Creep Failure Mechanism and Model of Granite under True Triaxial Loading and Unloading Conditions

The excavation of rock masses in deep engineering can induce stress concentration or unloading, which can significantly affect the long-term stability of engineering. To describe the creep mechanical behavior of deep hard rocks after stress adjustment, true triaxial creep tests were conducted through multistage loading and unloading. The effects of sigma 1 loading, sigma 2 unloading, and sigma 3 unloading on creep deformation of granite were investigated. Meanwhile, the changes in creep, acoustic emission (AE) counting, and energy release rates during creep stages were evaluated. The creep failure mode and mechanism of granite were revealed. The results of the true triaxial test showed that both sigma 1 loading and sigma 2 unloading accelerate the creep of granite, while sigma 3 unloading promotes the creep in the sigma 3 direction, but the creep in sigma 1 and sigma 2 directions is not promoted to any significant extent. In the first three creep stages of sigma 2 unloading, the strain rate of granite changes significantly. Upon approaching the stage of instability fracture, the changes in the AE count rate and energy release rate are more pronounced compared to the strain rate. By analyzing creep curves, a comprehensive three-dimensional nonlinear viscoelastic-plastic damage creep model specifically for granite was established. The consistency between the experimental data and the predicted result obtained from the model shows that the three-dimensional nonlinear viscoelastic-plastic damage creep model can demonstrate the creep behavior of granite under true triaxial sigma 1 loading and sigma 2 and sigma 3 unloading conditions, thereby serving as a valuable reference for assessing the long-term stability of deep rock masses.