# Prediction of Shallow Failure on a Slope Using Volumetric Water Content Gradient Characteristics
## 5 Conclusions  
This study aimed to develop a method to predict shallow failure using the VWC change characteristics that occur on a slope owing to rainfall infiltration. Considering the results of previous studies according to which the characteristics of the VWC can be used as a precursor to shallow failure, flume experiments were performed to identify the characteristics of the VWC gradient in the event of failure. Through the flume experiments, characteristics for each VWC sensor installation position were examined at various slope conditions. In addition, the characteristics of the VWC gradient according to the presence or absence of failure were compared through the VWC measurement data at actual sites where no failure occurred. It was found that the probability of failure increased as the VWC gradient increased. The results obtained from the analysis of the flume experiment and testbed data showed that the failure rate was: (a) less than 3% when the VWC gradient was less than 0.075, (b) less than 20% when it was between 0.075 and 0.1, (c) more than 66% when it was between 0.1 and 0.2, and (d) more than 80% when it was higher than 0.2. Based on these ranges, early warning criteria for these four stages were proposed. Considering that it was difficult to determine appropriate sensor installation positions in small-scale flume experiments, full-scale flume experiments were performed. For the full-scale flume experiments, a slope was created by simulating an actual site where failure occurred, and the shallow failure caused by rainfall infiltration was reproduced. In both the first and second full-scale flume experiments, small-scale and second failures finally occurred in the forms of shallow failures in the berm part. When the characteristics of the VWC gradient were investigated at each measurement position (berm, crest, inclined slope, and toe parts) and depth (0.5 m, 1.0 m, 1.5 m, and, 1.8 m) before the occurrence of failure, it was confirmed that the lower part (toe part) and a shallow depth of 0.5 m were not suitable for measurements because water was concentrated due to surface loss and scour, and the VWC value rapidly converged regardless of failure. In addition, it is inferred that sensors need to be installed at a depth of 1.0 m in the upper part (crest part) to predict small-scale failure in the berm part, where a sign of failure can be identified. Finally, for the prediction of shallow failure (second failure) that occurs at the boundary of the impermeable layer, the upper and middle parts at a high depth (1.8 m) or positions immediately above the impermeable layer are considered to be the most suitable sensor installation positions. In Korea, monitoring is generally performed by installing sensors when there is a risk of failure on a slope. In many cases, however, signs of failure are only observed using displacement meters, closed-circuit televisions, and clinometers. It can be stated that VWC sensors are highly applicable to the field because they can be used as a means to determine the presence or absence of failure in advance. The early warning criteria proposed in this study can be used as criteria for determining the possibility of the occurrence of shallow failure. Field verification is required for the early warning criteria proposed in this study. Considering that it is difficult to secure measurement data at sites where failure occurred, it is inferred that the applicability of the criteria to the field is secured as the results of the flume and full-scale flume experiments confirmed the possibility of predicting failure using the VWC gradient value. It is necessary to supplement the proposed measurement criteria by researching soils with various properties other than the weathered granite soil used in this study. Conceptualization, J.-W.S., S.-G.C. and M.-S.J. ; methodology, H.-S.K. ; formal analysis, J.-W.S., H.-J.K.; writing--original draft preparation, J.-W.S. ; writing--review and editing, H.-S.J. ; supervision, H.-[PERSON].