15 March 2001
the change of clogging, no geotextile should be specified with an AOS smaller than the openings
of a U.S. Standard Sieve No. 100. When possible, it is preferable to specify a geotextile with
openings as large as allowed by the criteria. Geotextiles should not be used for soils with 85
percent or more passing the No. 200 sieve.
NOTE: Methods for determining the AOS and gradient ratio of geotextile are given in ASTM
Specifications D 4751 and D 5101, respectively.
NOTE: Percent open area is defined as the summation of the open areas divided by the total
area of the geotextile (refers to woven geotextiles only).
12.6. Subsurface Drain Installation. Subsurface drains are typically installed using trenchers
or other suitable equipment (Figure 12.7.). Grade control and elevations are normally obtained
from the pavement surface. A minimal slope of 0.15 foot (0.04 meters) in 100 feet (30.4 meters)
is recommended for subsurface drains.
A minimum thickness of 6 inches (152 millimeters) of filter material should be placed around all
types of subsurface drains. Filter material must not become segregated or contaminated prior to,
during, or after installation. Segregation results in zones of material too fine to meet the
permeability requirements and other zones too coarse to meet the stability requirements.
should feed into existing storm drainage manholes where practicable. Outlets, 12 inches
(304 millimeters) in diameter and smaller, not terminating in a manhole, should be protected with
rodent screens located to prevent surface water from entering the system and protected and
marked to prevent damage by mowers or other equipment. The upstream end of drainage pipes
not terminating in a structure should be capped or plugged.