## 3 Investigation Results
### Failure Characteristics of the Parallel Pilot Tunnel  
The surrounding rock damage during the excavation of the parallel pilot tunnel is shown in Figure 8. The main failure phenomena include floor uplift, cracking and local dislocation, bulging and cracking of the concrete spray layer, steel frame buckling and deformation, and slab slope of the concrete spray layer.  
As can be seen from Figure 8, there are multiple failures in the parallel pilot tunnel, the damage degree is relatively serious, and the spatial location of the damage is relatively dispersed. Within a few days after excavation, no obvious cracks were observed in the concrete spray layer and no obvious deformation was observed in the steel arch. With increase in time, the face of the palm continues to advance, and the concrete spray layer 60-80 m away from the rear of the face exhibits many cracks and peels. Further surrounding rock cracks extend, and the concrete spray layer is stretched to form deep cracks (Figure 8a). In more serious cases, steel frame buckling occurs, resulting in floor dislocation and through cracks (Figure 8b). After the initial concrete shotcrete support, the slab wall occurs in the parallel section after the excavation of the backward tunnel (Figure 8c); the thickness of the slab wall of the concrete shotcrete layer is generally 20-30 cm, with the maximum thickness reaching 50 cm. After the initial support is destroyed, concrete supplementary spraying is adopted in time, and cracks and spalling of concrete still occur several days after supplementary spraying. Although the phenomenon is manifested as the deformation and failure of the concrete spray layer, it actually reflects the damage to the internal surrounding rock. The internal surrounding rock exhibits tensile or dilatancy failure, resulting in the cracking or falling off of the concrete surface.  
In terms of space, the main failure characteristics of the arch are the spalling of the initial concrete spray layer and the exposure of the steel frame, while the main failure characteristics of the bottom plate are cracking of the arch, with a crack length of 3-5 m. The damage characteristics of the side walls on both sides are more complex, and the damage degree of the side walls on both sides is more serious than that of the arch. The damage degree of the left side wall is more serious than that of the right side, and the  
Figure 7: Interface of surrounding rock pressure data results.  
main damage characteristics of the side walls on both sides are the sheet wall caused by tensile stress. Swelling of the concrete spray layer is common in the left arch shoulder and arch, and the swelling and cracking of the concrete spray layer has obvious segmenting characteristics (Figure 8a,c). The cracks are generally 0.5-2.0 m long and up to 9 m long, and the opening width is generally 0.2-2.0 cm and up to 8 cm. The distribution patterns of cracks are different--mainly developing horizontally and extending in a zigzag pattern of intermittent bending (Figure 8a). The shedding phenomenon of the concrete spray layer is widely distributed, the shedding area varies, and the shape is irregular, including long strip, sawtooth, groove, trapezoidal plane, etc. The bending of the steel frame can be seen at the part of the concrete spray layer falling off (Figure 8d), which reflects the large tangential stress of the surrounding rock at this part.