TM 5-818-8/AFJMAN 32-1030
Figure l-5. Needle-Punched Nonwoven Geotextile.
t o long term clogging potential of geotextile
placed in contact with and down gradient of soil to
drains. They are known to be effective in short
be drained. The plane of the geotextile is normal
to the expected direction of water flow. The capac-
duration applications.
ity for flow of water normal to the plane of the
d. Erosion Control. In erosion control, the geo-
geotextile is referred to as permittivity. Water and
textile protects soil surfaces from the tractive
any particles suspended in the water which are
forces of moving water or wind and rainfall ero-
smaller than a given size flow through the geotex-
sion. Geotextiles can be used in ditch linings to
tile. Those soil particles larger than that size are
protect erodible fine sands or cohesionless silts.
stopped and prevented from being carried away.
The geotextile is placed in the ditch and is secured
The geotextile openings should be sized to prevent
in place by stakes or is covered with rock or gravel
soil particle movement. The geotextiles substitute
to secure the geotextile, shield it from ultraviolet
for and serve the same function as the traditional
light, and dissipate the energy of the flowing
granular filter. Both the granular filter and the
water. Geotextiles are also used for temporary
geotextile filter must allow water (or gas) to pass
protection against erosion on newly seeded slopes.
without significant buildup of hydrostatic pres-
After the slope has been seeded, the geotextile is
sure. A geotextile-lined drainage trench along the
anchored to the slope holding the soil and seed
edge of a road pavement is an example using a
in-place until the seeds germinate and vegetative
geotextile as a filter. Most geotextiles are capable
cover is established. The erosion control function
of performing this function. Slit film geotextiles
can be thought of as a special case of the combina-
are not preferred because opening sizes are unpre-
tion of the filtration and separation functions.
dictable. Long term clogging is a concern when
e. Sediment Control. A geotextile serves to con-
geotextiles are used for filtration.
trol sediment when it stops particles suspended in
c. Drainage. When functioning as a drain, a
surface fluid flow while allowing the fluid to pass
geotextile acts as a conduit for the movement of
through. After some period of time, particles accu-
liquids or gases in the plane of the geotextile.
mulate against the geotextile, reducing the flow of
Examples are geotextiles used as wick drains and
fluid and increasing the pressure against the
blanket drains. The relatively thick nonwoven
geotextile. Examples of this application are silt
geotextiles are the products most commonly used.
Selection should be based on transmissivity, which
fences placed to reduce the amount of sediment
is the capacity for in-plane flow. Questions exist as
carried off construction sites and into nearby
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