Soil anisotropy and some field methods for measuring permeability

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Abstract
Three field methods—piezometer, auger hole, and tube—and the conventional undisturbed‐core method for measuring soil permeability are compared theoretically and experimentally. Natural channels in the soil, such as cracks, roots, and worm holes, cause extreme variations in results by the undisturbed core method and cause the rate of rise, in small‐diameter piezometers, to deviate from the theoretical. This deviation becomes negligible as the pipe diameter is increased. The size of sample associated with each method differs from one method to the other. In non‐uniform soils this causes differences in measured permeability values. In anisotropic soils the piezometer and auger methods, using the usual inflow cavities which are long compared to their diameter, measure essentially the horizontal permeability; whereas the tube method, where length of cavity equals zero, measures essentially the vertical. The undisturbed‐core method may be used to measure either the horizontal or vertical permeability depending upon the direction in which the cores are taken. The piezometer method appears to be convenient and valuable for general use because of its suitability for measurement of permeability at any depth in both uniform and non‐uniform soils. The auger‐hole method is well adapted for measuring permeability in unstratified soils and appears particularly advantageous in soils having many root holes and other natural channels. The tube method, a special case of the piezometer method, with large diameter and a cavity length of zero, is especially useful in measuring essentially the vertical permeability of anisotropic soils. All the methods as described here apply to water‐saturated soil below a water table. The core method in its application to unsaturated soil, where it is perhaps most useful, is not a part of this study.