## 4 Results and Discussion
### Analysis of Excavation Methods
#### 4.2.3 Force Analysis  
(1) Force analysis of the support structure  
The initial support structure was essential for the timely reinforcement and stability of the surrounding rock during tunnel construction. As a compressive element, it was not only required to ensure that the axial force met the design requirements but also to avoid excessive local bending moments, which could have caused excessive structural deformation or even damage. Figure 21, Figure 22, and Figure 23 showed the axial force, bending moment, and shear force distribution of the initial support under the two excavation schemes, respectively. Due to the asymmetrical pressure, the internal forces of the tunnel structure exhibited an asymmetric distribution in both excavation schemes. Regarding the axial forces, as shown in Figure 21a,b, the axial forces for both excavation schemes were larger at the top, shoulder, and foot of the vault. Numerically, the maximum axial force for the Three-bench Excavation Method was 2.455 MPa, while the maximum axial force for the Center Diaphragm Excavation Method was 2.338 MPa, with little difference between them. Concerning the bending moment, both excavation schemes resulted in a larger bending moment at the arch footing area. It should be noted that, when the Center Diaphragm Excavation Method was used for the excavation, as shown in Figure 22a, a larger bending moment would be generated during the combination of the temporary support and the initial support at the arch top, which was not reasonable for the initial support structure. On the other hand, when the three-step excavation was used, as depicted in Figure 22b, a large bending moment was generated at the joint part of the middle partition wall and the arch shoulder, as the pressure of the surrounding rock was transferred to the middle partition wall through the initial support after the excavation of the upper and middle steps was completed. In terms of the final values, the absolute values of the maximum bending moment for the Center Diaphragm Excavation Method and Three-bench Excavation Method were 0.225 MPa and 0.157 MPa, respectively, indicating that the Three-bench Excavation Method was better than the Center Diaphragm Excavation Method. For the shear force, as shown in Figure 23, its distribution followed a similar pattern to that of the bending moment; therefore, further elaboration was unnecessary. The absolute values of the maximum shear force for the Center Diaphragm Excavation Method and the Three-bench Excavation Method were 0.256 MPa and 0.173 MPa, respectively, confirming that the Three-bench Excavation Method was still superior to the Center Diaphragm Excavation Method.  
(2) Stress analysis of the middle partition wall  
The middle partition wall, as the key-bearing structure of the double-arch tunnel, was of utmost importance. It had to withstand not only the load from the upper rock but also the load transmitted by the lining on both sides. The burial depth of the strata, asymmetrical pressure, construction method, and construction disturbance all resulted in asymmetrical pressure in the diaphragm wall, which had a significant impact on tunnel stability. Figures 24 and 25 present the maximum and minimum principal stresses in the middle partition wall after the construction of the two excavation methods, respectively. The maximum tensile stress in the middle partition wall was 6.514 MPa for the Center Diaphrag Excavation Method and 7.070 MPa for the Three-bench Excavation Method, with the latter slightly outperforming the former. Both excavation methods exceeded the tensile strength of C35 concrete, indicating the need for the reinforced strengthening of the bottom part of the middle partition wall. Regarding the maximum compressive stress, it was 12.242 MPa for the Center Diaphragm Excavation Method and 12.480 MPa for the Three-bench Excavation Method, with a small difference between them.