## 1 Introduction  
Rainfall-induced slope failure is one of the most destructive natural disasters that occur in shallow natural slopes. The impacts of such catastrophic events are known, but these recurring natural hazards still result in many significant casualties and economic losses [1; 2]. This study deals with a slope consisting of a fine layer and an intermediate coarse layer. The presence of soil layers with different unsaturated hydraulic conductivities in a shallow depth of soil affects the process of water infiltration, distribution, and pore water pressure in the slope, which results in different failure modes [3]. Rainfall infiltration water is due to the build-up of capillary barriers [4], which accumulate at the interface between fine and coarser soil layers. The capillary barrier effect has been widely studied in terms of its use in cover systems in waste disposal sites [5; 6], whilst more recent research has focused on slope stabilization [7]. Capillary barriers can maintain a high degree of saturation in the soil above them, which results in different failure parts in a multi-layer slope [8]. These phenomena are related to the capillary tension, which limits the downward movement of the wetting front from a finer soil into underlying coarser soil. In an inclined interface, under continuous water infiltration from the slope surface, the accumulated water above the interface between the fine and the coarse material leads to the formation of a gravity-driven sub-surface water flow along the interface. This depends on the geometrical or boundary variations, at a certain distance downslope, often reported as the \"diversion length\", which can be estimated using a model proposed by [PERSON] [9]. In natural slopes, characterized by a length in the order of hundreds of meters and an irregular layer geometry, the conditions leading to the penetration of infiltrating water into the underlying coarse layer are mainly governed by the materials, slope angle, and infiltration rate [10]. In some field investigations, a multi-layered slope with different hydraulic conductivities is a common situation in layered hillside slopes. Such a slope consists of fine sand deposits and medium-coarse sand deposits that are a few meters thick [11]. Additionally, the inclined angles range from 10 to 35 degrees. The coarse sand deposits involved have been found to have large pores and a higher hydraulic conductivity in a saturated condition, which have an important influence on the subsurface water flow and water content distribution during the infiltration, steady percolation, and drainage [12; 13]. A schematic of a three-layer distribution in a slope is presented in Figure 1.  
Many previous field investigations have been conducted and have found that layered soil influences the process of rainfall water movement and the distribution of water content, which determines the slope failure. However, clarifying the influence of an intermediate coarse layer on slope failure is still a complex issue, since many uncontrollable factors exist in a natural slope, such as the slope angle, rainfall intensity, etc. Hence, slope model experiments replacing field monitoring have been conducted, as these allow the same slope in different conditions to be investigated, reduce the cost by reducing the duration, and consider variable slope types [14]. To investigate the infiltration process in a layered soil profile in simplified and known geometrical and boundary conditions, physical model experiments have been performed by many researchers. Infiltration experiments have been conducted and have indicated that the wetting front temporarily stops above the interface of the fine layer and coarse layer, and the infiltration rate slows down [15; 16]. Additionally, the pore water pressure head of the finer layer above the soil interface could not increase when the water started to infiltrate into the coarse layer [17]. The presence of a coarser lower-most layer may confine the infiltration within the upper finer layer up to a high degree of saturation.