EXPERIMENTAL EVALUATION OF GEOMETRICAL SHAPE FACTORS FOR SHORT CYLINDRICAL PROBES USED TO MEASURE SOIL PERMEABILITY TO AIR

Permeability of soil has become recognized as an important parameter i n determining the rate of transport and entry of radon from the soil i nto indoor environments. This parameter is usually measured in the fie ld by inserting a cylindrical tube with a short porous section into th e soil and measuring the Bow 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 mat hematical approximation to describe flow through porous cylinders brea ks down when the lengths of the cylinders approach their radii. It is also shown that this problem can be avoided by approximating short por ous cylinders as ellipsoids, This study compared side-by-side measurem ents 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 s ection of the probe) are used. This study also looked at the effects o f moisture profile in the soil on the permeability obtained by differe nt measurement methods. It is shown that the effective permeability (e quivalent value for a uniform, isotropic medium) in the soil differs b y two orders of magnitude in a 1-m deep soil column when the measureme nt locations differ by only 35 cm. The effective permeability was obta ined by inverting the arithmetic average of the reciprocal values of t he position-dependent permeability. Copyright (C) 1996 Elsevier Scienc e Ltd.