# Excavation Method Comparison and Optimization for a Super Large Cross-Section Tunnel
## 3 Comparison of Construction Methods
### Surrounding Rock Displacement Analysis  
The key monitoring points and lines within the tunnel are organized as depicted in Figure 9. Four crucial lines, designated as X1, X2, X3, and X4, have been established at each of the tunnel vaults for intensive monitoring. These lines are positioned 10 m apart from each other. The displacement patterns under the three excavation methods are illustrated in Figure 10. Following tunnel excavation, the displacement outcomes are as follows: For the three-benching seven-step method, the settlement at the arch top is 17.7 mm and the bulge at the arch bottom is 12.8 mm. The CRD method records a settlement of 15.3 mm at the arch top and a bulge of 12.1 mm at the arch bottom. Meanwhile, the double-sided drift method shows a settlement of 14.6 mm at the arch top and a bulge of 11.5 mm at the arch bottom. Comparatively, the double-sided drift method achieves a 26% reduction in displacement at the base compared to the CRD method, which itself shows a 23% reduction in base displacement relative to the three-benching seven-step method. Among the evaluated methods, the double-sided drift method results in the least displacement, indicating its effectiveness in minimizing structural deformation. Figure 8: Grid diagram of the three working methods. (**a**) Double-sided drift method; (**b**) CRD method; (**c**) three-benching seven-step method. Figure 9: Layout of monitoring points and monitoring lines. (**a**) Distribution of monitoring points; (**b**) horizontal monitoring line position. Figure 10: Surrounding rock displacement cloud map under different working methods. (**a**) Double-sided drift method; (**b**) CRD method; (**c**) three-benching seven-step method. During the excavation and support of the tunnel, the relationship between the vertical displacement at each monitoring point and the corresponding construction step is depicted in Figure 11. Given that the maximum vertical displacements typically occur at the top and bottom of the arch during tunnel section support, these locations are chosen as the primary monitoring targets for the study. Comparative analysis of the displacement data reveals that the three-benching seven-step method results in the largest vertical displacements. This is attributed to the method's initial full excavation of the upper step, which disturbs the surrounding rock significantly and leads to more pronounced deformation. In contrast, the double-sided drift method starts by excavating the left soil, facilitating a more rapid release compared to the right soil, which in turn minimizes the vertical deformation of the surrounding rock and results in a lesser overall vertical displacement. Additionally, as the number of construction steps increases, there is a consistent decrease in the displacement at the top of the arch at the end of the support phase across all three methods. Figure 12 presents the surface settlement values at the monitoring points X1, X2, X3, and X4, illustrating the impact of the three excavation methods on the ground surface near the tunnel vault. An increasing trend in settlement values is observed as proximity to the tunnel vault increases, while settlements decrease with greater burial depths at the tunnel's edge. Furthermore, the burial depth locations are symmetrically arranged along the tunnel's central distance line, with the maximum settlement point at each depth located on the tunnel's centerline, consistently reaching peak values at X1. For the three-benching seven-step method, the maximum surface settlement recorded at X1 is 11.73 mm, with the subsequent values at X2, X3, and X4 decreasing with depth to 8.55 mm, 6.81 mm, and 6.31 mm, respectively. The CRD method exhibits a maximum settlement of 11.45 mm at X1, with the surface settlements at X2, X3, and X4 showing a similar decreasing trend with increasing burial depth, registering at 8.97 mm, 8.06 mm, and 7.64 mm, respectively.