Soil Air Permeability: Threshold Gradient and Anisotropy

In this study, a triaxial confinement chamber and mass flow meter were used to measure the air permeability of compacted clayey sand specimens. Specimens were extracted at different angles with respect to the vertical direction to measure the anisotropy. The soil was compacted at high water contents below the optimum.

At lower gradients, deviations from Darcy´s law were observed. Threshold gradients were found below which the flow rate-gradient relationship was nonlinear and flow was very small. Measurements of anisotropy in air permeability revealed an order of magnitude difference in permeabilities of the specimens extracted at 0 and 90° with respect to the direction of compaction. The square root of permeability varied in the principal directions according to an ellipse function, although there was significant scatter about the ellipse. The air permeability values obtained from these tests were consistent with previously published results.

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