Asperity degradation characteristics of soft rock-like fractures under shearing based on acoustic emission monitoring

Abstract

Shear failure of rock masses along pre-existing discontinuities is one of the predominant failure modes of rock slopes and underground tunnels. The monitoring and prediction of the impending shear failure is of great significance to ensure the stability of the rock structures and the safety of the workers. In this study, direct shear tests under normal stress ranging from 0.5 to 10 MPa are conducted on rock fractures (analogous to soft rock discontinuities), which are obtained by artificial splitting, during which the AE parameters are monitored. Test results show that the AE parameters (hit, energy and events) increase with the shear stress, and peak at or near the peak shear strength, after which the AEs gradually decay with the decrease of shear stress. The number of AE events first increases and then decreases with increasing normal stress, which may be associated with the ductile deformation of the porous structure of cement mortar under higher normal stress. The degradation of asperities on the fracture surface can be inferred from the accumulative AE hits and events, which are characterized by “S” shapes and can be divided into slow growth, fast growth and slow growth stages. Conceptual and mathematical asperity damage models are proposed respectively based on the temporal characteristics of AE events and the curve fitting, which can be used to predict the asperity damage for a given shear stress curve. The AE b-value fluctuates and remains high in the shear process. There is no strong correlation between the shear stress and the b-value, which indicates that the b-value may not be an effective index to predict the quasi-static shear failure of jointed rock masses possessing similar properties like cement mortar. Results of this study will not only contribute to a better understanding of the asperity degradation characteristics but also provide valuable knowledge for AE monitoring applications in jointed rock masses.