Shear Strength of Fractures in Phu Kradung Sandstone Under True Triaxial Stresses

Abstract

Abstract

True triaxial shear tests have been performed to determine the peak shear strengths of tension-induced fractures in Phu Kradung sandstone.  A polyaxial load frame is used to apply mutually perpendicular lateral stresses (s_p and s_o) to the 76´76´126 mm rectangular block specimens.  The normal of 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 (s_p) can significantly reduce the peak shear strength of the fractures.  Under the same normal stress (s_n) the fractures under high s_p dilate more than those under low s_p.  According to the Coulomb criterion, the friction angle decreases with increasing s_p/s_o ratio and the cohesion decreases with increasing s_p.  The lateral stress s_p has insignificant effect on the basic friction angle of the smooth saw-cut surfaces.  The fracture shear strengths under s_p = 0 correlate well with those obtained from the direct shear tests.  It is postulated that when the fractures are laterally confined by s_p, their asperities are strained into the aperture, and are sheared off easier compared to those under unconfined condition.