## 4 Laboratory SSCB Model Test under the Lateral No-Flow Condition
### Apparatus and Conditions of the SSCB Model Test  
The apparatus used for the CB model test included a water supply tank, a rainfall apparatus, and a soil tank (Figure 4a). The inside of the soil tank was 45.5 cm long, 47.0 cm high, and 15.0 cm wide; moreover, the thickness of the acrylic material was 20 mm. Figure 4b shows the rainfall apparatus, which was 500 mm long, 45 mm high, and 200 mm wide; furthermore, the thickness of the acrylic material was 5 mm. To properly reproduce the rainfall phenomenon, 161 needles were installed at 20 mm intervals (7 needles per row, over a total of 24 rows) in the lower part of the rainfall device. The sand and gravel layers were 20.0 cm and 17.5 cm high, respectively (Figure 4c); moreover, their initial dry densities (\(\rho_{di}\)) were 1.50 g/cm\({}^{3}\) and 1.64 g/cm\({}^{3}\), respectively. Differently from past studies conducted under the lateral flow condition, we installed some material (i.e., tarpaulin) to prevent drainage on the right side of the flow direction in the sand layer (Figure 4c). To ensure the drainage of water that infiltrated from the sand layer to the gravel layer, the right side of the gravel layer was set as a drainage condition. Thus, the experimental condition in the SSCB model test reproduces the extreme condition under which breakthrough forcibly occurs in the CB system as shown in Figure 1a, and which derives from the infiltration of rainfall from the sand layer. In other words, through the proposed SSCB model test, we expected to be able to quickly examine the performance of the CB system based on differences in water retention between the sand and gravel layers under certain experimental conditions. The diversion length measured in the SSCB model test is limited to the length of the soil tank, and the diversion length considered for the real CB system based on the results of the SSCB model test is discussed in detail in a later chapter.  
Four holes were created in the apparatus of the SSCB model test for installing the soil moisture sensor, allowing the measurement of the volumetric water content 50 mm above and below the interface between the sand and gravel layers. Moreover, the water level in the water supply tank was kept constant, and the water used to reproduce different rainfall intensity (I) conditions was supplied by controlling a valve located between the rainfall apparatus and the water tank. The rainfall intensity conditions were the following: 20 mm/h (Case 1), 50 mm/h (Case 2), and 100 mm/h (Case 3). Notably, the slope angle of the soil tank was 10\({}^{\circ}\). Four soil moisture sensors (EC-5, Decagon Devices Co.) were installed inside the soil tank to measure the variations in volumetric water content due to water infiltration. These sensors were able to measure soil moisture (i.e., volumetric water content) levels between 0 and 100%, and their precision was \(\pm\)3.0% in the case of sandy soil. The performance of the CB system was hence evaluated by setting the duration of rainfall to 6 h under extreme conditions. Although the rainfall for 6 h in this study was applied as an extreme condition for evaluating the water-shielding performance of the CB system, further studies on the CB system applying rainfall patterns in specific regions would also be needed. After 6 h of rainfall with intensity I = 20 mm/h, the amount of drainage water was expected to reach a sufficiently steady condition, which was confirmed through a preliminary model test. The conditions of the laboratory SSCB model test under the lateral no-flow condition are summarized in Table 3.