## 4 Discussion
### Analysis of the Asymmetric Failure Mechanism  
But the minimum principal stress direction intersects with the tunnel axis at a large angle, which is not conducive to the stability of the surrounding rock of the cavern. Figure 13: Pressure value of surrounding rock on monitoring section with the parallel pilot tunnel on 23 June 2024 (unit: MPa). In this paper, three-dimensional discrete element software (3 DEC V7.0) is adopted for numerical simulation, and the model size is 150 \(\times\) 100 \(\times\) 40 m (length, height, thickness). In order to reasonably control the density of the grid elements and the fineness of the model, the dimensions of the grid elements for the tunnel, the inner surrounding rock, and the outer surrounding rock are 1, 2, and 4 m, respectively. The block model adopts the strain softening model, and the joint model adopts the Coulomb slip joint model. Normal displacement constraints are imposed on the model boundaries, and initial stresses are directly applied to the blocks and elements. Within the tunnel excavation range, the maximum principal stress value is approximately 24 MPa. The maximum principal stress, minimum principal stress, and maximum shear stress of the surrounding rock after simulated excavation are shown in Figure 15. It can be seen from Figure 15 that the compressive stress concentration occurs on the left arch shoulder and the right arch foot of the tunnel, and the stress value can reach 39 MPa. Tensile stress concentration occurred within approximately 0.5 m of the surrounding rock on the surface of the tunnel, with the stress value reaching 1.4 MPa. Shear stress concentration also occurred on the left arch shoulder and the right arch foot of the tunnel, with the stress value reaching 35 MPa. Figure 14: Schematic diagram of tunnel plane and ground stress orientation. Figure 15: (**a**) Numerical simulation model and spatial distribution characteristics of (**b**) maximum principal stress, (**c**) minimum principal stress, (**d**) maximum shear stress. According to the above analysis, after the initial concrete injection, the pressure caused by the deformation of the surrounding rock gradually increases with increase in time. As a whole, the surrounding rock pressure of the tunnel valley side is greater than that of the mountain side, but the surrounding rock pressure of the arch is greater than that of the side wall on both sides. The tunnel passes through an area characterized by steep terrain and deep valleys, where a \"V\"-shaped valley intersecting with the tunnel body at large angle is extremely developed. This geological configuration has resulted in greater stress concentration on the left side of the tunnel compared to the right side. Therefore, when it is not affected by other geological structures, the surrounding rock pressure on the left side of the tunnel is relatively greater. Under the action of this uneven and asymmetric pressure, the steel arch is easy to deform and distort. In addition, due to the development of two groups of main joints in the surrounding rock of the monitoring section, the joints and rock layers cut each other, and the interlayer is generally combined. The two groups of main joints develop into a network cutting system, which divides the intact rock mass into diamond or wedge blocks, significantly reducing the overall compressive strength and self-stability of the surrounding rock. There is an unfavorable intersection relationship between the joint plane and the occurrence of rock strata, and it easily forms a penetrating structural plane combination, which provides a structural foundation for the block sliding. The combination of structural planes developed from the right waist to the top of the arch forms an \"X\"-type cross-joint system, whose intersection lines tend to the inner direction of the cave, which constitutes the spatial geometry conditions of the potential sliding surface.