The Testing of Geotextile Permittivity and Related Properties Using the Characteristic Flow Equation and Air Flow Measurements

The characteristic flow equation (CFE) is a quadratic formula consisting of two terms; one that is proportional to the flow velocity and one that is proportional to the velocity squared. The first order term is referred to as the Darcy friction term, while the second order term is the “Inertia” term. This latter variable is based on the equation that describes flow through an orifice which explains the curved shape of the flow rate versus head plots generated in both permittivity and transmissivity tests, countering the commonly accepted supposition that the concave downward shape is due to turbulence. In this paper, The CFE is used to provide increased accuracy and utility for hydraulic measurements of geosynthetic flow capacity. This paper summarizes the features of the standard permittivity test procedure, ASTM D4491, Standard Test Methods for Water Permeability of Geotextiles by Permittivity, and reports test results using the CFE formula and air and water flow measurements to determine the permittivity of the same test specimens.