## 4 Results and Discussion
### Analysis of Excavation Sequence
#### 4.1.2 Force Analysis  
(1) Stress analysis of the support structure  
The stresses in the support structure under different excavation scenarios are shown in Figure 12. Regardless of the excavation sequence, the stresses in the support structure of the deeply buried side were generally greater than those in the shallowly buried side. In addition, the stresses in the support structure were greater in the area where the arch shoulder was combined with the middle partition wall. This corresponded to the monitoring results and the fact that spalling and cracking were mostly observed in the combination of the middle partition wall and the arch shoulder. Therefore, in the design and construction process of the double-arch tunnel, the combination of the middle partition wall and the arch shoulder was considered the key control area. It is worth noting that different excavation sequences had an impact on the location of stress distribution in the support structure. Firstly, let us analyze the stress state of the support structure on the deeply buried side. Taking the two excavation sequences \"1234\" and \"1243\" as examples, as shown in Figure 12a,b, it can be observed that the maximum stress of the support structure in the \"1234\" sequence occurred at Tunnel 3, while the maximum stress of the support structure in the \"1243\" sequence occurred at Tunnel 4. Secondly, let us analyze the stress state of the support structure on the shallowly buried side of the arch tunnel. Taking the two excavation sequences \"1234\" and \"2134\" as examples, as shown in Figure 12a,c, it can be observed that the maximum stress of the support structure in the \"1234\" sequence occurred at Tunnel 1, while the maximum stress of the support structure in the \"2134\" sequence occurred at Tunnel 2. This situation was due to the fact that, during the construction of a double-arch tunnel, the later tunnel disturbed the earlier tunnel, thereby increasing the stresses on the support structure of the earlier tunnel. The maximum stresses of each support structure for different excavation sequences are shown in Table 3. The maximum stresses in the support structure of the double-arch tunnel on the shallow-buried side for different excavation sequences are shown in Figure 13a; moreover, for the deeply buried side, they are shown in Figure 13b. Since the stress in the support structure on the deeply buried side was high, the first consideration was to make the force state of the support structure on the deeply buried side more reasonable. According to Figure 13b, it can be observed that the excavation sequences \"1234\", \"2134\", \"3412\", and \"3421\" achieved better results; therefore, they were organized into set C.  
Figure 11: Horizontal deformation curves of the middle partition wall with excavation steps under different excavation sequences: (**a**) the middle partition wall on the shallowly buried side; (**b**) the middle partition wall on the deeply buried side. Then, the focus shifted to making the support structure on the shallow-buried side more reasonable in force. According to Figure 13a, it could be seen that the excavation sequences \"1234\", \"1243\", \"3412\", and \"4312\" achieved better results; therefore, they were organized into set D. By taking the intersection of set C and set D, it was found that the excavation sequences \"1234\" and \"3412\" were optimal. This also indicated that the force magnitude of the support structure was more likely to be influenced by the excavation sequence of the single-lane, double-arch tunnel itself. In other words, the respective excavations of the left-line tunnel and the right-line tunnel had less impact on each other.