## 4 Analysis and Discussion  
### Effect of Air Entry on Porewater Condition  
The results shown plotted in Figure 4 illustrate the different patterns realised throughout the pore water pressure distribution profile with the value of \"_a_\" equal to 0.1, 0.5, 1, 5, 10, and 20 kPa, respectively. These results were obtained for soils having similar saturated coefficients of permeability (\(k_{s}\)) of 1 \(\times\) 10\({}^{-5}\) m/s, \(n_{f}\) of 2, and \(m_{f}\) of 1. Rainfall flux (\(q\)) of 1 \(\times\) 10\({}^{-6}\) m/s (i.e., \(q/k_{s}\) of 0.1) was applied on the slope. Mechanism by Which Air Entry Affects Slope Stability  
The air-entry value (AEV) is a measure of the matric suction head at which air starts to enter the soil's pores, leading to a rapid drop in the soil's water retention capacity. It is a key parameter in the soil-water characteristic curve. Soils with higher AEVs tend to resist water infiltration for longer durations during rainfall events. This resistance means that the buildup of pore water pressures, which can reduce the soil's shear strength, is delayed. Conversely, a lower AEV implies that the soil becomes saturated more quickly, leading to a faster rise in pore water pressures, which can compromise slope stability. For the soil with \(a_{f}\) that is equal to 0.1 kPa, the negative pore pressure (or matric suction) near the ground surface is seen to decrease with time (Figure 4). There is a clear distinction between the curves which indicate progressive movement, suggesting a rise in the groundwater as the infiltration continues. It is also observed that although there are relatively different values of suction for infiltration occurring from 1 min up to 24 h (1 day) of rainfall above the water table, the level of groundwater rise seems not to change. However, the reverse seems to be the case as infiltration continues up until 7 days beyond which the suction seems to remain the same even though the level of groundwater rise is greater with the passage of time. Groundwater rises by about 1 m between the major rainfall conditions (1 min-24 h) and 7 days and by 4 m between the major rainfall conditions and 30 days. At greater values of \(a_{f}\) (also indicating higher air-entry value), the suction of the soil during rainfall infiltration above the groundwater table generally seems to increase, as could be observed from Figure 4. Also, a distinct \"vertical\" wetting front with a slightly sharp separation of pore water profiles of the antecedent rainfall from the major rainfall conditions is noticed with a reduction in the air-entry values of the soil. However, as the \(a_{f}\) value increases, the wetting front becomes less distinct, but with the gradient of the pore water pressure being greater for the soil with smaller air entry. As rainfall infiltration progresses, the level of groundwater rise seems to be almost equal at about 2 m for \(a_{f}\) values between 5 kPa and 10 kPa. However, there seems to be a slight distinction in the progress of infiltration and water levels at the \(a_{f}\) value of 20 kPa. In this regard, there is a small increase in the water level between 1 min of rainfall and that lasting for 1 day or 24 h. It is also observed from Figure 5 that the level of rise in the groundwater across the infiltration times are generally lower for higher air-entry values of the soil. Figure 6 shows more clearly the pore water pressure profile highlighting the differences in the matric suction for different air-entry values as the time of infiltration increases when re-cast with varying durations. It is obvious that higher matric suctions are accorded to the soils having higher air entry across all infiltration durations. However, as the time of rainfall progresses, the matric suction values tend to reduce with increasing \(a_{f}\) value. Figure 4: Pore water pressure distribution and duration of rainfall.