Energy characteristics and micro-cracking behaviors of deep slate rock under triaxial loadings.

Sichuan–Tibet railway tunnels are generally long and deep, and some of them pass through layered slate, which is prone to failure under high in situ stress. Insight into the mechanical behaviors of deep slate under different stress environments is critical to the safety of engineering practices in this area. To this end, we studied the energy characteristics and micro-cracking behaviors of slate samples under different triaxial loadings. The results showed that the confining pressure increased the strength, elastic modulus, Poisson's ratio and crack characteristic stresses of the studied slate. As the confining pressure increased, the storage of elastic strain energy during the prepeak stress stage increased, and its release during the postpeak stress stage was inhibited. Additionally, we obtained the slate failure precursors by analyzing the acoustic emission (AE) evolution characteristics, i.e., "a sudden increase in AE metrics, followed by a quiet period, and then a sudden increase again" in terms of the AE count rate and AE energy rate after obvious expansion of the sample. Then, we further observed that with an increase in confining pressure, a gradual transition of the failure pattern from compressive shear to tensile shear occurred. Finally, we discussed the influence mechanisms of confining pressure on the slate failure behaviors and concluded that the change in the effective shear stress along the foliation planes played an important role in the failure mode transition. [ABSTRACT FROM AUTHOR]