Geosynthetics April/May 2022 - 9

PERMEABILITY OF GEOMEMBRANES
TO METHANE GAS
Q: What is the least permeable
geomembrane to methane gas? Regarding
the permeability of geomembranes to
methane gas, could you send me some
technical guidelines?
A: High-density polyethylene (HDPE)
geomembranes are denser (more crystalline)
than linear low-density polyethylene
(LLDPE) geomembranes and less permeable.
The water vapor, solvent and methane
transmission (diffusion) of an HDPE barrier
is better (less) than an LLDPE. However,
sometimes it still may be too high to be
suitable as a barrier for some applications.
In such cases, ethylene vinyl alcohol
(EVOH) is used as a layer sandwiched
between polyethylene layers (plies) in the
geomembrane cross section to give even
lower gas permeability.
Refer to the Los Angeles County
Department of Public Works (2020)
geomembrane specification for additional
technical guidelines and a prescribed
testing protocol. Stark and Choi (2005) is
also useful in this regard.
Los Angeles County Department of Public Works.
(2020). " Gas hazard mitigation policy and standard. "
Feb. 3. https://dpw.lacounty.gov/epd/swims/docs/pdf/
methane/methane%20packet.pdf.
Stark, T. D., and Choi, H. (2005). " Methane gas migration
through geomembranes. " Geosynthetics International,
12(2), 120-26.
IMPREGNATION OF GCLS
Q: We are confused with the term
" impregnation " as it relates to seaming
geosynthetic clay liners (GCLs). There
are two types of impregnations: (1) Edge
impregnation, and (2) complete nonwoven
(cap layer) impregnation of GCLs. How does
a manufacturer do this?
A: The edge impregnation of GCLs (1)
is done in the field by an installer with a
bentonite paste or powder for sealing the
longitudinal or butt-seamed edges. The
supplemental bentonite would need to
be placed continuously along the edges
at a width of 5.9 to 7.9 inches (150 to 200
mm). The complete nonwoven (cap layer)
impregnation situation (2) is typically
accomplished with a polymer spread
coating applied in the factory.
WELDING HDPE GEOMEMBRANES
WITH BIOGAS PRESENT
Q: We are a Brazilian company specializing
in biogas plants. We have built a big lagoon
biodigester with a high-density polyethylene
(HDPE) geomembrane cover. Now the cover
is leaking liquid contained in the lagoon:
We are organizing the repair procedure, and
we would like to first investigate state-of-art
practices and procedures for welding HDPE
geomembranes with biogas present.
Under the cover, there is residual biogas
storage with the following composition:
CH4: 60%
CO2: 38%
H2S: 2%
Considering this, we cannot have sparks in the
gasholder area, and all the equipment used to
do the repair in this area should be explosion
proof. Can you give some advice or the
contact information for a specialist in welding
HDPE geomembranes with biogas present?
A: You are correct in staying away from all
seven of the leak detection methods covered
by ASTM International. All use electricity,
which presents a spark risk for your biogas
site. With that written, there are still two
nondestructive tests that have been used
successfully to detect leaks in storage facilities
such as yours.
The first is to use positive pressure in the
storage unit and a soap solution mopped or
sprayed on the geomembrane surface. The
solution will bubble profusely when it meets
escaping air.
The second is smoke testing the storage
facility as described in GRI GM26, " Practice
for Smoke Testing of Field Fabricated
Geomembranes. " It is easy, inexpensive, quick
and works great in applications like yours. I
would certainly consider it.
QUESTIONER RESPONSE
First, I really appreciate your fast reply. Thank
you for your availability and kindness.
Once we detect the leakage, our main
concern consists in the risk of realizing the
welding HDPE geomembrane with biogas
likely present below the cover geomembrane.
Please find a simple sketch below (Figure 1):
Of course, we will try to extract the highest
volume of biogas possible before realizing the
welding with the compressor that we use to
send the biogas to the engines; nevertheless,
we cannot guarantee that there is 100% no
biogas below the cover.
So, my questions are:
1. Is there some welding machine that can
be used in a designated Appareils destinés
à être utilisés en ATmosphères EXplosibles
(ATEX) [French for " Equipment intended
for use in explosive atmospheres " ] area?
2. Do you know alternatives to weld
geomembranes that do not require a
welding machine?
In case the answers to 1 and 2 are no, what is
the state-of-art practice used in the U.S. for
these cases?
A: I know this type of repair very well. It is
predicated on being able to walk on the cover
geomembrane or being lifted above it. This
is dangerous work: Please exercise extreme
caution. I know of an inexperienced installer
killed attempting this repair. When successful,
it is carried out in the following three steps
by a roped-off certified installer, typically in
a bucket suspended from a crane:
1. Deflate a beach ball and then insert
it through a hole and inflate it. This
provides a clean, dry, high and stable
working surface.
2. Use TITUS Industrial Group Inc.'s tape or
equivalent to create a temporary seal.
3. Extrusion weld to make the final patch.
There is typically limited construction quality
assurance (CQA) testing on the patch.
You will need a very skilled installer to do
this work.
hole in the cover
liquid leakage
FIGURE 1 Cross-sectional schematic showing hole in geomembrane cover
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Geosynthetics April/May 2022

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