Figure 12. Test pit showing blocky soil structure
Figure 11. SEM photograph of a crack caused by
caused by freezethaw cracking--Valley Trail test pad.
freezethaw in Parkview clay.
causes the cracks to close. This explains why pre-
after thawing. Excavation in the test pads after
vious studies employing thin-walled tube sam-
thawing was complete exposed structures (Fig.
plers have drawn the conclusion that freezing
12) consisting of many blockaggregates, approx-
imately 1 in.3 (1.6 cm3) in size, separated by verti-
and thawing does not affect the hydraulic con-
ductivity of clay covers and liners. Benson et al.
cal and horizontal cracks.
(1994) discuss these tests in detail.
We believe that the high hydraulic conductivi-
To show this stress effect, hydraulic conductiv-
ties measured after freezing and thawing are rep-
ity tests were also conducted on test specimens at
resentative of the field-scale hydraulic conductiv-
higher stress levels than normally used in the
ities of the test pads. The test results obtained
back-pressure permeameter--i.e., at stress levels
with the large-diameter samples and with the
up to 10 lb/in.2 (69 kPa) in comparison with the 1
samples taken frozen agree very well (Fig. 8, top
lb/in.2 (6.9 kPa) effective stress level normally
panels). Furthermore, the results of the laboratory
used in our tests. Figure 13 shows that increasing
freezethaw tests (Fig. 8, third panels) also agree
stress decreases the hydraulic conductivity of the
very well with the results obtained with the field
Parkview clay and that about 12 lb/in.2 (83 kPa)
samples, making the laboratory test method a
of stress is needed to decrease the hydraulic con-
good way to predict field response of hydraulic
ductivity to its value before freezing.
conductivity to freezing and thawing.
The increasing stress test results also show
However, the hydraulic conductivities deter-
where, in a landfill or hazardous waste site, freez-
mined from test specimens sampled with the
ing and thawing has its greatest impact. In covers,
thin-walled tube after freezethaw (Fig. 8, bottom
even with several feet of frost protection, the max-
panels) do not agree at all with the other field and
imum effective stress will not generally exceed 3
laboratory test results. The values for hydraulic
or 4 lb/in.2 (21 or 28 kPa), whereas the stress level
conductivity after freezethaw were not signifi-
in a clay liner can easily exceed 12 lb/in.2 (83 kPa).
cantly changed from the values obtained before
The stress level in a cover will, thus, in most cases
freezing and thawing. It appears that the pressing
not be great enough to close the cracks and reduce
of the thin-walled tube sampler into the dense
the hydraulic conductivity to an acceptable value,
clay matrix increases the stress on the soil and
11
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