Shear Behavior and Properties of Granite Fractures Under Different Pore Water Pressure Conditions

Understanding and predicting the shear mechanical characteristics of rock fractures subjected to different pore water pressures is vital for the safety and the stability of the reservoir bank slope. A series of direct shear tests were conducted on granite fractures covering various reservoir impoundment-relevant pore water pressures. The results reveal that as pore water pressure increases, the peak shear strength, residual shear strength, shear stiffness, and peak shear displacement show a decreased trend, while the peak dilation increases with the pore water pressure. The shear-induced reductions in fracture surface parameter ( Z 2 ) and the sheared-off volumes of asperities decrease with the increase in pore water pressure. An empirical function is presented to successfully describe the evolutions of the shear parameters and the volume of sheared-off asperities with pore water pressure. Under drained conditions, it is found that shear failure will cause a transient drop in pore water pressure, and the pore water pressure fluctuation coefficient can be used as an early warning indicator for potential damage of fractures. In addition, a singular-value decomposition method is employed to fit the peak and residual shear strength with pore water pressure, which indicates that the effective stress principle can satisfactorily explain the hydro-mechanical mechanism of rock fractures. Before the shear failure, the effective stress coefficient is close to unit, while it is reduced after the shear failure due to the increase in the actual contact area caused by asperity degradation and gouge formation.