Anisotropic time-dependent deformation and damage constitutive model of rock under true triaxial compression

Time-dependent characteristics are key factors for the instability and safety in underground engineering projects. Due to geological structure and manual excavation, any rock mass is subjected to true triaxial stress. Therefore, true triaxial tests under the monotonous loading were conducted on sandstones first under different intermediate principal stresses along with a constant pore pressure to simulate rock in situ, and subsequently, the creep experiments were performed at different stress proportions under corresponding conditions. The experimental results show that anisotropic deformation and rates vary with stress levels and the intermediate principal stress. Combined with literature data, the lateral deformation ratio of the intermediate principal strain ( ε _2 ) to the minimum principal strain ( ε _3 ), is closely related to the reciprocal of the side stress ratio for instant and creep increments in the penultimate stage. The intermediate principal stresses ( σ _2 ) have a significant effect on the creep behavior of sandstone. Based on the experimental results, a new anisotropic creep-damage model is subsequently developed, and anisotropic damage is defined based on statistical damage theory. The parameter values can be obtained using the method of Universal Global Optimization (UGO). The theoretical predictions show good consistency with laboratory data and field data, respectively. The model is reliably used to simulate creep behavior and gives a good account of the effect of intermediate principal stresses. Therefore, it could provide a better understanding of the long-term stability of engineering projects.