## 4 Analysis and Discussion
### Effect of Soil Consolidation on Porewater Condition  
The effect of volume change either from external loading or self-weight of soil is modelled using the coefficient of volume change (mv), often regarded as the volumetric change in a soil per unit increase in effective stress. Figure 6: Pore water pressure for different rainfall durations and \(a_{f}\) values with (**a**) 1 min duration, (**b**) 1 h duration, (**c**) 4 h duration, (**d**) 1 day duration, (**e**) 7 days duration, (**f**) 30 days duration. Mechanism by Which Coefficient of Volumetric Compression Affects Slope Stability  
When water infiltrates a slope, the resulting increase in pore water pressures leads to a decrease in the soil's effective stress. Soil with a higher \(m_{v}\) will undergo a more significant volume reduction (or compression) for a given increase in pore water pressure. This compression can lead to a decrease in the soil's shear strength, making the slope more susceptible to failure, especially in fine-grained soils. The results in Figure 7 were obtained for soils having similar \(q/k_{s}\) of 0.1, \(a_{f}\) of 0.5 kPa, \(n_{f}\) of 2, and mf of 1. The pore water pressure distribution profiles shown correspond to soils with an mv of 0.2, 0.02, 0.002, 0.0002, 0.00002, and 0.000002 (kPa\({}^{-1}\)) which invariably represent a broad range of pressure that could be experienced by the slope. For an mv of 0.2 kPa\({}^{-1}\), the pore water pressure profile is very similar for infiltration occurring under 1 day of major rainfall. However, with decreasing mv, the distinction in the profiles seems clear for infiltration occurring under major rainfall (i.e., between 1 h up to 24 h/1 day) and antecedent rainfall afterwards. Beyond 1 day, the suction in the unsaturated zone tends to approach zero for antecedent rainfall experienced by the slope up to 30 days. The pore water pressure profiles all tend to converge to the initial antecedent rainfall stages at suction values of about 10 kPa. Generally, it is noticed that an increase in soil compressibility does tend to result in a decrease in soil suction above the rising groundwater level although the difference in the pore water profiles seems less for mv values below 0.002 kPa\({}^{-1}\). Also, similar to an earlier behaviour with the effect of air entry considered, a distinct \"vertical\" wetting front with a slightly sharp separation of pore water pressure profiles of the antecedent rainfall from the major rainfall conditions can be observed for mv values of 0.002 kPa\({}^{-1}\) and lower. Moreover, for mv values lower than 0.2 kPa\({}^{-1}\) (i.e., increased soil compressibility), the rise in groundwater level is clearly noticeable for antecedent rainfall or infiltration occurring beyond days (please see Figure 8). Although the mv for a soil is not an intrinsic property but depends on the stress range from which it is calculated, it is clear that a reduction in the volumetric compressibility of a soil due to stress change results in the reduction in soil suction in the unsaturated zone during rainfall infiltration, irrespective of whether the advancing groundwater level for rainfall is less than the saturated permeability of the soil. Increased soil compression means a gradual reduction in volume; therefore, the interaction between the re-arranged soil particles and the infiltrated rainwater (which does merit further investigation at a microscopic level) tends to lead to less suction with prolonged rainfall. It should be borne in mind that reduced soil volume incontrovertibly results in a decrease in pore spaces in the soil which, in an unsaturated soil, would mean increased suction. However, as this study suggests, although some antecedent rainfall may cause the suction to be eventually wiped out, for the period within which suction can still be maintained in the unsaturated zone, further investigations are required at the microscopic level to enable an understanding of the phenomenon of pore arrangements during volumetric changes under rainfall.