An Experimental Study on Normal Stress and Shear Rate Dependency of Basic Friction Coefficient in Dry and Wet Limestone Joints

Among all parameters that affect the friction of rocks, variable normal stress and slip rate are the most important second-order parameters. The shear-rate- and normal-stress-dependent friction behavior of rock discontinuities may significantly influence the dynamic responses of rock mass. In this research, two limestone rock types, which were travertine and onyx marble with slickenside and grinded #80 surfaces, were prepared and CNL direct shear tests were performed on the joints under various shear conditions. The shearing rate varied from 0.1 to 50 mm/min under different normal stresses (from 2 to 30 % of UCS) in both dry and wet conditions. Experiments showed that the friction coefficient of slickensided and ground #80 surfaces of limestone increased with the increasing shear velocity and decreased with the increasing normal stress. Micro-asperity interlocking between ground #80 surfaces showed higher wear and an increase in friction coefficient (µ) compared to slickensided surfaces. Slickensided samples with moist surfaces showed an increase in the coefficient of friction compared to dry surfaces; however, on ground #80 surfaces, the moisture decreased the coefficient of friction to a smaller value. Slickenside of limestone typically slides stably in a dry condition and by stick–slip on moist surfaces. The observed shear-rate- and normal-stress-dependent friction behavior can be explained by a similar framework to that of the adhesion theory of friction and a friction mechanism that involves the competition between microscopic dilatant slip and surface asperity deformation. The results have important implications for understanding the behavior of basic and residual friction coefficients of limestone rock surfaces.