# Geosynthetics February/March 2020 - 10

```THE

GMA TECHLINE

Moderated by George R. Koerner and Y. "Grace" Hsuan

THE GEOSYNTHETIC
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10

Geosynthetics | February March 2020

DARCIAN PERMEABILITY

PP TOXICITY

Q: I am aware of the section in your book
regarding geomembrane "permeability"
and get from it that it aims to calculate an
equivalent Darcian permeability using vapor
diffusion via Fick's law. Can one use the
derived permeability then when calculating
flow using Darcy's law? A 60 m head gives
a hydraulic gradient equal to 60/0.0015 =
40000! Using this gradient, the resulting
leakage is still small but by no means
negligible. Maybe I am overanalyzing the
matter or fail to appreciate that it is inferred
that the derived Darcian permeability applies
to Darcian flow. Further to the above, can one
argue that it is then clean water only and that
the majority or perhaps all dissolved metal
salts such as Ca, Na stay behind?

Q: Are there any comprehensive tests (or
a general synopsis) of the environmental
toxicity of leachate from nonwoven
polypropylene fabrics when aged in
freshwater or saltwater solution? Regulatory
and municipal bodies are requesting more
information about products that are used
in riparian and coastal environments.

A: Yes, the Darcian permeability goes right
into Darcy's formula for the resulting flow
coupled with the low diffusion value works
out reasonably well. Incidentally, any liquid
diffusing through a geomembrane as a vapor
only to recondense on the other side as a
liquid is the cleanest liquid ever produced!
The contaminants simply don't travel along
with the vapor.

A: As you might be aware, there is nothing
in polyethylene resin to cause toxicity. It is
simply carbon and hydrogen in a repeating
structure. Aged in fresh- or saltwater makes
no difference. However, most geosynthetic
materials are formulations of the parent resin
and an additive package containing some
carbon black (no problem) and antioxidants.
The latter are very complex structures but
only present in trace amounts, e.g., less than
0.5%. I have no idea if they can leach out of
the structure and, if so, what concern they
may be.
The above said, all geosynthetics can
be evaluated by elevated temperature
incubation in deionized, fresh- or saltwater
baths and subsequent testing of the
incubating liquid, but it will take years
for results and at an enormous cost.

```
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# Geosynthetics February/March 2020

Geosynthetics February/March 2020 - Cover1
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