## 4 Seepage Control System in the Xigou Tailings Dam with an Increased Height
### _Pureatic Surface of the Tailings Dam_  
To obtain the initial hydraulic properties and the phreatic surface of the tailings dam, 16 boreholes were arranged at the dam site (with the locations of the boreholes as shown in Figure 6. According to the geological engineering survey results, the initial permeability coefficients of each material are shown in Table 2, as the permeability coefficients of the tailings materials and foundation are different from depths to depths and from locations to locations [32]. A limitation of the borehole tests is that only part of the region's permeability coefficients is estimated [33]. The borehole testing results could not be used to calculate a numerical model. To obtain the precise permeability coefficients used for calculations, the MPSO-BP method and the in-house made finite element code were used to back-calculate the initial seepage field [34].  
Along the main channel, 16 piezometers were installed to continuously measure the phreatic surface and its range of variation. Using the observation wells in the boreholes, the steady-state groundwater level was measured during the exploration of the Xigou tailings dam. Figure 3 presents the location of the phreatic line at the typical cross-section. The groundwater level reflected the normal conditions of the tailings dam and was used as the initial seepage field because the seepage control system was designed according to this condition. The distribution of the water head of the initial seepage field could be used for back analysis. The basic idea of the MPSO-BP method is that the difference between the measured water heads at the boreholes and the fitness water heads is minimized. The objective function can be written as  
\[\text{min}f=\sum_{i=1}^{n}w_{i}(h_{i}^{c}-h_{i}^{m})^{2} \tag{6}\]  
where \(n\) is the total number of piezometers, \(w_{i}\) is the weight at the \(i\)th borehole, and \(h_{i}^{c}\) and \(h_{i}^{m}\) are the calculated and measured water heads at the \(i\)th borehole, respectively.  
Figure 9: The construction of a drainage substructure.  
The iterative process of the back-calculation was implemented with the MPSO-BP method based on a modified particle swarm optimization algorithm and a neural network simulator. The algorithm parameters were as follows: the maximum iterations is 1000, the number of particles is 20, weight function \(\omega\) = 0.4-0.9, \(c_{1}\) = 3.3, and \(c_{2}\) = 1.7. The back-analysis results showed that when the permeability coefficients of all the materials at the site were taken in Table 2, the value of the objective function was minimized at all observation boreholes. Figure 10 compares the calculated and measured groundwater levels at each borehole, indicating that the initial seepage field was reasonably simulated using the back-calculated permeability coefficients. The range of error could be controlled to below 5% between the calculated and measured phreatic surface, which also verified the rationality of the inverse results. Therefore, the back-calculated permeability coefficients were acceptable for a further performance assessment of the seepage.