Influence of the soil-structure interface on seepage characteristics in uniformly-grained soils: Microscopical insights from transparent soil experiments

Seepage failure of hydraulic structures is more likely at the soil-structure interface due to factors such as lower compaction and differing flow paths; however, detailed evidence is scarce. In this research, internal structures and flow velocities in uniformly-grained soils are successfully observed using transparent soil techniques. Experimental results showed that both porosity and flow velocity were highest at the interface. Significant variations in these parameters occurred within a range roughly equal to the average particle size from the interface, indicating that the interface effect is due to particle arrangement at the boundary, with minimal impact further away. Furthermore, three-dimensional modelling of flow paths based on flow velocities was conducted, and flow path distributions and their tortuosity were analyzed. The tortuosity at the interface was low (<1.2), and widely connected pores were observed. Fluid at the interface flowed upward along the two-dimensional plane, while within the soil, it meandered three-dimensionally along particle edges. In summary, this research revealed through microscopic observation using transparent soil that the pronounced flow at the boundary is due to the soil-structure interface having larger pores and straighter flow paths.