Excavation-induced fault instability: Possible causes and implications for seismicity

•Excavation-induced fault rupture is related to magnitude and orientation changes of principal stresses.•Principal stress change results in damage transition from matrix tensile failure to fracture shear failure.•Fewer small magnitude AE in rock matrix and more large magnitude AE along fractures cause smaller b value. Fault rupture and associated seismicity have been reported frequently in underground excavation. The mechanism that drives the human-induced geohazards still eludes explanation, and uncovering this mechanism relies heavily on our understanding of the failure characteristics of fractured rock. We carried out the triaxial compression testing and discrete element modelling on intact and fractured granite samples to consider the controllable change of the angle β between the maximum principal stress orientation and pre-existing fracture orientation and to interpret the complicated mechanism of fault rupture adjacent to rock excavation. Our results show that the reduction of the angle β leads to the failure pattern of fractured samples changing from the tensile failure of rock matrix to the shear failure along pre-existing fractures. This transition results in relatively small b value, as small magnitude AE events decrease in the rock matrix and large magnitude AE events concentrate along pre-existing fractures. The change of b value in the pre-peak stage is independent to the angle β, implying the challenge in rock failure prediction. Our study also indicates that fault rupture adjacent to rock excavation is due to the changes of magnitude and orientation of principal stresses, and the associated seismicity is likely dependent on the portions of rock matrix and fractures involved in the failure process.