10-3
Test results from specimens that were
taken from a region more than 1 ft (0.3 m)
Parkview Clay
below the maximum frost depth in test
10-4
pads PV-3 and VT-4 did not show any
15
increases in hydraulic conductivity.
Shrinkage cracks and ice lens cracks were
Field Frozen
10-5
4
not obviously present in the soil that was
well below the reach of the frost zone and,
Lab Frozen
therefore, an increase in hydraulic conduc-
10-6
1
tivity would not be expected. However, in
the zone within 6 in. (15 cm) of the freezing
front, the hydraulic conductivity was
-7
10
about 1 106 cm/s in the Valley Trail test
0 Freeze-thaw Cycles
section (Fig. 8). Thus, it appears that freez-
10-8
ing affects the hydraulic conductivity some
0
2
4
6
8
10
12
small distance below the freezing front.
Effective Stress (psi)
This is consistent with the prevailing ice-
Figure 13. Impact of effective stress on the hydraulic conductivi- segregation theory that the shrinkage
cracks are formed in the unfrozen soil
ty of Parkview clay after freezing and thawing.
beneath the freezing front because of the
whereas in a liner, there can easily be sufficient
high moisture suction that develops there.
overburden stress to close the cracks and reduce
The effects of stress on crack closure led us to
the hydraulic conductivity to an acceptable value.
examine whether the damage to the clay material
About 25 ft (7.6 m) of overlying soil and waste
caused by freezing could be repaired in place
material would be required to close the cracks in a
without its being removed. Three passes of a
liner constructed with the Parkview and Valley
rubber-tired scraper were made over the test pads
Trail soils. This is an overburden readily achieved
to increase the stress level. Observations of the
in most landfills, except at the margins.
clay structure in the test pads made after the clay
The test specimens taken with the thin-wall
was remolded with the scraper showed that this
sampling tubes after freezing and thawing had
apparently destroyed the blocky structure by
lower hydraulic conductivities than the block and
compressing the shrinkage and ice lens cracks.
core specimens. The hydraulic conductivities for
Blocks caused by shrinkage cracks and ice lenses
these specimens were similar to the values exist-
were not as visible.
ing before freezing (Table 2). One explanation for
Large-diameter blocks were carved from these
this difference is that a tube sampler disturbs the
samples in July 1994. Tests on large blocks
unique soil structure formed during freezing, the
removed from the remolded area showed that the
cutting and inside wall resistance that develops
hydraulic conductivity values obtained before
as the tube is pushed into the ground causing the
freezing were nearly recovered.
increase in the stress on the sample. As was dis-
cussed in the previous paragraph, about 12
Sandbentonite
2 of stress is needed to close the cracks in the
lb/in.
In contrast to the natural clay, comparisons of
clay soils examined in this project. For the tube
our laboratory and field tests on the sand
samples taken from the Parkview and Valley Trail
bentonite mixture showed no effect on hydraulic
test pads, considerable stress was applied to
conductivity from freezethaw. The test results
cause the sampler to penetrate the compacted
are shown in Table 2. The hydraulic conductivity
clay. Apparently, this stress is sufficient to close
measured in field test-pad permeameter SB-1 was
less than 5 108 cm/s in June and July 1994 after
the cracks and reduce the hydraulic conductivity
to the values existing before freezing. This was
two winter seasons. This is roughly the same as
confirmed, as visual inspection of the samples
was measured in the CRREL laboratory tests (be-
removed from thin-wall sampling tubes did not
fore and after freezing) on specimens prepared in
reveal any of the usual blocky structure observed
the laboratory from sandbentonite samples
in the hand-carved large-diameter samples.
mixed in the field.
These results and the effect of sampling technique
The sandbentonite in test pad SB-1 showed
are discussed further by Benson et al. (1994).
none of the blocky structure that was present in
12
Previous Page
