# Excavation Method Comparison and Optimization for a Super Large Cross-Section Tunnel
## 5 Conclusions  
Three excavation methods--namely, the double-sided drift method, the CRD method, and the three-benching seven-step method--were selected for numerical simulation studies, with the results subsequently compared against actual monitoring data. The following conclusions can be drawn:  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Working** & **Sequence** & **Sequence** & **Sequence** & **Sequence** & **Sequence** & **Sequence** \\
**Condition** & **I** & **II** & **III** & **IV** & **V** & **VI** \\ \hline Top of arch & & & & & & \\ settlement (mm) & 14.74 & 14.75 & 14.89 & 14.91 & 14.92 & 14.72 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Vertical displacement under different sequences.  
Figure 16: Surface settlement map under different sequences.  
(1) Comparison of Numerical Simulation Results: The CRD method, the three-benching seven-step method, and the double-sided drift method were analyzed for displacement at various key points. The three-benching seven-step method yielded the highest vertical displacement at the vault, followed by the CRD method, while the double-sided drift method exhibited the lowest vault settlement displacement. At the arch base, the double-sided drift method showed the smallest increase in displacement, whereas the three-benching seven-step method recorded the largest.  
(2) Implications of the Double-Sided Drift Method: The use of the double-sided drift method in the excavation sequence facilitates a greater release of support stress in the surrounding rock compared to the other methods, which can lead to stress concentrations. Therefore, it is advisable to complete the initial support promptly and quickly form the large section of the hole into a ring to minimize the risk of collapse.  
(3) Optimization and Construction Sequence Comparison: After further optimization of the double-sided drift method, six different construction sequences were compared. The findings indicate that the construction sequence of the center guideway significantly impacts the displacement of the surrounding rock, whereas altering the sequence of the left and right guideways has a minimal effect on the overall displacement of the surrounding rock. Analysis of the stress and displacement fields in the perimeter rock reveals that Sequence VI demonstrates the most stable performance overall. Additionally, this sequence involves fewer steps, thereby enhancing construction efficiency. Based on these observations, it is recommended that Sequence VI be implemented for the excavation construction using the double-sided drift method in similar large cross-section road tunnel projects.  
Conceptualization, Y.H.; software, Y.H.; validation, T.F.; investigation, T.F.; data curation, N.W.; writing--original draft preparation, Y.H.; writing--review and editing, T.F.; visualization, T.F.; supervision, N.W.; project administration, N.W.; funding acquisition, Y.H. and N.W. All authors have read and agreed to the published version of the manuscript.  
This research was supported by the National Natural Science Foundation of China, grant numbers 52168047, 52368071, 52068027, 52062016, and 41972291.  
Not applicable.  
The authors confirm that the data supporting the findings of this study are available within the article.  
The authors declare that they have no conflicts of interest.