## 4 Slope Stability Analysis and Discussion  
### Pore Water Pressure  
Since the first factor that causes changes during rainfall is the seepage field in the body of the slope, the mechanical field changes as the seepage field changes. Here, the flow field of the slope is analyzed first. Through the study of the seepage field within the slope, to understand the distribution of water pressure within the slope, a qualitative and quantitative analysis of the flow field of the slope body during rainfall is performed, which is mainly intended to study the positive and negative pore water pressure peaks and the regional distribution of the law of change. The change in the seepage field in the slope body after rainfall under different cycles is shown in Figure 3. The red area of Figure 3 represents the maximum value of the pore water pressure and the blue area represents the minimum value. At 0 wetting-drying cycles, i.e., under the condition of no rainfall on the slope, the maximum positive pore water pressure within the slope was located at the bottom of the slope body. With the wetting-drying cycles, both the maximum positive pore water pressure on the slope surface and the maximum negative pore water pressure inside the slope body were increased, and the matrix suction gradually became smaller. This indicates that more and more water penetrates deeper into the slope with the drying cycle--not only into the surface layer but into the whole slope. From the peak zone of maximum positive pore water pressure on the slope surface until the value of pore water pressure is 0, i.e., matrix suction is 0, a transient saturation zone occurs. Below the transient saturated zone is the unsaturated zone; the extent of the unsaturated zone decreases with the number of wetting-drying cycles and the pore water pressure below the wetting front is redistributed according to a gradient. It can be inferred that the slope surface is the first to reach the saturated state during the initial rainfall. In addition to saturating the slope surface, the continuous infiltration of rainfall increases the overall water content within the slope, leading to a decrease in matrix suction, i.e., the adsorption of rainwater is reduced, and the rainwater flow is more fluid, which accelerates the expansion of the upper transient saturated zone until the seepage field reaches equilibrium. Thus, the rainfall infiltration under the wetting-drying cycles makes the slope surface gradually saturated through two factors together. On the one hand, the rainfall causes the slope surface to enter the saturated state. On the other hand, the overall increase in the water content in the slope body will also cause the infiltration line to move downward rapidly. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Number of Cycles** & **0** & **1** & **2** & **3** & **4** & **5** \\ \hline Cohesion/(kPa) & 47.23 & 45.54 & 41.11 & 39.75 & 38.52 & 38.17 \\ \hline Angle of friction/(\({}^{\circ}\)) & 17.32 & 17.27 & 17.39 & 17.31 & 17.25 & 17.21 \\ \hline Permeability coefficient/ (m-s\({}^{-1}\)) & \(8.7\times 10^{-8}\) & \(9.2\times 10^{-8}\) & \(9.7\times 10^{-8}\) & \(13.0\times 10^{-8}\) & \(9.4\times 10^{-8}\) & \(9.1\times 10^{-8}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Waste dump material strength at different wetting–drying cycle times. In addition, comparing the distribution of the pore water pressure at the bottom of the lowest local slope and other locations, it can be seen that the pore water pressure at the bottom of the slope is affected by rainfall under different cycle times in a larger area than other areas, and it increases continually until it merges with the water table line, which shows that the rainfall intensity is greater than the saturated infiltration coefficient, leading to rainfall collecting not only along the bottom of the slope but also along the surface of the slope, forming runoff.