Experimental investigation on rockburst process and characteristics of a circular opening in layered rock under three-dimensional stress conditions

To investigate the influence of the bedding angle on the failure process and characteristics of circular hole sidewalls, true-triaxial lab tests were conducted on cubic phyllite specimens with different bedding angles. When the tunnel axis direction was parallel to the bedding plane, the initial failure vertical stress of the hole sidewall first decreased and then increased with an increase in the bedding angle β. A V-shaped notch was formed in the surrounding rock, and the characteristics of the V-shaped notch boundaries were such that one side was approximately parallel to the bedding plane, and the other side crossed the bedding plane. Three failure modes of surrounding rock under different bedding planes were observed: large-scale slip failure along the bedding plane (β = 0°, 15°, and 30°), large-scale shear failure across the bedding plane (β = 45° and 60°), and local rock fragment exfoliation failure (β = 75° and 90°). The large-scale slip (or shear) failure of the surrounding rock generated high acoustic emission (AE) counts, whereas the local failure of the surrounding rock generated relatively low AE counts. By comparison, the initial failure vertical stress was much lower than that when the bedding plane was perpendicular to the direction of the hole axis. Therefore, the tunnel layout method perpendicular to the bedding plane is beneficial for improving the stability of the surrounding rock in underground engineering and reduces the risk of rockburst occurrence.