indicate that the sediment size ranges down to
ductivity (permeability) at the site had to be char-
0.001 mm and less, with 50% coarser than 0.003
acterized. To do this we conducted infiltration tests
mm (by weight) (Lawson and Brockett 1993). Set-
both in the laboratory and in situ.
tling out these fines in a reasonable time frame in
the area allotted is not possible, so a large number
Laboratory tests
of very fine particles may be decanted off along
We hypothesized that the sediments from the
with the supernatant. Along with these particles,
dredging operation would quickly clog the voids
particles of larger size may become resuspended
in the gravel pad and significantly reduce the hy-
during the decanting process. Concern over recon-
draulic conductivity. It was expected that the sub-
tamination of the Flats by these larger particles was
sequent buildup of sediment would further reduce
expressed, so filtering investigations using sedi-
the infiltration rate into the EOD pad. To gain ad-
ments from the Flats were conducted at CRREL
ditional insight on the sediment's effect on infil-
(Henry and Hunnewell 1995, Henry et al. 1996).
tration, we conducted laboratory tests to deter-
After extensive testing using a modeling flume
mine the hydraulic conductivity of the sediment.
built at CRREL, a candidate fabric was chosen that
The following relationships were used in deter-
effectively filters particles 0.1 mm and larger.
mining hydraulic conductivity:
Filtering efficiency of the selected fabric is 73%,
Havg
= Average height of sediment (cm)
and when the fabric was incorporated in a system
∆t
= Elapsed time (sec)
that allowed for sedimentation of the mixed spoils,
h1
= Initial head of water at t = 0 (cm)
efficiencies approached 99.8% (Table 4). These ef-
= final head of water at t = ∆t (cm)
h2
ficiencies were attained even when scraping the
upstream side of the fabric to enhance flow rates.
Havg
.
k=
Filtering efficiencies will vary according to settle-
h1
∆t ln
(7)
ment time, and attempting to filter particles with
h2
diameters smaller than 0.1 mm is impractical due
to clogging of the fabric and low flow rates. One
These tests were performed in the sedimentation
important use of the silt fence is as a secondary
tube (Fig. 2) on the sediments formed during the
impoundment component in case the weir fails
previous laboratory sedimentation tests. The re-
(Fig. 4). This is important in ensuring that the Flats
sults are shown in Table 5 and Figure 10. In all
do not become recontaminated in the case of catas-
four tests, the hydraulic conductivity initially ex-
ceeded 1 104 cm/s in the first three or four hours
trophic weir failure.
and gradually fell to about 2 105 cm/s within
eight to 24 hours. This confirmed our expectation
Site characterization
With the basics of the retention basin concep-
that the sediment itself might not reduce the hy-
draulic conductivity below the targeted 1 105
tualized and laboratory test results supporting our
cm/s level.
design considerations, the surface hydraulic con-
Table 4. System filtering efficiencies using Texel GEO 9 filtering fabric (From
Henry and Hunnewell).
Final total
Filtering
Retained on
#200 (74 m)
Flow rate
suspended
efficiency:
[(m3/m2)/min]
Test #
Geotextile
solids (mg/L)
system (% )
sieve (%)
1
No
DNM*
9249
95.3
DNM*
3
No
DNM*
6360
96.8
1
7
No
DNM*
14466
92.7
3
2
Yes
0.021
654
99.7
<0.1
4
Yes
0.046
1185
99.4
<0.1
6
Yes
0.028
1465
99.3
DNM*
*Did not measure.
13
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