## 4 Analysis, Results, and Discussion  
### Displacement Analysis  
#### 4.1.1 Surface Settlement  
According to the construction plan, the left tunnel adopted the ring reserved core soil method, whereas the right tunnel adopted the CD method. In order to simplify the running program and improve the calculation efficiency, the excavation footage was set to 2.5 m. After the left tunnel was excavated for 50 m, the right tunnel was excavated. In order to analyze the surface settlement in the calculation process of the model excavation and support, the monitoring points were arranged at the surface of y = 1, and each monitoring point was horizontally separated by 1 m. A total of 90 monitoring points were set up, extracting the surface settlement data after every 5 m excavation. At the same time, in this section, at y = 1, 12 monitoring points were set in the inner profile of the tunnel, which were located at the arch crown, arch shoulder, arch foot, and arch bottom of the inner contour of the tunnel. The surface subsidence curves and the distribution of the monitoring points in the inner contour are shown in Figure 7.  
\begin{table}
\begin{tabular}{l c} \hline
**Cable Properties** & **Value** \\ \hline Anchor rod section area/mm\({}^{2}\) & 490.9 \\ Anchor rod elastic modulus/GPa & 200 \\ Anchor density/(kg\(\cdot\)m\({}^{-3}\)) & 7850 \\ Anchor rod tensile strength/MN & 0.2 \\ Mortar shear modulus/GPa & 9.5 \\ Unit-length mortar stiffness \(k_{g}/\)GPa & 17.556 \\ Mortar bonding force \(c_{g}/\)(kg\(\cdot\)m\({}^{-1}\)) & 159.40 \\ Slot outer circumference/m & 0.139 \\ Cement slurry friction angle/(\({}^{\circ}\)) & 25 \\ \hline \end{tabular}
\end{table}
Table 3: Parameters of the anchor rod.  
As shown in Figure 7, during the early excavation, the surface settlement curve above the left portal of the tunnel showed different degrees of a V-type distribution, and the maximum settlement value appeared on the right side of the arch crown of the left tunnel. The settlement value gradually increased with the excavation, whereas the maximum settlement position remain unchanged. The maximum displacement settlement difference during the left tunnel excavation occurred in the first 5 m and 10 m. The maximum cumulative settlement values were 4.012 mm and 7.993 mm, respectively, and gradually stabilized with the decrease in the surface settlement. After the left tunnel was excavated for 50 m, the largest ground subsidence was 10.467 mm, and the influence range of settlement was 43 m. At the beginning of the excavation, a slight uplift in the ground to the left of the  
Figure 7: Surface settlement curve: (**a**) surface settlement of the left tunnel excavation; and (**b**) surface settlement of the right tunnel excavation.  
left tunnel and towards the right of the center of the right tunnel was observed. When the right tunnel was excavated, the left side of the maximum settlement displacement point of the surface did not change significantly, and the right side started settling. Due to the supporting effect of the middle rock, the settlement above it was minuscule, and the surface settlement curve began to change from a V-type distribution to a W-type distribution. As the right tunnel was excavated by the CD method, the settlement value was small when the left part of the tunnel was excavated. A large settlement displacement occurred when the right part was excavated. The maximum settlement difference was about 4.5 mm, which then gradually decreased. Finally, the maximum settlement value above the right tunnel was 10.740 mm, and the influence range of surface settlement was expanded from 43 m to 100 m. The ground surface deformation did not exceed 30 mm as given in the _Code for Design of Highway Tunnels (JTG 3370.1-2018)_[26], indicating that the settlement control standard could be reached if the construction was carried out according to the CD method.