Abstract:

The true triaxial loading and unloading tests on granite were conducted to obtain the mechanical characteristics of the granite in the construction area of an underground cavern. The stability of the surrounding rock, the characteristic stress, the failure mode and the brittleness characteristics were mainly analyzed in this work. In addition, the evolution laws of total energy and dissipated energy with axial strain were explored. 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. It is found that the brittleness of granite is higher under unloading conditions. 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 exceeds the elastic strain energy, which becomes the main energy consumption. The energy dissipation value of the granite sample 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 produce damage, which is safer than the unloading path. The particle flow PFC^3D is used to simulate the true triaxial loading and unloading test of granite in this study. The failure mode and the crack distribution of the numerical simulation results are basically consistent with results of the laboratory test. Then the numerical model is used to simulate the rapid unloading failure process of granite. It is found that the rock sample particle ejection damage will occur at the end of the model, similar to a rock burst.

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