6.1.1 The apparatus must be capable of maintaining a
constant head of water on the geotextile being tested, or
6.1.2 The apparatus must be capable of being used as falling
head apparatus.
6.2 In addition, the apparatus must not be the controlling
agent for flow during the test. It will be necessary to establish
a calibration curve of volumetric flow rate versus head for the
apparatus alone in order to establish compliance with this
requirement (see 11.7).
6.3 Refer to Fig. 1 for a schematic drawing of a device that
conforms to all of the above requirements. The device consists
of an upper and lower unit, which fasten together. The
geotextile specimen is positioned in the bottom of the upper
unit. There is a standpipe for measuring the constant head
value. The rotating discharge pipe allows adjustment of the
head of water at the bottom of the specimen.3See ADJD4491.
NOTE 3—The location of the manometer for measuring the headloss in
either the constant head or falling head method shall be located directly
beneath the specimen. For the device shown in Fig. 1, this may be
accomplished by drilling a small (3mm; 1⁄8 in) diameter hole in the top
plate of the bottom reservoir tank directly beneath the specimen, and
attaching the manometer to this plate.