## 3 Investigation Results
### Failure Characteristics of the Left Line Tunnel
#### 3.4.1 Comparison of Spatial Failure Characteristics  
The comparison reveals that the failure locations of each tunnel exhibit spatial asymmetry, with the valley side experiencing significantly more severe damage than the mountain side, as shown in Figure 11. The causes and mechanisms of failure vary across different locations. Cracking in the tunnel arch and floor is primarily attributed to the convergence of unloading deformation and stress concentration under high ground stress conditions. The failure of the left wall and left arch shoulder is primarily caused by the stress associated with the deep valley on the left side and the stress concentration resulting from high ground stress. Conversely, the failure of the right arch shoulder is related to the presence of unfavorable structural planes extending from the right side to the arch top. The right wall is caused by excavation unloading and construction blasting disturbance after the excavation of the right line tunnel. The failure location of the right line is relatively concentrated in the left wall and left arch shoulder, which is mainly caused by the convergence of excavation unloading deformation and stress concentration under the condition of high ground stress. The failure location of the left line is relatively concentrated in the arch and the left arch shoulder, which is mainly caused by the convergence of excavation unloading deformation under high ground stress. It is worth noting that the left arch shoulder steel arch deformation is caused by excessive tangential stress concentration, while the surrounding rock deformation towards the free face makes it difficult to produce the above phenomenon. Therefore, it can be inferred that the failure of the surrounding rock in this part is caused by high stress concentration, rather than deformation, and the failure is a typical high ground stress failure.  
By comparing the failure sections, it is found that the deformation and failure of the parallel pilot tunnel are longer than that of both the right and left line tunnels. The statistical table of the paragraphs with serious deformation and failure is shown in Table 2. In the statistical sections of the parallel pilot tunnel, PK1193+130-PK1193+270, PK1193+600-PK1193+750 and PK1193+760-PK1193+900, the damage phenomenon occurs again after the spray layer is damaged, and the upper damage frequency is more than two times.  
Figure 10: Failure characteristics of surrounding rock in the left line tunnel, including: (**a**) Cracked concrete and exposed steel frame at the arch; (**b**) Concrete bulge cracking at the left arch shoulder.  
In terms of failure intensity, the cracks in the parallel pilot tunnel are generally 0.5-2.0 m long and up to 9 m long, and the opening width is 0.2-2.0 cm and up to 8 cm. In the severely damaged section, the area of the damaged area accounts for about 50% of the total area. Most of the cracks in the large-mileage tunnel on the right line are 0.5-1.5 m in length and up to 2.5 m in length, and less than 0.5 cm in width. In the severely damaged sections, the area of the damaged area accounts for about 20% of the total area. Most of the cracks are less than 0.5 m in length and less than 0.5 cm in width in the left line large-mileage tunnel, and the area of the damaged area in the severely damaged section accounts for about 10% of the total area.