Abstract: The true triaxial loading and unloading tests were conducted to obtain the characteristic stress, failure mode and energy evolution characteristics of the granite in an underground cavern. Results show that under the true triaxial loading and unloading stress paths, the failure modes of granite are both tensile and shear composite failure, and characteristics of high damage stress and brittleness are obviously observed. A new brittleness index is proposed to evaluate rock brittleness by using volumetric strain curve. The brittleness of granite under unloading condition is higher than loading condition. In the true triaxial loading test, the change trend of total energy with axial strain goes through three stages: slow increase, rapid increase, and steady increase. In the true triaxial unloading test, the dissipated energy increases rapidly at the moment of unloading, and its proportion in the energy distribution increases, which becomes the main energy consumption. The energy dissipation value of the granite in the loading test is obviously greater than that in the unloading test. It indicates that more energy is required for samples under the loading path to cause damage. More elastic strain energy can be released under the unloading path, which is more dangerous than the loading path.

Key words: true triaxial test, mechanical properties, energy evolution