Creep properties of rock under high confining pressure and different pore water pressures and a modified Nishihara model

The creep rules of rock are analyzed based on the triaxial creep test results of fine sandstone under high confining pressure and different pore water pressures. With the increase of the pore water pressure, the viscoelastic modulus linearly decreases. The linear function and power function are adopted to fit the linear parameters which change over time. Then an expression for the viscoelastic modulus, which changes with water pressure and time, is derived. Furthermore, a modified Kelvin model is proposed. According to the accelerating creep results under different pore water pressures, a power function is selected for fitting. By introducing a nonlinear element, a two-component visco plastic creep model is obtained for reflecting the accelerated creep characteristics under different pore water pressures. By combining this model with the modified Kelvin model, a modified Nishihara model can be put forward. Using the 1stOpt software to identify the model parameters, the results are satisfactory. A comparison between the test and theoretical curves proves the validity of the proposed model.