# Instability Risk Assessment for Deep Excavation of Soil-Rock Combinations Containing Groundwater
## 3 Case Study
### Exavation Risk Assessment
#### 3.2.1 Evaluation Index System for Excavation Instability Risk
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Guideline Layer** & **Potential Risk Factors** & **UR** & **Judgements** \\ \hline  & Soil stress C\({}_{11}\) & 0.9 & Fit \\  & Microseismic grade C\({}_{12}\) & 0.81 & Fit \\ The environment in the excavation & Weak rock thickness C\({}_{13}\) & 0.67 & Fit \\  & Microseismic frequency C\({}_{14}\) & 0.67 & Fit \\  & Special strata C\({}_{15}\) & 0.3 & Unfit \\ \hline  & Thickness of overburden C\({}_{21}\) & 0.9 & Fit \\ The environment around the excavation & Deformation of neighboring buildings C\({}_{22}\) & 0.81 & Fit \\  & Surface settlement C\({}_{23}\) & 0.7 & Fit \\  & Water table C\({}_{24}\) & 0.67 & Fit \\ \hline  & Vertical deformation C\({}_{31}\) & 0.81 & Fit \\ The excavation support structure & Transverse deformation C\({}_{32}\) & 0.81 & Fit \\  & Anchor cable axial force C\({}_{33}\) & 0.83 & Fit \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Potential risk factors and UR values. then the entropy value and entropy weight value of each index factor were obtained. The subjective weights and objective weights were integrated and weighted to determine the integrated weights of each indicator factor. It can be seen from Table 4 that the comprehensive weights of the 11 potential risk factors were all greater than 0.1; therefore, they were selected as the risk evaluation index, as shown in Table 5.  
In Table 5, the risk levels are graded based on the different evaluation criteria. First, based on the control criteria defined in the code [47; 48], the maximum (minimum) value of the class IV risk was determined. Then, the maximum (minimum) value was subdivided into different ranges with the assistance of the experts, to correspond to different risk levels. In addition, appropriate management measures were set up based on the established evaluation criteria, as shown in Table 6. Finally, the risk evaluation system for deep and large excavation with soil-rock combination was established, as shown in Figure 7.  
Grey relational analysis was used to calculate the correlation between each risk index and the risk level, so as to verify the rationality of its selection. Taking the risk level as the parent series and the values of quantitative indexes as the sub-sequence, after the indexes were made dimensionless, the gray correlation coefficients between the sub-sequence and the parameters of the parent series were calculated as shown in Table 7. The results show that the correlation coefficient of each sub-sequence risk factor index was more than 0.6, which had a strong correlation with the risk level of the parent sequence, so the risk evaluation system for deep and large excavation with soil-rock combination had been selected reasonably. \begin{table}
\begin{tabular}{c c} \hline
**Risk Level** & **Management Measures** \\ \hline I & Continue construction and perform daily monitoring. \\ \hline II & Construction continues with more frequent monitoring and prohibit construction overruns. \\ \hline III & Construction is suspended with monitoring, while meetings are held to investigate the cause and prepare contingency measures. \\ \hline IV & Stop construction and all personnel leave, while analyzing the cause, make an early warning and develop measures according to the emergency plan. \\ \hline \end{tabular}
\end{table} 
Table 6: Risk classification of excavation.