Geosynthetics June/July 2019 - 13

NONWOVEN VS.
WOVEN GEOTEXTILES
IN FILTER APPLICATIONS
Q: I was rewatching your webinar on
geotextiles in filter applications and I came
up with what I think would be a superiority
of using nonwoven geotextiles. Namely,
since an AASHTO M288 Class 2 nonwoven
geotextile will have lower strength but also
higher elongation at failure (i.e., a much
lower modulus), I would think that if it is
used around a crushed-stone underdrain,
there would be a much greater chance to
achieve intimate contact, since it would
stretch more when backfill is placed above
it. Am I correct in this understanding? If so,
am I also correct that as long as it meets
the AOS and permittivity requirements for
the base soil that, other things being equal,
the nonwoven would be preferred over the
woven for this application?
A: Clearly, needlepunched nonwoven
fabrics will deform around crushed stone
quite readily, the amount depending on
the specific normal stress applied from the
surrounding soil. Depending on its mass per
unit area, the elongation at break is more
than 80%. For heat-bonded nonwovens it
is slightly lower, but not much.
In contrast, the wovens, both monofilament
and slit films, have deformation at break
values of less than 30%. As a result, AASHTO
M288 makes the distinction at 50% without
overtly stating woven and nonwoven.
Marilyn Ashley will send you a set of
experimental curves illustrating the above
behavior for different geotextile types.
She will also send a really old paper I
wrote titled "Should I Specify a Woven or
Nonwoven?" Interestingly, it was written in
1984. It covers all of the possible functions
but is at this point in time of historical
interest only. Thanks for your question.

THE STRONGER THE FABRIC, THE
LOWER THE SEAM EFFICIENCY
Q: Can you recommend some papers that
discuss seam strength efficiencies for highstrength woven geotextiles? Thank you.
A: There is available literature as to how to
make geotextile seams, but little published
as to the results. Some limited data is in
the book Designing with Geosynthetics, 6th
edition, giving results from two field projects,
one using high-strength polyester fabrics,
the other using high-strength polypropylene
fabrics. Results are shown on Figure 2.10 of
the book along with the accompanying test.
As expected, the stronger the fabric, the
lower the seam efficiency. Further, factory
versus field seaming is illustrated.
The information is attached.

INTERFACE FRICTION VALUES
Q: Do you have a reference for interface
friction values at low normal loads between
any of the following?:
* Smooth HDPE geomembrane vs.
finished concrete
* Smooth HDPE geomembrane vs.
smooth HDPE geomembrane
* Smooth HDPE geomembrane vs.
rubber matting

DR. ROBERT M.
KOERNER ANSWERS
YOUR GEOSYNTHETICS
QUESTIONS
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A: The interfaces you ask for are quite
unusual. We suspect that smooth to smooth
high-density polyethylene (HDPE) will be
the lowest of the three, but at low normal
pressures, the finished concrete will be low
also. The rubber interface will likely be the
highest of the three interfaces but be highly
dependent on the durometer. Textured
HDPE will obviously increase the values
considerably, particularly against rubber.
Best to do a few tests and, as a suggestion,
you might get reasonably good values
using a small shear box. G

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