# Prediction of Shallow Failure on a Slope Using Volumetric Water Content Gradient Characteristics
## 3 Early Warning Criteria Using VWC Characteristics  
In this study, an attempt was made to compare the characteristics of the VWC gradient according to the presence or absence of shallow failure. As it is challenging to secure the measured data during the failure process at actual sites, flume experiments that can be repeated can be an alternative. Therefore, the flume experiment results were used as measurement data for shallow failure cases, while the testbed measurement results were used as data for nonfailure cases. However, in the case of flume experiments, boundary conditions, soil characteristics, and rainfall reproduction characteristics are different from those in the field. Thus, it is difficult to directly compare them with the testbed results. To compare only the tendencies of the two control groups, considering the limitations of the measurement data secured from flume experiments, the VWC gradient values subject to the condition that caused slope failure in flume experiments and those of the testbeds were analyzed (Figure 8). While ECR was distributed below 50 mm, and the VWC gradient ranged from 0.072 to 0.309 when failure occurred in the flume experiment, the range of ECR was relatively wider and the VWC gradient was also distributed in a wide range between 0.01 and 0.32 at the testbed sites where no failure occurred. This shows a different tendency from the distribution of the testbed values below 0.1 in the study by [PERSON] and [PERSON] [21]. The VWC gradient increased in flume experiments as the VWC responded to high rainfall intensity of 50 mm/h, whereas the VWC gradient was gradually increased in the testbeds owing to the long rainfall duration despite the relatively low rainfall intensity. Among the VWC gradient values of the testbeds in which no failure occurred, some exceeded 0.1. This can be frequently observed during measurements at actual sites, meaning that the VWC gradient is not directly related to the occurrence of shallow failure. When no failure occurs, however, the VWC gradient rarely has a high value. This is because either: (a) the rainfall needed to saturate the ground and cause failure is not sufficient, or (b) the VWC does not increase  
Figure 8: Relationship between volumetric water content gradient (VWC) and effective cumulative rainfall (ECR) based on the data from testbed and flume tests. rapidly owing to the effective drainage in the ground. Therefore, when no failure occurs, it is highly likely that the VWC gradient is relatively low. The distribution of the WWC gradient values according to the presence or absence of failure is plotted in a histogram form (Figure 9). While the cumulative rate increased abruptly in the section with low VWC gradient values when no failure occurred, it increased continuously as the VWC gradient increased when failure occurred. The failure rate according to the VWC gradient was examined for a total of 282 datapoints secured in this study using the distribution characteristics of the VWC according to the presence or absence of failure (Figure 10). As the VWC gradient increased, the failure rate showed a tendency to increase. In particular, when the gradient exceeded 0.2, a high failure rate >80% was observed. The failure rate in this study does not have an absolute meaning because it was obtained based on the experimental and testbed data. The failure rate may change if failure case data obtained using flume experiments or nonfailure case data obtained using testbeds are added. As confirmed in this study, however, there will be no significant difference in the tendency of the failure rate to increase as the VWC gradient increases.