Geosynthetics October/November 2023 - 11

SEPTIC SYSTEM DRAIN FIELDS
Q: Should I sock the perforated pipe of my
septic system drain field and eliminate the
gravel rock/bedding around the pipe?
A: Unfortunately, only socking perforated (and
hopefully corrugated) pipe for a septic system
drain field works for very few instances over
time. We have seen it work in alluvial deposits
of free-flowing sand and gravel with perk test
less than1 inch/minute (2.5cm/minute) (i.e.,
this is a very fast draining material). In most
cases, homeowners are not so lucky when it
comes to hydraulic-geotechnical conditions
at the site. Traditional drain fields for septic
systems are typically made up of geotextilewrapped
trenches that are filled with gravel.
Perforated pipes are laid within the gravelfilled
trenches. Generally, a geotextile filter
surrounds the gravel to keep soil from
infiltrating into the trenches. The geotextile
filter surrounding the gravel trench needs to
remain free flowing over time (i.e., it cannot
clog). It usually is a needlepunched nonwoven
polypropylene geotextile with an ASTM
D4751 AOS of 70-80 and an ASTM D4491
permittivity of greater than one (1) sec-1.
Contemporary drain fields are made with
a synthetic infiltrator system, which is an
alternative to traditional gravel and pipe
drain fields. They use a total geosyntheticengineered
modular design. The gravelless
systems are made up of preassembled
units of a perforated-corrugated drain pipe
surrounded by polystyrene aggregate and
held in place with a knitted geonet, which can
be placed vertically or horizontally. It should
be noted that these contemporary drains do
not respond well to heavy construction traffic.
They need to be buried with at least 18 inches
(45 cm) of soil cover prior to experiencing any
traffic or they risk collapse.
Both of these systems need to be sized
properly and should be designed against
clogging and infiltration. Again, the site
hydraulic and geotechnical conditions of
each scenario need to be considered. You
probably want to reach out to a site civil or
geotechnical engineer familiar with your
project. One needs to also consider codes
that govern septic drain field designs at
either the state or local level.
SERVICE LIFE OF
EXPOSED GEOSYNTHETICS
IMPROVING GEOSYNTHETICS
SHIPPING
Q: Can we correlate laboratory versus
outdoor weathering of geomembranes?
I would appreciate your thoughts on this
matter because we are getting technical
challenges on this issue from regulators.
A: Yes, we do this all the time. The results
are location dependent based on ambient
temperature and solar radiation. We have
several papers on the subject ( https://
geosynthetic-institute.org/whitepapers.
htm ) that predict the service life of exposed
geosynthetics. The Geosynthetic Institute (GSI)
published a report in 2019 offering predictions
of the lifetime of exposed geosynthetics
using laboratory weathering devices. The
report shows that the service life of exposed
geosynthetic products can be predicted by
measuring the oxidative induction time (OIT)
depletion, strength and elongation half-life
after exposure to elevated temperature and
ultraviolet (UV) radiation. A QUV-fluorescent
weathering device using the ASTM D7238
Standard Test Method for Effect of Exposure of
Unreinforced Polyolefin Geomembrane Using
Fluorescent UV Condensation Apparatus
protocol was used for the laboratory portion
of the test. We tested at three temperatures
and then did Arrhenius modeling to compute
the half-life of the materials.
GEOTEXTILE CHARACTERISTICS
Q: I have two nonwoven heat-bonded
geotextile filters: one made of polyester
and the other made of polypropylene. They
look the same but are giving very different
hydraulic performance. Can you explain
this phenomenon?
A: Geotextiles are permeable fabrics
consisting of materials such as
polypropylene or polyester. Depending on
their manufacturing method, nonwoven
geotextiles have different physical,
mechanical, hydraulic and endurance
characteristics. While nonwoven geotextiles
are comprised of yarns that are joined
together randomly, there are multiphenotype
analysis of rare variants (MARV)
results associated with each. First off, looks
can be deceiving. I would test the two
geotextiles and then compare the results.
Second, remember that polypropylene
(PP) is hydrophobic while high tenacity
polyester (PET) is hydrophilic. This has a huge
influence on wettability and will affect field
performance significantly.
Q: We are having delivery problems this
year and shipping has gotten much more
expensive. Can you suggest how we can
improve the situation?
A: Geosynthetics are generally installed in
the early stages of the project. If they are
delayed, it can have a domino effect on the
project's timeline. Therefore, work in advance
and pick a financially secure supplier with a
good reputation and plenty of space to hold
stock to satisfy your needs promptly. Worstcase
scenario, warehouse product yourself
in advance of large projects. The major
causes for higher shipping costs are industry
consolidation, inefficiencies, shipping
container shortage globally, limited supply
of commodities and increased consumer
demand. Each of these factors adds pressure
on shipping rates. In addition, thanks to
rising inflation, carriers have seen their
operating costs increase significantly over
the past several years. This, combined with
efforts by carriers to reduce losses, has led
to substantial increases.
LLDPE YIELD POINT
Q: Can you tell me why my linear low-density
polyethylene (LLDPE) is currently showing a
yield point? In years past, we have not seen
this happen.
A: The yield point of HDPE is the first
observation of zero slope measured on a
stress-strain response curve. In most cases,
this occurs between 12% and 19% strain for
most HDPE. As you know, the major variation
between HDPE and LLDPE is density. In the
past, most fabricators converted either HDPE
or LLDPE in their respective formulations.
Now they are blending these two resins
and just staying under the threshold for
density. As a result, some " blended LLDPE " is
exhibiting a slight yield point whereas in the
past there was none. G
>> For more, search Techline at
GeosyntheticsMagazine.com
or email your technical questions
to gmatechline@textiles.org.
GeosyntheticsMagazine.com
11
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Geosynthetics October/November 2023

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