be directly linked to the fracture shear strength estimation in engineering design.