## 3 Numerical Simulation of Tunnel Excavation
### Analysis of Calculation Results  
#### 3.2.1 Force Characteristics of the Bifurcated Tunnel Envelope at Different Construction Steps  
The contour plots of the average effective stress distribution in the surrounding rock during the construction stages of the bifurcation tunnel are shown in Figure 9. After excavation, the stress concentration in the five-lane tunnel is located in the arch waist areas on both sides. The small-clearance sections of the two-lane tunnel and the three-lane tunnel are concentrated in the arch shoulder and arch waist, especially in the arch shoulder area near the central rock. This pattern conforms to the stress distribution in the surrounding rock of the super-large-section tunnel. The surrounding rock of the five-lane section experiences compression at the top and bottom of the arch, while tension is observed on both sides of the arch waist. The maximum average effective stress is 9.38 MPa for the RTM and 7.42 MPa for the ACM. During the actual construction stage, it is important to strengthen the protection of the central rock and optimize the timing for the completion of excavation and support closure in the central drift of the five-lane section.  
starts constructing the IPT, with a peak vertical stress of 2.32 MPa in the surrounding rock. At this stage, the RTM is still in the stepped excavation of the two-lane tunnel, with the peak vertical stress in the surrounding rock remaining the same as in Figure 10a at 2.96 MPa, which is 1.3 times higher than the value of the ACM.  
(2) Analysis of the forces on the vault of the five-lane tunnel  
Figure 11 depicts the stress-time curves of the crown in the horizontal and vertical directions at the monitoring section K1+890 of the five-lane section. With the increasing excavation steps of the five-lane super-large section, the stress redistribution in the surrounding rock occurs. The horizontal stress at the crown of the large section is in an unloading state. The main excavation steps that affect the redistribution of loads are the excavation of the initial drift and the first-layer excavation of the large section. The horizontal stress decreases gradually with the increase in excavation area until it reaches equilibrium. The final horizontal stress at the crown of the large section is 0.30 MPa for the ACM and 0.42 MPa for the RTM. The vertical stress increases and then decreases with the excavation steps, with the peak occurring during the excavation of the central drift in the first layer. The maximum vertical stress is 2.68 MPa for the ACM and 3.10 MPa for the RTM. In the stable state, the vertical stress in the surrounding rock for the ACM is 0.15 MPa, which is consistent with the results at the monitoring section K1+840. For the RTM, it is 0.46 MPa, approximately three times higher than the value for the ACM.  
Figure 11: Stress curve of five-lane surrounding rock vault: (**a**) horizontal stress; (**b**) vertical stress.  
Figure 10: Stress curve of two-lane surrounding rock vault: (**a**) at K1+850, (**b**) at K1+870.  
The stresses applied to the surrounding rock during the construction phase were greater for the RTM than for the ACM, the former being more demanding for the two-lane tunnel and the five-lane tunnel support structure.