## 3 Evaluation System and Grade Classification  
### Evaluation Indices and Evaluation System  
Generally, water intrush is caused by many complex risk factors. Additionally, in the actual project, the factors interact with each other. The selection of indicators should be based on engineering examples and previous research to select the most appropriate indicators for the evaluation system. Additionally, each indicator can reflect the risk characteristics of the water intrush. In this paper, the literature related to water intrush was analyzed, 40 different evaluation systems were found, and the occurrence times of the risk factors were counted. The literature data were collected and obtained by searching for the keyword karst tunnel water intrush evaluation in the Web of Science database [23; 24; 25; 26; 27; 28; 29]. For example, according to statistics, the number of occurrence times of the groundwater index is 26. After calculation, the percentage of the groundwater index is 65%. Other percentages of factors are calculated similarly. The results are shown in Figure 7.  
Figure 7 shows that some influencing factors account for more than 50% of the total. They are selected for the indicators of the tunnel water inrush evaluation system in this paper. Through systematic summary and case studies, a tunnel water inrush evaluation system is established, which consists of seven evaluation indicators, namely, formation lithology (I\({}_{1}\)), unfavorable geological conditions (I\({}_{2}\)), attitude of rock formation (I\({}_{3}\)), landform and physiognomy (I\({}_{4}\)), contact zones of dissolvable and insoluble rock (I\({}_{5}\)), layer and interlayer fissures (I\({}_{6}\)), and groundwater level (I\({}_{7}\)). (1) Formation lithology (I\({}_{1}\))  
It is the primary elements that affect the water inrush. Water inrush reflects the physical, chemical, and structural characteristics of rock. Rock solubility is the basis of karst geology formation. The higher the solubility of rocks in the tunnel and the thicker the rock layer, the better the development of the karst. It is described by parameter t [30] and is defined as:  
\[t=\sum_{i=1}^{3}A_{i}B_{i} \tag{25}\]  
where \(Ai\) and \(Bi\) are the weight and strength of the solubility lithologies, respectively. They can be found in Table 3. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Solubility Lithologies**} & \multirow{2}{*}{**Weight**} & \multicolumn{5}{c}{**Percentage of Soluble Rocks (\%)**} \\ \cline{3-6}  & & **0\(-\)20** & **20\(-\)40** & **40\(-\)60** & **\textgreater{}60** \\ \hline Weak & A\({}_{1}\) = 0.105 & IV & IV & III & III \\ Medium & A\({}_{2}\) = 0.259 & III & III & II & II \\ Strong & A\({}_{3}\) = 0.636 & II & II & I & I \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Lithology class classification. Figure 7: Statistics of influencing factors of water inrush. Unfavorable geological conditions (I\({}_{2}\))  
It is closely related to the source of the water inrush, such as underground rivers, dissolution caves, water bearing structures, etc. These areas are prone to water accumulation and the flow of water between unfavorable geological structures increases the probability of water inrush. Attitude of rock formation (I\({}_{3}\))  
It will affect the development of groundwater. In this paper, the attitude of rock formation is expressed by the strata inclination. The strata inclination (Table 4) has been modified for easy calculation [31]. Landform and physiognomy (I\({}_{4}\))  
The topography and geomorphology reflect the undulation of the landscape above the tunnel. The tunnel water inrush potential is different under different geographical and geometric conditions. If the terrain above the tunnel is undulating and has significant depressions, water rush is more likely to occur. Conversely, the likelihood is lower [32]. As the connection channel of groundwater, karst depression and precipitation leakage have a great influence on underground karst systems.