Permittivity is determined by either a constant or falling head test. Constant Head Test—A head of 50 mm (2 in.) of water is maintained on the specimen throughout the duration of the test. The quantity of flow is measured versus time. The constant head test is used when the flow rate of water through the geotextile is so large that it is difficult to obtain readings of head change versus time in the falling head test. Falling Head Test—A column of water is allowed to flow through the geotextile and readings of head changes versus time are taken. The flow rate of water through the geotextile must be slow enough to obtain accurate readings.
permittivity, flow rate, permeability