Nonlinear creep constitutive model of rock under high confining pressure and high water pressure

According to the triaxial creep curves of sandstone under high confining pressure and high pore water pressure, the creep rule of rock is discussed. The Burgers model is selected to describe the characteristics of creep in the low stress level, which can get nice result. When stress exceeds the yield level, the accelerated creep damage of rock will occur after a period of time. The accelerated creep curves are fitted by the mixed equation of power function and logarithmic function. Then, the method of analogy is used to propose a new nonlinear viscous component, which is in parallel connection with the plastic component to constitute a new nonlinear viscoplastic model. By connecting this model with Burgers model in series, a new six-component nonlinear viscoelastoplastic creep model is acquired. Consequently, the creep constitutive equation in three-dimensional state is deduced. With the test result, the algorithm of Universal Global Optimization is adopted to identify the model parameters, which shows that the proposed six-component nonlinear viscoelastoplastic creep model is available and reasonable.