# Prediction of Shallow Failure on a Slope Using Volumetric Water Content Gradient Characteristics
## 4 Measurement Position Setting Method  
### Full-Scale Flume Experiment  
Full-scale flume experiments were performed to verify the proposed VWC measurement criteria and identify effective measurement positions. Figure 11 shows a site where shallow failure damage occurred during the invasion of the 18 th typhoon Mitag in 2019. The shallow failure that occurred on the slope behind a house due to heavy rainfall damaged the wall of the house and caused two human casualties. In this study, an attempt was made to simulate this site through full-scale experiments subject to the assessment that this case is associated with a typical damage type that may occur during heavy rainfall.  
Figure 12 shows the cross section of the full-scale flume experiments. The slope was created by reflecting the soil depth and soil characteristics of the damaged area identified through an onsite investigation. In particular, considering that the failure surface occurred at the boundary of the bedrock at the site, an impermeable layer with a thickness of 0.5 m was created under the soil layer at a depth of 1.5 m to reproduce shallow failure that  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Volumetric Water Content Gradient** & **Level of Alarm** & **Description** \\ \hline \(<\)0.075 & Prealert & Very low failure rate (\(<\)3\%) \\
0.075–0.1 & Alert & Low Failure rate (\(<\)20\%) \\
0.1–0.2 & Prewarning & High Failure rate (\(<\)66\%) \\ \(>\)0.2 & Warning & Very high failure rate (\(>\)80\%) \\ \hline \hline \end{tabular}
\end{table}
Table 4: The early warning criteria using the VWC gradient.  
Figure 10: Rate of landslide occurrence according to the VWC gradient.  
occurred owing to the impermeable layer that served as an active surface. Sensors were installed in the berm, crest, inclined slope, and toe parts.  
A full-scale flume experiment was performed two times in Table 5. In the second experiment, some of the measurement positions were adjusted according to the feedback based on the results of the first experiment.  
Figure 13a,b shows the failure process of the first full-scale flume experiment. Approximately 5 h and 28 min after spraying artificial rainfall with a rainfall intensity of 50 mm/h, local small-scale failure occurred at the boundary of the berm part on the slope (Figure 14a). This is the position at which failure frequently occurs in the field. The position also appears to be most vulnerable to failure in this experiment. At 11 h and 16 min, the second collapse occurred; at this stage, the entire soil layer on the slope collapsed and shallow failure occurred.  
Figure 12: Schematic representation of full-scale flume experiment set up with soil profiles.  
Figure 13: Full-scale flume experimental results (first experiment): (**a**) small-scale failure; (**b**) second failure (shallow failure).  
Figure 14: Variations of the VWC before small-scale failure in the flume experiments: (**a**) berm; (**b**) crest; and (**c**) inclined slope parts.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Specification** & **First Experiment** & **Second Experiment** \\ \hline \multirow{4}{*}{Slope} & Wet density (Mg m\({}^{-3}\)) & 1.603 & 1.543 \\  & Water content (\%) & 19.6 & 18.8 \\  & Slope angle (\({}^{\circ}\)) & 35 & 35 \\  & Depth of soil (m) & 1.8 & 1.8 \\  & Depth of impermemeable layer (m) & 0.5 & 0.5 \\ \hline \multirow{4}{*}{Sensors} & Number & 20 & 22 \\  & Berm part: 0.35, 0.75 & Berm part: 0.35, 0.75 \\  & Depth of measurement (m) & crest part, inclined part, & crest part, inclined part, \\  & toe part: 0.5, 1.0, 1.5 & toe part: 0.5, 1.0, 1.8 \\ \hline \multicolumn{4}{c}{Rainfall intensity (mm/h)} & 50 & 50 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Soil condition and positions of sensors for full-scale flume experiments.