# Numerical Simulation of Seepage Surface and Analysis of
## 4 Analysis and Discussion  
The best solutions for the permeability coefficients of the tailings dams are presented in Table 4 and, comparing them with the permeability parameters obtained from the tests in Table 2, it can be observed that the fitted tailings permeability coefficients were all greater than the values of the indoor tests. The main reason for this situation was that the role of tailings impoundment drainage measures has been equivalently reflected in the best-fit model permeability coefficients. During the back analysis, it was found that although the inversion analysis model could equivalently reflect the effect of the exfiltration measures, the location of the fitted phreatic line was still somewhat higher than the actual phreatic line depth. On the other hand, it suggested that the use of equivalent permeability coefficients for calculations was on the safe side. The permeability coefficient was comprehensively considered as the engineering site conditions and could be used for three-dimensional seepage analysis of a tailings dam. The analysis results are consistent with the conclusions of reference [41; 42]. Therefore, the drilling data and exploration results should be used as much as possible so that the established finite element model can contain more real measurement information, which is especially important in the inverse analysis. In the process of establishing a finite element model, modeling should be based on a realistic situation, but appropriate model simplification is also necessary. The model generated by this software was consistent with the actual terrain and the calculated values were in good agreement with the measured values, which is consistent with the conclusions of references [23; 32]. Under both normal water-level and flood-level conditions, the phreatic surface did not intersect with the slopes of the initial dam and the accumulation dam, and seepage would not overflow from the dam slope. The results are consistent with the reference, indicating that the permeability coefficient of the starter dam is crucial for the stability of the tailings dam. Based on the actual measured data of the phreatic line, it can be concluded that under the existing drainage measures, the depth of the infiltration line can generally be maintained above 25 m. In the later stage, the control phreatic line of the tailings dam that will be required is 23 m. It is preliminarily judged that the requirements could be met with corresponding measures taken. When the tailings dam was raised to 210 m, the calculated phreatic line was 23-28 m, and the control phreatic line was required to be greater than 23 m, which could meet the requirement but the richness was not big. Therefore, the seepage discharge facilities in the reservoir area should be strengthened during the heightening process to ensure the safe operation of the tailings dam in the later stage. Under the flood-level condition, the distribution pattern of the phreatic line is similar to that of the normal water-level condition. However, due to the relatively high water level, the location of the phreatic surface of the dam is elevated compared to that of the normal water-level condition. The elevation of the phreatic line in the left dam abutment area is relatively obvious, and the calculation results are consistent with reference [14], with both showing the elevation of the phreatic line due to the influence of the mountain. Therefore, the area of 1220 m to 1250 m a.s.l. on the left bank shoulder was the key area for the control of the phreatic line, and the seepage discharge measures in this area should be further increased, and local horizontal drainage methods can be used to control the seepage line [27]. The main causes of localized phreatic line elevation are summarized below:  
1. The seepage from three valleys was superimposed in the common area of the three valleys in the tailings dam; therefore, the seepage amount in this area was larger than that in other areas. 2.