Experimental Investigation of the Formation of Rockburst Pits in Circular Tunnel Based on CT Scanning

The rockburst simulation experiments of granite samples with a circular hole under biaxial loading were conducted, and the samples were scanned by computed tomography (CT) at the end of experiment. Through a series of CT images, the failure characteristics of the circular hole wall were analysed to determine the types of rockburst pits, and the detailed formation process of different types of rockburst pits was studied in combination with the crack characteristics around them. The experimental results indicate that there are mainly two types of rockburst pits, namely, pan-shaped and V-shaped, which can occur symmetrically or asymmetrically on the left and right sidewall of the circular hole. The formation of rockburst pits is related to the cracks parallel and perpendicular to the principal stress. Cracks parallel to the principal stress can determine the depth of the rockburst pit and affect the type of the rockburst pit, and cracks perpendicular to the principal stress determine the width of the rockburst pit. There is a correlation between the formation process of pan and V-shaped rockburst pits. During the formation of a V-shaped rockburst pit, several rockbursts occur, and each rockburst forms a pan-shaped rockburst pit. In the process of developing from the tunnel wall to the deep rock, the width of the pan-shaped rockburst pit gradually decreases and a V-shaped rockburst with a stepped upper and lower boundary is formed.