Geosynthetics August/September 2019 - 9

GM, GCL OR BOTH?
Q: I am reaching out to you to get clarity
on pond waterproofing material(s).
I am an architect and we are designing a
large school campus for a social organization
in India. On the campus, the client wishes to
have a pond of around 27,000 square feet
(2,500 m2). We have a limited budget, as in
all projects.
The vendor is suggesting a geomembrane
over geosynthetic clay liner. They say this will
result in less chance of leakage and be longer
lasting. We are planning to bury the entire
base surface of the geomembrane with
12-18 inches (30-46 cm) of soil covered with
2 inches (5 cm) of sand, for plants to grow
into the water.
What do you think would be the most
appropriate material to use: a geomembrane,
a geosynthetic clay liner or is it a substantially
good investment to have both?
I appreciate your online material, and I am
looking forward to hearing from you.
A: Clearly your vendor is aware of the liner
status for landfills and ponds containing
unusual or hazardous liquids. In such cases,
a geomembrane (GM) over a geosynthetic
clay liner (GCL) is the preferred strategy.
Any leakage passing the GM is captured
(attenuated) by the bentonite of the GCL.
The two materials together are what is
called a composite liner system. For a
pool situation you, as the designer, have
to assess how the GM will be placed, the
possibility of leakage, the loss of water,
erosion beneath the GM, the possibility
of interaction with subsoil constituents,
the possibility of bubbles (called whales
or hippos) lifting the GM, damage from
maintenance equipment during service
lifetime, etc. As such, it is dicey to use the
GM by itself. If the budget is available, a
GM/GCL composite is the way to go.

RECONSTRUCTING
A FUSION SEAM

SIZE OF THE U.S.
GEOTEXTILE MARKET

Q: I was discussing with my boss the
viability of reconstructing a fusion seam
that failed destructive testing by cutting
off the overlap and extrusion welding over
the edge of the existing weld. My boss
said the second heating of the material
ultimately causes a weakening of the bond
between the adjacent panels, but if I could
find anything to the contrary, he would
take it into account. Are there any recent
studies that show it is acceptable to extrude
a fillet weld over an existing fusion one,
and does this vary between 40 and 60 mil
(1 and 1.5 mm) thicknesses of high-density
polyethylene (HDPE) and linear low-density
polyethylene (LLDPE)?

Q: Hello, I am an MBA student
conducting research on the current
geosynthetic market in the U.S. I've found
some interesting information regarding
geotextiles and their use in highway
construction and erosion control.
I'm having trouble finding a number
to gauge the size of this industry. I
understand this isn't a technical question,
but I saw your note about this contact
being open to the world and thought I
would ask. If you know of any resources
that would help, I'd love to be pointed
in the right direction.

Thank you for your consideration.
A: The concern over the regularly used
technique of extrusion welding over a
fusion seam previously made by a wedge
welder has been discussed often in light
of weakening of the original seam, its
geomembrane on either side of the fusion
weld, or if the geomembrane on either
side of the proposed extrusion weld has
been raised. However, this is not from the
concern over weakening the bond, but
instead over crystallizing the adjacent
welding area, possibly leading to stress
cracking sensitivity.
We did some testing after several melt
cycles and found no degradation of the
HDPE in the area of the fusion seams.
Since LLDPE is less crystalline than HDPE,
I would think it is not sensitive either.
While not common, and if acceptable by
the construction quality assurance (CQA)
inspector, I would say okay. Regarding
the testing of the new seam, the vacuum
box method could be upgraded to using a
copper wire in the weld area and passing an
electric wand over it, which might be more
comforting to all involved. Best wishes.

A: The last time the Geosynthetic
Institute surveyed the industry insofar
as geotextiles are concerned was in
2012. At that time the worldwide market
was approximately 1.4 billion m2 at
approximately $0.75/m2 for a total of
$1.05 billion. You can increase those
numbers by about 3-6% per year to
bring things up to date. G

>> For more, search Techline at
www.GeosyntheticsMagazine.com
or email your technical questions
to gmatechline@ifai.com.

www.GeosyntheticsMagazine.com

9
http://www.GeosyntheticsMagazine.com http://www.GeosyntheticsMagazine.com
Geosynthetics August/September 2019

Table of Contents for the Digital Edition of Geosynthetics August/September 2019
