Unsaturated hydraulic properties of heterogeneously packed sands: A pore-scale computational study

The macroscopic effects of soil textural heterogeneity and fine-scale soil layering on unsaturated flow remain poorly understood. In this study we used the Discrete Element Method (DEM) to numerically generate artificial particle packings of heterogeneous sands from which the pore structure can be extracted. Packings were generated from known grain size distributions following sand grain mixing and fine-scale profile layering. Five sands with different mean grain sizes were used to investigate the effects of heterogeneity on their pore structure, including changes in the permeability and soil water retention (SWR). Heterogeneous media were created by mixing a relatively fine sand with coarser sands. In addition, we created layered media having sharp as well as transitional interfaces between the two sand samples. Mixing fine and coarse sand caused reductions in the average pore body and pore throat sizes of the coarser sands, and hence also in their intrinsic permeability. The layered media with a transitional interface showed lower porosities at the interface because of penetration of small particles into the larger pores, while the porosities increased at sharp interfaces. The nonlinear relationships between permeability and the average pore body and throat radii were explored using different unimodal sands. Mixing fine and coarse sand caused a decrease in the capillary pressure at a given water content of the new medium, and hence larger values of the van Genuchten α parameter. The SWRCs of the layered soils with a sharp interface were best described using bimodal functions. Sharp interfaces caused a non-monotonic change in the drainage curve due to discrepancies between the pore throat radii of the two adjacent sands. This change was influenced by d50 of the sands used for the layered media. In contrast to the sharp interface, a monotonic (smooth) change in the drainage curve was observed for two sands having small differences in their d50 values. The study gave considerable insight into how particle heterogeneity and layering affects the hydraulic properties of unsaturated media.