# Prediction of Shallow Failure on a Slope Using Volumetric Water Content Gradient Characteristics
## 2 Materials and Methods
### Flume Experiment  
#### 2.2.1 Experimental Conditions and Equipment  
Our analysis of the VWC gradient characteristics, which can determine a sign of shallow failure on a soil slope, was based on the results of previous studies. Therefore, we performed flume experiments under specific experimental conditions, namely various slope inclinations, rainfall conditions, soil characteristics, and sensor measurement positions, which can all affect the VWC. Other factors such as soil texture, vegetation type and coverage, and topographic features may affect the occurrence of shallow failure. However, there is a limit to reproducing all these factors in the flume experiments. Therefore, in this study, the flume experiments were performed by simplifying the slope condition. A rainfall simulation device was used to reproduce soil failure; the wetting front moves from the surface of the soil slope to the base, which causes the surface layer to collapse.  
Most natural bedrock in Korea is composed of granite and gneiss [22]. Landslides in Korea are more related to weathered granite layers than other rocks as in granite areas small slides are dominant, there are few outcroppings, and the depth of weathering is evenly distributed [23]. In total, 3435 landslides that occurred after 1990 showed that granite was more vulnerable to landslides than other rocks [3]. Based on the above research results, in this study, the slope of the model experiment was created using weathered granite soil (Table 1). To maintain the bulk density of the slope at 1.67 Mg m\({}^{-3}\), a flume made of steel plates was divided uniformly into six layers, and compaction was performed by calculating the soil weight for each layer's volume. Different depths were applied to the crest, slope, and toe parts of the slope, and VWC sensors were installed to investigate changes in the VWC before and after failure (Figure 3). To secure measurements at various positions, the sensors were installed at depths in the range of 5 to 30 cm in the crest, slope, and toe parts of the created soil slope (Table 2).  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Property** & **Value** \\ \hline Specific gravity (g cm\({}^{-3}\)) & 2.60 \\ Moisture content (\%) & 18.95 \\ Bulk density (Mg m\({}^{-3}\)) & 1.67 \\ Effective grain size (D\({}_{10}\)) & 0.16 \\ Soil conductivity (cm s\({}^{-1}\)) & \(2.5\times 10^{-3}\) \\ Liquid limit (\%) & 22.56 \\ Plastic limit (\%) & 18.92 \\ Uniformity coefficient (C\({}_{\text{u}}\)) & 9.38 \\ Curvature coefficient (C\({}_{\text{g}}\)) & 0.84 \\ Cohesion(kPa) & 11.84 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Physical properties of the soil used in flume tests.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Slope Angle (\({}^{\circ}\))** & \multicolumn{3}{c}{**Measurement Depth (cm)**} \\ \cline{2-4}
**Slope Angle (\({}^{\circ}\))** & **Crest Part** & **Slope Part** & **Toe Part** \\ \hline
35 & 10, 20, 30 & 10, 20, 30 & 10, 20, 25 \\
40 & 10, 20, 30 & 10, 20, 30 & 10, 20, 25 \\
50 & 5, 15, 25 & 5, 15, 25 & 5, 10, 15 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Measurement depths at different slope angles.