This International Standard specifies two test methods for determining the water permeability characteristics of a single layer of geotextile or geotextile-related product normal to the plane: a) the constant head method; and b) the falling head method. NOTE If the full permeability characteristics of the geotextile or geotextile-related product have previously been established, then for control purposes it can be sufficient to determine the velocity index at a head loss of 50 mm only.