## 4 Analysis of the Numerical Calculation Results  
### Surrounding Rock Stress Analysis  
The surrounding rock of a tunnel prior to excavation exists in a state of crustal stress equilibrium, known as the primitive stress state [43]. The excavation sequence acts as an unloading process for the surrounding rock, altering its stress state. This alteration in stress conditions prompts stress redistribution within the surrounding rock, culminating in the formation of a new stress equilibrium state through the release of stresses. This stress release and redistribution process is essential to accommodate the geological changes induced by the excavation. The excavation-induced deformation and displacement of the surrounding rock alter the original distribution of ground stresses, ultimately leading to a new equilibrium state. This critical process of stress release and rebalancing is fundamental to ensuring the stability and safety of tunneling projects and, as such, necessitates thorough analysis and evaluation.  
Figure 6 shows the major principal stress cloud diagrams for the three construction methods. Based on the distribution of major principal stresses depicted in Figure 6, significant changes in tunnel stresses can be observed at specific locations, such as the arch crown, arch base, arch waist, and arch feet. After organizing and analyzing the simulation results, it was found that the maximum principal stress occurs at the position of the right arch foot for both the double-side drift method and the CRD construction method. Specifically, the double-side drift method resulted in a maximum principal stress of 0.451 MPa at the right arch footing, whereas the CRD method recorded a maximum principal stress of 0.216 MPa at the same location. The three-benching seven-step method, on the other hand, displayed a different stress distribution, with the maximum principal stress reaching 0.621 MPa at a  
Figure 5: Excavation sequence diagrams of the three methods. (**a**) double-sided drift method; (**b**) CRD method; (**c**) three-benching seven-step method. (Note: The numbers in the figure represent the excavation sequence.)  
distinct point in the upper part of the right arch. These findings illustrate the variability in stress distribution in the surrounding rock during tunnel excavation by different methods. These results highlight the differences in the distribution of surrounding rock stress during tunnel excavation across different construction methods. The double-side drift method and the CRD method exhibit relatively lower maximum principal stresses at the right arch foot, whereas the three-benching seven-step method shows higher maximum principal stresses at a specific point on the upper right arch.  
Based on the analysis above, the double-side drift method demonstrates certain advantages for large section tunnel excavations. Compared to the three-benching seven-step method and the CRD method, the double-side drift method exerts less impact on the surrounding rock stress. Throughout the tunnel excavation sequence, significant variations are observed in the stress changes and trends within the surrounding rock due to these three methods. However, the double-side drift method impacts the surrounding rock stresses to a lesser extent than the other two methods. While localized stress disturbances may occur during the excavation process, these disturbances have a minimal effect on the overall stress changes in the surrounding rock.  
Figure 6: Cont.