Abstract: Mining deep coal resources involves high stress geological environment and rock excavation unloading engineering environment. Research on the re-bearing capacity characteristics and damage mechanism of unloading-damaged rock is crucial for revealing the instability and rupture behavior of deep rock body. The damaged sandstone specimens were prepared by controlling the unloading point during triaxial loading. Transverse and axial strains of the specimens were monitored using the RMT-150C rock mechanics loading system and the Donghua DHDAS stress-strain acquisition system during triaxial unloading and uniaxial reloading. The test results show that according to the wave velocity differences before and after loading, the unloaded rock body can be classified into three categories: compact rock body, low-loss rock body and high-loss rock body. With the increase of unloading stress level, the damaged sandstone transitions from plastic-elastic-plastic deformation to plastic-elastic deformation, and finally to elastic-viscous deformation. The crack volume strain curve of the compact rock body during uniaxial reloading is basically the same as that of the standard sandstone. The elastic deformation stage of the low-loss rock body is obviously shortened, while the high-loss rock body fails after minimal deformation. The compact rock body exhibits Y-type diagonal shear damage characteristics. The low-loss rock body develops and penetrates through longitudinal cracks and fissures formed during the unloading phase, leading to master cracks causing damage. The high-damage rock body develops X-type diagonal shear damage on the basis of Y-type diagonal shear rupture. A mechanical analysis model of cracks under uniaxial compressive stress was established. The relationship between rock crack fracture angle  and crack angle  and friction coefficient f of crack surface was elucidated.

Key words: damaged sandstone, uniaxial re-loading, characteristic strength, crack volume strain, failure mechanism