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Effect of Layered Structure on Anisotropy of Unsaturated Soils

Zhu, Jianting

doi: 10.1097/SS.0b013e31824114f6
Technical Article

Abstract: Changes in anisotropy of hydraulic conductivity with 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 the 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 hydraulic gradient condition. For the regularly structured layers of repeated units, the crossover capillary pressure head of the distinct soil layers is important to determine 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 the formation. The anisotropy factor is usually a nonmonotonic function against the capillary pressure head, which displays a minimum anisotropy at an intermediate capillary pressure head value. Minimum anisotropy is more pronounced when the formation consists of only a few contrasting soil 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 nonlinearity of hydraulic conductivity in relation to the capillary pressure head.

Desert Research Institute, Nevada System of Higher Education, 755 E. Flamingo Rd., Las Vegas, NV 89119. Dr. Jianting Zhu is corresponding author. E-mail:

Received February 4, 2011.

Accepted for publication November 8, 2011.

Financial Disclosures/Conflicts of Interest: This study was funded by the Desert Research Institute’s Maki Chair program, U.S. Geological Survey under grant 06HQGR0098, U.S. Department of Energy under grant DE-FG02-06ER46265, and the Applied Research Initiative of Nevada.

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