# Excavation Method Comparison and Optimization for a Super Large Cross-Section Tunnel
## 4 Optimization for the Construction Sequence  
Considering the impact of construction sequences on the stability of the tunnel surrounding rock, the processes depicted in Figure 15 are rigorously simulated and analyzed. Given the oversized cross-section of the highway tunnel studied in this paper, and to ensure the safety of construction operations, the first four sequences are subdivided into three segments centered around the guide hole and excavated following the sequence indicated in the diagram. In the numerical simulation, the initial four construction sequences include a total of 18 stages, ranging from establishing ground stress equilibrium to the final incorporation of the second lining. To expedite the construction timeline, sequences V and VI are streamlined by combining both sides of the guide pits into a single construction step, reducing the total number of sequences to 16. Figure 14: Surrounding rock stress maps under three working methods. (**a**) Double-sided drift method; (**b**) CRD method; (**c**) three-benching seven-step method. The vertical displacements at the top and bottom of the tunnel arch under different construction sequences are detailed in Table 2. During tunnel support excavation, the bottom of the tunnel arch is influenced by self-gravity stress and the rate of loss of the bottom layer, which generates a certain degree of vertical upward tensile stress. This tension results in smaller bulge deformations at the bottom of the arch compared to those at the top. However, the various construction sequences significantly affect the timing of the peak values of arch bulge deformation; For example, Sequence III exhibits the largest cumulative base bulge value by the end of construction. Sequence IV records the maximum bulge value at the bottom of the arch during construction, whereas Sequence VI demonstrates the least cumulative bulging at this location. The cumulative vertical displacement of the vault in Sequence VI, along with the associated settlement during construction, is minimized, making this sequence shorter and more cost-effective. Sequence I features the most consistent changes throughout the construction processes, resulting  
Figure 15: Working sequence arrangement. (**a**) Sequence I; (**b**) Sequence II; (**c**) Sequence III; (**d**) Sequence IV; (**e**) Sequence V; (**f**) Sequence VI. in smaller final vault settlements but requiring a longer duration and presenting more construction challenges than Sequence VI. Both Sequence III and Sequence IV experience the greatest cumulative bulging at the arch bottom and the most significant bulge values. Overall, Sequence VI not only reduces the cumulative bulging at the arch bottom and the settlement at the arch top but also minimizes displacement during construction, offering the benefits of a shorter construction period, reduced construction difficulty, and improved economic efficiency. The surface settlement results illustrate the surface disturbance caused by the construction of different sequences in super-large cross-section tunnels. There is a documented correlation between the amount of surface settlement and the distance from the center of the tunnel, as detailed in Figure 16. At monitoring point X1, the trends of cumulative settlement changes under the six sequences are fundamentally similar. Owing to the symmetrical distribution of the tunnel along the central axis, the surface settlement exhibits a symmetrical concave pattern. Further analysis indicates that Sequence IV experiences the most pronounced overall cumulative settlement and the widest sinkhole. This is closely followed by Sequences III and II, with the trends for Sequences III and IV largely coinciding. Sequences V and VI show comparable levels of cumulative surface settlement, and their sinkhole warpage magnitudes are virtually identical. Notably, Sequence VI results in the least cumulative settlement of the surrounding rock.