Transcript Silt Fence - CMC Construction Services

CMC and Hanes Erosion Control
Overview
What is Erosion?
Erosion
• Raindrop impact is the major cause of soil
particle detachment which can result in the
particles moving down slope in flowing water
(as sheet erosion) during a rainfall event.
Flowing water can also detach soil particles if
the velocity is high enough, usually where
water starts to concentrate (rill and gully
erosion)
Erosion
How do we prevent Erosion?
• If the velocity of flowing water is reduced
sufficiently, particles will settle out. The
velocity at which settling begins is dependent
on particle size and density, as is the time
required for the particles to settle out. Large,
dense particles, such as grains of sand, settle
first. Fine clays settle out slowly and only in
relatively still water. Mechanical filtration is
also used.
Erosion Prevention
Why do we have to prevent Erosion?
• Runoff from construction sites is the major source of
sediment in urban areas under development. There are
two main reasons why construction activities increase
pollutant loads in runoff. First, the volume and rate of
runoff typically increase, providing a larger capacity to
transport pollutants to rivers and lakes. Second, the
vegetation is removed, leaving bare soil that is much
more vulnerable to erosion, resulting in more sediment
moving into receiving waters. Typical sediment loading
rates from construction sites vary from 100 to 200 tons
per acre per year and can range up to 1,100 tons per
acre per year.
But WHY do we have to prevent
Erosion?
• Let’s face it- there is not a product in this
category that customers buy because they
want to. They buy them because they have to.
• Erosion control measures are dictated by DOT
regulations, municipalities, and the NPDES
Guidelines.
• Annnnd they’re all different. KNOW. YOUR.
SPECS!
Spec Differences
• Austin specs are different from San Antonio
which are different from Corpus which are
different from TXDOT which are different
from USDA specs which are different from
Corps of Engineers.
• Best you can do is ask your customers- and
there is ALWAYS a spec for all of these
products. That being said, here are a few
differences…
Austin vs. San Antonio
• Silt fence: 6oz
nonwoven on 12.5ga
wire w/ 4’ 1.25lb/lf
posts
• Curlex required as
erosion blanket, even
when on TXDOT
projects where straw
would otherwise be
acceptable
• Silt Fence: 70gm woven
fabric on 14ga wire w/
4’ 0.85lb/lf posts
• Straw blankets accepted
for ECBs
KNOW YOUR SPECS!
• Have the Standard Specifications for
Construction and Maintenance of Highways,
Streets, and Bridges.
• Have your City and surrounding City’s specs
printed and in binders- on your desk AND in
your truck. Yes. They’re 600 pages each. Have
hard copies.
• It takes research and dedication. And it sets
you apart.
Common Products
Silt Fence
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Install before land is disturbed
Install on down slope side of site parallel to contour of the land
Stake posts on downhill side
Extended ends up slope enough to allow water to pond behind
fence
Turn ends of fence uphill
Bury 8 inches of fabric in trench
Leave no gaps. Overlap sections of silt fence, or twist ends of silt
fence together.
Inspect and repair once a week and after every rain. Remove
sediment if deposits reach half the fence height
Maintain until a lawn is established
Remove upon project completion and disturbed soil is vegetated
Silt Fence
Silt Fence
• COMMON MISTAKES AND PROBLEMS
• Not adhering to daily inspection/maintenance
schedule
• Fabric not tucked into trench
• Posts on uphill side of fence
• Silt fence used in areas of concentrated flow
• Not repairing silt fence after it has been
damaged or knocked down
Erosion Control Logs
• Common sizes: 9”, 12”, and 20” diameter
• Matrix: Straw, Aspen, Coconut Fiber, or Mulch
• Require stakes, generally 24”-36” wood, when
installed
• Generally required to overlap ends, so allow
for slight waste when calculating total logs
needed
Straw Logs on Hillside
Coir Logs on Stream banks
Erosion Logs: the Why
• Serve same purpose as silt fence, basically.
• Install before land is disturbed to prevent jobsite
runoff.
• Placed perpendicular to flow rate, aside from stream
bank establishment applications.
• Lower impact, lower installation cost, and
biodegradable so often do not require being removed.
• One warning- just like straw erosion blankets, straw
logs can be destroyed by pesky wildlife. Mention to
customers!
A Rose by Any Other Name…
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Erosion Control Blankets
Rolled Erosion Control Blankets
Vegetation Mats
Turf Reinforcement Mats
Erosion Control Rolls
“That green stuff TXDOT uses…”
ECBs and TRMs
• All of these products are the same concept: A
rolled blanket or mat that prevents erosion
before vegetation is established by growing
through the material itself.
• On permanent mats there is a
nondegradeable component that reinforces
the root zone to strengthen the vegetation’s
resistance to shear stress of water moving
across it.
ECB vs. TRM?
What is the difference?
• ECB stands for Erosion Control Blanket and
generally refers to any Temporary or
Biodegradable blanket. I.E. Straw mats or
curlex mats
• TRM stands for Turf Reinforcement Mats and
are generally permanent and withstand higher
flow rates
Why vegetation at all?
• Vegetative cover is normally the most effective and
practical control of erosion and sediment loadings but takes
time for establishment. Sodding can reduce the time
required for grass establishment and can be used to control
erosion in some channels in lieu of riprap or concrete. The
roots of vegetation, such as grass, binds soil particles
together to resist erosion. Vegetation helps absorb the
impact of raindrops to prevent detachment of soil particles.
Vegetation can flatten and, like shingles on a roof, allow the
runoff to flow above the soil. Stiff, standing vegetation can
slow runoff (which may allow larger particles to settle out)
and act as a filter to remove soil particles from flowing
water.
Curlex installed on River Banks
Straw Mats installed on Hillside
TRM installed on steep riverbank
Geotextiles
Filter Fabrics are always geotextiles, but
geotextiles are not always filter fabrics!
There are two main types of Geotextiles: Woven
and Non-Woven
Woven Geotextiles
• Woven geotextiles are composed of 2
different kinds of “thread”: Slit Tape and
Monofilament
• Slit Tape is less expensive and substantially
more common in the market, Monofilament is
much stronger
Woven Fabircs
Monofilament Fabrics
Nonwoven Fabrics
• Nonwoven fabrics look like felt: uniform but
without a visible weave pattern.
• Have a very high flow rate and are good for
separation and the stronger ones for
stabilization
• Nonwovens are described by their weight in
ounces per square yard.
• Used behind retaining walls, in French drain
systems, and under rock for separation
Nonwoven- Separation for French
Drain
Woven and Non-Woven Fabrics Can be
used for Road Reinforcement
Road Reinforcement: Which to Use?
• Woven Fabrics tend to be stronger with less
stretch, BUT can be punctured easily
• Non-Woven Fabrics have more “give”, so are
more resistant to punctures, but don’t provide
the permanence of woven fabrics
• Nonwoven Fabrics have a higher flow rate
• Sooooo….
Road Reinforcement: Which to Use?
• Woven Fabrics are good for soft and saturated soils: i.e.
sandy and muddy conditions. Generally used along the
coast more than in central Texas
• Nonwoven fabrics are good for conditions with rock:
i.e. stabilized construction entrances, temporary roads,
etc.
• Woven Monofilament will always have very specialized
applications: i.e under articulated block, in jetties, etc.
Least common geotextile.
• This is ALWAYS an engineered product unless
contractor is slapping something together for private
work
Most Common Geotextiles
• Terratex SD- TXDOT Type 1 approved Filter Fabric.
Used under small rock (4”), French Drains, and
behind retaining walls. 4.2oz Nonwoven Fabric
• Terratex N08- TXDOT Type 2 approved Filter
Fabric. Used under gabions and large riprap, also
in Construction Entrances. 8oz. Nonwoven Fabric
• Terratex GS- (stands for Ground Stabilization)
Most common stabilization fabric for sandy and
soft soils. Equivalent to Mirafi 500x
Biaxil Geogrid
Geogrid
Geogrid
• Geogrid- used for aggregate containment and
reinforcement under roads
• Used either to strengthen the base material OR
to reduce amount of aggregate needed for
desired strength.
• Rectangular aperture Geogrids are the TXDOT
approved geogrids, triangular aperture are NOT!
• Type 1 Terragrid RX1100- used 90% of the time
• Type 2 Terragrid RX1200- used predominantly in
ports and freight yards. Stronger and less
expensive than TX5!
Other Products
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Gabions
Turbidity Barriers
Drainage Mats
Pond Liners
Hydraulically Applied Products
Rock Berm Wire
Seed
Safety Fence
Etc.
Questions?