# Failure Behavior Attributed to Internal Erosion Caused by Conduit Cracks in Homogeneous Embarment
## 3 Results and Discussion  
### Embankment Failure Shape  
One of the reasons for the failure of deteriorated earthen embankments is the movement of soil particles at the conduit-embankment interface; this failure initiates internal erosion when conduit cracks and joints are deformed. As the leakages from the cracks in the conduit flow downstream along the conduit boundary, the downstream embankment can become a risk zone for failure. Estimating the risk zones provides important information for future embankment pre-reinforcement plans. In this study, the risk zone was estimated based on the failure process and shape at the downstream slope. Figure 4 shows the process from the onset of deformation on the downstream slope (\(t=155\) min), owing to leakage, to the time the embankment failure occurred (\(t=180\) min). After reaching the full-water level (FWL), the time (\(t=0\)-\(155\) min) at which the deformation was not visible on the downstream slope was excluded (see Figure 4). The process was divided into the following: (a) initial (\(t=155\) min), (b) progress (\(t=157\) min), (c) rapid development (\(t=158\) min), (d) dam crest failure (\(t=165\) min), (e) overflow (\(t=168\) min), and (f) complete failure (\(t=180\) min) stages. Table 2 presents an overview of the failure process on the downstream slope according to the progression of the internal erosion, which was based on the height (\(h=20\) cm), at the site where the conduit was located. The pattern of slope failure due to the presence of conduit defects was characterized by the gradual extension from the lower area of the embankment, where the conduit was located to the dam crest side. By contrast, the slope failure caused by overtopping was completely different from that of the failure due to the presence of conduit defects, as the slope failure occurred from the dam crest to the lower part of the embankment. As the strength of the embankment was maintained at the initial stage of slope failure owing to overtopping, gradual step-like erosion appeared on the downstream slope [5]. However, in this experiment, gradual step-like erosion was not observed in the overtopping phase (\(t=168\) min). From these results, it is presumed that the strength of the embankment was already lost owing to the many deformations that occurred on the downstream slope before overtopping. Based on these results, the following can be inferred:  
1. Leakage due to the presence of conduit defects primarily weakens the strength of the downstream slope. Accordingly, when secondary overtopping occurs, the slope failure can be accelerated;
2. From a disaster warning and response point of view, a failure due to the presence of conduit defect may allow for more adverse conditions to respond to a disaster than failure due to the fact of overtopping. Figure 3: Position of the downstream slope in the initial state and the location of the settlement gauge at the dam crest. In the case of slope failure owing to overtopping and conduit defects, qualitative comparisons cannot be easily conducted, because the physical properties and environment of each reservoir are different with respect to failure time, failure scale, and deformation level. However, failure due to the presence of conduit defects suggests that the potential risk may be relatively high compared with overtopping disasters because deformations originating inside the embankment are not identified. Figure 5 presents the process of forming the vulnerable part of the downstream slope based on the erosion depth (D), erosion width (W), and the length of expansion attributed to erosion (L). Herein, the erosion depth is the maximum perpendicular distance to the slope when the slope fails owing to deformation. The erosion width is the maximum distance when deformation occurs along the horizontal direction on the slope where the conduit is located. The erosion length is the maximum distance when deformation occurs