Download EvTEC - the tommy™SILT FENCE MACHINE

Properly installed silt fence is the most effective
temporary sediment control device available
100 ft. of effective silt fence can retain 180 tons of sediment.
Properly installed silt fence detains water
for sedimentation to occur
Sediment control devices
primarily collect larger
particles of soil because clays
and silts are very small. Large
particles cause further erosion
– thus a multiplying effect -
once introduced into a stream.
Goal of silt fence
 Silt fence, for maximum efficacy, should
detain water for sedimentation to occur.
 Occasionally it may be used to divert
stormwater to a storage area.
 Silt fence, at a minimum, should retain large
soil particles and prevent loss from the site.
Two most common problems
 Placement issues – won’t pond water or
insufficient quantity for the area
 Installation issues - inadequate backfill
and/or compaction – one weak area
allows the fence to fail
Effective silt fence works
like a chain…
Placement
Installation
Quantity
Support
Compaction
Attachment
Each link must work;
if one link fails,
the system fails
A failure on any segment
of the system makes the
entire silt fence a complete
waste of money!
PLACEMENT
refers to specific locations on
each construction site, and to
specific designs (layouts)
at each location.
The ends of a “smile” must always be long
enough to pond water and sediment.
Designs called
J-hooks insure
water & sediment
pond behind
each silt fence.
Silt Fence Placement
Correct
Incorrect
Correct Silt Fence Placement
Incorrect Silt Fence Placement
 Streets that have
been excavated,
but not paved,
are prime areas
for sediment
discharge.
 Recommend
silt fence to
control sediment
and velocity.
Silt fence should pond water to
be considered effective.
Myth – flat sites do not
need protection
Stop sediment before it reaches
the pavement.
Ditch checks
 Do not install silt fence in a continuous flow
situation, or on the discharge of an outlet.
 Install on the inlet side of a culvert.
 A properly installed, compacted, and
supported silt fence will retain
significant sediment in a ditch situation.
 Shallow posting (wood posts?) and poor
compaction will not be effective.
25 tons of sediment captured per
fence in a ditch situation
Concentrated flow is standard
on many SWPP’s.
Proper installation can withstand
concentrated flows
Leave room for sediment storage
at toe of slope
Protect area intakes with larger
silt fence structures, if possible
Proper placement should insure
sedimentation.
On-site adjustment is mandatory
 Grading plans and PPP’s are often drawn in an
office many months prior to disturbance.
 The ESC contractor must adjust the plan to
properly control the site, or at a minimum notify
the site managers of potential problems.
Do not use silt fence:
 Where you can not create a storage
area for runoff, such as on steep slopes,
on small areas, or in short segments.
 Where you can not prevent water
running around the end
 In a V-shaped or shallow channel

Effective silt fence works
like a chain…
Placement
Installation
Quantity
Support
Compaction
Attachment
Each link must work;
if one link fails,
the system fails
QUANTITY relates to area of control.
The volume of water from a large site
can not be controlled in one run of silt
fence. Multiple locations or storage
areas are often required.
Large areas often
need additional
runs installed in
the interior to
reduce the volume
of water reaching
the perimeter fences.
Long runs should be avoided. They tend to
accumulate sediment in one area,
causing premature overflow.
Long runs often concentrate water
and then overflow
Breaking up a run more than
doubles storage area
Multiple runs slow runoff velocity
and add storage area on site.
Proper Quantity
100 ft. of silt fence per 10,000 sq. ft.
of disturbed area seems to be an
adequate rule of thumb for sediment
control and economics.
Effective silt fence works
like a chain…
Placement
Installation
Quantity
Support
Compaction
Attachment
Each link must work;
if one link fails,
the system fails
INSTALLATION relates to how the
fabric is placed in the soil, appropriate
depth of placement, and appropriate
backfill for an effective silt fence.
Improper installation often leaves
silt fence blowing in the wind.
No compaction = washouts
DOT job with inspector does
not insure proper installation.
Mechanical installation
reduces many labor-related
installation problems.
Mechanical installation
provides consistent,
dependable silt fence.
The slicing method disturbs,
but does not excavate soil,
creating an optimal
condition for compaction.
Effective silt fence works
like a chain…
Placement
Installation
Quantity
Support
Compaction
Attachment
Each link must work;
if one link fails,
the system fails
COMPACTION
relates to soil permeability. Loose or
trash-filled backfill is easily saturated
with water and washed out under the
silt fence. Compacted soil resists
saturation, and thus washouts.
Silt Fence Study
EPA co-sponsored research has
shown compaction is the critical
factor in silt fence effectiveness.
 Trenches must be over-backfilled and
mechanically compacted –
BEFORE installing posts
 Slicing installation must be mechanically
compacted before installing posts
Poor compaction means washouts
EPA co-sponsored research
ASTM and ASHTO
specifications are vague on
backfill, compaction, and
chronological order of posting resulting in poor performance.
TRI Environmental, Inc. national
field study of random
installations
 70% of trench installations had no backfill
or no compaction
 The remaining 30% of trench installations
had an average compaction of only 45% of
the undisturbed soil strength
TRI Environmental, Inc. national
field study of random
installations
 Less than 8% of sliced installations
were not compacted.
 The remaining 92% of sliced
installations had an average compaction
of 86% of the undisturbed soil strength.
Effective silt fence works
like a chain…
Placement
Installation
Quantity
Support
Compaction
Attachment
Each link must work;
if one link fails,
the system fails
PROPER POST SPACING
AND POST DEPTH
relates to a support system that
will not fall over under the load
of a full silt fence.
Support Post
Spacing
24”
above
ground
Post should be spaced
a maximum of 6 ft.
apart and driven
24”
below
ground
60" steel
T-post (1.33# per foot)
24 inches into
compacted ground.
EPA co-sponsored research
Trench installations were adversely
affected by the inability to compact
effectively when the posts were
installed first and when insufficient
backfill was placed in the trench.
Properly supported silt fence will carry
24 inches of sediment.
Wood post spacing
Wood posts may be more difficult to
drive an adequate depth into
compacted soil, requiring closer
post spacing to achieve adequate
silt fence support.
wood posts are difficult to install
to a proper depth
ATTACHMENT
Proper attachment is the glue of the
system, combing the strength of the
fabric and the support posts into a
unified structure. It must be adequate
to support 18 inches of sediment.
Attachment must be adequate to
support a fully loaded silt fence
Attachment
 For steel posts, recommend 3 plastic
ties per post, located in the top 8
inches of the fabric, with each tie hung
on a post nipple, placed diagonally to
attach as many threads as possible.
 For wood posts, recommend several
staples per post using a wood lath to
overlay the fabric for extra strength.
A properly installed silt fence can retain
several tons of sediment, thus saving
millions of tons across the country.
Tell-tale sign of a problem
If there is minimal sediment or
water behind a silt fence, either
water is running around the
end, or there is a washout
needing repair.
Wire-backed fence problems
 Doubles the cost of silt fence
 Creates voluminous, expensive disposal
 Disposal creates environmental harm
 Modern fabrics do not need wire support
 Improper post spacing for fence
support is the real problem
Disposal pile of wire-backed silt
fence from just one job
Posts too far apart, wire no help
Alternatives to wire-backed fence
 Possibly more runs of silt fence to
reduce the potential volume/load
 Use of steel posts spaced 4 feet
apart and 24 inches deep
Maintenance of silt fence
 Once the fabric is clogged with sediment, it no
longer functions as designed – so removing
sediment does not result in a performing fabric.
 What is to be done with the removed sediment
and how do you physically handle it?
 Solution – leave the sediment in the silt fence
and build a new silt fence above or below to
collect additional sediment.
Cleaning simply creates a low
volume sediment basin
Fabric can not be restored
Inspector Responsibilities
 Inspectors must know proper placement
and installation concepts
 Must not approve improper installations for
payment.
 Improper installation includes silt fence that
does not pond water.
Effective silt fence works
like a chain…
Placement
Installation
Quantity
Support
Compaction
Attachment
Each link must work;
if one link fails,
the system fails
A Study of
Silt Fence Installation Techniques
and Associated Efficiency and
Performance Issues
Silt Fence Study
EvTEC
Environmental Technology
Evaluation Center
Silt Fence Study
Verification of Performance
• The field evaluation included 51 test
segments reflecting different soil
types, installation methods, and
hydraulic conditions.
• Various amounts of backfill, degrees
of compaction, spacing of posts,
volumes of runoff, and types of soil
were evaluated.
Silt Fence Study
 Additionally, installation sequence, such
as installing posts before versus after
compaction, was evaluated.
 Performance, as measured by water
retention, and efficiency, as measured
by installation time, were evaluated.
Silt Fence Study
Nuclear Density Measurements
Silt Fence Study
Silt Fence Study
Indication of Piping
Failure
Silt Fence Study
Typical High Volume
Retention Test
Silt Fence Study
High Volume Water Application
Silt Fence Study
Piping failure
Silt Fence Study
The static slicing method was found to
provide storm water retention as good as
or better than the ‘Best’ trenched
installation and far superior to common
installations.
Silt Fence Study
 The ‘best’ installation far exceeded the
standard ASTM and ASHTO
specifications which are vague as to
backfill, compaction, and chronological
order of posting.
Silt Fence Study
 The conclusion was clear that when the
enhancements of the ‘Best’ trenched
installations were not performed - the
trenched installation performed poorly.
Silt Fence Study
 Trench installations were adversely
affected by the inability to compact
effectively when the posts were installed
first and when insufficient backfill
material was placed in the trench.
Silt Fence Study
The ‘Best’ trenched installation requires
nearly triple the installation effort to achieve
similar effectiveness as static slicing.
Trenching techniques meeting only minimum
or marginally enhanced specification
requirements fared quite poorly.
Silt Fence Study
 Performance trends provide a clear
indication that a greater level of
compaction (i.e. higher density obtained)
corresponds to better performance (i.e.
greater water retention).
 There was a significant correlation
between the cone penetrometer readings
and the nuclear density measurements.
Silt Fence Study
 Poorly performing test segments
experienced excessive seepage and, in the
worst case, subsequent “blow-out” of soil in
the trench.
 Segments installed using slicing or the
“best” trenching techniques experienced no
blowouts.
 Those segments installed using the
minimum specification requirements
experienced both excessive seepage and
blowout.
Silt Fence Study
Additionally, the static slicing method of
installation was found to be much more
efficient, and therefore, cost effective
technique for silt fence installation when
compared to a range of traditional
trench-based procedures.
Silt Fence Study
 Static slicing ranged from 1.75 to 4
times faster than all trench-based
installation techniques.
 As far as installation efficiencies go, the
static slicing method provided much
quicker installations than any trench
method installation attempted.
Silt Fence Study
 The static slicing method offers practical
advantages over traditional trenchingbased methods, such as maneuverability,
minimal soil-handling and hand labor,
consistent depth and compaction, and
ease of installation in windy conditions, on
steep side slopes, through rocky soils,
and in saturated soils.
Silt Fence Study
 Mechanical installation by static slicing
minimizes the hand labor requirements,
as well as the potential backfill and
compaction problems associated with
trenching. As a result, static slicing can
be expected to provide uniform,
dependable installations.
Silt Fence Study
Conclusions
 There appear to be two possible ways to
achieve maximum silt fence performance –
static slicing or the “best” trench-based
installation.
 The combination of maximum performance
and maximum productivity can be achieved
in one method – static slicing. The static
slicing method is included in ASTM D 6462.
Silt Fence Study
Conclusions
 In all cases, static slicing produced silt
fence installations as good as or better
than the best trench-based installations.
 Both static slicing and the best trenchbased installations substantially
outperformed “typical” trench-based
installations.