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.