## 3 Early Warning Index System
### Early Warning Index System  
Ten experts in the field of subway project management were interviewed to construct the early warning index system for the construction safety risk of subway stations. Basic information about the ten experts is reported in Table 1.  
The ten experts who participated in the interview were all from units related to subway station projects, which indicates they had a good professional background. Their average number of working years was 22.5 years, and nine experts had senior professional titles, which indicates that they had rich experience. The average number of subway station projects attended by the ten experts was 55.7, which also demonstrates that they had rich experience in the project management of subway stations.  
By combining the various impact factors described in Section 3.1, the interview results of the ten experts, and previous research results [25; 26], the early warning index system for the construction safety risk was designed and presented in Table 2.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**No.** & **Position** & \
\begin{tabular}{c} **Length of** \\ **Work Years** \\ \end{tabular} & **Title** &
\begin{tabular}{c} **Number of Subway** \\ **Projects Involved** \\ \end{tabular} \\ \hline (1) & Contractor & 16 & Senior engineer & 47 \\ (2) & Contractor & 21 & Senior engineer & 38 \\ (3) & Contractor & 28 & Senior engineer & 27 \\ (4) & Contractor & 25 & Senior engineer & 30 \\ (5) & Contractor & 10 & Senior engineer & 35 \\ (6) & Design & 35 & Senior engineer & 147 \\ (7) & Design & 38 & Senior engineer & 205 \\ (8) & Government & 12 & Senior engineer & 7 \\ (9) & Government & 5 & Intermediate engineer & 5 \\ (10) & Academy & 35 & Professor & 16 \\ \hline \hline \end{tabular}
\end{table}
Table 1: The information about the ten experts.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Primary Index** & **Secondary Index** & **Unit** \\ \hline \hline \multirow{4}{*}{\(R_{11}\): Rate of operation violation} & \% \\  & \(R_{12}\): Rate of technical failure & \% \\  & \(R_{13}\): Emergency handling & - \\  & \(R_{14}\): Rate of monitoring error & \% \\  & \(R_{15}\): Proportion of old workers & \% \\ \hline \multirow{4}{*}{\(R_{2}\): Machine} & \(R_{21}\): Rate of mechanical quality failure & \% \\  & \(R_{22}\): Rate of mechanical installation failure & \% \\  & \(R_{23}\): Rate of mechanical maintenance failure & \% \\ \hline \multirow{4}{*}{\(R_{3}\): Material} & \(R_{31}\): Qualified rate of concrete & \% \\  & \(R_{32}\): Qualified rate of steel & \% \\  & \(R_{33}\): Rate of material supply & \% \\  & \(R_{34}\): Rate of material stacking error & \% \\ \hline \multirow{4}{*}{\(R_{4}\): Environment} & \(R_{41}\): Maximum deformation of foundation pit & mm \\  & \(R_{42}\): 24-h maximum rainfall & mm \\ \cline{1-1}  & \(R_{43}\): Poor geological conditions & - \\ \cline{1-1}  & \(R_{44}\): Poor geomorphic conditions & - \\ \cline{1-1}  & \(R_{45}\): Extreme weather conditions & - \\ \hline \multirow{4}{*}{\(R_{5}\): Management} & \(R_{51}\): Efficiency of communication & - \\ \cline{1-1}  & \(R_{52}\): Team cohesion & - \\ \cline{1-1}  & \(R_{53}\): Rate of personnel change & \% \\ \cline{1-1}  & \(R_{54}\): Rationality of organization & - \\ \hline \hline \end{tabular}
\end{table}
Table 2: The early warning index system.  
The index system shown in Table 2 is hierarchical and includes five primary indexes and 21 secondary indexes. \(R_{43}\), \(R_{44}\), \(R_{45}\), \(R_{51}\), \(R_{52}\), and \(R_{53}\) are qualitative indexes, which refer to data obtained by qualitative methods. Other indicators are quantitative indicators, which refer to the data obtained through quantitative methods. \(R_{13}\), \(R_{31}\), \(R_{32}\), \(R_{33}\), \(R_{45}\), \(R_{52}\), \(R_{53}\), and \(R_{54}\) are cost indexes, which refer to those for which the lower the index score, the higher the security risk warning level. Other indicators are benefit indexes, which refer to those for which the higher the index score, the higher the security risk warning level.