## 4 Discussion  
### Evolution Law of Formation, Expansion, and Penetration of Slab Buckling Circle  
The cutting effect of tension cracks will lead to the formation of a slabbing structure. The occurrence of slabbing failure causes the rock within its influence range to be broken, and the displacement value of rock mass is usually large, with an irregular displacement direction. From the free face to the interior of the surrounding rock, the displacement value of the surrounding rock decreases with a gradual increase in the thickness of the rock plate formed by the cutting of the plate crack surface; there are obvious gradient changes on both sides of the outermost measuring point of the plate crack structure. The area where the displacement curve of the rock mass in the radial range of the surrounding rock shows obvious bending or an abrupt boundary is defined as the \"slab buckling circle\". Therefore, the slab buckling rockburst is finally formed by the buckling deformation of the rock mass during the rockburst disaster in the slab buckling circle and the occurrence of bulging or even burst ejection under certain induced conditions. Therefore, through the identification and determination of the slab buckling circle, the exploration of the evolution law of the slab buckling circle is of great value for obtaining precursor information about the rockburst disaster. In this paper, with the help of an \"image drilling\" method proposed by [PERSON] et al. [40], taking the center of the tunnel as the center of the circle, 12 image \"boreholes\" are arranged radially at 15\({}^{\circ}\) intervals along the periphery of the tunnel. The displacement of the pixel measuring point on the \"borehole\" in each experimental stage is calculated by PhotoInfor software, and the approximate range of the thickness of the slab buckling circle can be determined by the whole curve of displacement-hole depth. The bending or mutation point of the curve can be accurately located by local amplification; that is, the boundary point of surrounding rock in slab buckling circle, which can be smoothly connected to obtain the range of the slab buckling circle. The identification process of the surrounding rock in the slab buckling circle is shown in Figure 12, and the setting interface of the identification parameter and the arrangement of the borehole survey line of the surrounding rock in the slab buckling circle are shown in Figure 13.  
Figure 11: Comparison of the shape of rockburst pits between rockbursts on site and simulation experiment. (**a**) The morphology of rockburst pit in ZK49 + 223.2; (**b**) The morphology of rockburst pit in slab buckling rockburst experiment. In general, there is an inconsistency between the generation of the slab buckling circle and the formation of slab cracks. The slab buckling circle is closely related to the propagation and penetration of slab cracks. However, the use of high-precision digital photographic measurement technology reveals that the formation of the slab buckling circle starts from the displacement gradient change that we cannot see with the naked eye. It can be seen from the following figure that the formation of the slab buckling circle has gone through three stages. In the first stage, the initial formation process of the slab buckling circle (Figure 14a,b) shows that the right side starts from the arch roof to the spandrel, while the left starts from the arch footing to the haunch, which represent the stress concentration area as well as the weak point of the horseshoe tunnel and begin to spread to the left and right sides of the tunnel from the above positions. In the second stage, in the process of the propagation and penetration of the slab buckling circle (Figure 14c,d), the thickness and range of the slab buckling circle are increasing, while the variation of the arch roof and the arch bottom is smaller than that of the left and right sides.