## 4 Engineering Case Analysis
### Determination of Safety Risk Level  
The evaluation team conducted on-site field surveys of the surveyed tunnels and communicated and exchanged information with safety managers from relevant organizations. They gained an understanding of the segments within the evaluated tunnels that are prone to accidents and the current state of operation and management. Additionally, they  
collected various materials, including design drawings, management systems, emergency plans, structural condition management records, and maintenance and rectification records. After categorizing and organizing the above data, specific scores for each indicator in the risk assessment index system for each evaluated object were determined. The compiled indicator assessment scores are presented in Table 6. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Criteria Layer**} & \multicolumn{2}{c}{**Indicator Layer**} & \multicolumn{2}{c}{**Indicator Value**} \\ \cline{2-7}  & **Entrance Section** & **Middle Section** & **Method** & **Entrance Section** & **Middle Section** & **Exit Section** \\ \hline \hline \multirow{7}{*}{Tunnel condition} & Skid resistance performance of road surfaces \(U_{11}\) & Technical condition of road surfaces \(U_{11}\) & Information search and on-site inspection & 64.6 & 87 & 68.2 \\ \cline{2-7}  & Tunnel portal geometry & Minimum radius of planar curve \(U_{12}\) & Information search & 148 & 1500 & 140 \\ \cline{2-7}  & Maximum slope \(U_{13}\) & Maximum slope \(U_{13}\) & Information search & 0.5 & 0.5 & 0.5 \\ \cline{2-7}  & Civil engineering structural techniques \(U_{14}\) & Civil engineering structural techniques \(U_{14}\) & Information search and on-site inspection & 78.5 & 78.7 & 80.0 \\ \cline{2-7}  & Electromechanical facilities technical condition \(U_{15}\) & Electromechanical facilities technical condition \(U_{15}\) & Information search and on-site inspection & 98 & 98 & 98 \\ \cline{2-7}  & Technical condition of other facilities \(U_{16}\) & Technical condition of other facilities \(U_{16}\) & Information search and on-site inspection & 86 & 86 & 86 \\ \cline{2-7}  & & Cross passages and emergency stopping lanes \(U_{17}\) & Information search & - & 90 & - \\ \cline{2-7}  & & Tunnel length \(U_{18}\) & & & 1392 & \\ \hline \multirow{7}{*}{Traffic characteristics} & Operating speed \(U_{21}\) & Operating speed \(U_{21}\) & On-site testing & 18.9 & 4.8 & 7.7 \\ \cline{2-7}  & Coordinated nature of operating speeds \(U_{22}\) & Coefficient of variation of vehicle speed \(U_{22}\) & On-site testing and model prediction & 13.2 & 0.11 & 2.9 \\ \cline{2-7}  & Service level \(U_{23}\) & Service level \(U_{23}\) & Information search & 0.44 & 0.44 & 0.44 \\ \cline{2-7}  & Proportion of large vehicles \(U_{24}\) & Proportion of large vehicles \(U_{24}\) & Information search & 6.04\% & 6.04\% & 6.04\% \\ \cline{2-7}  & Overload ratio \(U_{25}\) & Overload ratio \(U_{25}\) & Information search & 2\% & 2\% & 2\% \\ \hline \multirow{7}{*}{Operational environment} & \multirow{2}{*}{CO concentration \(U_{31}\)} & Information search and on-site inspection & 47 & 62 & 53 \\ \cline{2-7}  & Visibility \(U_{32}\) & Visibility \(U_{32}\) & Information search and on-site inspection & 5.2 & 6.0 & 5.2 \\ \cline{2-7}  & & & Information search and on-site inspection & 11.7 & 5.5 & 11.0 \\ \cline{2-7}  & & & Information search and on-site inspection & 81 & 93 & 83 \\ \cline{2-7}  & Proportion of adverse weather \(U_{35}\) & Proportion of adverse weather \(U_{35}\) & Information search & 47.4\% & 47.4\% & 47.4\% \\ \cline{2-7}  & & & Information search & & & \\ \cline{2-7}  & & & Information search and on-site inspection & 47.4\% & 47.4\% & 47.4\% \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Evaluation indicator scores for Sijiao’an Tunnel. The individual indicator membership degrees to each level were determined using Equations (13)-(17).