Journal of Rock Mechanics and Geotechnical Engineering (Jun 2023)
Experimental investigation on frictional properties of stressed basalt fractures
Abstract
The frictional strength and sliding stability of faults are crucial in interpreting earthquake mechanisms and cycles. Herein, we report friction experiments on basalt fractures, using a self-designed triaxial apparatus that allows direct shear of samples under coupled hydro-mechanical conditions. Velocity-stepping (VS) and slide-hold-slide (SHS) experiments are performed on both bare and gouge-bearing surfaces of Xiashan basalt subjected to cyclic shear velocities at 1–30 μm/s, effective normal stresses of 1–5 MPa, and pore pressures of 70–300 kPa. The measured basalt friction coefficients are in the range of 0.67–0.74, which is sensitive to gouge thickness, normal stress, and water. Specifically, a reduction in friction coefficient is observed with an increment in gouge thickness, normal stress, and pore pressure. Based on the microscopic observation of the pre- and post-shearing sliding surfaces, this weakening effect in friction coefficient can be attributed to powder lubrication. Furthermore, the VS test results reveal predominantly velocity-strengthening behavior at investigated slip velocities, and this velocity strengthening behavior does not appear to be influenced by variations in normal stress, gouge thickness, and water. However, changes in sliding velocity and normal stress can lead to a shift between stable and unstable sliding. Specifically, stable sliding is favored by high sliding velocities and low normal stress applied in this study. Finally, we analyze the experimental data by calculating the rate-and-state parameters using the rate- and state-dependent friction (RSF) theory. Importantly, the calculated friction rate parameter (a-b) supports the velocity-strengthening behavior. Both frictional relaxation (Δμc) during hold periods and frictional healing (Δμ) upon re-shearing are linearly proportional to the logarithmic hold time, which may be attributed to the growth in true contact area with hold time. This study sheds light on the roles of sliding velocity, and gouge thickness in controlling frictional strength and stability of basalt fractures.
