## 4 Conclusions  
For the purpose of the sensitivity analysis of many parameters for the response of the mechanical characteristics of tunnel construction in karst areas, 11 kinds of influencing factors related to the mechanical characteristics of tunnel construction in karst areas as well as the levels were confirmed, and the orthogonal test design method was applied to scientifically establish the orthogonal test table of 11 factors and 3 levels for karst tunnels, and 27 three-dimensional finite-element numerical models of karst tunnels were established. Three original databases for sensitivity analyses of engineering parameters should be formed with the vertical displacement, the first principal stress, and the principal  
Figure 11: Comparative bar chart of best combinations: (**a**) vertical displacement; (**b**) S1; (**c**) S1–S3. stress difference as target variables, respectively. Comprehensively adopting the two methods of orthogonal experimental design method and a random forest algorithm, these \(11\) influencing factors were ranked hierarchically according to the degree of sensitivity from the highest to the lowest, and the optimal level of each influencing factor was selected. The research conclusions have been shown below:  
(1) To ensure the representativeness and generality of the tunnel measurement points, the arch vault and the arch bottom 30 m away from the tunnel entrance were selected as the tunnel measurement points, with vertical displacement as the target variable. With the first principal stress as the target variable, the location of the tunnel haunch at a distance of 30 m away from the tunnel entrance was chosen as the measurement point. Taking the principal stress difference as the target variable, one of the four points on both sides of the tunnel, such as the spandrel and the skewback of the tunnels, 30 m away from the tunnel entrance, was selected as the tunnel measurement point;  
(2) Taking vertical displacement as the target variable, the parameters with the highest feature importance were A6 (tunnel diameter) and A10 (tunnel buried depth). The random forest algorithm could satisfactorily avoid the misjudgement of the true dispersion degree of data caused by data outliers. Taking the first principal stress as the target variable, the most important influencing factors were A10 (tunnel buried depth) and A9 (location of the karst cave). In the analysis of variance (ANOVA), the results observed in A10 (tunnel buried depth) were statistically significant. Taking the principal stress difference as the target variable, the most important influencing factors were A10 (tunnel buried depth) and A6 (tunnel diameter). In the analysis of variance (ANOVA), the results observed in A10 (tunnel buried depth) were statistically significant;  
(3) On the basis of the random forest algorithm and the range analysis method, the optimal combination of each influencing factor for tunnel construction in karst area was derived. Taking vertical displacement as the output variable, the amount of vertical displacement output from Case B2 was 42.99% smaller than that output from Case B1. With the first principal stress as the output variable, the value of the first principal stress output for Case B3 was 17.54% smaller than that output for Case B1. Taking the principal stress difference as the output variable, the principal stress difference output for Case B4 was 30% smaller than that output for Case B1. Nevertheless, after comparing the calculation results of the three output variables: the vertical displacement, first principal stress, and principal stress difference, the optimal level combination of 11 factors influencing the safe construction of tunnels in karst areas with the principal stress difference as the target variable is more balanced. The research results have selected the key parameters for the construction of karst tunnel engineering and laid a theoretical foundation for the optimization of the construction parameters of karst tunnel engineering. Conceptualization, B.W. and W.S. ; methodology, B.W.