## 3 Results and Discussion
### Effect of Rainfall Duration on the Safety Factor  
Table 3 and Figure 13 illustrate how rainfall infiltration causes a reduction in the slope's safety factor. This behavior is explained by the interaction between the increase in soil saturation and the decrease in suction, which weakens the soil's internal structure and reduces its ability to resist shear stresses.  
For SW-SM soil, a reduction of up to 73.7% in the safety factor (S.F.) is observed as rainfall duration increases. Initially, without rainfall, the S.F. is 4.329, indicating high stability. However, with 8 h of rainfall, the S.F. decreases to 3.229 and continues to drop progressively, reaching 1.140 after 24 h of rainfall. This suggests that, although SW-SM soil can retain stability under moderate infiltration conditions, its S.F. significantly decreases with prolonged rainfall, increasing the risk of slope failure.  
For SP-SM soil, the initial safety factor without rainfall is considerably lower (1.126), indicating limited stability even without rainfall conditions. When analyzing the effects of rainfall, there is less variation in S.F. compared to SW-SM soil. After 8 h of rainfall, the S.F. remains almost constant at 1.123 and slowly decreases to a minimum of 1.063 after 18 h of rainfall, stabilizing at 1.078 after 24 h. This low sensitivity to infiltration is due to SP-SM  
Figure 13: Safety factor vs. time for both soil types.  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & \multicolumn{2}{c}{**Safety Factor (S.F.)**} \\ \cline{2-3}  & **SW-SM** & **SP-SM** \\ \hline No rainfall & 4.329 & 1.126 \\ Rainfall duration 8 [h] & 3.229 & 1.123 \\ Rainfall duration12 [h] & 2.229 & 1.085 \\ Rainfall duration18 [h] & 1.427 & 1.063 \\ Rainfall duration24 [h] & 1.140 & 1.078 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Safety factors obtained for SW-SM and SP-SM soils.  
soil's limited suction retention capacity; however, its generally low S.F. indicates that the slope is inherently less stable and remains at risk even with short-duration rainfall.  
This differentiated evolution of the factor of safety between the two soils can be mainly attributed to the effects of grain size distribution on permeability. Recent studies have shown that grain size distribution and the coefficient of uniformity directly influence the soil's ability to allow water flow, altering pore connectivity and the hydraulic behavior of the soil profile [52, 53]. The SW-SM soil, due to its better gradation and higher fines content, has lower permeability, which favors greater moisture accumulation at depth, decreases effective suction, and gradually reduces the S.F. In contrast, the SP-SM soil, with a more uniform grain size distribution and lower fines content, facilitates faster but shallower infiltration, maintaining higher suction in deeper zones and, therefore, more consistent stability over time--though there is a generally lower S.F. value. This behavior highlights the critical influence of grain size distribution on infiltration dynamics and overall slope stability.  
Overall, SW-SM soil, while initially more stable due to its higher fine content and cohesion, experiences a significant reduction in S.F. with prolonged rainfall. In contrast, though less affected by rainfall, SP-SM soil consistently exhibits low S.F., making it more vulnerable overall. Both soil types display distinct behaviors in response to saturation, emphasizing the importance of considering soil composition and exposure duration to water in slope stability analysis.