The effect of unloading stress levels on the unloading failure mechanism is explored from both macro- and microscopic perspectives by carrying out tests to load axial compression and unload confining pressures on marble samples under different unloading stress levels and with the simulation of particle flow. The results reveal four points. First, the lower the prepeak unloading level, the lower the peak bearing capacity of rock samples after unloading; while the closer the unloading level to the peak bearing capacity, the later the occurrence of inflection point for the negative increase of volumetric strain. Second, bond energy and strain energy mainly fluctuate due to changes in unloading levels. The lower the prepeak stress level, the lower the inflection point where bond energy first increases and then decreases and the more obvious the fluctuations in strain energy from the unloading point onwards. Third, the acoustic emission (AE) event count rate increases after unloading from different stress levels. The lower the prepeak unloading stress level of specimens, the smaller the axial strain corresponding to the maximum AE event count rate. Fourth, the evolution of cracks with such a slow–rapid–slow increase does not vary with the changes in unloading stress levels. Moreover, the damage to failure caused by tensile cracks in specimens is more serious than that caused by compression-shear forces during unloading failure. There are a finite number of secondary failure surfaces inside the model at high unloading levels and the cracks caused by tensile failure are distributed over a large area. Relevant experimental results can provide theoretical basis for surrounding rock deformation control of rock underground engineering.

The excavation of deep tunnel in rock mass undergoes complex loading and unloading stress paths, resulting in rib spalling, flaking, and even severe rockburst disasters. Based on the variation law of the stress path of the surrounding rock, laboratory tests of rock mechanics are designed, and the deformation and strength behavior of marble with different initial confining pressure and unloading rates are systematically studied. By introducing strain increment, the characteristic stress, and the dilatancy index, the rock’s dilatancy and brittleness under different unloading conditions are quantitatively analyzed. During unloading, the energy transformation mechanism of rock is described, and the law of deformation and failure is discussed based on characteristic energy. The rock failure strength fitting formula is given by applying the Mogi–Coulomb criterion and elastic strain energy criterion. The advantages of the elastic strain energy criterion are theoretically explained. This study shows that comprehensive consideration of the complex stress paths, confining pressure levels, and the loading‐unloading rates of surrounding rock is an effective way to accurately study unloading rock characteristics. The results can provide theoretical basis for stability analysis of high‐stress underground engineering.