Mechanical behaviour and damage evolution of Beishan granite considering the transient and time-dependent effects of excavation unloading

A series of triaxial unloading tests on integrated rock, triaxial compression tests and creep tests on unloading-cracked rock were carried out on dry and saturated specimens. The spatial and temporal distribution of microcracking process were obtained with the help of 3D acoustic emission (AE) monitoring system. The experiment data revealed that the saturation had a degeneracy effect on the transient failure strength of integrated granite, and the failure strain at high confinement. The creep characteristics of unloading-cracked granite in lateral direction were found to be greater than in axial direction. At high confinement, the critical strain of creep failure, the creep deformation and the lateral creep strain rate were shown to be degraded by the saturation. When the applied stress surpassed the level of 90% of the compressive strength of unloading-cracked specimen, the variation of AE event showed a consistent tendency with the evolution of lateral strain, and the AE events were found to be mainly accumulated along the macroscopic failure surface. The AE signal was noticed to be more sensitive than the macroscopic deformation to the occurrence of creep failure. Before the appearance of large increase in lateral strain, the rapidly increase of AE activity had occurred.