Geosynthetics August/September 2021 - 10

DESIGN OF A CCR COMPOSITE
LINER SYSTEM
Q: We are designing a composite liner system
for a coal ash landfill. From the bottom-up, the
cross section is subgrade soil, geosynthetic
clay liner (GCL), geomembrane, composite
drainage layer with nonwoven geotextiles on
both sides and then ash from coal combustion
residuals (CCR). Do you have an opinion on
the design, particularly on 3:1 slopes in a
seismically active zone?
A: Wow, this a big question for the
GMA Techline. I think you need to get a
professional engineer involved. That written,
there are considerations to this design that
have presented themselves in the past
that I can offer. Three points of interest are
outlined below.
THE GEOSYNTHETIC
INSTITUTE ANSWERS
YOUR QUESTIONS
gmatechline@ifai.com
View past questions and answers at
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free, direct technical answers.
You might want to consider a single-sided
textured geomembrane with textured side
down. This will ensure no tension in the
geomembrane. This is a result of the friction
on top being less than that on the bottom of
the geomembrane.
One is going to need to do a good job with
the filter design of the geotextile for the
drainage geocomposite. If fines from the CCR
(particularly if it includes fly ash) pipe through
the geotextile of the drainage geocomposite,
this will cause trouble. There may be a chance
that the friction on a contaminated interface
becomes greater than the bottom-side textured
geomembrane to the GCL. If this is the case, the
geomembrane will be in tension, which will be
problematic for long-term stability.
Depending on how the CCR is placed, the
range of shear strain or displacement needed
to trigger the residual strength of these
interfaces is very low. It is probably in the
3%-5% strain range. Therefore, the interface
will slide suddenly with little warning as
opposed to other more elastic interfaces in
your cross section.
Please be very careful with this. These points
deserve much more discussion beyond casual
questions and answers in the GMA Techline.
You need a professional engineer for this
design and an accredited lab to run direct
shear tests to establish real answers for such
a challenging application.
A GEOTEXTILE FILTER
FOR AN EDGE DRAIN
Q: I am doing a geotextile filter design for a
highway edge drain. All I have from the site is
a grain size analysis of the sandy subgrade soil.
Can you point me in the right direction?
A: First you are going to need to calculate
an estimate of hydraulic conductivity for the
sandy soil by the Hazen method based on the
grain size distribution of the soil as illustrated
in the following:
k=Cd_10^2
k = hydraulic conductivity (cm/sec)
C = factor depending on grain size and
sorting (well-graded vs. uniform)
i.e., very fine soil poorly sorted (C=40-80),
coarse sand, well-sorted uniform clean
(C=120-150)
d_10 = the grain diameter for which 10%
of distribution is finer (usually " effective grain
size " between 0.05 and 0.5 cm for sands)
After that, it is rather straightforward by adhering
to the five steps for filter design that follow:
1. Permeability criteria
2. Retention criteria
3. Clogging check
4. Survivability assurance
5. Long-term flow check
If you want to pursue this further, please
consult the following references. They are
good ones.
Holtz, R. D., Christopher, B. R., and Berg, R. R. (1997).
Geosynthetic Engineering, BiTech Publishers Ltd.,
Richmond, B.C., Canada.
Luettich, S. M., Giroud, J. P., and Bachus, R. C. (1992).
" Geotextile filter design guide. " Jour. of Geotextiles and
Geomembranes, 11(4-6), 19-34. G
>> For more, search Techline at
www.GeosyntheticsMagazine.com
or email your technical questions
to gmatechline@ifai.com.
10
Geosynthetics | August September 2021
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Geosynthetics August/September 2021

Table of Contents for the Digital Edition of Geosynthetics August/September 2021

Geosynthetics August/September 2021 - Cover1
Geosynthetics August/September 2021 - Cover2
Geosynthetics August/September 2021 - 1
Geosynthetics August/September 2021 - 2
Geosynthetics August/September 2021 - 3
Geosynthetics August/September 2021 - 4
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Geosynthetics August/September 2021 - Cover3
Geosynthetics August/September 2021 - Cover4
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