Experimental evaluation of geometrical shape factors for short cylindrical probes used to measure soil permeability to air

Abstract

Permeability of soil has become recognized as an important parameter in determining the rate of transport and entry of radon from the soil into indoor environments. This parameter is usually measured in the field by inserting a cylindrical tube with a short porous section into the soil and measuring the flow rates that result from a range of applied pressures. A variety of mathematical relationships have been used to analyze the resulting data. It is demonstrated that a commonly used mathematical approximation to describe flow through porous cylinders breaks down when the lengths of the cylinders approach their radii. It is also shown that this problem can be avoided by approximating short porous cylinders as ellipsoids. This study compared side-by-side measurements of soil permeability for a number of porous cylinders and spheres with different sizes and orientations. It is shown that all the data can be analyzed with a single curve when the appropriate shape factors (geometrical factor that describes the shape and size of the porous section of the probe) are used. This study also looked at the effects of moisture profile in the soil on the permeability obtained by different measurement methods. It is shown that the effective permeability (equivalent value for a uniform, isotropic medium) in the soil differs by two orders of magnitude in a 1-m deep soil column when the measurement locations differ by only 35 cm. The effective permeability was obtained by inverting the arithmetic average of the reciprocal values of the position-dependent permeability.