The weakening effect on shear behavior of granite with discontinuous joints under normal cyclic disturbances: insights from acoustic emission analysis

To investigate the shear failure mechanism and precursory characteristics of jointed rocks under cyclic disturbances, conventional and disturbed shear tests were conducted on jointed granite under different stress conditions. The effects of the disturbance and normal stress on evolution of characteristic parameters such as AE energy rate, energy release rate, rise time/amplitude–average frequency (RA-AF), and b-value during shear fracture process are discussed. The research results show that as the normal stress increases, the proportion of AE events with low AF and high RA increases, indicating an increase in the shear contribution. Compared with conventional shear tests, frequent disturbances promote the formation of secondary cracks near the main crack, thus forming a surface-shedding zone. In addition, the disturbance will induce the jointed rocks to exhibit the shear fracture weakening effect, which reduces its resistance to deformation and load. Before shear fracture failure, the jointed granite shows precursors including an obvious AE quiet period, accelerated energy release, and constant reduction of b-value. The precursors of shear fracture failure under normal disturbances exhibit the following significant differences from conventional shear tests: the AE quiet period has a shorter duration and is relatively unquiet; the accelerated release of AE energy is more significant; and the descent stage of the AE b-value starts earlier. The research results are of important significance for identifying fracture failure precursors of deep jointed rocks.