The effects of anisotropy on in situ air permeability measurements

Previous research has established a correlation between air permeability (k(a)) and saturated hydraulic conductivity for agricultural soils based on ex situ air permeability (k(ex situ)). In situ air permeability (k(in situ)) measurements in nonagricultural soils, however, have shown a decrease in correlation that may be attributed to soil anisotropy. Our objectives were: (i) to examine the effects of anisotropy on k(in situ) using a three-dimensional air flow model; (ii) to develop a method to identify anisotropy using k(in situ) and k(ex situ) measurements; and (iii) to determine the sample volume of an air permeameter as a function of the permeameter design and the anisotropy ratio. Numerical results showed that the k(a) measured in situ in anisotropic media results in some average of the horizontal and vertical permeabilities. The averaging depends on the degree of anisotropy and the ratio of the diameter to the insertion depth of the permeameter. Therefore, a shape factor developed for an isotropic soil can give unreliable results. We determined that paired in situ and ex situ permeability measurements can be used to infer the anisotropy ratio. This approach is more accurate if the vertical permeability, k(az), is higher than the horizontal, k(ax). The sample volume does not extend outside of the air permeameter for high k(ax)/k(az). It is stretched vertically for low k(ax)/k(az). A field experiment showed qualitative agreement with model predictions, but anisotropy alone was not able to fully explain the difference between k(in situ) and k(ex situ).