The Failure Characteristics of Granite under True Triaxial Unloading Condition

Abstract

The brittle failure (rock burst) of rock is generally triggered by excavation under high stress in hard rock mass with the characteristics of local violent failure due to a large amount of elastic energy release suddenly around the surrounding opening. For better understanding and monitoring the behavior of hard rock, we have done a series of experiments on samples of Laizhou granite to simulate brittle failure of rock under excavation. Deformation and acoustic emission (AE) were monitored during the tests, which were conducted in a static servo electro-hydraulic controlled true triaxial test machine at a loading rate of 0.5-1.0 MPa/s in three directions and an unloading rate of 30 MPa/s on one side of the samples. The failure characteristics include thin spalling and slabbing due to tensile stress, blocky fragments with irregular shapes dominated by to shear stress or their coupling effect. The authors observe the slabbing or irregular blocky fragments for granite is closely related to the stress path and the boundary conditions, the higher stress before unloading, the more blocky fragments generated after unloading. The existence of free boundary of the rock mass is of benefit to form slab structure near the opening under a certain stress state to permit the stress redistribution in a period of time. Violent increase of AE events is an indication of rock failure. The testing results are indicative of the failure nature of this granite rock which implies stress mitigation is one method to decrease the disasters caused by brittle failure of rock mass due to excavation in deep underground engineering.