# Numerical Simulation of Seepage Surface and Analysis of
## 3 Results
### Analysis of Controlled Position of the Phreatic Line  
The controlled position of the phreatic line meets both the requirement on the criticized position of the phreatic line and the requirement on the minimum buried depth of the phreatic line on the downstream slope of the embankment. When the height of the tailings dam reached 210 m and the storage capacity was less than 100 million cubic meters, according to 3.3.1 of the \"Code for Design of Tailings Facilities\" (GB50863-2013), the tailings dam was classified as a second-class storage, and the stability of the controlled position of the phreatic line was executed according to the second-class storage standard. The minimum safety factor for anti-sliding stability of dam slopes required by the specifications is shown in Table 6.  
Three fitting phreatic lines were selected for calculating the controlled position of the phreatic line, with burial depths of 15 m, 20 m, and 25 m. The control depth was based on the phreatic line depth when the safety factor of the dam body's anti-sliding stability was slightly greater than the standard value. Taking the stability calculation of Section A-A and Section B-B as examples (the distribution of profile positions is shown in Figure 1), the stability test of the dam body was conducted under normal water-level conditions for different burial depths of the phreatic line. The computational models for Section A-A and Section B-B are shown in Figure 10 and the test results are shown in Figure 11. In Figure 11, the blue solid line represents the factor of safety calculated using the Bishop method and the orange solid line represents the factor of safety calculated using the ordinary method. The blue dashed line represents the minimum factor of safety required in the code for the Bishop method and the orange dashed line represents the minimum factor of safety required in the code for the ordinary method. When the solid line is above the dashed line, it indicates that the dam could meet the stability requirements, and when the solid line is below the dashed line, it indicates that the dam does not meet the stability requirements under this controlled position of the phreatic line. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Calculation Method**} & \multirow{2}{*}{
\begin{tabular}{c} **Operating** \\ **Condition** \\ \end{tabular} } & \multicolumn{4}{c}{**Grade of Dam**} \\ \cline{3-6}  & & **1** & **2** & **3** & **4, 5** \\ \hline Simplified Bishop & & & & & \\ method & Normal operation & 1.50 & 1.35 & 1.30 & 1.25 \\ Ordinary method & Normal operation & 1.30 & 1.25 & 1.20 & 1.15 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: The minimum safety factor for anti-sliding stability of dam slopes. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Operating Mode**} & \multirow{2}{*}{**Position**} & \begin{tabular}{c} **Critical Seepage** \\ **Gradient** \\ \end{tabular} & \
\begin{tabular}{c} **Calculated** \\ **Seepage Gradient** \\ \end{tabular} &
\begin{tabular}{c} **Analysis** \\ **Results** \\ \end{tabular} \\ \hline \multirow{2}{*}{**Normal water-level conditions**} & At the foot of the & & & \\ starter dam slope & & 1.02 & 0.679 & Steady \\ \multirow{2}{*}{**Flood-level conditions**} & At the foot of the & & 1.02 & 0.690 & Steady \\ \multirow{2}{*}{**Flood-level conditions**} & & & & \\ starter dam slope & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Results of seepage stability analysis. According to the analysis results, it can be seen that:  
1. For section A-A, when the controlled position of the phreatic line was at a buried depth of 20 m, the ordinary method calculation result under normal operation was 1.265 and was greater than the minimum safety factor specified of 1.250, while the simplified Bishop method calculation result was 1.406 and greater than the minimum safety factor specified of 1.350.