Roles of Coarser-Grained Soil Layers in Capillary Barrier System

Water diversion occurring at the interface between finer- and coarser-grained soil layers is referred to as capillary barriers, which are used for soil covers to restrict water infiltration into waste facilities and improve slope stability. However, methods for selecting optimum soils for capillary barriers have not been established. This study aims to develop measures to select optimum soils for capillary barriers by focusing on the roles of coarser-grained soil layers. Water diversion occurring at the bottom of a sandy soil layer exposed to the atmosphere, which corresponds to a capillary barrier when the pore spaces of the underlying coarser-grained soil layer are infinitely large, is investigated via rainfall tests and numerical simulations. The results reveal that the porewater pressure at the soil–atmosphere interface where water diversion occurred is close to or less than the air entry value but remains negative. This indicates that the negative porewater pressure is a key component for the water diversion at the soil–atmosphere interface, thus providing a novel perspective on the mechanism of capillary barriers. Capillary barriers can be caused by negative porewater pressure generated at non-contact zones between finer- and coarser-grained soil layers. Essentially, the role of coarser-grained soils can make a soil–atmosphere interface by installing spaces under the finer-grained soil layer. This study is useful for reconsidering the mechanism of capillary barriers and developing reasonable measures to select optimum soils.