## 3 Results and Discussion
### Results of Numerical Simulation
The thickness of tailings at point B is greater than that at points A and C, and the influence of rainfall infiltration on point B is slower than that of points A and C. Positive pore pressure also appeared at point B after 5.75 years, indicating that a complete saturation line began to appear in the dry-stack tailings pond in 5.75 years. The same phenomenon is found in Figure 4b,c. The difference is that under the condition of annual rainfall of 800 mm, the positive pore water pressure occurred at points A and C after 3.25 years of rainfall, and at point B after 4.25 years of rainfall. Under the condition of annual rainfall of 1200 mm, the positive pore water pressure occurred at points A and C after 2.5 years of rainfall, and at point B after 3.25 years of rainfall. This indicates that the occurrence time of the saturation line in the dry-stack tailings pond is related to the annual rainfall. The higher the annual rainfall rate, the earlier the occurrence time of the saturation line, but it has little to do with the rainfall pattern. Stage 2: the pore water pressure at each point in the tailings pond changes slowly and begins to approach a stable state. The growth value of the pore water pressure at each point in this stage is shown in Table 3.  
Stage 3: the pore water pressure of each point in the tailings pond tends to be relatively stable and fluctuates within a certain range. The growth value of the pore water pressure in this stage is shown in Table 4.  
In this stage, the pore pressure of points A, B, and C remained basically stable but fluctuated within a certain range. Referring to Figure 5a, the point C in Case II-a has the largest fluctuation with 0.6 kPa, ranging from 74.6 kPa to 75.2 k, which is a small fluctuation range. Hence, it can be considered that the seepage field is in a relatively stable state. Referring to Figure 5b, the point C in Case II-f has the largest fluctuation with 1.3 kPa, ranging from 153.2 kPa to 154.5 kPa. Compared with the pore water pressure value, the fluctuation range is not large, therefore it can be considered that the seepage field is in a relatively stable state. Referring to Table 2 and Figure 5c, the fluctuation of point C in Case II-i is the largest, with it fluctuating between 235.8 and 241.8, and the fluctuation range is 6.0 kPa. Compared with the pore water pressure value, the fluctuation range is small. The main reason for the fluctuation lies in the influence of the rainfall curve within one year, that is, the greater the rainfall rate, the greater the fluctuation of the relatively stable seepage field. However, compared with the total pore pressure value, the fluctuation is not large, so it can be considered that the seepage field is in a relatively stable state. 1.2 Evolution Process of the Saturation Line in the Dry-Stack Tailings Pond under Multi-Year Rainfall  
The equipotential surface with a pore water pressure of 0 is called the saturation surface in three-dimensional space, which is correspondingly called the saturation line in two-dimensional profile. The saturation line is known as the lifeline of tailings ponds, which is the basis of seepage drainage design and the stability analysis of tailings pond. As shown in Figures 6-8, the evolution process of the saturation line in the dry-stack tailings pond under multi-year rainfall with different working conditions was analyzed. Figure 6: The spatial–temporal evolution law of the saturation line in the tailings pond (Taiyuan, 400 mm): (**a**) Taiyuan 2018 (caseII-a); (**b**) Taiyuan 2019 (caseII-b); (**c**) Taiyuan 2020 (caseII-c). Figure 7: The spatial–temporal evolution law of the saturation line in the tailings pond (Zhengzhou, 800 mm): (**a**) Zhengzhou 2018 (caseII-d); (**b**) Zhengzhou 2019 (caseII-e); (**c**) Zhengzhou 2020 (caseII-f). From Figure 6a-c, it can be seen that in Taiyuan, when the annual rainfall is 400 mm, the saturation line appeared in the local area of the dam after 5 years of rainfall, which is mainly distributed in the thinner position of the tailings sand.