## 5 Conclusions  
1. Taking the bifurcated section of the Liantang Tunnel, the largest cross-section highway tunnel currently, as the research object, two excavation schemes for transitioning from a small cross-section to a large cross-section tunnel were proposed. Numerical simulation was used to analyze the stress and deformation characteristics of the surrounding rock during the excavation process in the bifurcation section, and on-site verification was conducted. The calculation results provide a theoretical basis for the design of the transitional excavation scheme, optimization of excavation steps, and support recommendations. They are of significant reference value for the future formulation of design specifications for super-large cross-sections with more than four lanes.
2. A three-dimensional computational model for the excavation of a large cross-section bifurcation tunnel was established using the MIDAS GTS NX software, and the calculation results of the two schemes were analyzed. During the construction process, the total displacement of the crown in the large section and the small-clearance section in the RTM was 1.2 to 1.4 times that of the ACM. After construction stabilization, the vertical stress at the crown in the ACM was approximately three times that of the RTM. During the excavation of the large section, the maximum shear stress of the intermediate rock in the RTM exceeded the plastic failure threshold of 5.92 MPa for rock, while the ACM remained within the safe range. The calculation results indicate that the ACM is the optimal excavation scheme.
3. The on-site monitoring results of the Liantang tunnel show that after adopting the ACM, the large section was excavated using the double sidewall method. The maximum cumulative displacement in the large-section tunnel was 23.7 mm, and for the two lanes, it was 8.5 mm. The peak pressures of the initial support and secondary lining were concentrated in the waist areas on both sides, reaching 0.4 MPa for the large section and 0.58 MPa for the two lanes, located near the shoulder and waist areas close to the intermediate rock. The crown sinking and convergence displacements of the surrounding rock in the Liantang bifurcated tunnel were within the allowable displacement safety range for highway tunnels, confirming the rationality of the ACM.  
Conceptualization, X.W.; methodology, X.W. and Y.L.; software, Y.W.; validation, H.W.; formal analysis, M.G.; investigation, H.W.; resources, M.G.; data curation, Y.W.; writing--original draft preparation, X.W.; writing--review and editing, Y.L. and H.W.; visualization, X.W. and Y.L.; supervision, M.G.; project administration, M.G.; funding acquisition, M.G. and X.W. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China, grant number 51934001 and the China Postdoctoral Science Foundation, grant number 2022 TQ0025.  
Not applicable.  
The data used to support the findings of this study are available from the corresponding author upon request. The data are not publicly available due to personal information and privacy about the data.  
We would like to acknowledge the editors and reviewers for their invaluable comments.  
The authors declare that they have no known competing financial interest or personal relationships that could have appeared to influence the work reported in this paper.