Creep characteristics and nonlinear creep damage model of Bocigou formation slate in Kangding-Xinduqiao section of West Sichuan

The creep characteristics of rock are closely related to the long-term stability of rock mass. With the excavation of deep buried tunnels in west China, in order to ensure the safety of engineering and the long-term stability of underground buildings, it is particularly important to study the creep characteristics of rock under complex stress conditions. The traditional creep constitutive model is difficult to accurately describe the accelerated creep stage of rock, and the existing creep model is difficult to fit the creep characteristics of the Pocigou Formation slate. This study takes the Bocigou Formation slate in the Kangding-Xinduqiao section of west Sichuan as the research object. Based on the identification of geological environment background and rock mineral composition, the unloading creep test is carried out. The deformation characteristics of the Bocigou Formation slate under unloading conditions are analyzed, and the creep characteristics of the slate and the damage evolution law during unloading are revealed. Considering the cumulative damage effect in the unloading creep process, the damage variable is introduced to improve the Newtonian element in the traditional Nishihara model, and a creep damage model which can describe the accelerated creep stage is established. The results show that under unloading conditions, the deformation of slate is dominated by instantaneous elastic strain, and the creep phenomenon is significantly enhanced with the increasing deviatoric stress level. The long-term strength of slate is reduced by 20.2%–27.1%. The 1-stOpt is used to identify the parameters of the nonlinear creep damage model. The fitting theoretical curve is in good agreement with the experimental value, and the correlation coefficient reaches 0.945. The improved nonlinear creep damage model after the introduction of damage variable is more reasonable for description of the unloading creep characteristics of slate in the study area, which provides a theoretical basis for the stability analysis of surrounding rock under relevant working conditions.