## 2 Project Overview, Problems, and Solutions
### Construction Design for the Bifurcation Tunnel
#### 2.3.1 The Characteristic of RTM  
As shown in Figure 5, the advance tunnel is initially excavated with two lanes, progressing normally until reaching the intersection with the maximum section. The intersection surface serves as the starting face for the IPT construction. In the early stage of large-section excavation, the IPT is constructed using stepped sections on the two-lane side. The IPT is constructed with a variable section until reaching the end of the large section. At this point, the section of the IPT aligns with one side of the large section's single-side wall section. The area above the IPT where the large section has not yet formed is excavated in reverse until reaching the design contour. With this, the construction of the transitional section for the bifurcation tunnel's variable section is completed. Subsequently, horizontal excavation is carried out to form the other side of the dual-side wall and the central drift, completing the excavation construction.  
Figure 4: Construction methods for different sections of bifurcated tunnels (unit: m).  
This method has the following characteristics:  
1. The angle of the IPT is a fixed value determined by the length of the large section. The climbing angle is a fixed value determined by the relationship between the two-lane section and the position of the large section on the plan. The shorter the length of the large-section segment, the steeper the climbing angle.
2. The difficulty lies in controlling the drilling angle, and there is a high risk factor for workers during construction. In step c of Figure 5, irregular arc-shaped thin rock layers require multiple blasts for excavation. When workers drill holes, the bottom of the drilling rig needs to be treated for debris, and a horizontal working platform needs to be installed. If horizontal drilling and blasting are used, it is difficult to control the drilling angle, which may lead to slabbing. If vertical drilling is adopted, controlling the depth of the borehole is challenging, and the downward-facing borehole opening is not conducive to charging and blocking, resulting in a high risk factor for workers.
3. Blasting in the vault area causes cumulative damage to the top of the large section, and the resulting debris blocks the tunnel entrance. During the excavation by reverse top heading blasting, the surrounding rock of the large section in this area undergoes multiple cumulative damages, affecting the stability of the surrounding rock of the large-section tunnel. At the same time, the direction of the blasted rock fragments from the top will be downwards, and the resulting debris will block the IPT entrance, forming a sealed space that hinders subsequent operations and affects the construction schedule of follow-up work.
4. Changes in the original zoning plan for the super-large section result in changes to the sequence of support operations. Changes in the partitioning plan for the dual-side walls of the large section lead to changes in subsequent work processes and the sequence of support operations. For large sections over 400 m\({}^{2}\), the synchronization of construction sequences and support plays an important role in the stability of the surrounding rock.