###### Abstract  
Capillary barrier (CB) systems consisting of a fine-grained soil layer placed over a coarse-grained soil layer can generally provide a water-shielding effect, increasing the slope stability of soil structures during rainfall. In order to improve the water-shielding performance of CB systems, laboratory model tests have been previously conducted under various conditions; notably, large-scale model tests are especially required. The inefficiency in increasing the production time of CB models until now explains their high cost. In this paper, we propose a laboratory small-scale CB (SSCB) model test for a quick and efficient evaluation of the function of a CB system. In this model test, differently from previous studies, a side drainage flow in the direction of the inclined sand layer was set as the no-flow condition; moreover, the laboratory SSCB model tests were performed by considering three rainfall intensities (i.e., 20, 50, and 100 mm/h) under the lateral no-flow condition. The results showed that the larger the rainfall intensity, the shorter the diversion length was of the CB system. To evaluate the effectiveness of the SSCB model test proposed in this study, the diversion length was estimated by an empirical equation under the lateral flow condition based on hydraulic conductivity functions and the soil water characteristic curves of sand and gravel and then compared to the results of the SSCB model tests. It was hence demonstrated that the water  
the layers. Based on the results of numerical modeling, they also showed that the geometry of the CB model is one of the key issues to be considered to properly analyze the behavior of CBs used for controlling water infiltration [9]. [PERSON] et al. [21] proposed a three-layer CB landfill cover system for minimizing rainfall infiltration in humid climates. To evaluate the effectiveness of the new system, they used a flume model (3.0 m long, 1.5 m high, and 1.0 m wide). Their experimental and back-analysis results indicated that no water seeped through the tested three-layer barrier. [PERSON] et al. [22] performed laboratory CB model tests to observe the response of a three-layer CB system under a continuous heavy rainfall of 70 mm/h. Their inclined model box was 2.0 m long, 1.0 m high, and 1.2 m wide. The authors reported that the three-layer CB system did not allow deep percolation under heavy rainfall; moreover, the correlation between the physical and numerical modeling results indicated an anisotropic behavior of the system with respect to hydraulic conductivity in the unsaturated sand layer. [PERSON] et al. [23] considered the use of a three-layer barrier for improving the water-shielding performance of the CB system. They hence performed a series of laboratory CB model tests using a large-scale model box (3.0 m long, 1.5 m high, and 1.0 m wide). The effect of each layer's thickness change on the system function was investigated. It was hence found that the measured results and those estimated through the empirical equation of [PERSON] et al. [28] (for upper (silica sand No. 6) and middle (silica sand No. 8) layers with thicknesses of \(\geq\) 50 cm and \(\geq\) 30-50 cm, respectively) were similar. In the laboratory CB model tests of those mentioned above, a large-scale model with a lateral length \(\geq\) 2.0 m was used: in this case, the water infiltration from the upper sand layer to the lower gravel layer occurred randomly. This large-scale model test requires a lot of time for its functioning; moreover, its fabrication is expensive, and the corresponding testing time is high, making it difficult to perform many CB model tests under different conditions. Additional methodologies such as the BoSG (Boolean Stochastic Generation) and the Monte Carlo technique to characterize the infiltration and geomechanical parameters by means of random approaches in the numerical modeling for the slope stability may be needed [29; 30; 31].