# 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
As the excavation and unloading proceeded, the surrounding rock showed obvious displacement changes, particularly at the intersection of the #2 edit and the main tunnel and at the intersection of the leftward excavated section and the fault, indicating that the stress concentration at the intersection of the tunnel and weak structural planes were the main factors affecting tunnel stability. The analysis of the leftward excavated section reveals that the deformation was large due to the greater burial depth of this section compared with other sections, and this area was dominated by gravity load, implying that the deformation of the leftward excavated section was primarily caused by the higher ground stress. Similarly, the excavation process of the diversion tunnel was broken down into sections. Similar to the spillway tunnel, the initial excavation destroyed the stress state of the primary rock and caused complete stress self-adjustment in the surrounding rock. Moreover, the tunnel deformed at the intersection of the tunnel due to unloading, with the maximum deformation being slightly larger than that of the spillway tunnel, reaching 18 mm. The displacement of the shallow surrounding rock was mostly between 10 mm and 15 mm, which was related to the larger excavation section of the diversion tunnel. The displacement occurred at the intersection of the fault and the tunnel toward the free face when the diversion tunnel was excavated to the vicinity of f54 (step 3 excavation), and the deformation of the shallow surrounding rock was small in the exposure area of the fault. According to the analysis, the hanging wall of the fault was disturbed by excavation and unloading, and the surrounding rock on the hanging wall  
was under the contact force of that on the footwall, resulting in only minor deformation. However, when the first layer was excavated through the fault and the second layer was excavated near the fault, the surrounding rock of the diversion tunnel at stakes 0 + 630 m to 0 + 650 m exposed two free faces and a fault plane on the other side and lost the bearing layer. As a result, the displacement vector developed in the direction of the two free faces, with a value close to 20 mm. The deformation of the surrounding rock near the first layer of the tunnel face was inconspicuous, and the deformation of the shallow surrounding rock was less than 7 mm. At this time, the deformation in the leftward excavated section was generally greater than that in the rightward excavated section due to relatively high ground stress. The surrounding rock deformed slowly during the subsequent excavation, and only a few areas were slightly deformed. The deformation started to increase significantly once the excavation reached the f27 and f31 areas. When the tunnel passed through f27 and f31, the deformation in the exposed area of the fault increased. Moreover, on the roof, the deformation on the hanging wall of the fault was less than that on the footwall, whereas, on the floor, the deformation on the hanging wall of the fault was greater than that on the footwall. According to the mechanical structure of the fault, on the roof, the surrounding rock on the footwall bore the weight of that on the hanging wall, resulting in a small deformation on the surrounding rock of the hanging wall, while the surrounding rock on the hanging wall pressed that on the footwall in the direction of the free face, resulting in a large displacement of the surrounding rock on the footwall toward the free face. On the floor, the surrounding rock on the footwall was pressed by that on the hanging wall in the opposite direction of the free face, resulting in inconspicuous deformation and displacement. The pressure received by the surrounding rock on the hanging wall from that on the footwall, in contrast, was directed toward the free face, resulting in a greater displacement of the hanging wall than the footwall.