Woven Slit Film Geotextiles: A
Highly Efficient and Cost Effective Alternative to Polypropylene Geogrids for
Base Stabilization Applications.
Fueled by effective promotion and well-documented
research, rigid polypropylene geogrids have become the civil engineer's
geosynthetic of choice for base stabilization. Well meaning specifiers have
placed their faith in a successful, yet proprietary product, misunderstanding
punched and drawn polypropylene geogrids unique physical properties to be an
actual technology. As a result, preferential testing methodology and design
'standards' have stunted the development of performance based design criteria
and discouraged good faith competition.
Specifications currently in use effectively exclude the
use of viable design alternatives such as high-strength woven geotextiles. Many
of these woven geotextiles had been used successfully for soil reinforcement
nearly a decade before the development of geogrids. In particular,
high-strength, high-modulas woven slit-film geotextiles have been virtually
ignored as a design option.
Many
geotechnical engineers practicing worldwide understand the type of geosynthetic
used is irrelevant to the design of a reinforced soil structure. What is
important is the geosynthetics' in-situ performance. As such, woven slit-film
geotextiles are stronger and can offer a highly efficient and cost effective
alternative to rigid, punched-and-drawn polypropylene geogrids.
In addition to superior strength
and lower cost, slit film geotextiles offer an added advantage over any
geogrid: separation. Geogrids rely on 'interlock,' a process whereby the soil
particles or aggregate 'lock' into place within the geogrid's open structure or
apertures. Base stabilization geogrid openings are typically 1.0' x 1.3'. As
such, it is critical that the soil particles or aggregate be of sufficient size
to properly interlock with the geogrid. Often, the actual particle size is not
known and/or varies widely. As a result, the amount of interlock provided can
be in question.
While the slit-film geotextiles
openings are large enough to allow the passage of water, they are small enough
to impede all but the most minute soil particles. Therefore, separation of the
weak and potentially wet soils from your base material is assured.
Specifically, the geotextile eliminates the pumping of weak material into your
base that can occur with a geogrid.
One
reliable and quantifiable measure of performance among all types of reinforcing
geosynthetics is Ultimate Tensile Strength (ASTM D 4595/6637). With this
criterion, slit-film geotextiles outperform polypropylene geogrids by as much
as 40%. Other design properties including geosynthetic direct shear, soil
pullout and reduction factors for creep, installation damage and biological /
chemical degradation are all relevant.
Often, proprietary properties
such as junction efficiency or index tests such as tortional
rigidity, are included in
specifications to the benefit of no one except a particular manufacturer's
bottom line. Ultimate strain @ 2% and 5% relates to the rate in which a
particular polymer reaches its final elongation and does not necessarily
preclude one product or another.
When considering what type of
geosynthetic to use in a base stabilization application, consideration must be
given to the products' in-situ performance. Performance-based designs allow the
contractor to pick from a variety of solutions taking into account additional
factors such as ease of installation, pricing and product availability. Often,
woven slit-film geotextiles provide a highly efficient and cost effective
alternative to geogrids for base stabilization applications.
Bill Knueven
US Fabrics Inc.
Cincinnati, OH
