Experimental and numerical investigations on the mechanical behavior of sandstone subjected to gas undrained triaxial compression

In this study, mechanical behavior of sandstone is investigated through laboratory triaxial loading tests under various confining pressures (5, 15, and 20 MPa) and pore gas pressure (0, 1, and 2 MPa). Rock strength and its failure mode are compared and analyzed on the basis of the experimental results. It is seen that the pore gas pressure induced the diminution of compression peak strength, cohesion (5.3-9.1%), and friction angle (4.6-9.7%) and led to local brittle failure at both ends of the rock. With the increase of the confining pressure, the brittle-ductile transition of the mechanical behavior took place; meanwhile, significant plastic deformation and volumetric dilatation are noticed. It is indicated that the failure and nonlinear mechanical behavior of red sandstone are essentially related to the coupling effect between the damage and plastic deformation. Accordingly, a coupled elastoplastic damage model for the sandstone under different drainage conditions is proposed along with a non-associated plastic potential function. The coupling between the damage and plastic deformation is established by introducing the independent damage variable in the plastic yield criterion. The results show that the model is capable to describe the main features of the mechanical behaviors of the sandstone. The strength, deformation characteristics, and the pre-peak plastic hardening characteristic are in good agreement with the experimental descriptions.