TM 5-818-8/AFJMAN 32-1030
(4) Placement of cover material on geotextile.
Table 6-2. Pin Spacing Requirements in Erosion Control Appli-
cations.
For sloped surfaces, placement of the cover stone
or riprap should start from the base of the slope
Pin Spacing
Slope
moving upward and preferably from the center
feet
outward to limit any partial movement of soil
2
Steeper than 1V on 3H
because of sliding. In no case should drop heights
3
1V on 3H to 1V on 4H
which damage the geotextile be permitted. Testing
5
Flatter than 1V on 4H
may be necessary to establish an acceptable drop
V = vertical; H = horizontal.
height.
welding, or gluing adjacent panels, either in the
6-3. Precipitation Runoff Collection and Diver-
factory or on site, are preferred to lapping only.
sion Ditches
Sewing has proved to be the most reliable method
of joining adjacent panels. It should be performed
A diversion ditch is an open, artificial, gravity
using polyester, polypropylene, kevlar or nylon
flow channel which intercepts and collects precipi-
thread. The seam strength for both factory and
tation runoff, diverts it away from vulnerable
field seams should not be less than 90 percent of
areas, and directs it toward stabilized outlets. A
the required tensile strength of the unaged geotex-
geotextile or revegetation mat can be used to line
tile in any principal direction. Geotextiles may be
the ditch. It will retard erosion in the ditch, while
held in place on the slope with securing pins prior
allowing grass or other protective vegetation
to placing the cover material. These pins with
growth to take place. The mat or geotextile can
washers should be inserted through both strips of
serve as additional root anchoring for some time
the overlapped geotextile along a line through the
after plant cover has established itself if UV
midpoint of the overlap. The pin spacing, both
resistant geotextiles are specified. Some materials
along the overlaps or seams, depends on the slope,
used for this purpose are designed to degrade after
as specified in table 6-2. Steel securing pins, 3/16
grass growth takes place. The geotextile can be
inch in diameter, 18 inches long, pointed at one
selected and specified using physical properties
end, and fitted with a l.5-inch metal washer on
indicated in table 6-1 and the filter criteria of
the other have performed well in rather firm soils.
chapter 3. Figure 6-4 shows a typical example.
Longer pins are advisable for use in loose soils.
6-4. Miscellaneous Erosion Control
The maximum slope on which geotextiles may be
placed will be determined by the friction angles
Figures 6-5 and 6-6 show examples of geotextile
between the natural-ground and geotextile and
applications in erosion control at drop inlets and
cover- material and geotextile. The maximum al-
c u l v e r t outlets and scour protection around
lowable slope in no case can be greater than the
bridges, piers, and abutments. Design criteria sim-
lowest friction angle between these two materials
ilar to that used for bank protection should be
and the geotextile.
used for these applications.
Figure 6-2. Pin Spacing Requirements in Erosion Control Applications.
6-3
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