Geosynthetics April/May 2021 - 29

the 32 tested products provided greater
than 80% filtration efficiency, and the
majority of the tested products provided
greater than 90% filtration efficiency.
However, this category of product is also
the category with the lowest flow rate, in
general. These results are intuitive, as the
low flow rates in this product category
inherently require greater sediment retention or filtering efficiency as a function of
sediments settling out of suspension, binding against the upstream face of the fabric,
or simply being unable to pass through the
tightly woven matrix of the slit tape fabric.
Woven slit film geotextile silt fence
is the most widely used silt fence fabric,
based on the number of products tested
to satisfy DOT Qualified Products List
requirements, and is composed of a weaving of flat two-dimensional tape-like plastic fibers that create a textile with very
small openings for sediment and water
to escape. It should be noted by end users
and designers that while slit film fabrics
provide exceptional filtering efficiency,
the requisite low flow rate of the product
category leads these fabric types to produce
excessive ponding. Excessive ponding may
be an acceptable risk when slit film fabric is
used to protect sensitive water bodies, but
may be an unnecessary risk along roadways when excessive ponding could lead
to roadway flooding and potential risk to
motorists. It must be noted that slit film
fabrics, when deployed as two-dimensional
silt fences, must be adequately supported
by robust posts with frequent post spacing
to reduce the risk of fence failure due to the
load associated with high-volume ponding.
As with all SRD products, market
price must be considered. Slit film fabric is
generally understood to be the most economical of all silt fence fabrics and is often
sourced from overseas. When considering
whether slit film fabric is the appropriate
SRD filtration component for a construction project, the designer must balance the
economic implications, potential flooding/

ponding risk and exceptional filtration
capabilities of this product type.
Woven monofilament geotextile silt fence
Six products from the woven monofilament geotextile silt fence category were
tested in this research. The data suggests a product category that provides
moderate filtration efficiency with all
tested products (excluding one outlier),
providing between 65% and 85% filtration efficiency and moderately high, yet
relatively uniform, flow rate within the
product category. In contrast to slit film
fabric, monofilament fabric is composed
of rounded plastic fibers, similar to fishing line, woven together to form a fabric
matrix with larger open areas and greater
flow capabilities. The differences in manufacturing practices that produce fabrics
with more fibers per square inch can
lead to greater filtration efficiency while
allowing similar volumes of water to pass.
This product category may be exceptionally valuable to designers in areas
where the risk of excessive ponding and
roadway flooding would lead to unnecessary motorist risk, but where downstream
impairment is of relatively low risk. Often
within the erosion and sediment control industry, SRD products nominally
termed " high flow " will be composed of
a filtration component made with monofilament geotextile.

Woven slit film
geotextile silt fence is
the most widely used silt
fence fabric, based on
the number of products
tested to satisfy DOT
Qualified Products List
requirements, and is
composed of a weaving
of flat two-dimensional
tape-like plastic fibers
that create a textile with
very small openings for
sediment and water
to escape.

Nonwoven geotextile silt fence
Five products from the nonwoven geotextile silt fence category were tested in
this research. The data suggests that the
product category provides excellent filtration efficiency, with all tested products
providing greater than 95% filtration efficiency and moderately high, yet uniform,
flow rate. Nonwoven fabrics are different
from woven fabrics in that they have randomized open spaces. The vast array of
nuance in nonwoven fabric manufacturing can, however, lead to products that
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Geosynthetics April/May 2021

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