# Numerical Simulation of Seepage Surface and Analysis of
## 2 Materials and Methods  
### Basic Information about Tailings Dam  
A tailings dam is an upstream valley tailings dam and is located at the confluence of three gullies, which are numbered as the first branch gully, the second branch gully, and the third branch gully from the left bank to the right bank. The average specific slope of the river valley is 8-10% and the slope of the mountains on both sides is 50-90\({}^{\circ}\). The height of the reservoir area is 1100-1500 m a.s.l, with a relative height difference of 400 m. There are no landslides, mudslides, subsidence, and other adverse geological conditions within the tailings dam area. The development of karst in the tailings dam area is dominated by shallow karst, while the deep karst is not developed and the shallow karst has no influence on the tailing dam. The seismic intensity of the area is 6 degrees and the design basic seismic acceleration value is 0.05 g. There are artificial layers (Q\({}^{m}\)) and slightly weathered dolomitic marble (jxb\({}^{3}\)). The main types of tailing sand layers are fine sand tailings, silty sand tailings, silt tailings, and silty clay tailings. The physical and mechanics index of rock and tailing sand layers are presented in Tables 1 and 2. The rainfall is concentrated from June to August every year for the tailings dam. The maximum rainfall in this region is 1386.6 mm, the minimal rainfall is 403.3 mm, and the average annual rainfall is 746.3 mm. The reservoir area belongs to a medium-high mountain area, with a relatively large height difference and steep slopes. The bedrock is exposed, the valleys are developed, and the vegetation is sparse, which is conducive to the natural discharge of atmospheric precipitation and surface runoff. The source of replenishment water in the reservoir area is insufficient and the hydrogeological conditions are simple. The layout of the tailings dam is shown in Figure 1.  
The tailings dam is dammed by the upstream embankment method. The starter dam is a permeable rockfill dam with a height of 60 m, and the embankment height is 150 m, the total dam height is 210 m, and the total storage capacity is approximately 59.9 million cubic meters. The tailings dam is classified as second class, with a current dam height of 160 m and an elevation above sea level of 1250 m. The final embankment has an elevation of above sea level of 1300 m. For the complicated geography condition that the tailings dam was constructed in three gullies at the same time, the three-dimensional seepage numerical simulation was used for back analysis of the seepage flow field and seepage field mutual influence law analysis, to reveal the key areas and reasons why the tailings dam's phreatic line was prone to be uplifted under complicated geography conditions. In the meantime,  
Figure 1: Layout of the tailings dam (Sections A-A and B-B are typical). (**a**) Layout of the tailings dam and distribution of drilling positions. (**b**) Aerial view of the tailings dam. (**c**) The 3D model diagram of the tailings dam and surrounding mountains.  
seepage stability analysis and controlled phreatic line control research were performed to put forward countermeasures for seepage control.