Abstract
The confined tri-axial extension test is defined by a compressive stress system in which the maximum principle stress (σ1) is equal to medium one (σ2). Using cylindrical specimens of Kimachi sandstone, the tests were carried out in which radial stress was driven by liquid confining-media as σ1 (=σ2) and axial stress was driven by solid pistons as minimum principal stress (σ3). Test specimens were failed with both shear and tensile fractures, more than 70 MPa radial stress. The surface topography of the fracture was digitized by the stereo-photogrammetry. Using a pair of overlapping digital images of a fracture surface, a three dimensional morphology of the surface can be produced. From the morphology of shear fractures, it is 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, shows that the one with high axial stress is smoother than the one with low axial stress. Therefore, the formation of tensile fracture follows the shear one and it is influenced with the minimum principle stress (axial stress), which is perpendicular to the tensile fracture surface.
