to soak the gravel, and then a mixture of four parts
Figure 14 illustrates how hi and H for each of the
water to one part wet sediment was poured into
barrel tests were measured.
the barrel and allowed to settle overnight. The
The EOD pad barrel percolation test results are
barrel (Fig. 14b) was then filled to a depth of about
shown in Figures 15 and 16 and Table 8. Figure 15
30 cm and water from the Flats was used to esti-
shows that the estimated hydraulic conductivity
mate the hydraulic conductivity of the sediment.
for the bare gravel pad was in the range of
1 102 to 1 103 cm/s, very high as we expected.
To determine the effectiveness of peaty silt in lim-
iting the infiltration of water into the gravel pad,
This result validated our assumption that the wa-
a layer about 14 cm thick was compacted directly
ter pressure at the bottom of the barrels in the
on the gravel inside a test barrel (Fig. 14c). A wa-
gravel pad was roughly zero. One dose of sludge
tersediment mixture was then poured into the
(sedimentwater mixture) reduced kest to less than
1 103 cm/s and a second dose further decreased
barrel over the peaty silt and allowed to settle. The
kest to about 1 104 cm/s, still short of reaching
hydraulic conductivity was then estimated using
the target level of 1 105 cm/s. Figure 16 shows
the same procedures as for the first two barrel
tests.
that a layer of compacted peaty silt will help
The infiltration rates in the EOD pad barrel tests
achieve that goal. In this case the sludge was
were used to estimate the hydraulic conductivity
poured over the peaty silt in the bottom of the
of the gravel, the sediment, and the peaty silt. The
barrel. The resulting estimated hydraulic conduc-
tivity was about 4 106 cm/s. This result vali-
Darcy's equation for laminar flow:
dated the laboratory observations for the peaty silt.
To further substantiate these positive results,
q = ki
(8)
barrel infiltration tests were also conducted di-
where
rectly on the compacted peaty silt liner in the re-
q = the velocity of flow (cm/sec)
tention basin. The locations of these tests are
i = the hydraulic gradient (= h/H)
shown in Figure 17. The peaty silt was compacted
h = the pressure loss across the sample (cm)
with several passes of a smooth vibratory drum
H = the thickness of the sample (cm).
roller on a loose lift about 25 cm thick. The water
contents and dry densities for each of the test lo-
The infiltration tests on the EOD pad were con-
cations are given in Table 9. The barrels were in-
ducted with a falling head, i.e., the elevation of
stalled in the peaty silt liner as shown in Figure
the water surface fell during each test. The result-
18. We conducted one test with just Flats water in
ing solution for the hydraulic conductivity for a
the barrel and found the hydraulic conductivity
of the peaty silt to be just slightly greater than 1
test with a falling head is
105 cm/s (Fig. 19, Table 10). At two other loca-
h - h0
H
k=
ln 1
(cm/sec)
(9)
tions we used a sludge mixture as the permeant.
∆t h2 - h0
Figure 19 shows that with a sedimentwater mix-
where
ture similar to that expected from the dredging
∆t = time interval between making the h1
operation, the hydraulic conductivity was reduced
to 1 105 cm/s. Furthermore, it continued to
and h2 readings (sec)
h1 and h2 = water elevation heads at the start and
decrease with time.
end of the test (cm)
The hydraulic conductivities obtained from
h0 = water pressure head at the bottom of
field test results with the peaty silt are slightly
the barrel (cm).
greater than the results obtained for low stress in
the laboratory. The reason for this is that the wa-
For the percolation tests in the barrel tests, the
ter content in the field was much higher than ex-
k value determined is an estimated value because
pected, about 60% rather than 38%. This can be
the pressure head at the bottom of the barrel is
seen in Figure 20, where the hydraulic conductivi-
indeterminate. We assume that the water is free-
ties and dry densities are plotted versus the water
draining at the bottom of the barrel (in the gravel)
contents for both the laboratory and the field tests.
The higher-than-expected water content is prob-
and the water pressure is zero at the bottom of the
ably due to the considerable rainfall that occurred
barrel, i.e., h0 = 0. Thus, eq 9 is reduced to
after the peaty silt was stockpiled in the basin area.
h
H
kest =
ln 1 (cm/sec).
(10)
Nonetheless, the goal of achieving a hydraulic
∆t h2
19
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