Geosynthetics June/July 2021 - 11

GEOTEXTILE UNDER ARTICULATED
SYSTEMS IN SLOPE ARMORING
Q: I have a question about the geotextile
placed under articulated systems in slope
armoring. Based on my understanding, a
drainage layer is recommended between the
geotextile and the armoring structure, but,
since the armoring structure has some spaces
in between the armoring blocks, won't the
particles of the drainage layer be washed out
through these spaces? How can we make sure
that the drainage layer will not be eroded?
Or is it assumed the water will flow in parallel
to the drainage layer and will not leach out
through the armoring blocks? I guess that
a minimum thickness for the drainage layer
should be considered in this case.
A: In extreme cases, one may need to build
a graded filter (i.e., layered with multiple
geotextiles and possible sand). Geotextiles have
been used beneath erosion control structures
since the late 1950s. Depending primarily on
the care exerted by the contractor in placing
the riprap, a sand cushion may be needed to
protect the geotextile from impact damage
during installation or abrasion damage during
its lifetime (due to wave action agitating the
rock riprap). If precast concrete blocks are being
used, a sand layer is often used, not so much
as a cushion (since these blocks are placed by
hand or carefully lowered into position), but
now as a pore water dissipator since a major
part of the fabric can be directly covered by
the blocks. This would be the geotextile filter
in place with a riprap cover and also a failed
system of an articulated precast block system.
The system you are describing may need a
needlepunched nonwoven (NPNW) geotextile
on the silt subgrade, uniform sand, a woven
monofilament geotextile and then, finally,
the slope-armoring system. Please make sure
the armoring system has enough open area
to dissipate pore water pressure from rapid
drawdown. The U.S. Army Corps of Engineers
has fantastic details on such a design.
OIT FOR UV AND OVEN AGING
Q: I am contacting you in regard to the
requirements for oxidative induction time (OIT)
(standard [STD] and high pressure [HP]) in the
GRI-GM13 specification for both ultraviolet light
and oven aging. Do you need to conduct both
OIT tests or just one or the other?
A: I would strongly recommend reading
the EuroGeo 4 keynote paper " Long-Term
Performance and Lifetime Prediction of
Geosynthetics " by Hsuan et al. from 2008 in
Edinburgh, United Kingdom.
To properly understand and assess the longterm
behavior of geosynthetic materials,
it is necessary to investigate the various
types of possible degradation mechanisms.
Degradation of geosynthetics depends on
the polymer type, formulation and quality
of the components. For example, polyolefins
are vulnerable to oxidation. The oxidation
reactions of polyolefins are rather well known.
The sequential steps of the oxidation chain
reaction include initiation, propagation,
chain branching and finally the termination
reactions. Furthermore, the influence of
temperature, pressure and ultraviolet light
on the service life are critical.
The OIT value indicates the amount of, but not
the type, remaining in the test specimen of
antioxidant. Many researchers use this method
to monitor the depletion of antioxidants
because of its simplicity. However, for different
antioxidant formulations (types and amount),
direct comparisons can be misleading. There
are two OIT tests, standard OIT (STD-OIT) and
high pressure OIT (HP-OIT), which are carried
out using a differential scanning calorimeter
(DSC) with standard and high-pressure
cells, respectively. The STD-OIT is performed
according to ASTM D3895. The test specimen
is subjected to 5-psi (35-kPa) gauge pressure
of oxygen at 392˚F (200˚C). The HP-OIT test
is performed under 51-psi (350-kPa) gauge
pressure of 302˚F (150˚C) condition according
to ASTM D5885. The determination of the OIT
value is typically based on the STD-OIT test for
GRI-GM13. The main application of the HP-OIT
is for antioxidants that have low efficiency
temperature range, such as hindered amine
light stabilizers (HALS). Therefore, the answer
to your question is complex and you need
to guess (assume) how your high-density
polyethylene (HDPE) geomembrane was
formulated. There is no sense in running
the expensive HP-OIT test if the HDPE
geomembrane doesn't have any HALS in it.
Hsuan, Y. G., Schroeder, H. F., Rowe, K., Muller, W.,
Greenwood, J., Cazzuffi, D., and Koerner, R. M. (2008).
" Long-term performance and lifetime prediction
of geosynthetics. " Proc., EuroGeo 4, the 4th European
Geosynthetics Conference, IGS UK, Edinburgh, United
Kingdom, 5-24.
INDUSTRY STANDARD FOR
WALL DRAINAGE SYSTEMS?
Q: I just watched your recorded video of
mechanically stabilized earth (MSE) wall
inspection. Thank you for the presentation;
it was very informative. I do have a follow-up
question for you. I was recently the site
engineer for a precast panel MSE wall and now
have the opportunity to inspect construction
of the wall that has just begun. The question I
have for you is on industry standard MSE wall
drainage systems.
The wall vendor shop drawings propose an
underdrain pipe that's installed at the back
of the wall straps. This type of wall drainage
is typical to what I've seen in Connecticut
and California, and is what I detailed in the
wall plans. During the design, the geotech
has included another wall drainage detail
that proposed an 8-inch (20-cm) thick sand
blanket underneath the wall along with a
slightly different underdrain. I attached both
details for reference. In your experience,
is there an industry standard or do the
wall drainage systems vary based on the
geotech/region?
A: The short answer is yes, we see slightly
different MSE underdrain systems (base,
back outlets) depending on availability,
geotechnical condition and region of the
United States. I have attached a PDF and
PowerPoint presentation that I think are
expansions of what you saw earlier. The
three National Concrete Masonry Association
(NCMA) scenarios are informative and
probably realistic. Unfortunately, one only
really knows once the cut (excavation) is
open. It is often a tough call for a young
engineer or technician.
In the northeast, we see base drains of
geotextile-wrapped gravel with embedded
perforated polyvinyl chloride (PVC) pipe.
Back drains are almost exclusively
drainage geocomposites. G
>> For more, search Techline at
www.GeosyntheticsMagazine.com
or email your technical questions
to gmatechline@ifai.com.
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Geosynthetics June/July 2021

Table of Contents for the Digital Edition of Geosynthetics June/July 2021

Geosynthetics June/July 2021 - Cover1
Geosynthetics June/July 2021 - Cover2
Geosynthetics June/July 2021 - 1
Geosynthetics June/July 2021 - 2
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