Properly installed silt fence is the most effective

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 Quantity Installation Compaction Support 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 Quantity Installation Compaction Support 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 Quantity Installation Compaction Support 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 Quantity Installation Compaction Support 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 Quantity Installation Compaction Support 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 Quantity Installation Compaction Support 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 Ev. TEC 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.