## 4 Seepage Control System in the Xigou Tailings Dam with an Increased Height
### Numerical Results  
#### 4.4.1 Performance Estimation of the Seepage-Proof System  
In this section, the performance assessment of the proposed seepage control system after the height of the dam was increased was described under extreme operating conditions using the previously proposed method. Under this condition, the water level of the tailings impoundment was 1029.0 m \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Parameters**} & \multirow{2}{*}{**Permeability Coefficient (cm/s)**} & \multicolumn{2}{c}{**Corrected Permeability**} \\  & & & \multicolumn{2}{c}{**Coefficient (cm/s)**} \\ \cline{2-5}  & **Horizontal \(\mathbf{k_{x}}\)** & **Vertical \(\mathbf{k_{y}}\)** & **Horizontal \(\mathbf{k_{x}}\)** & **Vertical \(\mathbf{k_{y}}\)** \\ \hline Moderately weathered crystal tuff & \(2.3\times 10^{-7}\) & \(2.1\times 10^{-7}\) & \(2.5\times 10^{-7}\) & \(2.3\times 10^{-7}\) \\ Strongly weathered crystal tuff & \(5.7\times 10^{-5}\) & \(4.8\times 10^{-5}\) & \(5.7\times 10^{-5}\) & \(4.8\times 10^{-5}\) \\ Initial dam & \(3.3\times 10^{-3}\) & \(3.3\times 10^{-3}\) & \(1.5\times 10^{-3}\) & \(2.1\times 10^{-3}\) \\ Silty sand tailings & \(5.0\times 10^{-4}\) & \(4.5\times 10^{-4}\) & \(5.1\times 10^{-4}\) & \(4.3\times 10^{-4}\) \\ Silty soil tailings & \(8.0\times 10^{-5}\) & \(6.4\times 10^{-5}\) & \(8.2\times 10^{-5}\) & \(7.1\times 10^{-5}\) \\ Silty clay & \(1.4\times 10^{-5}\) & \(1.2\times 10^{-5}\) & \(3.2\times 10^{-5}\) & \(2.5\times 10^{-5}\) \\ Artificial clay I & \(5.1\times 10^{-5}\) & \(5.1\times 10^{-5}\) & \(5.1\times 10^{-5}\) & \(5.1\times 10^{-5}\) \\ Artificial clay II & \(3.3\times 10^{-3}\) & \(3.1\times 10^{-3}\) & \(5.0\times 10^{-2}\) & \(4.3\times 10^{-3}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The comparison between the measured and calculated permeability coefficients. Figure 10: The comparison between the measured and calculated values of the water head at the boreholes. from the flood prevention calculation. With the calculation of the finite element model, the distribution of the water head and the free surface of the typical cross-section were compared under the two conditions of setting drainage and without drainage, as shown in Figure 11. Initially, no additional seepage control structure was set up during operation. The distribution of the water head changed along the axis of the dam, and the equipotential line was relatively sparse upstream of the dam, while the equipotential line was highly dense near the junction of the heaping dam and the initial dam. The phreatic surface was smooth in the upstream tailing zone, but it was out of the dam slope downstream. When it reached the initial dam, the phreatic line showed a significant depression due to the high-permeability rockfill material, which then traveled downstream in a nearly horizontal state. The predicted maximum seepage gradient was 1.50, which appeared at the junction of the heaping dam and initial dam. To decrease the phreatic surface, the new seepage control system was arranged in the process of increasing the dam height. The predicted seepage field of the finite element simulation is illustrated. Figure 11 plots the phreatic line and pressure head distribution of the whole seepage field (with the real line representation). Clearly, the phreatic line was reduced, the variation in the water head was gentle, and the free surface no longer escaped from the downstream slope. By calculating the seepage gradient, the maximum seepage gradient decreased from 1.50 to 0.83, which occurred at the same location as in the system without drainage. Figure 12 presents the three-dimensional free surface and the contour map of the groundwater level distribution and shows that due to the sufficient seepage-proof system, the distribution of the seepage flow field was more uniform in the tailings area, and the overall fluidization was smoother compared with the absence of the drainage system.