# Excavation and Construction Technology of Diversion Tunnel under Complex Geological Conditions
## 6 Design of Excavation Construction Methods
### Tunnel Excavation and Support Construction  
The excavation and support of the tunnel body are completed by three working groups: the drilling and blasting group, the slag removal working group, and the support group. Without affecting the construction, the slag removal working group and the initial support group can work in parallel to minimize the construction period. The excavation and support process of the tunnel body is shown in Figure 3.  
In the support construction of this study, the excavation method is mainly used for construction. During construction, it is necessary to directly adopt the operation mode of underground excavation in the tunnel in order to achieve a close combination of initial support and secondary lining for the weak surrounding rock at the tunnel entrance. The main construction process of trapezoidal cutting is shown in Figure 4.  
Figure 4: Main construction process diagram of trapezoidal cutting. Figure 3: Tunnel excavation and support process flow. In Figure 4, the main construction of trapezoidal cutting is completed in the order of improving temporary drainage, excavation and protection of side and front slopes, and arch protection construction. Among them, the inner mold of the arch protection is reshaped. The installation of arch protection formwork, pouring of arch protection concrete, backfilling construction, and other steps are all within the scope of arch protection construction. 1. Improve temporary drainage. Firstly, construct a temporary drainage ditch to divert the right tunnel gully water system to the portal drainage system. After backfilling is completed, construct a circular intercepting ditch outside the tunnel to introduce the natural water system. 2. Excavation and protection of side and front slopes. Open excavation construction will be carried out on the muddy soil and silty clay at the top of the tunnel arch. The excavation range is 120\({}^{\circ}\) from the center, the longitudinal length of the excavation is 30 m, the excavation height is 5.18 m below the arch top, and the permanent slope rate is 1:075. Anchor rod frame beams will be used for protection, and the temporary side and front slopes will be protected with anchor rods (C25 sprayed concrete+) \(\Phi\)20 roll anchor rods+ \(\Phi\)8 steel mesh. Before excavation, the excavation contour line of the side and front slopes should be set out, with one excavation point set out every 3 m to ensure that the excavation contour line of the side and front slopes is linear, smooth, and beautiful. And monitoring and measuring points should be set up at the top of the slope, with a longitudinal spacing of 5 m. Each monitoring and measuring point should be buried with a steel head of no less than 50 cm and firmly fixed with concrete and pasted with reflective stickers. 3. Arch protection construction. Step 1: Testing the bearing capacity of the foundation of the arch protection. Before the construction of the arch protection, the bearing capacity of the foundation should be tested. When the bearing capacity is less than 200 kPa, steel flower pipe grouting reinforcement should be carried out at the platform, and the steel pipe should be \(\Phi\)105 \(\times\) 5 mm hot-rolled seamless steel pipe (section length 6.2 m), with a horizontal spacing of 80 cm and a vertical spacing of 80 cm, arranged in two rows in a plum blossom shape. Step 2: Temporary drainage. A ditch of 60 cm \(\times\) 20 cm shall be constructed outside the platform on both sides of the arch guard, and the water system shall be discharged to the drainage system of the entrance. Step 3: Shape the inner mold of the arch protection. Preliminary trimming of the inner formwork of the arch protection is carried out through measurement, setting out, and mechanical coordination. After the preliminary trimming is completed, \(\Phi\)42 steel pipes are used as longitudinal positioning measures. After pouring the protective arch cushion layer, a ring arch frame is assembled at a distance of 5 m for horizontal positioning.