Geosynthetics June/July 2023 - 10

Q &
THE GEOSYNTHETIC
INSTITUTE ANSWERS
YOUR QUESTIONS
gmatechline@textiles.org
View past questions and answers
at geosyntheticsmagazine.com/
category/gma-techline.
Email gmatechline@textiles.org anytime
for fast, free, direct technical answers.
A
GMA TECHLINE
MODERATED BY
George R. Koerner
HYBRID TMAX CALCULATING
Q: As of 2020, there are new, updated designs
for mechanically stabilized earth walls in
AASHTO. In short, there are new methods
to calculate the maximum force (Tmax) that
incorporates design methods for extensible
reinforcements (i.e., geosynthetics). They
include limit equilibrium and stiffness
methods. I was wondering if you can mix and
match concepts from the two methods to
form a hybrid way of calculating Tmax?
A: There was a robust short course at
Geosynthetics 2023 in Kansas City, Mo., that
addressed this question. Dr. Dov Leshchinsky
(University of Delaware), ADAMA Engineering
Inc., and Daniel Alzamora (U.S. Federal
Highway Administration) put on a fantastic
short course that provided an overview of
the American Association of State Highway
and Transportation Officials (AASHTO) 2020
methods for the design of geosynthetic
reinforced walls. They discussed the stiffness
method and the limit equilibrium method
at length. While discussing internal stability,
geometry, surcharge loads, external stability
and seismic design, they clearly stated that
you cannot mix concepts from the two
design methods. The Transportation Research
Board's (TRB) presentation " Updating Designs
for Mechanically Stabilized Earth Walls in
AASHTO, " October 8, 2020, is outstanding and
available at the following:
https://www.nationalacademies.org/documents/
embed/link/LF2255DA3DD1C41C0A42D3BEF0989ACAECE3053A6A9B/file/D99B6772C21D189FAC81E6636
FFD682C922117A2652F?noSaveAs=1
10
Geosynthetics
| June July 2023
GEOMEMBRANE RESIDUAL STRESS
Q: We are getting a lot of uncertainty with
our ASTM D5397 single-point notched
constant tension load (SP-NCTL) stress crack
testing. Can you shed some light on the
situation? This uncertainty is frustrating our
field personnel and holding up a rather large
HDPE geomembrane liner installation for a
massive landfill liner.
A: You may be experiencing residual stress
in the geomembrane from fabrication.
This often occurs on thin geomembranes
0.75 mm (29.5 mil) thick or less that are run
fast in production and where stretching
may occur during the cooling and take-up
operations. Residual stresses are those
stresses that remain in a product even in
the absence of external loading. In extreme
cases, residual stresses result in plastic
deformation, leading to distortion of the
geomembrane. One can get rid of residual
stress in geomembranes by annealing
the material to relieve or redistribute the
residual stresses. This is generally done to
a SP-NCTL sample prior to testing in the
conditioning phase of the method at 212
degrees Fahrenheit (100 degrees Celsius) for
one hour in a forced air oven.
http://www.geosyntheticsmagazine.com/ https://www.nationalacademies.org/documents/
Geosynthetics June/July 2023

Table of Contents for the Digital Edition of Geosynthetics June/July 2023
