# Case Study and Risk Assessment of Water Inrush Disaster in Qingdao Metro Line 4
## 3 Analysis on the Causes of the Collapse
### Numerical Simulation for the Collapse
#### 3.2.2 Analysis of Settlement Results  
Figure 10 shows the settlement of the ground during excavation. The slightly weathered tuff is the dominant lithology from ZDK25+395 to ZDK25+360, and the grade of surrounding rock is III. Figure 10a,b shows the settlement of the tunnel during the excavation from ZDK25+395 to ZDK25+360. It can be seen from these pictures that the influence zone on the surrounding rock caused by excavation is minor, while the ground surface is basically unaffected. During this period, the excavation caused minor vault settlement, with a maximum settlement value of \(-0.16\) mm. The strongly weathered tuff is the dominant lithology from ZDK25+360 to ZDK25+340, and the grade of surrounding rock is V. When excavated to ZDK25+355, the tunnel is located at the junction of slightly weathered tuff and strongly weathered tuff. During the excavation here, the tunnel vault in the strongly weathered tuff area settled significantly with a settlement value of \(-7.77\) mm, as shown in Figure 10c. Meanwhile, the influence zone of the tunnel excavation started to extend towards the surface. When excavated to ZDK25+350, the surrounding rock of the tunnel consisted entirely of strongly weathered tuff. At this point, tunnel excavation caused significant settlement and had a large impact. Surface settlement was the largest factor, with a settlement value of \(-31.82\) mm, as shown in Figure 10d, which exceeded the warning value of \(30\) mm required for tunnel monitoring. When excavated to ZDK25+340, the collapse zone, the surface settlement value increased to \(52.47\) mm, as shown in Figure 10e. After excavation, near ZDK25+343, a cave-in with the diameter of \(30\) m appeared in the model, as shown in Figure 11. The actual collapse location is ZDK25+343, and the diameter of the collapse pit is about \(30\) m. The simulation results are consistent with the actual situation, indicating that the simulation is reliable. \begin{table}
\begin{tabular}{c c c c c c c} \hline  & _E/kPa_ & _μ_ & \(e\) & _K/(m/Day)_ & _C/kPa_ & _\(\varphi\)_ \\ \hline Plain fall & 8000 & 0.2 & 0.9 & 30 & 0 & 15 \\ Silty clay & 5671 & 0.33 & 0.718 & 0.05 & 8.2 & 12 \\ Sand & 6070 & 0.33 & 0.5 & 0.05 & 13.9 & 12.5 \\ Strongly weathered tuff & 20,000 & 0.3 & 0.8 & 5.1 & 3.0 & 30 \\ Moderately weathered tuff & 50,000 & 0.25 & 0.8 & 1.728 & 3000 & 45 \\ Slightly weathered tuff & 5.00 \(\times\) 10\({}^{6}\) & 0.22 & 0.09 & 0.026 & 11,500 & 55 \\ C\({}_{25}\) concrete & 2.20 \(\times\) 10\({}^{7}\) & 0.2 & 0.01 & 8.64 \(\times\) 10\({}^{-5}\) & 20,000 & 60 \\ \hline \end{tabular}
\end{table} 
Table 3: The empirical parameters of concrete. Figure 10: Settlement of the ground. Before excavation, many monitoring points were set up at the surface. The distribution of monitoring points near the disaster area is shown in Figure 12. As can be seen from the figure, there are three groups of monitoring points located in the collapse area, namely ZDBC67, ZDBC68 and ZDBC69. The surface settlement of these three groups of monitoring points 7 days before the collapse is shown in Figure 13. From the figure, it can be seen that the collapsed area had a large settlement before the accident, and the maximum value of surface settlement was about 30 mm at ZDBC69-1. In particular, ZDBC67-2 showed a significant settlement abruptness with a deformation rate of 4.50 mm/d on 23 May. Corresponding to the locations of the three groups of monitoring points, the surface deformation data of the model during the excavation from ZDK25+355 to the collapse zone were extracted, and the results are shown in Figure 14. It can be seen from the figure that when excavated to ZSK25+355, i.e., at the intersection of strongly weathered tuff and slightly weathered tuff, the surface settlement was already large, and the maximum surface settlement was about 28 mm at ZDBC69-1. As the tunnel was excavated, ground deformation gradually increased. When excavated to the collapse zone, the maximum value of surface deformation was about 30 mm, which reached the warning value.