## 3 Results and Discussion
### Effect of Rainfall Duration on Matrix Suction  
In Figures 9 and 10, it can be observed that, as soil saturation increases, matric suction decreases significantly. Additionally, the longer the rainfall duration, the greater the depth affected on the slope, further reducing matric suction.  
Figures 11 and 12 present graphs of matric suction versus time, measured at the control points shown in Figure 4.  
Figure 11 shows how the effective suction of the SP-SM soil varies over time at five levels of the slope (12 m, 9 m, 6 m, 3 m, and 0 m). Each curve represents a specific elevation, highlighting the reduction in suction as a consequence of the advancing infiltration. The data should be interpreted from the top to the bottom of the slope: the higher the elevation, the earlier the loss of suction occurs, reflecting the progressive movement of water through the soil profile.  
Figure 12, like the previous one, shows how the effective suction varies--this time for the SW-SM soil--over time at five levels of the slope (12 m, 9 m, 6 m, 3 m, and 0 m). Each curve represents a specific elevation, showing a progressive decrease in suction as infiltration advances, although in a more gradual manner compared to the SP-SM soil. The data should be interpreted from the top to the bottom of the slope, reflecting the gradual movement of water in a soil with higher retention capacity and lower permeability, such as SW-SM.  
Significant differences in suction behavior are observed between the two soil types. This reflects the fines content [19] and the well-graded or poorly graded granulocyte.  
Figure 10: Reduction in suction for SW-SM soil under rainfall durations of 8, 12, 18, and 24 h. Grid lines indicate the finite element mesh used for the numerical simulation. Blue and green zones denote areas of significantly reduced suction, reflecting high saturation levels.  
Figure 9: Reduction in suction for SP-SM soil under rainfall durations of 8, 12, 18, and 24 h. Grid lines indicate the finite element mesh used for the numerical simulation. Blue areas represent zones where suction approaches zero due to full saturation.  
For SP-SM soil, the curves show a rapid loss of suction in the upper areas of the slope (H = h and H = 0.75 h) within the first 4.8 h, reaching very low suction values in a short time. In the slope's intermediate and lower areas, the suction loss is more gradual, indicating a limited water retention capacity in this soil due to its lower fines content.  
In contrast, SW-SM soil initially exhibits higher suction values (around 120 kN/m\({}^{2}\)) and a more gradual decrease in the upper areas. Effective suction decreases significantly within the first 5.6 h at heights H = 0.5 h and H = 0.75 h but remains elevated in the lowest zone (H = 0) until the end of the analysis, around 100 kN/m\({}^{2}\). This behavior indicates a greater suction retention capacity, attributable to its higher silt content, allowing it to store and retain moisture better than SP-SM soil.  
Figure 11: Graph of effective suction vs. time for SP-SM soil at heights H = 12 [m], H = 9 [m], H = 6 [m], H = 3 [m], and H = 0 [m].  
Figure 12: Graph of effective suction vs. time for SW-SM soil at heights H = 12 [m], H = 9 [m], H = 6 [m], H = 3 [m], and H = 0 [m].  
In summary, SW-SM soil retains suction for a longer period. It demonstrates a greater ability to limit saturation in the lower areas of the slope, which could result in greater stability under prolonged infiltration conditions. On the other hand, SP-SM soil experiences a rapid loss of suction, especially in the upper areas, which could reduce slope stability under rainfall conditions. These behaviors illustrate how gradation and fine content affect slope water and suction retention capacity.