## 3 Results and Discussion
### Measurement Results  
Figure 11 shows the PWP and displacement measured in the overtopping prevention structure. In this experiment, after establishing the water level upstream of the reservoir dam, the supply of additional water for maintaining the water level was limited to identify changes in the displacement behavior of the overtopping prevention structure and the PWP inside the dam body. During the flood level (S1, 44 cm), the PWP on the upstream slope (P1) gradually decreased over time after the water level increased. This decrease was attributed to an upstream water level reduction caused by water infiltrating the dam body, indicating that the infiltration process proceeded smoothly. The slope reductions of PWP under flood (S1, 44 cm) and dam crest (S2, 51 cm) water level conditions were compared. Here, the slope of the PWP is associated with the operation of drainage structures such as filters or toe drains within the dam. The results show that the dam crest water level (S2) decreased the slope of the PWP by approximately 1.8 compared to that of the flood level condition (S1). These results suggest that seepage water is not adequately drained by the filter and toe drain, thereby posing a potential risk to dam stability owing to the seepage water retention. During the period of the overtopping water level (S3: 54 cm), the decreasing trends of PWP at the upstream slope (P1) and the bottom of the dam center (P2) were greater than that at the dam crest water level (S2, 51 cm), indicating the effective operation of the filter and toe drain. Figure 10: Deformation of the stepped gabion retaining wall. (**a**) initial state, (**b**) shrinkage phenomena, (**c**) blockage of seepage water, and (**d**) steady state. Meanwhile, the PWP at the bottom of the dam center (P2) increased rapidly during the dam crest water level (S2, 51 cm). This rapid increase might be because the saturated area formed in the upstream fill zone extended to the bottom of the center of the dam. In this experiment, the upstream water level was reached in 1 h, and there was no gradual saturation process at the bottom of the dam. The results suggest that dam body seepage indicates a potential saturation at the top rather than at the bottom during the rainy season as water flows into the reservoir and upstream water levels rise rapidly. Moreover, this result indicates that, in terms of sudden water pressure fluctuations, the dam crest water level (S2, 51 cm) may show greater instability than the flood (S1, 44 cm) and overtroping (S3: 54 cm) water levels. Conditions like those of the dam crest water level (S2) were maintained at the bottom of the downstream fill zone (P3), below the dam crest core (P4), and at the top of the downstream fill zone (P5). The PWP recorded below the dam crest core (P4) barely affected the seepage water pressure when the water level increased, and the seepage water drained through the vertical filter efficiently. In contrast, the top of the downstream fill zone (P5) exhibited the lowest PWP throughout the experiment, indicating that the dam crest core effectively blocked water leakage. Consequently, optimizing the design parameters, including the slope, thickness, and permeability coefficient of the vertical filter, may be necessary to reduce the likelihood of forming a potential saturated zone below the dam crest core at the dam crest water level (S2). The observed stability in pore pressure changed at each water level, indicating a generally stable trend, highlighting its potential practical application as an alternative for preventing overtopping problems. The upstream horizontal displacement (H1) gradually increased from the flood level (S1) to the overtopping water level (S3) and moved downstream. The maximum value of this displacement was measured to be approximately 4 mm, suggesting that the upstream water levels directly affected the displacement. In contrast, the downstream horizontal  
Figure 11: Measured values of the PWP and displacement.