## 4 Discussion
### Rockburst Mechanism  
The existing views on the formation mechanism of slab buckling rockburst are shown in Table 3. The expressions of the scholars in the table all mention that the formation of slab buckling rockburst has experienced the stage of slabbing failure, and finally, it is manifested as the ejection of rock mass, but the expressions of the second stage are different. Some focus on the role of \"shear\", but some focus on the result of failure. In this paper, it is believed that the importance of slabbing structure in the process of rockburst failure should be fully emphasized in the study of the formation mechanism of slab buckling rockburst, and the law of the formation, propagation, and penetration of slab cracks should be started. Therefore, on the basis of the above research and drawing on and fully combining the views of domestic scholars, it is concluded that the inoculation mechanism of slab buckling rockburst in deep-buried horseshoe-shaped tunnels is divided into two stages, as shown in Figure 15. The first stage is the formation stage of the slabbing structure, with the occurrence process shown as the initial equilibrium state, initiation, and propagation of shear cracks on the inner wall, initiation and propagation of tension cracks on the outer surface, propagation, interlacement, and penetration of two types of cracks, and initiation, propagation, and compaction into plates, as well as bending and breaking into blocks of shear cracks on the outer surface. The second stage is the rockburst stage, and the occurrence process is from particle ejection to full burst. The specific process is as follows:  
1. Initial equilibrium state Figure 15a. Before the tunnel excavation, the rock mass unit on the excavation boundary is in the true triaxial state, which satisfies the in situ stress condition of the engineering site. 2. The initiation and propagation of shear cracks on the inner surface (Figure 15b). The stress redistribution effect caused by excavation will significantly change the stress state of the rock mass unit within a certain range from the excavation boundary. The stress of the original rock will be transferred, and the tangential stress will be concentrated. Due to the free face of the rock mass, the stress concentration and transfer effect will cause the horseshoe-shaped tunnel to start from the corner, and the shear cracks will initiate and propagate along the inner wall of the tunnel. 3. The initiation and propagation of tension cracks on the outer surface (Figure 15c). With the increase in surrounding rock stress and affected by the horseshoe-shaped structure effect, the outer surface is characterized by the initiation and propagation of tension cracks. Affected by the heterogeneity of the rock mass, the timing of the occurrence of the tension cracks on the left and right sides of the surrounding rock is not the same, but the propagation of the direction and angle is basically the same. 4. The propagation, interlacement, and penetration of two types of cracks (Figure 15d). When the tangential stress increases, the existing cracks extend and expand. In the process of expansion, they intersect and penetrate each other, and the micro-crack expands and penetrates along the approximate parallel free face. 5. The initiation, propagation, and compaction into plates, and then bending and breaking into blocks of shear cracks on the outer surface. At this stage, cracks deflect and compact into plates and evolve to the internal rock plates. The surrounding rock near the free face of the tunnel is cut into a fractured rock mass containing wedges of different blocks, that is, the generation of the slabbing structure, the bending of the rock plate, and then breaking into blocks (Figure 15e). 6. The ejection of the article and the spalling of rock sheet. When the combination of tensile and shear rock plates reaches the energy storage limit, it begins to enter the second stage of rockburst, and the energy is suddenly released, accompanied by particle ejection and rock flake spalling (Figure 15f).