ABSTRACT
ABSTRACT: The confined triaxial extension test is defined as a compressive stress system in which the maximum principal stress (σ1) is equal to the intermediate one (σ2). Using cylindrical specimens of Kimachi sandstone, tests were carried out in which radial stress was applied by liquid confining medium as σ1 (= σ2) and axial stress was applied by solid pistons as minimum principal stress (σ3). Test specimens failed with shear and tensile fractures, at radial stresses higher than about 70 MPa. The surface topography of the fracture was digitized using stereo-photogrammetry. Using a pair of overlapping digital images of a fracture surface, a three-dimensional morphology of the surface was produced. From the morphology of the shear fractures, it was shown that the fracture angles increase with radial stress. Tensile fractures initiate from the shear fractures. In addition, fracture roughness, which is calculated from the morphology of tensile fractures, showed that the one with high axial stress is smoother than the one with low axial stress. Therefore, the formation of tensile fracture follows the formation of shear fracture and the tensile fracture is influenced by the minimum principal stress (axial stress), which is perpendicular to the tensile fracture surface.
