# Stability Analysis of Jinchuan Hydropower Station Hydraulic Tunnels during Excavation and Unloading
## 4 Numerical Modeling of the Excavation of Adjacent Tunnels
### Mechanical Response of the Surrounding Rock during Step-by-Step Exavation
#### 4.4.1 Deformation Characteristics of the Surrounding Rock
\begin{table}
\begin{tabular}{l c} \hline \hline
**Excavation Steps** & **Excavation Position** \\ \hline Step 1 excavation & Adits and adjacent main tunnels at the inlet and outlet \\ Step 2 excavation & Stakes 0 + 160 m to 0 + 240 m and 0 + 724 m to 0 + 824 m in the first layer \\ Step 3 excavation & Stakes 0 + 240 m to 0 + 340 m and 0 + 655 m to 0 + 724 m in the first layer \\ Step 4 excavation & Stakes 0 + 630 m to 0 + 655 m in the first layer \\ Step 5 excavation & Stakes 0 + 323 m to 0 + 414 m and 0 + 530 m to 0 + 630 m in the first layer, and stakes 0 + 724 m to 0 + 824 m \\ Step 6 excavation & Stakes 0 + 464 m to 0 + 530 m in the first layer \\ Step 7 excavation & Stakes 0 + 160 m to 0 + 240 m in the second layer, and stakes 0 + 414 m to 0 + 424 m in the first layer \\ Step 8 excavation & Stakes 0 + 424 m to 0 + 464 m in the first layer \\ Step 9 excavation & Stakes 0 + 390 m to 0 + 490 m in the first layer \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Step-by-step excavation of the spillway tunnel. leftward excavation caused significant floor heave deformation. According to the analysis, the first layer of excavation formed a cross-section with an arch on the upper part and a plane on the lower part because the pressurized section was circular. The arch material could transform the upper load into transverse pressure along the arch ring when it was under pressure, and the rock had better compressive performance, so smaller deformation was observed on the upper part, whereas a larger displacement was observed on the lower-part free face toward the unstressed plane. Moreover, no obvious floor deformation was discovered during site surveys, due mainly to the site being repeatedly crushed by heavy-duty machinery, and no obvious floor heave deformation was observed. The displacement impact area of leftward excavation and rightward excavation intersected and caused deformation at f31 and f27, which were not excavated. According to Figure 8, the diversion tunnel had greater deformation in the vicinity of its inlet than the spillway tunnel, owing to the larger span of its left and right side walls. Before excavation in both directions reached f27, no obvious deformation was observed, and the deformation of the surrounding rock did not differ significantly from that observed during the excavation of the previous section. When rightward excavation reached f27, the displacement of the tunnel face increased sharply on the hanging wall of the fault, reaching 25 mm. Furthermore, the relaxation depth was greater, and a 10 mm displacement was still present 8 m from the tunnel face. Because the fault was disturbed by excavation, the mechanical performance of the surrounding rock deteriorated, and the surrounding rock on the free face lost the bearing layer, resulting in significant deformation and displacement in a direction far from the tunnel face. As a result, excavation of this section should be closely followed by advanced support to prevent microscopic deformation or even collapse of the surrounding rock near the tunnel face. During step 8 of the excavation, the first layer of the spillway tunnel was penetrated, resulting in a large deformation of the surrounding rock, up to 15 mm between f27 and f31. The next two steps were to excavate the diversion tunnel so that the surrounding rock of the spillway tunnel completed stress adjustment without obvious displacement changes, and only the area affected by deformation extended slightly into the deep. The deformation evolution characteristics of the surrounding rock in the diversion tunnel excavated to the penetration of the first layer were obtained using FLAC3D after a 3D numerical calculation, as shown in Figure 9.  
Figure 9 shows the deformation evolution law of the surrounding rock of the diversion tunnel during the excavation and unloading process. In general, the deformation was small, but it was greater than in the spillway tunnel, due mainly to the wider span of the diversion tunnel and the greater heights for layered excavation.