SHEARING RESISTANCE OF FRACTURES IN SATURATED PHRA WIHAN SANDSTONE

Abstract

Triaxial shear tests are performed to assess the effects of water saturation and confining pressure on shear strengths and dilations of tension-induced fractures and smooth saw-cut surfaces prepared in Phra Wihan sandstone.  A polyaxial load frame is used to apply confining pressures between 1 and 18 MPa with displacement velocities ranging from 1.15×10^-5 to 1.15×10^-2 mm/s.  The results indicate that water saturation of the rock can reduce the peak and residual shear strengths of the fractures.  Shearing resistances of smooth saw-cut surfaces tend to be independent of the displacement velocity and water saturation.  Under each confinement the peak and residual shear strengths and dilation rates of rough fractures increase with displacement velocities.  The sheared-off areas increase when the confining pressure increases, and the displacement rate decreases.  An empirical criterion that explicitly incorporates the effects of shear velocity is proposed to describe the peak and residual shear strengths of fractures in saturated sandstone.  The criterion fits well to the test results, which may be used to predict the shear strengths of the saturated sandstone under in-situ condition.