Triaxial Compression Testing at Constant and Reducing Confining Pressure for the Mechanical Characterization of a Specific Type of Sandstone

In this study, constant confining pressure (CCP) and reducing confining pressure (RCP) triaxial compression tests were conducted for mechanical characterization of a sandstone material. These triaxial experiments were carried out within a range of confining pressure from 0 to 50 MPa. The experimental results revealed that the sandstone exhibits a significant ductile-semibrittle stress degradation behaviour in the post-peak region. It was found that the plastic flows are mostly perpendicular to the initial, ultimate, fracture, and residual yield surfaces. This demonstrates that the associated flow rule can be adopted to describe the macroscopic plastic deformation of the sandstone. With increasing confining pressure, the brittleness decreases and causes a transformation of the failure mode from tension-splitting failure to mixed tension-shear failure, and then to shear failure. The sandstone exhibits the same deformation and strength behaviours along the CCP and RCP stress paths, however, deformation and failure were more brittle in the RCP tests than in the CCP tests. This experimental investigation can provide an important technical reference for geotechnical engineering projects involving sandstone.