Shear strengths of sandstone fractures under true triaxial stresses

True triaxial shear tests have been performed to determine the peak shear strengths of tension-induced fractures in three Thai sandstones. A polyaxial load frame is used to apply mutually perpendicular lateral stresses (σp and σo) to the 76 × 76 × 126 mm rectangular block specimens. The normal to the fracture plane makes an angle of 59.1° with the axial (major principal) stress. Results indicate that the lateral stress that is parallel to the fracture plane (σp) can significantly reduce the peak shear strength of the fractures. Under the same normal stress (σn) the fractures under high σp dilate more than those under low σp. According to the Coulomb criterion, the friction angle decreases exponentially with increasing σp/σo ratio and the cohesion decreases with increasing σp. The lateral stress σp has insignificant effect on the basic friction angle of the smooth saw-cut surfaces. The fracture shear strengths under σp = 0 correlate well with those obtained from the direct shear tests. It is postulated that when the fractures are confined laterally by σp, their asperities are strained into the aperture, and are sheared off more easily compared to those under unconfined condition.