Geosynthetics February/March 2021 - 13

USING POSTCONSUMER WASTE
PE IN GEOMEMBRANES
Q: Do you think it is possible to include other
material besides high-density polyethylene
(HDPE) and linear low-density polyethylene
(LLDPE) in the cross section of a multilayer
geomembrane? Suppose we can run on three
layers cast line, then put recycled material in the
middle. This may be good for a sustainability
purpose, if we can include industrial waste, or
later postconsumer waste in the middle layer
. . . for certain geomembrane grade, perhaps
the less sensitive one.
A: In response to your question of utilizing
postconsumer waste polyethylene in
geomembranes, I do not think this is a good
idea. Geomembranes are critical barrier
materials used in containment systems with
very harsh geoenvironmental conditions. They
need to last hundreds of years in a flawless
manner. Durability is a critical property, which
has to be proven with statistical process control
data for both base resin and masterbatch.
That written, there are always off-specification
products that are used for noncritical projects.
One needs to be very careful in regard to
marketing these products.

GEOMEMBRANE THICKNESS
IMPACTS ON WRINKLE
DEVELOPMENT
Q: As some states continue to review and
" improve " their regulations governing base
liner design, the use of 80-mil geomembrane
in both primary and secondary liner systems
is being considered. At the same time, the
maximum height of wrinkles may also be
regulated. The " stiffer " 80-mil product should
help with wrinkles but comes with more
thermal mass, which may be an issue for black
product. Does GSI have any information on
geomembrane thickness impacts on wrinkle
development or propagation?
A: In regard to your question on high-density
polyethylene (HDPE) wrinkles as they relate
to thickness, I have attached two pages from
Te-Yang Soong's 1996 Drexel University Ph.D.
thesis entitled " Behavior of Waves in HDPE
Geomembranes. " I think they answer your
question directly and well. Unfortunately,
thickness is not a major factor affecting wrinkles.
To achieve intimate contact with the subgrade
(i.e., reduce wrinkles), we recommend the
following for HDPE:
1.	 Push/accumulate/cut/seam

Furthermore, we are seeing advances in polymer
cleaning and pelletizing systems, which can
sometimes yield a very good product; however,
consistency and traceability are issues.

2.	 Fixing berms or advance soil in piles with
a track hoe or gradall excavator with
articulating bucket

INSTALLING CCR LINER
LOW TO HIGH?

4.	 Textured or scrim-reinforced sheet

Q: I am an engineer for a coal combustion
residual site, and I want the installer to install
the liner system starting at the sump and
progress to higher elevations. Please advise.
A: What you are recommending is unusual,
and I am surprised that an installer would
agree to it. I have seen most, if not all, liner
systems installed from high to low gradient.
There are many stakeholders involved in the
panel layout and sequencing of geomembrane
installation. The installer, general contractor,
regulator, designer and owner all have
a schedule and ideas on the matter. The
decision must consider the tasks of subgrade
preparation, approvals, access, geometry,
facility size, weather and groundwater. Know
that if you install a liner system low to high and
have a precipitation event, you are going to
have a mess on your hands because water will
get under the liner system. Furthermore, if the
liner system is a composite one (i.e., GM/GCL),
the clay component will saturate, hydrate,
swell and need to be reworked prior to sign-off
and approval.

3.	 White geomembranes or light color
geotextiles to reduce surface temperature
5.	 Morning- or night-controlled backfilling
when temperatures are low

BIOLOGICAL DEGRADATION
OF PE AND PP
Q: Can you please enlighten me on the
biological degradation of polyolefin
polyethylene (PE) and polypropylene (PP)?
A: Within the various plant forms of biological
life, i.e., bacteria, actinomycetes, fungi and
algae, polymer degradation is essentially
nonexistent due to high molecular weight of
the common resins used in PE geomembranes.
In order for such degradation to occur, the
chain ends must be accessible, and this is
highly unlikely for molecular weights greater
than 1,000, let alone 10,000 to 30,000, which
is common for geomembrane resins such as
polyethylene or polypropylene (Koerner et
al. 2007, Rowe and Sangam 2002, RestrepoFlórez et al. 2014). Biological degradation
might be possible for plasticizers or additives
compounded within polyvinyl chloride (PVC),

but information is not authoritative on this
subject and many current plasticizers can be
made biologically resistant (Rollin et al. 2005,
ASTM G160 and D3082, ISO or EN 12225).
Within the higher forms of biological life, i.e.,
protozoa, spiders, insects, moles, rats and small
mammals, polymers are not a food source and
thus are unlikely to be consumed. It is possible,
however, that an animal may try to penetrate
the geosynthetic for access to the opposite
side. In this case, hardness of the predator's
teeth enamel versus the geomembrane's
hardness is the key comparison. While such
events are possible, authoritative information
is not known by the author.
Verification of biological resistance is
confirmed by soil, sewage or sludge burial tests
(ASTM G160 and D3082, ISO or EN 12225). It is
usually carried out for long exposure times, at
nearly neutral pH and at elevated temperature.
The test specimens are periodically removed
from the soil and tested for changes in
properties. The extent of the degradation is
also examined by way of surface microscopy
and various fingerprinting techniques.

References
ASTM D3083. Specification for Flexible Poly (Vinyl
Chloride) Plastic Sheeting for Pond, Canal, and
Reservoir Lining (Withdrawn 1998).
ASTM G160. Standard Practice for Evaluating Microbial
Susceptibility of Nonmetallic Materials by Laboratory
Soil Burial.
ISO, EN 12225:2000. " Geotextiles and geotextile-related
products: Method for determining the microbiological
resistance by a soil burial test. "
Koerner, G. R., Hsuan, Y. G., and Koerner, R. M. (2007).
" Durability of geosynthetics, " chapter 3, Geosynthetics
in civil engineering, ed. by R. W. Sarsby. Woodhead
Publishing, CRC Press, Cambridge, England, pp. 36-61.
Müller, W. (2007). HDPE geomembranes in geotechnics.
Springer-Verlag, Berlin, Germany, p. 384.
Restrepo-Flórez, J. M., Bassi, A., and Thompson, M.
(2014). " Microbial degradation and deterioration of
polyethylene-A review. " International Biodeterioration
and Biodegradation, 88, pp. 83-90.
Rollin, A., Pierson, A., Lambert, M., and Christopher, B.
(2005). " Geomembranes: A guide to material selection. "
Proc., Geo-Frontiers 2005, Austin, Texas, ASCE Press, pp.
313-327.
Rowe, R. K., and Sangam, H. P. (2002). " Durability
of HDPE geomembranes. " Jour. of Geotextiles and
Geomembranes, 20, pp. 77-95. G

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