Bentonite aggregates appeared to collect and
cover and liner systems. The Parkview clay is a
glacial till deposited during the last glacial
build up in the space between the auger and its
advance over southeast Wisconsin. It occurs in
protective cover. To correct for this problem, dry
dense deposits at the test site and contains peb-
bentonite was added to the stockpile containing
bles, cobbles, and occasional boulders. The Valley
the lean sandbentonite mixture to make up for
Trail clay is a glacio-lacustrine clay from east-
bentonite stuck in the mixing machine. These
central Wisconsin. It is varved and relatively free
components were blended on the ground with a
of large particles. Nearly all of the Valley Trail
loader bucket to create a homogeneous mix with
clay passes the no. 200 sieve (less than 0.074 mm),
the desired bentonite content. Unfortunately,
whereas only 80% passes for the Parkview clay.
additional QC testing after construction showed
Compaction curves for these soils were devel-
that, even with these efforts, the sandbentonite
oped using ASTM D698 procedures. The maxi-
mixture varied in bentonite content throughout
mum dry unit weights for these soils are: Park-
the test section.
view clay--117 lb/ft3 (1874 kg/m3), and Valley
Trail clay--111 lb/ft3 (1778 kg/m3). The optimum
Geosynthetic clay liners
Three different GCLs (Claymax, Bentomat,
water contents for the Parkview and Valley Trail
Gundseal) were used in this study. These GCL
clays are 13.7 and 18.0%, respectively.
materials were provided by the CPAR partners.
Each uses dry granular bentonite as the moisture
Sandbentonite
The sandbentonite (SB) mixture was pre-
barrier medium to prevent flow through the GCL.
pared in the field so that it represented a typical
However, the geosynthetics used to contain the
sandbentonite liner material. The base was a
bentonite varied. Claymax and Bentomat each
clean mortar sand purchased from a local con-
use two geotextiles whereas the Gundseal uses a
crete supplier. It was classified as poorly graded,
single HDPE membrane. The backing materials
clean, medium to fine sand (SP). More than 90%
for the Claymax and Bentomat GCLs are porous
of the sand particles passed the no. 30 sieve (0.420
and they allow moisture to pass through to the
mm) and less than 5% passed the no. 200 sieve
bentonite. The bentonite in the Claymax and Ben-
(0.074 mm). A granular sodium bentonite (Ameri-
tomat is held in place in between the two geotex-
can Colloid CS-50) with no polymer additives
tiles by cross stitching. The bentonite granules are
was used as the admixture to control hydraulic
bonded to the HDPE membrane in the Gundseal
conductivity. The theory is that the bentonite,
product, the membrane itself being a barrier to
when well mixed with the sand, will hydrate on
water flow. The Claymax and Bentomat GCLs
exposure to water to swell and block the flow of
function as barrier to moisture flow by allowing
water through the continuous void paths in the
moisture to flow through the porous geotextiles
sand. A target mixture containing 9% bentonite
to the bentonite. The bentonite clay then hydrates
was selected on the basis of laboratory hydraulic
and swells, forming a putty-like filling between
conductivity tests. Compaction curves for the se-
the two layers of geotextiles. This hydrated ben-
lected mixture indicate that the maximum dry
tonite then forms the hydraulic barrier. In the
unit weight is about 108 lb/ft3 (1730 kg/m3) and
Gundseal product, the HDPE membrane is the
optimum water content is 16%. The index proper-
hydraulic barrier. The bentonite is there to ensure
ties for the sandbentonite (SB) mixture are given
that the membrane does not leak should a hole or
in Table 1.
slit form during the manufacturing or construc-
The sand and bentonite were combined inside
tion process. Should a leak form in the mem-
a mobile concrete mixing truck in an attempt to
brane, the bentonite on the Gundseal GCL will
produce a homogeneous, uniform mixture. The
hydrate and swell to form a moisture barrier.
mobile concrete mixer fed dry sand and bentonite
All three of the GCLs come in long rolls that are
at a controlled rate onto a belt to a mixing auger.
rolled out on site by special equipment. The GCL
Water was added as the dry materials were drop-
sheets are overlapped during placement to pro-
ping into the auger. The auger was about 2.5 m
vide a continuous barrier. Granular bentonite
long and encased by a rubber boot. The mixer
clay is placed in the overlap seam to prevent leaks
worked well, but initial quality control (QC) tests
from forming between adjacent sheets. Further
showed that some of the mixture contained a
description of these GCLs is provided by Estor-
lower percentage of bentonite than was desired.
nell and Daniel (1992).
4
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