Waku OSHITA

Microscopic observation of seepage flow at the boundary between different grain sizes using transparent soil

Yutaka FUKUMOTO

The objective of this study was to understand the characteristics of seepage flow by directly observing seepage flow in multiple cross-sections at the boundaries of different-sized bedrock such as fill and fill dams. The experimental results showed anisotropy in the observed cross-sectional seepage flow characteristics for single-grain and dissimilar-grain bedrocks. The mean velocity <Vx> was found to be larger for the larger pore size, and the mean velocity <Vy>, normalized by the mean velocity <Vx>, was found to be smaller for the heterogeneous ground regardless of the observed cross section. Unlike the horizontal seepage flow, the vertical upward seepage flow in the different grain size beds flows in the vertical direction to the layer, and the gap between the small grains in the upper layer and the large grains in the lower layer rapidly decreases at the boundary where the small grains in the upper layer and the large grains in the lower layer intersect. Therefore, it was found that the seepage flow is dammed at the boundary and flows perpendicularly to the direction of flow velocity. Hydraulic tortuosity, which represents the tortuosity of the seepage flow, was studied from this fiscal year, and it was found that the tortuosity decreases with increasing flow rate because the simulation does not take into account the formation of a new seepage flow path near the transition zone from laminar to turbulent flow. The actual phenomenon is that the degree of bending temporarily increases near the transition zone from laminar to turbulent flow. In addition, the results of multiple cross-sectional photographs of horizontal seepage flow focused on the layer boundary of a different-sized bedrock showed experimentally that the value of hydraulic tortuosity increased near the layer boundary.