Abstract: To explore the failure mechanism of deep-buried hard rock, a series of experiments was performed, including conventional triaxial tests and the compression tests by decreasing the confining pressure under constant or variable axial stress. The crack volumetric strain is selected as the key analysis variable, and using volumetric strain, crack volume strain and equivalent plastic strain, the rock failure process is analyzed under different stress paths. The phenomena and characteristics of rock failure process are clarified by using the volume strain-axial strain curve of the cracks, the equivalent plastic strain-axial strain curve and the axial stress-strain curve. It is shown that the crack volumetric strain can be used to reflect the extent of rock initial damage, the crack closure has an important influence on the calculation of elastic modulus of rock, and to obtain accurate modulus, the crack closure needs to be divided into appropriate stages to eliminate the influence of the crack closure stage on the elastic modulus calculation. In addition, at the moment that the confining pressure decreases, the cracks of rock sample are closed suddenly, leading to a mutation of the volumetric strain-axial strain curves of the cracks. There exists hysteresis in the crack growth during the unloading failure of rock. The above results can help understand the process of rock failure, while providing a theoretical basis for preventing the failure of hard rock under high stress conditions.

Key words: crack volume strain, deep-buried hard rock, marble, failure mechanism, loading and unloading