Abstract: To investigate the triaxial creep mechanical properties of unloading-damaged sandstone, some sandstone samples with different unloading damage degrees were prepared by controlling the unloading amount using the MTS816 rock mechanics test system. Triaxial creep mechanical tests with graded loading were carried out on the aforementioned unloading-damaged sandstone to analyze the influence of initial unloading damage degree on the strain characteristics, creep rate, deformation modulus, long-term strength, and failure mode of sandstone. The results show that: (1) Under the condition of graded creep, both the axial and radial strains of sandstone with different initial unloading damage degrees stepwise grow. The axial instantaneous strains exhibit an almost linear growth trend with increasing loading levels, whereas the radial instantaneous strain shows a nonlinear growth trend. Moreover, as the initial unloading damage degree increases, the increment of axial instantaneous strain caused by increasing unit loading continuously increases. (2) The unloading-damaged sandstone undergoes three stages in the non-destructive stage: instantaneous deformation, deceleration creep, and steady creep. The steady creep rate increases exponentially with the increase of initial degree of unloading damage. In the failure stage, the sandstone experiences four stages: instantaneous deformation, deceleration creep, steady creep, and accelerated creep. Among them, the accelerated creep stage takes a very short time but can produce significant expansion deformation, leading to unstable failure of the rock sample. (3) The total creep time, average deformation modulus, and long-term strength of sandstone all show a non-linear decreasing trend as the initial unloading damage degree increases. When the initial unloading damage degree surpasses 70%, there is a sharp decline in the total creep duration, average deformation modulus, and long-term strength of sandstone, with the maximum reductions being 24.77%, 33.28%, and 21.79%, respectively. (4) As the initial unloading damage degree increases, the creep failure mode of sandstone gradually transitions from a single shear failure to a complex tension-shear mixed failure.

Key words: rock mechanics, unloading damage, graded creep, long-term strength, failure mode