3) The flume was set at a 1% (vs. 8%) grade to
Part II
Part II test conditions simulated field use of the
simulate the slope of the dredged material
silt fence. The soilwater mixture had a TSS value
being deposited from a 1-m-high dredge out-
equal to the expected maximum TSS of the
let pipe located 125 m from the weir.
dredged material. The mixture was made with
4) At about eight minutes and 16 minutes after
brackish water, then allowed to settle before re-
the water mixture was released, the down-
leasing the top layer of relatively clear water. Six
stream side of the geotextile was scraped
tests were conducted: three without a geotextile
with a spatula to increase the water flow
and three using geotextile B.
through the geotextile. A practice like this
Following is a list of the changes made to the
will be performed in the field in order to
ASTM procedure for Part II tests:
maintain water flow through the silt fence
1) The TSS of the soilwater mixture was 2.0
as long as possible. The silt fence will either
105 mg/L (67 times that prescribed by ASTM
be backflushed with water or scraped with a
D 5141 1992) and the salinity of the water
long-handled squeegee.
used was 4.5 ppt (vs. fresh water).
5) After the TSS samples were collected, the
2) A 28-cm-high impermeable gate was in-
remaining soilwater mixture was poured
stalled 23.5 cm from the back of the flume to
through a #200 sieve (0.075-diameter open-
allow the soil to settle out of the mixture.
ing size) so that the effectiveness of the
The top 13 cm of the gate was removable to
geotextile in retaining particles of 0.1 mm
allow decantation in a manner similar to the
and larger could be estimated.
lowering of the weir on the settling pond.
These changes were implemented as follows:
For the first test, the soilwater mixture
RESULTS
was released from the top container into the
back of the flume. It consisted of 46.6 L of
A complete list of results from Parts I and II is
water, 233 g of sea salt, and 9248 g of oven-
included in Appendix B. The filtering efficiency is
dry soil. This mixture settled until there was
defined as the percentage of soil particles retained
a clear water/sediment interface located 5
by the silt fence. The "system filtering efficiency"
cm below the removable portion of the gate
in Part II tests was determined analogously by
(approximately two hours). The test was con-
comparing the final TSS of the sediment-laden
ducted by quickly lifting the top portion of
water (whether or not a geotextile was present) to
the gate and releasing the upper layer of
the original TSS of the water allowed to settle in
water all at once.
the back of the flume.
Due to lack of soil and time, it was not
desirable to remake the entire 50 L of soil
Parts I and II
water mixture for the second, third, and
Table 2 is a summary of Part I results. Christo-
fourth tests. Therefore, the amount of soil
pher and Holtz (1985) recommend a minimum
and water lost in the first test was deter-
filtering efficiency of 75% and flow rate of 0.012
mined* (222.0 g and 24.0 L, respectively, with
a salt content of 111.7 g), then added to the
tively well, and geotextiles C and D performed
soil and water that had remained behind the
poorly. However, only geotextile B was selected
gate. The entire soilwater mixture was
for further study, because it had the desirable char-
stirred vigorously by hand for one minute.
acteristic of being reinforced and because the time
After stirring, the soilwater mixture was
required to perform the Part II tests was limited.
Table 3 summarizes the results from Part II.
allowed to settle until the soil/water inter-
The tests that used geotextiles achieved system
face reached a level of 5 cm below the re-
filtering efficiencies of 99%, and the geotextile fil-
movable portion of the gate. Then the next
ter reduced the final TSS values by a factor of 10.
test was conducted.
The "average effective filtering efficiency" for the
geotextile, determined by comparing the final TSS
values for the geotextile tests to those from the
tests without geotextiles, was 89%.
* This was calculated by averaging the total suspended
Comparing the final TSS measurements of tests
solids found in the three samples and multiplying by
3 and 7 with test 1 in Table 3 suggests that the
the volume of water lost.
5
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