## 3 Results and Discussion
### Seepage Characteristics and Displacement Distribution Analysis  
Figure 7 illustrates the seepage lines for each of the three (overtopping, dam crest, and flood) main water level distributions in the dam. On reaching the flood level (S1, t = 30 min), a seepage line was formed from the center of the dam, extending through the lower part to the slope end, indicating that seepage had not occurred. At the flood level (S1, t = 24 h), the upstream fill zone exhibited a certain degree of saturation; at this point, the seepage line drained to the slope end through a point slightly lower than the center of the vertical filter. At the same flood level (S1, t = 24 h), dam crest water level (S2), and overtopping water level (S3), the seepage line descended linearly toward the center of the vertical filter, highlighting the significant effect of the filter on the reduction of the seepage line. This result indicates that a difference as small as 24.5 times in the permeability coefficient between the vertical filter and fill zone can significantly impact the drainage efficiency. Figure 8 shows the PWP distribution. When the flood water level (S1) was continuously maintained, the water pressure affected the gabion structure area. The ranges of the PWP at the dam crest water level (S2) and overtopping water level (S3) exceeded 74% of those in the gabion structure area. However, the PWP changes in the dam body and downstream slope are insignificant. These results suggest that the drainage system incorporated with the vertical filter designed beneath the gabion retaining wall operated effectively. Additionally, the dam crest core suppresses infiltration, thereby limiting the water pressure expansion in the gabion structure and downstream slope.  
Figure 9 illustrates the changes in the horizontal (\(x\)-axis) and vertical (\(z\)-axis) displacements. The displacement values are extremely low because only stress changes due to water pressure were considered in the analysis, unlike experiments that reflect continuous phenomena, such as material inhomogeneity and scour progress. As the reliability of the quantitative values could not be determined, this analysis focused on qualitatively analyzing the overall displacement trend.  
At the flood level (S1), the initial displacement began at the top of the upstream side of the gabion. At the dam crest water level (S2), the displacement area expanded from the top of the upstream-side gabion to the bottom of the downstream-side gabion, indicating that saturation progressed in the downstream fill zone. At the overtopping water level (S3), the displacement further increased, showing a pattern of gradual settlement toward the downstream side. These results indicated that the upstream recharge zone was already  
Figure 7: Seepage lines at overtopping, dam crest, and flood water levels.  
saturated, and no further deformation occurred, whereas the gabion sunk downstream as saturation progressed in the downstream recharge zone.  
Overall, gabion walls, dam crest cores, and vertical and horizontal filters are considered effective design methods for preventing the upper saturation of the downstream slope by reducing seepage lines.