of allowing the dredged sediment to settle before
remain after exposure to ultraviolet radiation ac-
decanting water. This alone resulted in system
cording to ASTM D 4355 (1992). Although most
filtering efficiencies in excess of 90%.
geotextiles have some ultraviolet stabilizing ad-
In tests 2 and 4 of Part II, a small fraction of soil
ditives incorporated during manufacture, the re-
was retained on the #200 sieve from the water
sults of ASTM D 4355 (1992) are not available for
that flowed through the geotextile. Since the
geotextile B. Furthermore, geotextile B is usually
geotextile also had the effect of reducing the final
used for some combination of reinforcement, sepa-
amount of TSS in the water by a factor of 10, the
ration, and stabilization, i.e., in situations where
percentage of soil with a minimum diameter of
it is exposed to sunlight for only a short time
0.1 mm was clearly negligible when the geotextile
during construction (Geotechnical Fabrics Report
filtered the suspension. The Part II results also
1993). Thus, the silt fence should be carefully moni-
clearly demonstrate the importance of allowing
tored during use, and replacement material should
the spoils to settle before decanting the water. This
be readily available and properly stored at the
practice resulted in system filtering efficiencies in
site in the event that unacceptable strength degra-
excess of 90%.
dation of the material occurs.
Applying laboratory test results
CONCLUSION
to the field use of the silt fence
The estimated velocity of flow over a weir is
Among the four geotextiles tested, geotextile B
is the best geosynthetic candidate available to per-
V=
2 gh
(1)
form the filtering of WP particles in ERF. It effec-
tively retained particles of 0.075 mm and larger.
where V is the velocity of the water flow, g is the
acceleration of gravity, and h is the head of the
water on the weir (Roberson and Crowe 1975).
RECOMMENDATIONS
Because the head of water on the weir was the
same in the laboratory as expected in the field, the
The following recommendations are made to
initial flow rates over the weir in the field and the
help ensure the proper functioning of geotextile B
as a silt fence in Eagle River Flats:
gate in the laboratory are approximately equal. In
1. The quantity of material with diameters
the field, the velocity of water flow through the
larger than 0.1 mm passing through the silt
silt fence might be somewhat less due to effects of
fence should be monitored during field op-
end contractions on the weir, turbulence created
eration.
when water falls onto the concrete pad, and fric-
2. The silt fence should be carefully visually
tion drag from the concrete pad as the water flows
inspected frequently to detect any signs of
over it. However, the length of time that the water
strength loss due to ultraviolet degradation
is flowing at relatively high velocities will prob-
or other damage.
ably be longer than that experienced in the labo-
3. Replacement geotextile should be readily
ratory test. Thus, there are unquantified differ-
available and properly stored at the site.
ences in water flow rates and time of water flow
Proper storage includes wrapping in black
between the laboratory and the field that could
plastic and storing in a dry area indoors
have opposite effects on the long-term filtering
where it is not exposed to freezing tempera-
efficiency of the silt fence. Therefore, the quantity
of material with diameters larger than 0.1 mm
tures.
passing through the silt fence should be moni-
4. The silt fence should be backflushed with
tored in the field to ensure that the silt fence is
water or rubbed with a squeegee regularly
performing as intended.
to help ensure proper flow rates across it.
Geosynthetics are known to experience strength
5. According to Christopher and Holtz (1985),
degradation when exposed to ultraviolet light for
soil retained on the upstream side of the silt
fence should never exceed one-half the
more than a few days. When left exposed for peri-
height of the fence (0.6 m) due to the poten-
ods of about a year, material loss has also been
tial danger of water overtopping the fence.
observed. Koerner (1990) noted that standard
Thus, if soil buildup behind the fence
specifications for geotextile silt fences require that
a minimum of 70% of the original tensile strength
reaches this height, it should be removed.
7
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