# Stability Analysis of Jinchuan Hydropower Station Hydraulic Tunnels during Excavation and Unloading
## 3 Study of the MS Activity of the Surrounding Rock
### Temporal Evolution Characteristics of MS Events  
Figure 3 shows the temporal distribution of daily MS events and their cumulative energy in the spillway tunnel project of the Jinchuan Hydropower Station. It can be shown that, despite the uneven distribution of MS events over time during the monitoring period, these events could generally be divided into three stages. To be specific, the first stage saw a general increase in the number of MS events and a gradual increase in the cumulative energy; the second stage saw a significant increase in the cumulative energy and a wide range in the number of MS events, from zero to six; in the third stage, the number of MS events was generally one or two and remained low while the cumulative energy initially increased significantly and then tended to level out. Figure 2: Overall layout of the underground powerhouse caverns of the Jinchuan hydropower station. The number of daily MS events remained at between zero and four in the first stage of MS monitoring in the spillway tunnel (2 August to 11 September 2020), typically below two, and the cumulative energy of MS events gradually increased. In this stage, the main tunnel was being slowly excavated while the #1 adit of the spillway tunnel was already finished. This stage mainly involved supporting the intersection of the #1 adit and the main tunnel and conducting blasting and excavation for the spillway tunnel. The excavation of the upper portion of the main tunnel was gradually accelerated with the completion of the excavation and support of the intersection with the #1 adit. In addition, due to the disturbance from the construction of the adjacent diversion tunnel, micro-fractures developed in the surrounding rock of the spillway tunnel, and MS activity increased gradually. In the second stage (12 September to 8 November 2020), the number of daily MS events fluctuated between one and six, reaching or exceeding five events on several days. In particular, the number of daily MS events reached a low point at the end of September before rising dramatically from 1-2 to 4-6 events, peaking at six events on October 9 and October 22. The cumulative energy of MS events grew slowly before October 7. Following October 7, it surged for 5 days before climbing at a steadily slower rate. In this stage, MS activity diminished as the support for the section from the intersection of the #1 adit to the tunnel face of the main tunnel was completed. After the support was completed, the upper portion of the main tunnel was dug out more quickly, with an average of two blasts per day and a maximum of three. Furthermore, due to the intersection of the spillway tunnel with two faults close to the working chamber, the damage to the surrounding rock from the intersection of the spillway tunnel and the #1 adit (stake \(0+135\) m) to the working chamber (stake \(0+450\) m) worsened, especially near the working chamber (stakes \(0+400\) m to \(0+450\) m). Internal stress adjustment, the initiation and development of micro-fractures, and a rise in MS activity were all brought on by this. Following that, tunneling was slowed down and MS activity gradually decreased once the excavation of the pressurized section was completed. In the third stage of the MS monitoring period (9 November to 16 December 2020), the number of daily MS events showed an overall decrease and an increase for some periods. The number of daily MS events remained between zero and three but was usually one. Despite this, until early December, the cumulative energy of the MS events appeared to rise progressively before leveling off after December 5. This stage mainly involved supporting the working chamber and penetrating the first layer of the spillway tunnel, which resulted in fewer MS events. However, the blasting and excavation of the adjacent  
Figure 3: Temporal evolution of the number of MS events and cumulative energy released.