Energy evolution characteristics of hard rock during triaxial failure with different loading and unloading paths

Triaxial compression tests were conducted on fine-to-medium–grained granite specimens, with initial confining pressures of 10, 20, 40, and 60 MPa, under various loading and unloading stress paths. The energy evolution characteristics of granite specimens from a quarry in Miluo city (China) were studied in the triaxial deformation and failure process of the rocks. The results show that the time history curves of the total strain energy, elastic strain energy, and dissipative strain energy exhibit significant stage features. In particular, the ratio of the dissipative strain energy to the total strain energy can be used to describe the deformation and degree of damage to rock specimens during the triaxial loading and unloading processes. Under the same initial confining pressure, the maximum values of the total strain energy, elastic strain energy, and dissipative strain energy occur in the conventional triaxial compressive testing of group I, and the minimum values occur in test group II with constant axial stress and decreasing confining pressure. The total strain energy, elastic strain energy, and circumferential strain energy all increase as the initial confining pressure increases, whereas the dissipative strain energy does not. During the process of unloading the confining pressure, the increase of the circumferential strain is considerably larger than that of the axial strain. Under unloading conditions, rock bursts may occur more easily for hard rocks than under conventional triaxial loading conditions, especially under the conditions of test group III with increasing axial stress and decreasing confining pressure. The micro-difference in the granite micro-cracks was identified using a scanning electron microscope (SEM) combined with an energy dispersive spectrometer (EDS). Shear failure characteristics were observed in a conventional triaxial test, and the combined tension and shear failure was identified through unloading confining pressure tests. The tensile failure characteristics of the granite in group III are more pronounced than those of group II. This indicates that the triaxial failure of rock results from the development of micro-extension cracks and volumetric expansion in the granite specimen under unloading confining pressure tests.