## 4 Slope Stability Analysis and Discussion
### Slope Infiltration Patterns  
To better illustrate the distribution of and variation in pore water pressure within the slope body, the central step of the Bang Nau dump was chosen to set a probe line with a fixed vertical direction line segment, in order to reveal the relationship between the pore pressure and depth (Figure 4). As can be seen from Figure 4, with the increase in the number of wetting-drying cycles, the slope's pore water pressure increases with the increase in the number of wetting-drying cycles, at the surface of the single-step slope, which is the same as the overall infiltration pattern of the slope in Section 4.1. The water level of the groundwater at this location is almost unchanged with the rainfall cycle, which is located at a depth of about 32.1 m below ground. Through the slope saturation line under different wetting-drying cycles, it can be seen that the infiltration line decreases with the number of wetting-drying cycles. The depth of the step saturation line under different wetting-drying cycles was 3.2 m, 4.1 m,  
Figure 3: Slope pore pressure under wetting\(-\)drying cycles. (**a**). Slope pore pressure under 0 wetting\(-\)drying cycle. (**b**). Slope pore pressure under 1 wetting\(-\)drying cycle. (**c**). Slope pore pressure under 2 wetting\(-\)drying cycles. (**d**). Slope pore pressure under 3 wetting\(-\)drying cycles. (**e**). Slope pore pressure under 4 wetting\(-\)drying cycles. (**f**). Slope pore pressure under 5 wetting\(-\)drying cycles. 4.9 m, 5.6 m and 6.2 m. In the early stage of rainfall infiltration, due to the dryness of the soil body of the drainage field, which resulted in a large gradient of matrix suction, the saturation line moved down faster, but, with the wetting-drying cycles, the rate of descent was reduced. It can be seen that although the infiltration line becomes lower and lower with the rainfall cycle, the rate of decrease in the infiltration line becomes slower and slower. This is because, as the rainfall cycle progresses, the infiltration coefficient of the transient saturated zone out of the surface changes after each rainfall event, and the infiltration location changes in the time taken during subsequent rainfall. Due to the first few cycles, the saturated permeability coefficient of the transient saturated zone will increase; in the next rainfall event, infiltration into the same depth takes less time, i.e., it will leave more time to infiltrate deeper, causing the transient saturated zone range to increase. The infiltration line decreases, but, due to the saturated permeability coefficient in the first two cycles, the saturated permeability coefficient grows faster, so the infiltration line of the first two rainfall cycles decreases faster. The saturated permeability coefficient does not change significantly after two to four rainfall cycles, so the infiltration line decreases more slowly in the third to fifth rainfall events. The permeability coefficient of the rock mass after the fifth rainfall event is \(9.1\times 10^{-8}\), which is reduced compared to the value of 9.8 \(\times\) 10\({}^{-8}\) for the fourth, and rainfall infiltration is more difficult compared to the previous rainfall. In the subsequent wetting-drying cycles, the saturated permeability coefficient of the rock mass will be further reduced, and the slope infiltration line will not be able to reach the infiltration line in the fifth rainfall event. From the intersection of the pore water pressure-depth curve and the curve without rainfall, after different rainfall cycles, it can be seen that the range of the rainfall-affected zone is always larger than the range of the transient saturated zone after each rainfall event. Moreover, the range of the affected zone continues to increase with the rainfall cycles, and the different times of the wetting-drying cycles are 8.4 m, 9.5 m, 11.3 m, 12.6 m and 14.3 m respectively. Meanwhile, the negative pore water pressure within the affected zone becomes smaller, being \(-2.5\) m, \(-2.6\) m and 14.3 m, respectively.