## 5 Field Monitoring
### The Flow of the Underground River Monitoring  
In order to achieve a clear understanding of the underground river flow, 130 m to the left of d3k279 + 938 and 70 m to the left of d3k279 + 970 were taken as observation sections in the underground river in the karst cave to observe and measure the flow in the tunnel. The statistics and the amount of rain on that day are plotted in Figure 11.  
The two black lines in Figure 11 represent the flow of the underground river in the karst cave and the outlet flow, respectively. It can be seen that the outlet flow is significantly greater than the flow of the underground river in the karst cave, indicating that there are multiple tributaries in Yujing Mountain, and because the limestone belongs to soluble rock, it forms many connecting drainage routes, resulting in the collection of the whole water at the outlet. The change in rainfall and the change in underground river flow in the karst cave are relatively slight, but they have little correlation with the flow at the outlet of the underground river. Therefore, the water system is complex, and so we should pay attention to the monitoring of groundwater to prevent the water level of the underground river rising quickly.  
Figure 10: Range analysis of the comprehensive evaluation method. (**a**) The worst backfill material parameters. (**b**) The optimal backfill material parameters.  
There is a deep pool at the outlet of the karst cave. According to the flow monitoring data for the outlet, the maximum water volume of the outlet is \(Q=68.75\) m\({}^{3}\)/s, which is far less than the calculated maximum water inflow and underground river water inflow. Therefore, in the rainy season, the water level in the karst cave must rise. Based on the static water sediment on the surface of the accumulation at the bottom of the cave and the drilling, it is judged that the water level of the underground river in the cave once rose to the elevation of 990 m (an increase of about 30 m). Due to the great uncertainty of the drainage property of the underground river channel, it is very likely that the underground river pipeline will collapse and block the river channel during the tunnel operation, which will further aggravate the rise of the underground river in the karst cave.  
In order to avoid the backfill blocking the underground river channel and adversely affecting the operation, the underground river in the karst cave section was diverted before the karst cave treatment, as shown in Figure 12.  
The underground river channel mainly uses drainage and blocking to prevent the rise of underground river water level in the rainy season from having a great impact on the safety of the tunnel in the giant karst cave. After the construction of the discharge tunnel, the underground river shall be blocked upstream and downstream. At the same time, the underground river flow in the karst cave section shall be led downstream of the underground river through the NO. 4 and 5 discharge tunnels.  
Figure 11: Rainfall and flow monitoring map in the cave.  
Figure 12: Schematic diagram of underground river diversion.  
In addition, a new channel is to be constructed at the intersection between the underground river and the tunnel, which will be able to maintain the rapid flow of the underground river. At the foot of the slope of the karst cave, the new channel follows the flow direction of the underground river and is about 30-50 m away from the tunnel wall on the right side, and a circuitous discharge tunnel is set to divert the underground river section so as to direct the upstream water of the underground river downstream through the discharge tunnel. The discharge tunnel is about 450 m long, 7.2 m wide and 6.0 m high.