## 2 Project Overview, Problems, and Solutions
### Construction Design for the Bifurcation Tunnel
#### 2.3.2 The Characteristic of ACM  
The excavation of the two-lane tunnel stops at a certain position before reaching the large section. The IPT is excavated in advance from the two-lane stepped section to the end face of the large section, aligning with one side wall section, as shown in Figure 6. The climbing of the advance ramp causes the elevation of the advance tunnel's climbing section to be higher than the design elevation of the two-lane section. Additionally, the profile of the stepped section differs from the single-side wall profile of the large section.  
Figure 5: Construction steps of the RTM.  
Therefore, backfilling and support operations are required in the climbing section after excavation is completed. For sections with different profiles, the climbing section is divided into two parts: the regular climbing section and a variable-section construction approach in the vicinity of the large section. The variable-section construction is used to connect with one side wall working face of the large section, completing the transitional section for the bifurcation tunnel's variable section. After the single-side wall of the large section is formed, horizontal excavation is carried out to create the other side wall and the central drift, followed by excavation and support operations.  
Compared to the first method, this method has the following characteristics:  
1. The starting point of inclined pilot tunnel is flexible. The slope is started in a reasonable location on the stepped section. Through analyzing the impact of the starting point on the stress of the surrounding rock and the backfill volume, the climbing angle is quantitatively calculated to ensure the stability of the surrounding rock. This method has broader applicability in similar projects.
2. The original excavation zoning scheme of a super-large cross-section tunnel will not change. Multi-stage excavation and the unloading of energy have an impact on the surrounding rock and support structures. Therefore, precise control of blasting technology is required for excavation. Synchronization of the support process is also crucial. After the completion of the climbing section excavation, the construction sequence of the large section is aligned with the design plan, ensuring the safety of tunnel construction.
3. Horizontal expansion will increase the number of workfaces, which is beneficial to improving the construction progress. After horizontal excavation, the subsequent construction processes can be easily connected, allowing for the addition of two to three working faces, effectively improving the progress of subsequent works.  
In conclusion, there are fundamental differences between the two methods in terms of construction and support plans, construction progress, and control blasting technology. The RTM completes the section transition by constructing auxiliary IPT within the large section, with simple construction connections and lower requirements for section shape. However, the top heading process is more complex, and support operations cannot be carried out using the dual-side wall drift method. On the other hand, the ACM plans the starting point for climbing in advance and constructs the auxiliary IPTs outside the large section up to one side crown. The construction sequence of the two-lane section in the advance tunnel smoothly connects with the large section's sequence, and subsequent excavation works  
Figure 6: Construction steps of the ACM.  
in the large section can be carried out according to the design plan. To evaluate the stress and deformation patterns of the surrounding rock during the construction stages of the two methods, numerical simulation modeling and analysis are employed.