Geosynthetics August/September 2019 - 8

THE

GMA TECHLINE

answers all

Moderated by Robert M. Koerner, George R. Koerner and Y. "Grace" Hsuan

A GEOTEXTILE FOR UNPAVED
DRIVEWAY/ROAD?

DR. ROBERT M.
KOERNER ANSWERS
YOUR GEOSYNTHETICS
QUESTIONS
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Q: Is there any point in using a 150 g/m2
(4.4 oz/yd2) nonwoven geotextile in building
an unpaved gravel driveway or gravel road?
If so, what should be the most important
characteristics of the fabric and what are
the benefits? The site conditions are average:
the soil is not sandy, no high clay content,
no high moisture, no slope.
Could you, please, recommend any resources
to find answers to the questions asked by
customers when I sell geotextiles?
A: The answer to your question is definitely
"yes." The geotextile, even though it is very
lightweight, will act as a separator, keeping
the subgrade soil from contaminating
the overlying gravel, which, in this case,
is the wearing surface. This maintains the
base course's permeability and drainage
to the sides of the roadway. It is extremely
important in winter to prevent heave within
the soil-contaminated saturated base course
(without the geotextile) from creating
uneven surfaces and/or potholes. It's the
least expensive "insurance" one can get.
For further reading on geotextiles as
separators, look at the book Designing with
Geosynthetics, available through Amazon
or Barnes & Noble or through us at www
.geosynthetic-institute.org/publications.htm.
P.S. If the base course is quarried gravel
(hence, angular) and is thin (6 inches [15 cm]
or less), I would go to a heavier geotextile
than you suggest.

8

Geosynthetics | August September 2019

ASPERITY HEIGHT IN GRI-GM13
Q: I have a question about the GRI-GM13
standard, and I am glad to have this channel
for clarification.
In 2014 the minimum geomembrane asperity
height was changed from 0.25 to 0.40 mm
in the GRI-GM13 standard for high-density
polyethylene (HDPE) geomembranes.
I did not find more details about this revision
in the GSI White Paper #32 "Rationale and
Background for the GRI-GM13 Specification."
Is there any report with the reasons why it
was concluded that an asperity height of
0.25 mm was not enough? I'm looking for
the rationale of this change.
A: Thank you for your question regarding
asperity height in our GRI-GM13
specification. The original height was indeed
set at 0.25 mm, which we thought gave a
reasonable interface friction value for most
situations. As time progressed higher asperity
heights, particularly for geomembranes
produced by flat die methods, came into
being. While possible to achieve in blown film
production, there is a tendency to decrease
core thickness unless more polymer is used.
This trade-off led to the 0.40 mm height
and is admittedly difficult, but we wanted to
accommodate both types of texturing.
The above said, if an interface with a
particular material is satisfied at the 0.25 mm
asperity height, one should indicate that the
geomembrane meets GRI-GM13 (modified),
where the modification is substantiated
by site-specific direct shear tests. As
such, we don't mind modifications to our
specifications if properly explained.
https://www.tinyurl.com/GMATechline http://www.geosynthetic-institute.org/publications.htm
Geosynthetics August/September 2019

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