Effect of unloading rate of normal stress on the frictional slip behaviour of fractured rocks

Abstract

Unloading disturbances encountered during excavations in fractured rock masses significantly affect the safety of deep underground structures. However, the influence of the unloading rate on the frictional slip behaviour of fractured rocks is not fully understood. In this study, a series of fracture slip tests induced by unloading normal stress were carried out, where the effects of unloading rate, initial normal stress, and initial shear stress were considered. Acoustic emission (AE) technique was employed to monitor damage in the fractured rock specimens. The results reveal that the unloading rate played a significant role in the frictional slip behaviour and failure mode of the fractured rock specimens. At lower unloading rates, the stress path approached the failure envelope horizontally, whereas at higher unloading rates, the stress path followed a diagonal trajectory. The near-critical stress state and near-critical stress ratio were introduced to describe the normal and shear stresses during the fracture activation slip. The results show that lower unloading rates (≤0.010 MPa/s) consistently placed the near-critical stress state closer to the failure envelope compared with higher unloading rates. In addition, the near-critical stress ratio decreased with an increase in the unloading rate. The AE count exhibited a distinct peak before failure of the fractured rock. This initial peak in the AE signals corresponds to the violent rupture of the surface asperities, making it a valuable indicator for monitoring and predicting potential failures in fractured rock masses. An empirical model was developed to describe the relationship between the unloading rate and near-critical stress ratio.