TM 5-818-8/AFJMAN 3 2 - 1 0 3 0
CENTER LINE PROFILE OF GEOTEXTILE-LINED DITCH
Figure 6-4. Ditch Liners.
h (fig 6-7) behind the silt fence with sediment. The
the sediment-filled water through the geotextile.
height of the silt fence geotextile should not
(4) Required geotextile properties. The geotex-
exceed 3 feet.
tile used for silt fence must also have:
(2) Design for maximum particle retention.
Geotextiles selected for use in silt fences should
to prevent damage by floating debris and to limit
have an AOS that will satisfy the following equa-
tearing where attached to posts and fence.
tion with a limiting value equal to the No. 120
sieve size.
and biological, chemical, and thermal actions for
the desired life of the fence.
(5) Construction considerations.
(eq 6-l)
(a) Silt fences should be constructed after
the cutting of trees but before having any sod
A minimum of 90-pound tensile strength (ASTM D
disturbing construction activity in the drainage
4632 Grab Test Method) is recommended for use
area.
with support posts spaced a maximum of 8 feet
(b) It is a good practice to construct the silt
apart.
fence across a flat area in the form of a horseshoe.
(3) Design for filtration efficiency. The geotex-
This aids in the ponding of the runoff, and in-
tile should be capable of filtering most of the soil
creases the strength of the fence. Prefabricated silt
particles carried in the runoff from a construction
fence sections containing geotextile and support
site without unduly impeding the flow. ASTM D
posts are commercially available. They are gener-
5141 presents the laboratory test used to deter-
ally manufactured in heights of 18 and 36 inches.
mine the filtering efficiency and the flow rate of
6-5
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