# Stability Improvement Method for Embankment Dam with Respect to Conduit Cracks
## 3 Results and Discussion  
### Investigation of Erosion of Current and Improvement Models  
In order to compare the erosion phenomena in the current model and the improvement model, the embankment model requires conditions close to a steady state. Here, steady state means that most of the infiltration has occurred, and there is no change in the water pressure. The experiment was initiated after the embankment model reached a steady state through preliminary experiments, and the pore water pressure was measured for 48 h (2 days) at the full water level (H = 35 cm). In Case 1--Crack, soil loss was observed at the outlet and inside the conduit, as shown in Figure 3a, indicating no resistance to erosion. The downstream slope was slightly dry, as shown in Figure 3b. This dry state indicates that the soil and water in the embankment \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Model** & **Case Number** & **Crack Location** & **Section** \\  & & **of Conduit** & **Location** \\ \hline \multirow{3}{*}{Current model} & Case 1—Crack & Downstream side & Section A \\  & Case 1—Reinforcement & - & - \\ \cline{1-1}  & Case 2—Crack & Downstream side & Section A \\ \cline{1-1}  & Case 2—Reinforcement & - & - \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Experimental cases for the current model and the improvement model. Figure 2: Comparison between conduit models: (**a**) current model, (**b**) proposed improvement model. zone were flowing within the conduit through its cracked area. Therefore, the seepage water did not reach the downstream slope, and the downstream slope remained dry. In Figure 3c, it can be observed that cavitation occurred due to erosion in the clay layer near the crack in the conduit. In this experiment, the cause of cavitation is considered to be the erosion of clay particles into the conduit cracks. It is assumed that the initially formed small cavitation gradually expanded into the left and right fill zones and the upper soil layer as erosion progressed. In general, it is believed that if the embankment zone has a high degree of compaction, cavitation is less likely to occur owing to the high density of soil. The model embankment constructed in this study had a compaction degree of approximately 95%, similar to that of the actual embankment dam. Experimental results clearly show that even if the compaction or strength of the embankment is high, structural defects, such as conduit cracks, can create a special environment wherein cavitation can occur. Cavitation is suggested as one of the causes that can lead to the failure of an embankment dam [19]. Generally, the causes of embankment accidents and failures are investigated, but they are difficult to specify because they are diverse. However, the results obtained in this study are considered important for the field of embankment dams, as cavitation due to conduit cracks was identified. Consequently, Case 1--Cracks did not lead to the failure of the embankment, but the current model clearly showed that the cavitation risk due to the internal erosion of the embankment zone could be high once conduit cracks occur. In the condition reinforced by a geomembrane (Case 1--Reinforcement), no soil loss or erosion occurred inside or outside the conduit, as shown in Figure 4a,b. However, a wide wet area in a loose state was observed on the downstream slope, as shown in Figure 4b. This indicates that the seepage line inside the embankment was formed on the top of the clay layer. These results show that the clay layer was functioning normally, but it should be noted that over a long period of time, structural defects occurred in the conduit, and a potential failure might form in the downstream slope. Figures 5 and 6 depict the leakage of soil into the conduit, the downstream slope, and the erosion of soil around the cracked area of the conduit.