Geosynthetics June/July 2020 - 18

Assessment of condition of an uncovered geosynthetic landfill bottom liner system

Low bentonite
content in GCLs
near toes of slopes
has not previously
been reported in
the literature.

Extensive quantitative analysis has
been conducted on the samples of the
geomembrane and GCL materials collected from the field. Results for oxidative induction time (OIT) for the
geomembrane are presented in Tian
et al. (2019), with tests on mechanical
properties to be presented elsewhere.
For the GCLs, preliminary data on mass
per area, water content, swell index and
hydraulic conductivity are presented
in Williams et al. (2018). Analysis on
cation exchange capacity and mineral composition of the bentonite in
the GCLs, membrane behavior of the
GCLs, and temperature and precipitation cycles at the site are to be presented elsewhere.

Conclusion
Condition of an uncovered single composite bottom liner system that consisted of two geosynthetic layers (geomembrane over GCL) is provided. The
liner system was exhumed after 12 years
of exposure to the environment at a
municipal solid waste landfill cell. The
sides of the cell were relatively steep
at 2H:1V slopes. The geomembrane
was observed to undergo diurnal cycles
of expansion and contraction due to
temperature changes with 6- to 8-inch
(150- to 200-mm) high gaps between
the geomembrane and the GCL due to
wrinkling along the base of the cell with
less space present between the geosynthetics along the slopes. The condition of the geomembrane was uniform
based on visual observation alone, with
testing required to evaluate the properties of the material. In contrast, the
condition of the GCL was observed to
be highly variable. The GCL was dry
near the top (with granular bentonite)
and relatively wet at the bottom (with
hydrated gel bentonite). Significant
migration of bentonite from the GCL

with accumulation between the geomembrane and the GCL, deposition
of bentonite over the GCL, and erosion of the bentonite within the GCL
were observed. At locations near the
slope crest and in the vicinity of a sump
near the toe, nearly all of the bentonite had eroded away from between the
cover and carrier geotextile sheets of
the GCL. The GCL panels underneath
the geomembrane were separated at
multiple places, where separation was
observed at eight locations along seven
separate seams (from a total of 43 seams
at the cell) with gaps approximately
0.8 to 8.7 inches (20 to 220 mm) in
width and 5.6 to 55.8 feet (1.7 to 17 m)
in length. Degradation of the condition of the GCL was apparent for the
exhumed liner system, and the observations herein highlight the vulnerability of GCLs under exposed conditions.
Covering the liner system could have
reduced the extent of the deterioration
of the material condition. Accumulation
of subgrade soil between the geomembrane and GCL near sump locations
identified in the investigation constitutes a new concern for GM-GCL liner
systems. This mechanism also may be
applicable to covered conditions and
requires further investigation.

Acknowledgments
Waste Connections Inc. and Cold
Canyon Landfill are acknowledged for
allowing site access and sampling of the
liner system. Dr. Amro El Badawy, Kyle
O'Hara, John Buringa, Sean Herman and
Spencer Jemes assisted with sampling.
Anthony Trujillo assisted with graphics.
References
Benson, C., Albright, W., Fratta, D., Tinjum, J.,
Kucukkirca, E., Lee, S., Scalia, J., Schlicht, P., and Wang,
X. (2011). Engineered covers for waste containment:
Changes in engineering properties & implications
for long-term performance assessment, NUREG/CR-

Geosynthetics | June July 2020

0620GS_p10-19.indd 18

5/20/20 5:38 PM

Geosynthetics June/July 2020

Table of Contents for the Digital Edition of Geosynthetics June/July 2020