Used in Unsaturated tests the high air entry porous disk is necessary to separate the pore-air and the pore-water so that differential pressures can be maintained. In a GDS system the HAEPD is bonded into the base pedestal. The pore-water connection is routed from outside of the cell to the underside of the HAEPD.

NOTE: It is important that the pressure on the underside of the HAEPD should not exceed the pressure on the top surface by more than 50 kPa. A 'high air entry' porous material has the particular ability (if it is kept saturated) to maintain a water pressure of xkPa on one side and an air pressure of (x+y)kPa on the other side without the air passing through the material. The value y is known as the 'air-entry value'.