under ideal conditions in the laboratory and in the
course, is the increase caused by the chlorine
bleach. The consequences of those results are im-
field, are not adversely affected by freezing and
portant reminders of the care required in conduct-
thawing. This is in sharp contrast to the behavior of
ing tests on GCLs and in using them in landfills.
compacted clay soils. Thus, there appears to be an
advantage of using a GCL or a sandbentonite mix-
ture in place of a compacted clay in that much of
GCL field test pond test results
The three ponds constructed in 1992 did not
the frost protection layer can be eliminated. How-
have working seepage collection systems because
ever, the advantages of using sandbentonite mix-
of unrepairable leaks in the leachate collection sys-
tures are not so great as using the GCL systems.
tems. Nonetheless, some useful observations were
With sandbentonite mixtures, costs will be saved
made of how they held water. When constructed,
by eliminating the frost protection layer, but costs
each pond had a seam and a slice that was located
will also increase because bentonite clay and the
over the seepage collection system. The slice was
bentonite with sand will have to be purchased. The
placed to investigate the effectiveness of self-
sandbentonite mixtures require 10 to 20 times as
healing of the bentonite in the GCL after freezing
much bentonite as there is in a GCL system, and the
and thawing.
mixture must be very uniform, as any regions of
The slice in the Gundseal product allowed a
low bentonite content can be a path of low resis-
high rate of seepage immediately upon filling of
tance for water flow. Thus, special equipment is
the pond. Initial attempts to fill the Gundseal GCL
needed to thoroughly mix the sand, bentonite, and
pond showed that it would not hold water. The
water prior to its being compacted in place.
sliced area was uncovered, and the slice was found
Thus, the advantage of using a GCL in place of a
to have widened from a slit to roughly 1.5 cm
compacted clay soil is not just in the frost resis-
across. It appears that after the warm HDPE was
tance, but it is also in the cost savings resulting
buried and water was added, it cooled and subse-
from the elimination of much of the frost protection
quently contracted, causing the slice area to open,
layer and from the increased storage capacity for
allowing water to seep out. A 10-cm-wide Gund-
waste material achieved. Geosynthetic clay liner
seal GCL patch strip was placed over the slice, and
systems can cost in place about the same as com-
the gravel cover was replaced. After the patch was
pacted clay liners, depending on the local price of
placed, the pond retained water.
compacted clay. Therefore, there may not be any
The pond lined with Bentomat GCL did not
savings in the hydraulic barrier itself. It is the expe-
hold water soon after construction. We found that
rience of the second author (AEE) that a GCL liner
some seams were constructed without sufficient
may cost more or less in place than a compacted
bentonite in the overlap. An attempt to excavate
clay layer, generally a little more. So, for this dis-
and repair the seams was made, but it was ineffec-
cussion, we assume that the costs are the same.
tive, and the pond continued to leak.
With a GCL, all but about 1 ft (0.3 m) of the soil
The Claymax GCL pond held water for the sum-
normally required for the frost protection layer can
mer of 1993 (the summer after the first winter), but
be eliminated. Some soil is still needed above the
did not hold water after the second winter. A rea-
cover barrier as a medium to grow grass and to
son for this change could not be found.
protect the GCL from mechanical and ultraviolet
The three GCL pond studies reveal some of the
damage. This layer is still protective, but its pri-
problems of conducting field studies with GCL
mary enemy is not frost. The GCLs are also much
barrier systems. Because the hydraulic conductiv-
thinner than the compacted clay layer for which
ity of the hydrated GCL is so low, no leaks in the
they can be substituted: the hydrated GCL is about
leachate collection system can be tolerated. Fur-
0.5 in. (13 mm) thick and the normal compacted
thermore, these studies show the limitations of
clay layer is 2 ft (0.6 m) thick. Figure 15 schemati-
using the GCL systems under field conditions. Any
cally illustrates the increased storage capacity
imperfections in the seams or stress on cuts or
gained by the elimination of the compacted clay
defects can lead to significant leaks.
and most of the frost protection layer.
Frost protection layer thickness required
COST SAVINGS USING GCLs
for a compacted clay barrier
We first determined how much frost protection
Cost savings rationale
We have shown that the hydraulic conductivi-
is required over the U.S. We used a freezing index
ties of both GCLs and sandbentonite mixtures,
map from TM 5-818-2, Pavement Design for Seasonal
16
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