Many of the damages to river levees due to seepage have occurred on levees built on multi-layered foundations, with a low-permeability covering soil layer on a highly permeable foundation. In our previous studies, we showed that foundation drainage works installed at the foot of the levee slope are highly effective as a countermeasure against the abovementioned damages due to seepage. In this paper, we verified the effectiveness of discretely installed foundation drainage works in the longitudinal direction of the levee by model experiments and seepage flow analysis. As a result, we showed that in the sections without countermeasures between foundation drainage works, the seepage surface seeps to the toe of the embankment, causing small-scale slides at the toe of the embankment, which can trigger large-scale slides, and that the combined use of the continuous drainage work installed at the toe of the embankment to suppress such slides is effective.

In this study, a centrifuge model test was conducted to investigate the infiltration of river water into a concrete-covered levee at high water levels, focusing on the presence or absence of drainage works at the landside slope toe. When a conventional concrete retaining structure was installed at the landside toe, river water infiltrated the levee body and increased the pore water pressure near the toe. Furthermore, replacing the concrete retaining structure with a gabion showed that incorporating a drainage function into the retaining structure helped mitigate the rapid increase in pore water pressure within the levee and prevented drainage failure near the slope toe. These results indicate that the toe drainage works and the gabion retaining structure act as a protective measure, effectively preventing soil erosion and structural deformation of the levee.

Thus far, previous studies on seepage failure, known as sand boil, have been mainly focused on the critical hydraulic gradient or critical seepage velocity at which the seepage failure occurs. However, in order to understand how the stability of soil structures changes after seepage failure starts, it is necessary to figure out the transport of soil particles during seepage failure. Herein, laboratory experiments were conducted to investigate the behavior of sand transported by seepage flow during the seepage failure of a sand due to vertical upward seepage flow. The experimental results have shown that the relative velocity between seepage water and soil skeleton follows the Darcy law, and the velocity of sand particles can be well estimated by the equilibrium of forces exerting onto the sand particles, in which the seepage force is given by a function of the hydraulic conductivity.

In the Kumamoto region, which depends on groundwater for almost 100% of its drinking water supply, the groundwater management has made progress from a qualitative understanding of groundwater flow toward a quantitative understanding and the future prediction of groundwater flow using groundwater models. This challenge has been based on data collection by the government, efforts by the academic field, and the need for private sector to make decisions based on quantitative understanding. This article reviews the history of groundwater model-based approaches, as also the latest activities for future groundwater management in response to the environmental changes.