TM 5-818-8/AFJMAN 32-1030
placement. This can be done by using small
amounts of aggregate to hold the geotextile in
through the perforated pipe is relatively ineffec-
place or using loose pinning and repinning as
tive on clogged geotextiles.
necessary to keep the geotextile loose. This method
3-7. Strength Requirements
of placement will typically require 10 to 15 per-
cent more geotextile than predicted by measure-
ment of the drain's planer surfaces.
have secondary functions (separation, reinforce-
ment, etc.) requiring high strength, the require-
(1) Secure lapping or joining of consecutive
ments shown in table 3-2 will provide adequate
pieces of geotextile prevents movement of soil into
the drain. A variety of methods such as sewing,
heat bonding, and overlapping are acceptable
Table 3-2. Geotextile Strength Requirements for Drains.
joints. Normally, where the geotextile joint will
Class A 1
not be stressed after installation, a minimum
12-inch overlap is required with the overlapping
inspected to ensure complete geotextile-to-geo-
textile contact. When movement of the geotextile
sections is possible after placement, appropriate
overlap distances or more secure joining methods
Class A Drainage applications are for geotextile installation
should be specified. Field joints are much more
where applied stresses are more severe than Class B applica-
difficult to control than those made at the factory
tions; i.e., very coarse shape angular aggregate is used, compac-
or fabrication site and every effort should be made
tion is greater than 95 percent of ASTM D 1557 of maximum
density or depth of trench is greater than 10 feet.
to minimize field joining.
2 Class B Drainage applications are for geotextile installations
(2) Seams are described in chapter 1. Strength
where applied stresses are less severe than Class A applica-
r e q u i r e m e n t s for seams may vary from just
tions; i.e., smooth graded surfaces having no sharp angular
enough to hold the geotextile sections together for
projections, and no sharp angular aggregate, compaction is less
installation to that required for the geotextile.
than or equal to 95 percent of ASTM D 1557 maximum density.
Additional guidance for seams is contained in
3-8. Design and Construction Considerations
AASHTO M 288. Seam strength is determined
a. Installation Factors. In addition to the re-
using ASTM 4632.
quirement for continuous, intimate geotextile con-
d. Trench Drains.
tact with the soil, several other installation factors
(1) Variations of the basic trench drain are
strongly influence geotextile drain performance.
the most common geotextile drain application.
Typically, the geotextile lines the trench allowing
(1) How the geotextile is held in place during
use of a very permeable backfill which quickly
removes water entering the drain. Trench drains
(2) Method of joining consecutive geotextile
intercept surface infiltration in pavements and
seepage in slopes and embankments as well as
lowering ground-water levels beneath pavements
and other structures. The normal construction
exposure to sunlight. Geotextile should retain 70
sequence is shown in figure 3-l. In addition to
percent of its strength after 150 hours of exposure
techniques shown in figure 3-1, if high compactive
to ultraviolet sunlight (ASTM D 4355).
efforts are required (e.g., 95 percent of ASTM D
1557 maximum density), the puncture strength
b. Placement. Pinning the geotextile with long
nail-like pins placed through the geotextile into
requirements should be doubled. Granular backfill
the soil has been a common method of securing the
does not have to meet piping criteria but should be
geotextile until the other components of the drain
highly permeable, large enough to prevent move-
have been placed; however, in some applications,
ment into the pipe, and meet durability and
this method has created problems. Placement of
structural requirements of the project. This allows
aggregate on the pinned geotextile normally puts
the designer to be much less stringent on backfill
the geotextile into tension which increases poten-
requirements than would be necessary for a totally
tial for puncture and reduces contact of the geotex-
granular trench drain. Some compaction of the
tile with soil, particularly when placing the geo-
backfill should always be applied.
textile against vertical and/or irregular soil
(2) Wrapping of the perforated drain pipe with
surfaces. It is much better to keep the geotextile
a geotextile when finer grained filter backfill is
loose but relatively unwrinkled during aggregate
used is a less common practice. Normally not used