## 4 Results
### Results of Mix Proportion Optimization by PSO  
Figure 8 shows the PSO optimization process of mix proportions of different wet-mix shotcrete. It can be seen that all the fitness curves converge within 61 iterations, indicating that PSO is efficient in finding the optimal mix proportions of wet-mix shotcrete. The consumed time for PSO optimization is shown in Figure 9. According to Figure 9, it took PSO less than 60 s to obtain the optimal mix proportion of wet-mix shotcrete corresponding to one type of concrete mark required by the mine. Compared with the traditional experimental optimization methods, the intelligent optimization method saves a lot of time and greatly improves the efficiency of mix proportion optimization.  
After the PSO optimization process, the optimal mix proportions of wet-mix shotcrete corresponding to three different types of concrete mark are tabulated in Table 3. Considering the convenience of field construction, the parameters of mixing proportions should be rounded. To verify whether the above optimal mix proportions of wet-mix shotcrete are reasonable, the physical experiments of 28-day UCS tests were carried out. The comparisons between experimental values and predicted values are shown in Figure 10. As shown, the maximum error is 0.6 MPa and the minimum error is 0.1 MPa, which indicates that the PSO optimization model is ideal, and the strength redundancy was properly avoided.  
Figure 7: The evaluation of predicted performance of ANN model: (**a**) comparison of experimental UCS and predicted UCS; (**b**) regression between experimental UCS and predicted UCS; (**c**) error distribution.  
Figure 8: The fitness curves during the PSO iteration.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & \multicolumn{4}{c}{**Optimal Mix Proportions**} & \multicolumn{4}{c}{**The Bounded Optimal Mixing Proportions**} \\ \cline{2-9}
**Types of** & \multicolumn{2}{c}{**Cement**} & \multicolumn{1}{c}{**Water–Came**} & \multicolumn{1}{c}{**Coarse**} & \multicolumn{1}{c}{**Reducer Content**} & \multicolumn{1}{c}{**Water–Came**} & \multicolumn{1}{c}{**Sand–Course**} & \multicolumn{1}{c}{**Water–Reducer**} \\
**Shatcrete** & \multicolumn{1}{c}{**Content**} & \multicolumn{1}{c}{**Ratio**} & \multicolumn{1}{c}{**Aggregate**} & \multicolumn{1}{c}{**Content**} & \multicolumn{1}{c}{**Ratio**} & \multicolumn{1}{c}{**(kg\(\cdot\)m\({}^{-3}\))**} & \multicolumn{1}{c}{**Ratio**} & \multicolumn{1}{c}{**Aggregate**} & \multicolumn{1}{c}{**Content**} \\ \hline C20 & 316.228 & 0.700 & 7.338 & 0.600 & 316 & 0.70 & 7.3 & 0.6 \\ C25 & 374.234 & 0.420 & 7.463 & 0.600 & 374 & 0.42 & 7.5 & 0.6 \\ C30 & 418.539 & 0.420 & 5.552 & 0.600 & 419 & 0.42 & 5.6 & 0.6 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Optimal mix proportions of wet-mix shotcrete.  
Figure 9: The consumed time of PSO optimization.  
On the other hand, it is necessary to test the fluidity of wet-mix shotcrete corresponding to above optimal mix proportions, so as to avoid the fact that--although it meets the requirements of mechanical properties--the fluidity is very poor, resulting in pipe blockage. Therefore, the slump test experiments of wet-mix shotcrete were carried out and the results are shown in Figure 11. The results show that the wet shotcrete slurry with corresponding optimum mix proportions has excellent flow performance and meets all mine flow standards (slurry slump is within 150-210 mm). The performance of UCS and slump values show that the optimal mix proportions of wet-mix shotcrete are satisfactory.  
Moreover, good performance had been obtained by using those optimal mix proportions of wet-mix shotcrete in the field engineering application experiments. Figure 12 shows the underground engineering experiments in the Jinchuan mine.  
Figure 11: The slurry slump of wet-mix shotcrete.  
Figure 10: Comparison of experimental UCS and predicted UCS.