To study the creep mechanical behavior of surrounding rock under the coupled effect of confining pressure unloading and deviatoric stress increase during the excavation of deep underground engineering, triaxial creep tests under stepwise unloading of confining pressure and loading of deviatoric stress were conducted using marble as the research object. The influences of different initial confining pressures, unloading amounts, and stress levels on the axial and circumferential creep deformation, creep rate, and failure characteristics of marble were systematically analyzed. The results indicate the following: (1) As the amount of confining pressure unloading increases, the creep failure stress of marble decreases significantly, and the test duration is markedly shortened. (2) Under conditions of small unloading amounts, the creep rate initially decreases and then increases with increasing stress levels, while under large unloading amounts, the creep rate monotonically increases with stress levels, indicating that confining pressure unloading significantly weakens the stability of the rock mass during creep. (3) Based on the test results, a three-dimensional non-stationary Nishihara creep model suitable for the stress path of stepwise confining pressure unloading and deviatoric stress loading was established. The model calculations agree well with the experimental data, effectively describing the entire creep process of marble under unloading conditions. The research findings can provide a reference for the long-term stability analysis of the surrounding rock after excavation in deep underground engineering.
Guo et al. (Sun,) studied this question.