Permeability of a geotextile must be substantially greater than that of the protected soil, so that water can pass freely from the soil through the fabric without build-up of hydrostatic pressure. A high value of the permeability of the geotextile also infers that partial clogging will not reduce the permeability. Both Constant and Falling Head Permeameters are generally used for measuring normal permeability known as permittivity. Permeability is defined as the volumetric rate of flow of water per unit cross sectional area under unit hydraulic head across the plain of the geotextile. ASTM 0 4491 specifies permeability test using Constant Head and Falling Head Permeameters. The constant head test is carried out using a head of 50 mm of water. Specimen diameter is 100 mm.
