Experimental investigation of the failure mechanism of deep granite under high seepage water pressure and strong unloading effect

For deep hard-brittle rock mass engineering in a complicated hydrogeological environment, deformation and damage of the surrounding rock during excavation are often driven by the combined action of the dynamic seepage water and the strong unloading effect. To investigate the failure mechanism of a deep-buried rock mass under high seepage water pressure and high unloading rate, a series of conventional triaxial and unloading triaxial compression tests at different seepage water pressures and unloading rates were conducted on Erlang Mountain granite. The results confirmed that the dynamic seepage water pressure has a great influence on the mechanical properties and failure mechanism of rock because its splitting and expanding effects could stimulate crack generation and accelerate fractures. As the water pressure increased, the granite entered the nonlinear growth phase of the permeability coefficient earlier and entered the energy softening stage later. While the compressive strength and energy storage capacity of the rock were weakened gradually by dynamic seepage water, the permeability, energy hardening properties, and brittle failure mechanism could be magnified. Moreover, the effect of the strong unloading on the mechanical properties and damage mechanism of deep-buried granite was similar to that of the dynamic water pressure. Although the peak stress, peak strain, and energy storage capacity were reduced under the strong unloading effect, the permeability, brittle fracture characteristics, energy hardening properties and energy release rate could be significantly enhanced. Under the combined action of high seepage pressure and high unloading rate, the rock gradually decreased in hardness and increased in brittleness, and these changes were conducive to the instantaneous release of elastic strain energy in the failure stage.