Permeability of a geo-textile 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 geo-textile 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 geo-textile. ASTM D- 4491-85 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

 

The Geo-textile Permeameter has been designed to meet the test requirements of water permeability of Geo-textile. It has been made keeping in view the growing requirement of testing of coir geo-textile. The geo-textile is tested in the uncompressed state for permittivity. The determination of permittivity of geo-textile is carried out, either by using constant head or falling head test procedures. 

The US Army Corps of Engineers established a direct measure of geo-textile clogging potential. They defined it as gradient ratio which is the ratio of hydraulic gradient through the geo-textile plus 25.4 mm of the soil to that of hydraulic gradient through the adjacent 50.8 mm of the soil. It is determined after 24 hours of flow. 

 

The percentage passing of different fractions determines the porometry of the geo-textile investigated. In the Hydrodynamic Sieving Method, the geo-textile specimen, loaded with a certain quantity of glass bead fraction, is continuously rotated in a water trough, forcing the glass beads to pass through the geo-textile openings. The apparatus consists of two test drums of 14 cm dia and 7 cm effective length with 16 nos. of 4 mm dia rods provided circumferentially at equal spacing to hold the geo-textile specimen in position. Two troughs to contain the test drums are supported on horizontal axis, facilitating free rotation and capable of being filled with distilled water to a level of 20 mm below the drum axis. 

This test is necessary for drainage application. The permeameters can be of parallel flow or radial flow type. In either case, flow occurs along the plane of the permeameters. In the apparatus, flow occurs radially outwards, from a central hole to the periphery of a circular specimen. The transmissivity is defined as the volumetric rate of flow per unit width of geo-textile and unit hydraulic head.