Geosynthetics August/September 2020 - 11

GEONET-TO-GEONET CONTACT
Q: I have come across an interesting scenario
on-site; however, this time it is to do with
geocomposites and how we connect their
end seams.
What I have noticed is that the way in which
the geocomposite is manufactured causes
the geotextile to have a strong bond to
the geonet core at the ends (sides are not
bonded). This is for good reason. However,
it then creates a difficult scenario on-site as
that bond needs to be broken to create the
overlap. I have noticed that process is very
difficult and, in some circumstances, tears the
geotextile, thus compromising the design.
Practically speaking I can see it costing the
industry a lot of time and money.
To ensure the geocomposite is allowing
the fluid to flow as designed, isn't being
compromised during installation and to assist
the workers on-site, I have been hypothesizing
a slight change in the design and would
very much appreciate your feedback. In our
scenario, we have an HDPE geomembrane
underneath the geocomposite and soil placed
on top. I'm of the opinion that installing the
geocomposite would still allow fluid to flow
with a similar velocity.
Your feedback would be appreciated.
A: I disagree. One needs geonet-to-geonet
contact along the butt seam overlap (L=about
a foot for a 3:1 slope) to maintain high flow
rates. The geotextile at the exit of the upslope
geocomposite will be a very high correction
factor (DCF) and be susceptible to clogging.
Your second scenario is not recommended for
the northeastern U.S. where we get significant
rain events. We have seen this scenario fail in
the past.

GEOTEXTILE TUBES
WITH FLOCCULANTS
Q: I just listened to the webinar today on
geotextile bags. We are planning a pilot
study for March for a paper mill site that is
a Superfund. Our plan currently adds the
flocculant before the bag and then we go
through bag filters before going through
carbon treatment. The idea of adding the
carbon to the bags and potentially eliminating
another step is very appealing. So, I was
wondering if the carbon addition that you
did was in isolation or combined with a
flocculant? We will be using a line addition
and will be using CO2 to adjust the pH after

flocculation before the carbon treatment.
The focus for the carbon is driven by PFAS/
PFOA. So, if you could point me toward any
additional information on what has been tried
with respect to the carbon addition to the
geobags, that would be appreciated.
A: Thank you for your GMA Techline questions
related to geotextile tubes and polyfluoroalkyl
substances (PFAS). PFAS are a group of
human-made chemicals that includes PFOA,
PFOS, GenX and many other chemicals. PFAS
have been manufactured and used in a variety
of industries around the globe, including in
the United States since the 1940s. There is
evidence that exposure to PFAS can lead to
adverse human health effects, and its cleanup
is one of the hot issues of our day. There have
been three areas related to slurry additives
with geotextile tubes: activated carbon, nano
clays and organo clays.
The idea of dewatering and decontamination
can be accomplished with geotextile tubes
used in conjunction with the above additives.
Success depends on type and concentration
of the pollutant. The key parameters are
solubility and sorption.

THE GEOSYNTHETIC
INSTITUTE ANSWERS
YOUR QUESTIONS

Much of the effort with geotextile tubes has
focused on river and harbor sediments. I would
recommend the following two references:

gmatechline@ifai.com

Moo-Young, H. K., Gaffney, D. A., and Mo,
X. (2002), "Testing procedures to assess the
viability of dewatering and geotextile tubes,"
Jour. of Geotextiles and Geomembranes, 20(5),
289-304.

View past questions and answers at
https://tinyurl.com/GMATechline. Email
gmatechline@ifai.com anytime for fast,
free, direct technical answers.

Mori, H., Miki, H., and Tsuneoka, N. (2002), "The
geo-tube method for dioxin-contaminated
soil," Jour. of Geotextiles and Geomembranes,
20(5), 281-288.
Activated carbon treatment has been studied
extensively for PFAS/PFOA removal. Activated
carbon is commonly used to absorb natural
organic compounds and synthetic organic
chemicals in drinking water treatment
systems. The problem with adding it to the
geotextile tube is that the fluid is turbulent
and not clean enough for it to be effective.
To work well it needs to be added during
finishing or a tertiary stage of water treatment.
Otherwise, one is using a lot of it inefficiently.
If the surface area of the activated carbon gets
coated with too many "other things," it won't
capture the PFAS/PFOA well. It is also very
light and does not distribute well in a slurry
without help.

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Geosynthetics August/September 2020

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