Determination of Anisotropic Saturated Hydraulic Conductivity of a Macroporous Slope Soil

Abstract

For a steeply sloped part of the “Heumoser”, a creeping landslide in Vorarlberg (Austria), it is postulated that subsurface macropore flow results in high pore water pressures at the base of the slope and thus triggers mass movement or may even cause slope failure. To quantify saturated hydraulic conductivity and anisotropy in a soil containing macropores, a novel method was required to obtain a “very large” (L/W/H ≈ 30/30/60 cm) undisturbed soil sample. A square column was carved out from the slope, wrapped in cling wrap, surrounded with timber walls on the four lateral sides and stabilized by injection of polyurethane foam. The sample was then cut off at the base and carefully transported to the laboratory where it was installed in a constant head permeameter to obtain values for vertical hydraulic conductivity (Kᵥ). After completing the vertical flow measurements, the sample was divided into two cubes with approximately 30 cm side length and the respective vertical hydraulic conductivities were determined. Then the cubes were turned by 90° to obtain the horizontal saturated hydraulic conductivities (Kₕ). This new method allowed thus for a directional characterization of the hydraulic conductivity of soils containing macropores. It could be shown that the vertical hydraulic conductivity decreased by a factor of two from the upper sample to the lower sample. A similar anisotropy factor (Kₕ/Kᵥ) of approximately 0.5 was determined for both layers, in other words, it was shown that macropores can reverse the expected “natural” anisotropy (Kₕ/Kᵥ > 1).