al. 1990, Sowers 1993). These authors found some
the GCLs works in much the same way as in the
sandbentonite mixture. In this case, the bento-
or no change in hydraulic conductivity from tests
nite is in a continuous layer, held in place by the
conducted on samples taken with thin-walled
geosynthetics, that hydrates and forms a barrier
tubes. However, the laboratory evidence has been
to moisture flow.
so convincing for compacted clay soils that regu-
The goals of the study were to improve the
lations and guidelines have frequently required
understanding of how freezethaw affects these
that compacted clay liners and covers be protected
materials, to improve the design and construction
against freezing.
process, and to reduce costs and increase produc-
To protect a compacted clay cover from freez-
tivity. The scope of the project was developed to
ing, the normal method employs a thick layer of
gather data to be used in answering the following
fill on top of the compacted clay layer. This fill
questions:
layer has several functions. It provides a medium
upon which to grow grass to control surface ero-
Do freezing and thawing increase the
sion; it provides a medium to keep the hydraulic
hydraulic conductivity of compacted clay
barrier moist and prevent it from drying and des-
soils, sandbentonite mixtures, or GCLs
iccating; it acts as a barrier to ultraviolet light for
under natural conditions?
geosynthetic components of the cover system; it
Do laboratory hydraulic conductivity tests
acts as a barrier to burrowing animals; and it acts
accurately predict the hydraulic con-
as an insulation layer to prevent freezing of the
ductivity of clay liners and covers in the field
clay component of the cover system.
after freezing and thawing?
Only 12 ft (0.30.6 m) of fill soil is required as
If laboratory tests can be used to predict the
the protective cover in regions where frost is not a
hydraulic conductivity in compacted clay
problem, the thickness primarily depending
covers and liners after freezing and thawing,
upon the type of grass used and the amount
then why have past studies failed to do this
of precipitation. The thickness of the frost-
and what can be done to improve the predic-
protection fill layer is usually estimated and is
tive methods?
often excessive because of uncertainties in the
If freezing and thawing do increase the hy-
estimate. If the compacted clay does freeze, for
draulic conductivity of these cover and liner
instance during construction of a liner or cover or
materials, how much protective soil cover is
during an especially severe winter, regulatory
required to prevent freezing?
rules typically require specific actions to show
What changes can be made in the design and
that there was no measurable frost damage. This
construction technology for soil liner sys-
issue has generated heated debate because
tems in cold regions that will reduce costs or
several studies have shown that no damage was
improve productivity?
caused by freezing and thawing. However, Ben-
What are the cost savings that the new tech-
son et al. (1994) and Chamberlain et al. (1990)
nology will bring to soil liner systems in cold
(and this report) show clearly that the negative
regions?
findings of frost damage are an artifact of the field
sampling method. This report tells us that large
increases in hydraulic conductivity are caused in
BACKGROUND
compacted clay soils by freezing and thawing,
Over recent years, a considerable debate has
and that the sampling and test methods are criti-
developed regarding the effects of freezing and
cal to accurately identifying these changes.
thawing on the hydraulic conductivity of the soil
In contrast to the results for compacted
components used in landfill covers and liners.
clay, past laboratory testing of GCLs and sand
While laboratory test results have consistently in-
bentonite mixtures has shown that freezethaw
dicated that freezing and thawing cause large in-
has no deleterious effect on hydraulic conductiv-
creases in the hydraulic conductivity of compact-
ity. Several projects by Chen-Northern (1988),
ed natural clay soils (e.g., Chamberlain et al. 1990,
Geoservices (1989), and Nelson (1993), and more
Kim and Daniel 1992, Bowders and McClelland
recent investigations by Kraus and Benson (1994),
1993, Othman and Benson 1993), information
show a statistically significant decrease in hy-
from definitive field studies has not been avail-
draulic conductivity after freezethaw testing of
able. Furthermore, the field evidence that is avail-
three GCLs. However, since field verification of
able is conflicting (e.g., Starke 1989, Paruvakat et
these results on GCLs and sandbentonite mix-
2
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