Influences of pore pressure on short-term and creep mechanical behavior of red sandstone

Short-term and creep tests for saturated red sandstone under different pore pressures were carried out by a rock servo-controlled triaxial equipment. Based on the experimental results, the influences of confining pressure and pore pressure on short-term mechanical behavior of red sandstone are firstly analyzed. The results show that the peak strength of dry red sandstone increases with the confining pressure, which can be better expressed by the nonlinear Hoek–Brown criterion than the linear Mohr–Coulomb criterion. But with the increase of pore pressure, the peak strength and elastic modulus of saturated red sandstone all decrease step by step. The short-term failure mode of red sandstone is dependent of the confining pressure, but independent of the pore pressure. And then, the influence of pore pressure and axial deviatoric stress on the creep mechanical behavior of saturated red sandstone is analyzed quantitatively. Creep contribution to rock deformation increases with the pore pressure, and the specimens show significant time-dependent effect at higher deviatoric stresses. However, the steady-state creep rate of saturated red sandstone increases nonlinearly but the viscosity coefficient decreases gradually with increasing axial deviatoric stress and pore pressure. An exponential function is suggested to characterize the relationship between the creep parameters (including creep strain, steady-state creep rate and viscosity coefficient) and the axial deviatoric stress, and pore pressure. The calculated curves using the proposed function are compared with the experimental results, which is in a good agreement with the experimental data. In the end, the mechanism that the pore pressure and deviatoric stress affect the creep deformation of saturated red sandstone is discussed in detail. The presented experimental results on short-term and creep mechanical behavior of red sandstone under different pore pressures are very significant for evaluating the long-term stability and safety of deep underground rock mass engineering.