## 2 Characteristic of the CB System  
In the stratified grounds, where a sand layer can naturally occur over a gravel layer, infiltrated water tends to be retained and to accumulate in the sand above the contact interface, due to the different water retention characteristics of the two types of layers (Figure 1) [1]. The interface between the sand and gravel layers, which acts as a wall (preventing the infiltration of water from the sand layer into the gravel layer), can be identified as a CB. Since the CB is capable of protecting certain areas of the interface below the sand layer from water infiltration, this type of barrier has been widely used in soil structures, such as burial mounds and reservoirs. The functioning of CBs is based on a continuous capillary mechanism, which involves soil layers with different thicknesses. In Figure 1b, the relatively thin capillary tube at the top (with diameter \(r\)) represents the void formed in the upper layer (made of fine-grained soil), while the relatively thick capillary tube at the bottom (with diameter \(2R\)) represents the void formed in the lower layer (made of coarse-grained soil). The water droplets that flow in the unsaturated state are maintained by the capillary force just above the junction of the thin and thick capillary tubes [32]. The height (\(l_{c}\)) of the water column, which is maintained in an equilibrium state, can be expressed by the following equation:  
\[h_{c}=\frac{2T}{\rho_{w}g}(\frac{1}{r}-\frac{1}{R}) \tag{1}\]  
where \(r\) and \(R\) are the radiuses of the upper thin and lower thick capillary tubes, respectively, \(T\) (0.074 N/m in water at 15 \({}^{\circ}\)C) is the surface tension of water, \(\rho_{w}\) is the density of water, and \(g\) is the acceleration of gravity. Since \(r\) is always smaller than \(R\), the larger the difference in size between the particle diameters of the fine and coarse soils, the more moisture will be maintained by the fine-grained soil layer containing the upper fine capillary tube. CB systems systematically used in geotechnical engineering can also be composed of a fine soil (i.e., sand) layer over a coarse soil (i.e., gravel) layer, as shown in Figure 1. Since natural soils can be used for the creation of CB systems, such systems are eco-friendly. The water-shielding performance of a CB system is usually determined by the difference in hydraulic conductivity functions and soil-water characteristic curves between the sand and the gravel used for its creation [33]. In particular, the difference in the relative water retention of soil in the unsaturated condition can be understood by studying the soil-water characteristic curve (SWCC) [34]. As explained above, because the water-retaining capacity of sand is relatively larger than that of gravel, water retained in the upper sand layer tends to flow along the interface between the sand and gravel layers and to accumulate in the bottom part of the sand layer. This generates a funneled flow in an inclined direction (Figure 1). However, as the degree of saturation of the sand layer increases due to continuous water infiltration at the top of the interface, the difference in water retention between sand and gravel gradually decreases. Specifically, as the suction of the sand layer decreases, the unsaturated hydraulic conductivities (\(k_{w}\)) of the two samples become the same [12; 35; 36; 37; 38]. At this point, water begins to inf  
Figure 1: Concept of the CB system and capillary tube model: (**a**) concept of the CB system; (**b**) capillary tube model. layer (i.e., the \"breakthrough\" phase), and the CB system finally loses its water-shielding performance. The above processes can be understood as a water retention function of the CB, or as a water-shielding performance that results in the retention of water in the upper layer and suppresses water infiltration to the lower layer.