## 5 Application Case
### Results of the Simulation
#### 5.3.3 Permeability Coefficient of the Impervious Curtain  
Considering the excellent results of the PSO simulation in terms of head and leakage volume, the change in curtain permeability coefficient by PSO was analyzed to justify the simulation further. Figure 7a demonstrates the variation in curtain porosity for leaching times of 25, 50, and 100 years, respectively. The contour distribution of porosity shows that the calcium leaching effect leads to greater porosity on the upstream side of the curtain than on the downstream side and greater porosity in the upper part than in the middle and lower parts. The difference in porosity distribution intensifies with increasing leaching time. Figure 7b shows the porosity evolution at the height of 63 m for leaching times of 25, 50, and 100 years. After a century of leaching time, the porosity on the upstream side of the curtain is 0.206, with an increase of 0.056 compared to the porosity at the initial moment. The porosity on the downstream side is 0.170, increasing 0.020 in comparison with the porosity initially. Figure 7c presents the porosity distribution at different elevations of the curtain (at the elevation of 54 m, 63 m, and 72 m) after 50 years of leaching time. There is little difference in porosity distribution at the elevation of 54 m and 63 m. The upstream and downstream side porosity at the elevation of 72 m is 0.018 greater than the other two locations. The higher the altitude, the more significant the difference in the porosity distribution.  
Figure 8a shows the distribution of the curtain permeability coefficient at 25, 50, and 100 years of leaching time, respectively. Similar to porosity, the permeability coefficient on the upper and upstream sides of the curtain increases significantly compared to other areas. The distribution difference of different parts is more evident with the increase of leaching time. Figure 8b shows the influence of different leaching times on the distribution of the curtain permeability coefficient at the altitude of 63 m. After 100 years of evolution, the permeability coefficient of the upstream side of the curtain is 2.60 \(\times\) 10\({}^{-6}\), two orders of magnitude higher than the initial permeability coefficient. The permeability coefficient on the downstream side of the curtain is 2.72 \(\times\) 10\({}^{-7}\), with an increase in one order of magnitude. Figure 8c presents the distribution of the curtain permeability coefficient at different elevations after the leaching time of 50 years. The permeability coefficient of the upstream side of the curtain at the elevation of 72 m is 1.45 \(\times\) 10\({}^{-6}\), while that of the downstream side is 3.45 \(\times\) 10\({}^{-7}\). The permeability coefficient at the altitude of 72 m is almost half an order of magnitude higher than the other two elevations. The higher the elevation, the more pronounced the increase in the permeability coefficient of the impervious curtain.  
Figure 7: Impervious curtain porosity evolution in the leaching process: (**a**) porosity distribution; (**b**) porosity evolution at 63 m elevation in a century; (**c**) porosity evolution of three positions.