Effects of Freeze/Thaw on Landfill Clay Covers
Introduction
This is an Environmental Systems term project written by John Burns, Tara
Cunningham, and Will Lindheimer. The assignment is based on undergraduate
research. The material is very general because much of the information
used was not yet published. Also, since this is ongoing research, many
results have not yet been tabulated. The project is a basic summary of
freeze/thaw effects on compacted clay covers.
Brief Overview
Compacted clay is widely used as a water or hydraulic barier in waste containment
structures. A major application is landfill cover
design. The basic principle behind hydraulic barriers are to minimze the
flow of water. The reason why water is damaging to landfills is because
that water has the ability to carry and transmit many hazardous chemicals
to other locations. The purpose of the landfill is to keep the waste confined.
To do this we must limit water from entering the landfill. Therefore the
permeability of the clay cover is very important.
Studies have shown that freeze/thaw effects can increase permeability
of clay liners one to two orders of magnitude. This could be very detrimental
to the landfill and the surrounding environment. The increase in permeability
is caused by the formation and expansion of ice in the soil. When the soil
thaws, cracks are left creating pathways for
water movement.
Simulation of Freeze/Thaw Effects in the Laboratory
To simulate the effects of freezing and thawing on clay liners, we took
soil samples from actual landfill sites. Next, we compacted the soil using
standard comapction techniques. We also maintained the water content of
the sample at approximately 19% for a realisic consistency. We then preceeded
to run permeability tests on a number of samples to compare to theoretical
values of permeability.
After the permeability tests were completed, we subjected the
samples to one-dimensional freezing. To obtain one-dimensional
freezing, we placed the samples in insulation with a heating pad placed
at the bottom to simulate the warm ground and cooled the air to a temperature
below freezing. Next, we used thermal couples to measure the temperature
at different heights in the clay sample to insure that freezing was occuring.
Finally, we thawed all samples and ran permeability tests to check
for increase in permeability.
Reality of Freezing in Actual Landfill Covers
The next step is to see if landfill covers actually freeze in field conditions.
To test the freeze/thaw potential we gathered information on two separate
landfills covers in the northeast. This information consisted of obtaining
temperatures at various heights within the clay layer. We found that at
both sites the covers have not yet frozen in the clay layer.
This makes us question if freeze/thaw is cause for concern in
our climate. In extreme climates the effects could be more damaging.
Conclusion
We must reiterate that the permeability of a clay liner is very important.
It prevents the spread of harmful contaminants into the environment. Test
results have shown that the freezing and thawing of clay substantially
increases the permeability of the liner. Although we have not completed
our lab tests, we can assume the same results. We have noticed a trend
that in certain moderate climates freeze/thaw effects may be negligable.
Extra Information