LAYERED STRUCTURE AND ANISOTROPY IN UNSATURATED SOILS

Variations in anisotropy of hydraulic conductivity with water capillary pressure affect flow and transport in unsaturated heterogeneous soils. In this study, we examine how the anisotropy is related to the layered structure of soil formation consisting of both randomly arranged layers and repeated unit cells of binary structure subject to a same capillary pressure head at both ends. In addition, the performance of commonly adopted conceptualization of harmonic and arithmetic mean approach in characterizing unsaturated soil anisotropy is also investigated. The anisotropy is determined by the fluxes in directions perpendicular and parallel to the layering subject to the same capillary pressure boundary condition. For the regularly structured layers of repeated units, the crossover capillary pressure head of the distinct soil materials is important in determining the capillary pressure distribution in the layered system and the associated anisotropy. The layered formation is generally less anisotropic when the more permeable layers are located on the top of formation. The anisotropy factor is usually a non-monotonic function against the capillary pressure, which displays a minimum anisotropy at an intermediate capillary pressure. Minimum anisotropy is more pronounced when the formation consists of only a few contrasting layers. The minimum anisotropy is mainly due to the fact that at some intermediate capillary pressure head, individual layers have similar hydraulic behavior around the crossover point as a result of non-linearity of hydraulic conductivity in relation to the capillary pressure head.