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Silt fence is a joke
on many construction sites. It often is seen just blowing in the
wind or sagging and broken down. Some specifiers won't utilize silt
fence on their sites because too often it is installed improperly
and not maintained. Yet the need for effective onsite sediment control
has never been greater.
Sediment-laden runoff
has been shown to result in the loss of in-stream habitats for fish
and other aquatic species, an increased difficulty in filtering
drinking water, the loss of drinking-water storage capacity, and
the negative impacts on the navigational capacity of waterways (EPA
833-F-00-001, January 2000, Storm Water Phase II Fact Sheet 1.0).
Sediment clogs stormwater conveyance systems and causes expensive
maintenance repairs. For these reasons it is unlawful to discharge
sediment into our waterways.
Ineffective silt fence
also is a waste of valuable environmental protection dollars. Specifications
exist to ensure that construction monies are properly spent and
that owners receive their money's worth. When specifications
are not followed or are inadequately written, our financial and
environmental resources are wasted.
Still, silt fence is
often the primary sediment control practice on a site in the early
phases of construction for logistical reasons–controls are
not practical on the interior until the site work has been completed.
When a site is first opened up, silt fence is used on the perimeter
and along waterways to minimize sediment loss from the site.
Silt fence also is used
along streets within a development to prevent sediment from entering
the stormwater system. Its somewhat temporary nature promotes its
use until more permanent stabilization practices can be utilized.
Problem: Installation
Practices
Numerous improper installation
practices associated with trench-based silt fence systems are the
cause of many of the problems associated with their use.
- Excavated soil from
the trench is intended to embed the silt fence, but often is neither
adequately backfilled nor properly compacted.
- The trench might not
be completely cleaned out prior to installing the fabric, leaving
debris that can interfere with fabric installation and backfilling.
- Posts are installed
in the trench prior to backfilling, preventing compaction equipment
from contacting the full width of the trench.
- The silt fence might
not be inserted to a uniform depth throughout the installation,
allowing shallow areas to "washout" more easily.
- And finally, cumbersome
trenching equipment might be hard to maneuver in many situations.
Some of these improper
installation problems derive from antiquated specifications that
do not provide sufficient detail on such things as:
- what trench depth
is effective and what is the relationship between the depth and
the backfill;
- what quantity and
what type of backfill is appropriate;
- in what manner, at
what time, and how much to compact the backfill;
- in what sequence to
install the posts.
In addition, common specifications
never have been tested for efficacy; there are no scientific data
supporting them.
The Introduction of
Static Slicing
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| After
a May 2000 rain event, this street in Ancaster, ON, was flooded
by runoff from a field. The field had been stripped bare and
graded the previous autumn (it was contoured and seeded in late
spring 2000). Silt fences were in place but ineffective. |
In the mid-1990s, an
effort to overcome many of the deficiencies associated with trench-based
silt fence installation culminated in a method called static
slicing. The concept originated in Iowa and was field-tested
on Iowa Department of Transportation (DOT) projects in 1997. The
Iowa DOT recognized the benefits of static slicing through this
testing and approved its use on all DOT projects that same winter.
Nebraska and Minnesota
followed suit within a year, and acceptance of the static slicing
method quickly caught on with erosion and sediment control contractors
in all three states. Since then the machines for mechanical static
slicing installation have been commercially successful in many areas
of the United States.
Static slicing is defined
as the insertion of a narrow, custom-shaped blade at least 10 in.
into the ground and the simultaneous pulling of silt fence fabric
into the opening created as the blade is pulled through the ground.
The blade imparts no vibration or oscillatory motion. The tip of
the blade is designed to slightly disrupt the soil upward, preventing
horizontal compaction of the soil and simultaneously creating optimum
soil condition for future mechanical compaction. Compaction follows
(four passes typically) using a tire on the tractor used to pull
the slicing machine. Post-setting and driving, followed with tying
the fabric to the post, finalizes the installation.
Evaluating Performance
The designer of the static
slicing method recognized the need for independent verification
of the benefits of static slicing to facilitate the change of specifications
to include the new technology.
The Environmental Evaluation
Technology Center (EvTEC), a program of the Civil Engineering Research
Foundation, the research and technology transfer arm of the American
Society of Civil Engineers, was contacted about their verification
process.
EvTEC is operated to
assist developers of new environmental technology in bringing their
technologies to the marketplace in a timely fashion. EvTEC has a
systematic prototype verification process. As part of that process,
an industry advisory panel of experts convened; in this case, state
DOTs, civil engineers, and the Environmental Protection Agency.
The panel, an EvTEC representative,
and the developer then meet to discuss the new technology, consider
the proposed solutions or benefits to be derived, and determine
what evaluations and verifications would be significant to specifiers
and users of the new technology.
The verification was
designed to provide baseline environmental data about both the trenched
and static-sliced silt fence installation methods by performing
actual field tests. The installation protocol outlined in ASTM D6462
Standard Practice for Silt Fence Installation was utilized to provide
the basis for comparison. TRI/Environmental Inc. supervised the
research as an independent third-party laboratory, under contract
to EvTEC.
The goal of the testing
was to provide potential users and purchasers with verification
information enabling informed decisions about the static slicing
method as an alternative to traditional silt fence installation
methods. This testing focused on methods rather than equipment so
as not to limit the future use of the verification protocol for
other installation technologies.
The three objectives
determined by the panel were to:
- determine the relative
performance of the static slicing method of silt fence installation
as compared to the trenching method,
- determine if static
slicing is more cost-effective to install than trenching,
- detail other practical
benefits, such as ease of operation and versatility of each method.
Performance was measured
in terms of the water retention capabilities of the different installations
when subjected to runoff. Excessive seepage under the fence (undermining)
was expected to adversely affect retention. And this undermining
was expected to be related to inadequate compaction of soil within
the trench and/or adjacent to the fabric. Therefore, water retention
and the degree of compaction achieved were evaluated for each soil
type and installation sequence.
Field Testing Program
This hands-on field research
project utilized 52 test segments reflecting different soil types,
different installation methods, and different hydraulic conditions.
The testing was completed in seven days. All fieldwork involved
full-scale equipment and experienced silt fence installers. One
primary site and three other sites were used to provide a variety
of installation conditions.
Testing primarily was
performed at one site with a predominately silty clay soil type.
Several alternative installation schemes were evaluated to define
the benefits of each installation type (slicing vs. trenching) under
a variety of conditions. Various amounts of backfill, degrees of
compaction, spacing of posts, volumes of runoff, and types of soil
were evaluated. 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.
Thirty tests were performed
on a gentle sloping area using "smile" configurations.
These 30-ft.-radius smiles created areas to impound or retain runoff.
Six 12-ft.-radius smiles were evaluated, and 10 straight 100—lin.-ft.
segments were constructed to evaluate installation efficiency. An
additional six runs were installed to evaluate the methods on steep
slopes, in rocky soils, and through wet spoils.
The depth was no less
than 10 in. for static slicing or 6 in. for trenching per the ASTM
D6462 specification. The vendor felt the extra depth wasn't
critical for slicing (6 in. vs. 10 in.) but provided a safety factor
for the machine operator, reducing the potential for a poor installation.
Deeper trenches were not tested because it was beyond the scope
of the project to test all trenching alternatives.
For both methods, a 6.5-ft.
post spacing primarily was used, as well as a woven slit-film textile,
36 in. wide, designated Amoco 2130.
A concentrated flow from
a 2-in.-diameter hose and 5-hp pump created the runoff (the standard
was 1,000 gal.), which was introduced within eight to 10 minutes.
The runoff hose was supplied by a tank reservoir that permitted
measurement (i.e., volumetric metering) of the outflow.
A nuclear density gauge
and a cone penetrometer measured compaction. The panel was hoping
to identify alternative, easy-to-use, economical means for determining
compaction effectiveness as an added benefit to this protocol.
The details and sequences
of tasks associated with "traditional" trenching installation
methods vary significantly from contractor to contractor. Therefore,
three general types of trench-based installations were identified
based on the likelihood of obtaining a fully backfilled and densely
compacted trench:
1. Minimum Installation
(Spec). Minimum silt fence specifications typically allow
for the following practices and typically are installed in this
order: trenching; post setting and driving; fabric installation
and attachment to the posts; backfilling (fill to level, if sufficient
excavated soil is available); compaction (required effort usually
not defined, detailed, or quantified in the specification).
2. "Better"
Installation (Spec+). A better-than-specified installation
of silt fence would include (1) fabric installation, use of only
available backfill, and then compaction before setting and driving
posts; (2) overbackfilling the trench; or (3) posting and then mechanically
compacting the filled trench.
3. "Best"
Installation (Spec++). This would include multiple enhancements,
such as hand-cleaning the trench prior to installing the fabric,
mechanical compaction of an overfilled trench, and posting as the
final action.
The minimum (Spec) installation
was included because the vendor felt it was a typical trenching
process in the real world. The vendor felt that many contractors
only backfill with available soil from the upstream side of the
trench for two reasons: (1) a trencher deposits excavated soil on
both sides of the trench, which would require them to hand-move
soil, and (2) contractors normally set the posts and hang the fabric
before backfilling or use prefabricated silt fence, which would
require them to shovel the excavated soil over the top of the 18-
to 20-in.-tall silt fence. These actions are inherently true with
all prefabricated silt fence materials.
Field Testing Results
In general, the static
slicing method was found to provide stormwater runoff retention
as good as or better than the best (Spec++) trenched installations,
and far superior retention to common (Spec) installations. Additionally,
the static slicing method of installation was found to be a much
more efficient, and therefore cost-effective, technique for silt
fence installation when compared to a range of traditional trench-based
procedures.
However, the Spec++ trenching
installation requires nearly triple the effort for similar performance
to static slicing. It also requires adequate, trash-free backfill
and sufficient soil moisture for compaction, as well as site conditions
enabling a trencher to maneuver. This is a demanding trench-based
installation: Failure to perform any of the tasks completely likely
will result in a significant reduction in efficacy.
Static slicing avoids
these concerns. Mechanical installation minimizes labor efforts
and potential backfill and compaction problems. Mechanical installation
produces a uniform installation and the greatest potential for an
effective silt fence with the least risk for some level of failure.
Retention Performance
Runoff retention tests
measured the ability of a "smile" of installed silt fence
to retain runoff. Segments installed using static slicing or the
Spec++ trenching techniques provided superior runoff retention.
Those segments installed using minimum effort generally experienced
both excessive seepage and washout.
It should be noted that
in this evaluation, trenches were hand-cleaned prior to fabric placement
and backfilling. This procedure was used to optimize trench-based
installation performance but commonly is skipped in real-world installations.
A clean trench allows the fabric to fit snuggly against the ground,
minimizing potential piping action that could occur when clods and
trash are left in the trench.
Installation Efficiency
It was clear from the
fieldwork that the installation time and manual labor associated
with any type of trenching installation are substantially greater
than that with the slicing technique.
One of the clearest advantages
to the static slicing method over all trench-based installations
evaluated was greater productivity. This productivity translates
into the ability to install a silt fence much faster and with a
smaller (typically two-man) crew than in trench-based methods.
Compaction Benefits
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). System comparisons showed that static slicing
provided installations that had both higher densities and greater
water retention than all trench-based installations.
Trench-based installations
were affected by the inability to compact effectively when posts
were installed first, when insufficient backfill material was placed
in the trench, or when inadequate compaction effort was provided.
Still, it should be noted that the installations using static slicing
also required reasonable compaction efforts to perform properly.
Compaction density was
measured with a nuclear density gauge and a handheld cone penetrometer.
There was a significant correlation between the cone penetrometer
readings and the nuclear density measurements. This might indicate
that the much easier–and less expensive–hand penetrometer
can be used effectively as a field quality-assurance tool.
Other Observations
The static slicing method
also offers practical advantages over traditional trench-based methods,
including maneuverability, and ease of installation on steep sideslopes,
through rocky soils, and in saturated soils.
Steep Slopes.
A 3:1 slope was available on another site to evaluate the relative
ease of installing silt fence by static slicing versus trenching.
In both cases, the steepness of the slope tended to encourage the
equipment to drift downslope, although much less for the static
slicing method. In comparison to trenching, static slicing provides
much straighter, faster installation of silt fence across steep
slopes. The spoil from the trenching operation also has a tendency
to fall back into the trench and down the slope–both of which
would decrease potential effectiveness.
Rocky Soils.
A third site provided very rocky soil conditions in which to
compare static slicing and trenching. While large buried rocks were
able to disrupt both installation methods, static slicing appeared
to be significantly more resistant to being "kicked" out
of the ground. In rocky conditions, the static slicing method provides
a better installation than does trenching.
Saturated Soils.
A fourth site had wet, organic soils and abundant vegetation
making it practically impossible to remove the soils from a trench,
install fabric, and then replace and compact clean soil in the trench.
Conversely, the static slicing apparatus was able to "insert"
the fabric deep into the wet soils without substantially disrupting
the area.
Specifications/Recommendations
By analyzing the field-testing
performed for this evaluation, there appears to be two possible
ways to achieve maximum silt fence performance–static slicing
or the very best trench-based installations. Yet there is no clear,
generally accepted specification with explicit installation details
to obtain this best trench-based installation.
In all cases, static
slicing produced silt fence installations as good as or better than
the very best trench-based installations. This finding provides
an important argument for toughening trench-based specifications
with more specific requirements for backfilling and mechanically
compacting the soil.
Still, the combination
of maximum performance and maximum productivity can be achieved
in one standard installation method: static slicing. This method
already is included in ASTM D6462 and therefore is easily incorporated
into project specifications.
Joel Sprague, P.E.,
is a principal with TRI/Environmental Inc. and Thomas Carpenter,
CPESC, is president of Carpenter Erosion Control.
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