Effect of Immersion Duration on Shear Behavior of Granite Fractures

An understanding of shear behavior of fractured rocks subjected to different durations of groundwater immersion is essential for practical rock engineering. Direct shear tests were conducted on fractured granite samples after different immersion durations of 1, 3, 6 and 12 months, and the initially dry granite fracture samples were also tested as reference. The results indicate that the peak shear strength, residual shear strength, shear stiffness and dilation at peak shear stress decreased with increasing immersion duration, while the peak shear displacement increased with increasing immersion duration. The shear induced reductions in fracture surface parameter (Z₂) and the sheared-off volumes of asperities increased slightly with increasing immersion duration. An empirical function was proposed to successfully describe the evolutions of the shear parameters and the volume of sheared-off asperities with immersion duration. In addition, micro-scale tests including atomic force microscope, X-ray photoelectron spectra, nanoindentation and so on were conducted to reveal the underlying mechanism of immersion-induced weakening of granite fractures, which is attributed to the formation of a thin water layer attached on granite surfaces and the decreasing mechanical properties of asperities.