EN ISO 11058  DETERMINATION OF WATER PERMEABILITY CHARACTERISTICS NORMAL TO THE PLANE, WITHOUT LOAD
In the constant head method a single unloaded layer of geotextile is subjected to a unidirectional flow of water normal to the plane under a range of constant heads.
Measuring the volume of water V (l) collected in a unit time t (s) under a fixed hydraulic head, at a standard temperature of 20 °C, is obtained the flow velocity v through the surface a (m^{2}) of the specimen :
v= V/(a x t ) in l/s.m^{2} or mm/s
In the falling head method a single unloaded layer of geotextiles is subjected to a unidirectional flow of water normal to the plane under a falling head, using an apparel as shown here below. With a series of measurements of hydraulic head dh related to time t, we obtain a curve that show the relation between loss of head and flow velocity.
The velocity index vih50 represents the value of flow velocity in correspondence to a loss of head dh=50 mm . Note : if t is the thickness of geotextile, the coefficient of permeability normal to the plane kn is obtained as follows:
kn = (vih50 x t )/50 mm/s
v
(mm/s)


