Study on viscoelastic-plastic creep model of rock based on creep parameter degradation

Establishing a reasonable rock creep model is of great significance for analyzing the creep characteristics of rock and predicting the long-term stability of engineering rock mass. Based on the analysis of the nonlinear creep characteristics of rock, this paper innovatively proposes an unsteady Kelvin body that reflects the nonlinear characteristics of the attenuation creep stage. Based on the negative exponential function relationship between the viscosity coefficient and the stress and time, a nonlinear viscoplastic body that reflects the nonlinear characteristics of the accelerated creep stage is proposed. A viscoelastic-plastic creep model of rock considering the deterioration of creep parameters is constructed, and the creep equations of the creep model under one-dimensional stress state and three-dimensional stress state are derived. The model in this paper is used to fit and analyze the creep test data of sandstone and carbonaceous shale under different confining pressures. The results show that the fitting degree of the test curve and the model curve is good, with the coefficient of determination R2 generally above 0.98 and low RMSE/MAE values, indicating that the rock creep model considering the deterioration of creep parameters established in this paper can well describe the creep mechanical characteristics of rock. At the same time, the fitting effects of the model, the classical Nishihara model and the fractional Nishihara model are compared to further illustrate the correctness and advancement of the established model. The research in this paper not only enriches the theory of rock creep model, but also has certain theoretical reference value for the study of long-term stability of rock mass engineering.