## 1 Introduction  
Climate change is predicted to trigger more fluctuations in the patterns of rainfall in the long term [1]. The alterations in the frequency, intensity, and amount of precipitation can result in the deformation of soil slopes leading to instability. Slope failures and general landslides are responsible for thousands of fatalities and millions of dollars' worth of damage to critical infrastructural assets such as railways, roadways, and flood defence systems [2]. In geotechnical engineering, the effects of infiltration and seepage on the stability of soil slopes are normally addressed through the determination of the factor of safety or in some instances, critical depth [3]. This type of analysis lends itself to saturated steady-state flow occurring over a given depth. To account for the worst-case scenario of rainfall infiltration, it is usually presumed that the groundwater table (or the phreatic surface) would gradually rise to coincide with the surface of the slope, hence rendering a completely saturated condition [4]. In a fully saturated slope, additional seepage or infiltration may not be feasible, hence, precipitation will have no further effect on the stability of the slope. However, it has been proven through in situ measurements of matric suction that negative pore water pressures in a soil slope may not be entirely destroyed following a rainstorm [3, 5]. In this regard, it is possible for some amount of shear strength to be sustained in such slope conditions.  
Previous studies have demonstrated that the hydraulic responses of a slope to precipitation can be broadly divided into two major stages, namely: (1) the propagation of a wetting front and (2) the rise of groundwater table [4; 6; 7]. Based on the theory of infiltration and seepage through an unsaturated-saturated soil system, certain conditions under which soil suction can be maintained in a slope with an advancing wetting front resulting from rainfall has been established in the past [8; 9; 10]. Under steady state conditions, one of the most important factors that can contribute to the permanency or maintenance of matric suction in soil slopes is the amount of precipitation experienced by the soil. This magnitude of precipitation (or rainfall flux) can also be expressed as a ratio or percentage of the saturated coefficient of permeability of the soil [11]. For soils with low saturated coefficient of permeability (or larger water storage capacity), the negative pore water pressure may require a sufficient amount of time to dissipate in order to ensure that it is maintained over a longer time duration than the rain is likely to last, even if the rainfall intensity (or ground surface flux) is greater than or equal to the saturated coefficient of permeability [12; 13].  
Previous studies have also shown that under transient conditions of seepage, pore water pressure is dependent on the magnitude of rainfall, saturated coefficient of permeability, desaturation coefficient, slope geometry, and unsaturated soil property functions such as the air-entry value of the soil and the water storage function [5; 14; 15; 16; 17; 18]. Although not explicitly considered in most previous research in the recent past, it is proposed that slope stability analysis should also incorporate the behaviour of pore water pressure of a slope that is subjected to consolidation from external forces (or under its own self weight) during a rainfall event.  
Hence, the present study aims to examine and evaluate, through an unsaturated -saturated infiltration and seepage model, the matric suction response and the stability of a consolidating slope subjected to rainfall. The conditions leading to the maintenance of negative pore water pressure in the unsaturated zone of the slope are studied under transient-seepage conditions using finite element numerical modelling. Furthermore, semi-parametric analysis is carried out to include a range of unsaturated soil and volume conditions with an investigation of the stability of the slope under those conditions.