Effect of Unloading Stress Levels on Macro- and Microfracture Mechanisms in Brittle Rocks

Abstract

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.