Transfer mechanism and criteria for static–dynamic failure of granite under true triaxial unloading test

In the construction procedure of infrastructure, the excavation disturbance significantly changes the initial stress state of rocks in deep geotechnical engineering, and a rock burst would occur accompanied by the ejection of rock block. In the present study, a true triaxial unloading test and its corresponding numerical analysis based on the three-dimensional particle flow code (PFC3D) are conducted on granite specimens to acquire characteristic information of static and dynamic failure during unloading and to explore the criteria for determining static and dynamic failure. The results indicate that the failure modes of granite under true triaxial unloading can be classified into three categories: ① no obvious coalesced crack, ② a main coalescence shear crack, and ③ a main coalescence shear crack with a V-shaped pit. The instantaneous kinetic energy in static failure is characterized by a sharp increase and sporadic sharp increases, while that in dynamic failure shows a sharp increase, a short quiet period, and a substantial increase. The critical stress combination for the transformation of static–dynamic failure of granite is proposed, which is consistent with the experimental results. The number of shear cracks gradually increases in static failure, while it suddenly increases in dynamic failure. During the transformation from static failure to dynamic failure, the ratio of a tensile crack number to that of shear crack number decreases. The proportion of shear cracks in particle bonding increases, while the trend of tensile failure weakens. However, tensile failure remains predominant in granite, accompanied by shear failure. Article highlights The failure modes of granite under true triaxial unloading are classified into three categories: ① Few particles are ejected; ② Some particles are ejected at the upper end near the unloading surface; ③ A great number of particles are ejected near the unloading surface and a V-shaped pit is formed. The instantaneous kinetic energy upon static failure is characterized by a sharp increase and sporadic sharp increases, while that upon dynamic failure is characterized by a sharp increase, a short quiet period, and a substantial increase. A judgement criterion is proposed for distinguishing static or dynamic failure of granite specimen under true triaxial unloading test.