Most construction sites in the United States today are regulated under provisions of the federal Environmental Protection Agency (EPA) National Pollutant Discharge Elimination System (NPDES) permit program, which prevents eroded sediment carried by storm water runoff from entering the nation's water supplies.

If construction disturbs surface soil in any way—through excavation, trenching, grading, stockpiling of materials, or other activities—and the site is one acre or larger, NPDES regulations apply. Smaller sites that are part of larger developments also are included.

Federal NPDES regulations are minimum requirements for states to follow in developing their own land-disturbance regulations. If an individual state has a federally approved program, the Federal EPA will delegate the responsibilities for issuing permits and monitoring and enforcing compliance to the state. Regulations vary from state to state, and they may even be different between cities in the same state.

The owner-operator is responsible for acquiring a permit. Owner-operators include developers of residential and commercial projects. Public utilities usually obtain permits for water and sewer construction; electrical, natural gas, and telecommunications companies are responsible for permitting for their projects. Erosion-control plans are typically developed by the project's consulting engineering firm, which also handles permitting.

Sediment-control work may be performed by the general contractor or subcontractors. Today there are contractors who specialize in sediment control, have knowledge of NPDES and local regulations, and have specialized equipment to perform their work.

Many methods are employed to meet compliance, including check dams, contouring the land, hydro mulching, erosion-control blankets, straw or rice wattles, and seeding/fertilization to establish and maintain ground cover to hold soil in place. Walls constructed of concrete bags stabilize and protect vulnerable areas from erosion. Erosion-control fences control sediment by forming small ponds that allow sediment to settle out of suspension instead of being carried offsite in runoff.

Each site is different, so each plan must take into account topography of the terrain to determine which erosion-control methods are needed. It is essential to develop an erosion-control plan that complies with NPDES and local regulations while correctly utilizing the different erosion-control techniques available. However, plan development is beyond the scope of these guidelines, which focus solely on methods for constructing sediment-control fences, the most common type of erosion control used on construction sites.

TEMPORARY SEDIMENT-CONTROL FENCES

Many different types of erosion-control methods are employed at construction sites, but one of the most common is the temporary sediment-control fence, or silt fence, which is removed after a construction project is complete.

A silt fence is composed of a geotextile fabric stretched between steel or wooden stakes. The bottom edge of the fabric is buried in the ground with posts on the uphill side. Geotextile fence material is available in rolls with widths of 36 inches. Fence material with a wire mesh backing also is available. The wire adds rigidity to the fence.

Contrary to common belief, the fabric is not used to filter out sediment. Its purpose is to slow the flow of storm water runoff, allowing water to "pond" so that suspended sediments can settle. A fence's effectiveness depends on its location relative to terrain features. Ideally, the fence is installed on the contour of the site; however, in many cases this is not possible, so the next-best practice is to install sections of fence in j-hook configurations that create ponding areas for sedimentation settlement. Silt fences usually are placed along perimeters of work sites, but also can be used at other locations as barriers and to direct runoff water flow.

Many silt fences are improperly constructed and maintained. Breaks in fences, collapsed sections of fabric, and gaps between the ground's surface and the bottom of the fabric may allow water carrying sediments to flow freely past the barrier. These violations of NPDES standards can result in penalties ranging from a few hundred dollars to substantial amounts and can result in an order to stop work.

Traditionally, the basic methods for installing fence fabric in the ground have been trenching and static plowing, also called slicing. More recently, the technique of vibratory plowing has been adapted for silt fence installation.

Whatever method is used, the first step in constructing a silt fence is to secure the bottom edge of the fence fabric into the ground. Staking is done after the fabric is in place.

INSTALLATION BY TRENCHING

When silt fences first came into use, the procedure to install fabric was to dig a trench, place the fabric in the ground, fill the trench and compact the fill to hold the fabric in place. Specifications today typically call for fabric to be placed at a depth of six to eight inches in the ground, depending on site plan specifications. A trench for short sections of fence can be dug by hand, but manual digging is time-consuming, and most trenches are dug by a compact walk-along or riding trencher, or skid-steer loader with trenching attachment. Whatever the equipment, the trench's depth must be sufficient to meet specifications of the project, and the width of the trench should be as narrow as practical to limit the amount of backfilling and compacting.

Fabric is unrolled adjacent to the trench for placement. After fabric is secured in the trench, the excavation is filled and compacted. Depending on soil conditions, driving the trench-digging machine several times over the filled excavation may sufficiently compact the soil. Hand-held air or hydraulic tampers do a better job of compaction.

Stakes are driven in the ground; fabric extending from the ground is secured to the stakes to form the fence.

The trenching installation method can be difficult and must be done properly to avoid problems. Stakes must be driven after trenching so as not to interfere with compaction, the trench must be cleaned out before placing fabric, fabric should be inserted in the trench at a uniform depth, and backfilling or compaction must be done properly.

INSTALLATION BY PLOWING

There are two plowing methods used to place silt fencing in the ground: static plowing (slicing) and vibratory plowing.

Proponents say that slicing is more efficient than trenching, reduces labor costs, and keeps fabric tight as it is buried in the ground, preventing sags in the fencing. Because no trench has been dug, restoration also is faster and less costly. Fabric for fencing can be installed as fast as the terrain of a site permits the tractor or compact tool carrier to travel. In most cases, all that is required for compaction is to drive the host vehicle over the slit created by the plow blade. Depending on soil conditions, one or several passes may be necessary. Both standard rolls of fabric and fabric with mesh backing can be installed by this method.

Static Plowing. In the mid 1990s, installation of fence fabric by static slicing was developed as a way to avoid problems associated with trenching installation methods. Installations are made with a plow attachment mounted on a tractor or skid-steer loader. Either machine can be equipped with a reel to carry the supply of geotextile fabric, and a narrow, custom-shaped plow blade. Fabric is threaded through a chute in the blade and placed in the ground.

To install the fabric, the blade is lowered into the ground. As the machine moves forward, fabric feeds through the chute and into the soil. The blade is designed to keep fabric tight as it is buried to prevent sags in the fencing. The blade generates no vibration or oscillatory motion; the blade's tip disrupts the soil upward to prevent horizontal compaction, while simultaneously preparing the soil for future mechanical compaction.

Vibratory Plowing. A recent enhancement to the slicing method is vibratory plowing, a proven technology that has been used for decades to install electrical and communications cable and small-diameter piping.

Installations are made with a vibratory plow attachment mounted on a tractor, compact tool carrier or a Zahn® tool carrier. A vibratory plow uses a vibratory drive unit and blade attachment to produce a slice in the soil. The vibrator rapidly oscillates, moving the blade a short distance vertically; this vibrating action creates a fracture in the ground in front of the blade, substantially reducing the draft force required for the blade to cut through the ground, as well as reducing resistance on the sides of the blade.

A Ditch Witch® silt fence vibratory plow consists of a shaker component, special plow blade, guide disk, and compaction wheel. A 1,500-foot supply of fencing fabric is placed on the reel and fabric is fed through the chute of the special blade. The vibratory plow attachment trails the machine feeding the fabric through the blade into the ground. The disc inserts the fabric into the slit in a j-hook configuration at its lowest point. In most soils, the narrow slit created by the blade results in the soil closing back onto the fabric, firmly securing it at the required depth in the ground. Positioned behind the blade, the small compaction wheel rides on top of the ground to minimize disturbance caused by the blade. Both standard and wire-mesh-backed fencing fabric can be installed by this method.

In most soil conditions, vibratory plowing creates less surface damage than static plowing, which drags a blade through the ground by pulling force alone. The vibrating action typically reduces by half the drawbar force necessary to pull the blade through the ground, making it possible to use more compact equipment for installations. Because its oscillating blade cuts through soil faster than a static blade, the vibratory plow is, in most conditions, faster than a static plow.

Ditch Witch vibratory plow silt fence components can be factory- or dealer-installed on a variety of equipment models and sizes.

STAKING

Once geotextile fabric is secured in the ground and the soil is compacted, the final step in fence installation is to attach the fabric to stakes.

Fabric should always be installed on the uphill side of the posts, and the distance between stakes should range from five to eight feet, depending on site conditions and project specifications. Either wooden or steel stakes may be used and should be driven at least 12 inches into the ground. Stakes must be long enough to support fencing material and accumulated sediment at the specified vertical height.

Fence material should be stretched so there are no sags and secured to each post. Staples may be used to secure fabric to wooden stakes. Steel posts should have projections to facilitate securing fabric to stakes. "Zip" ties—at least three to a post—can be used to quickly fasten fabric to steel stakes. If fabric pulls from the ground during the process of attaching it to posts, then it has been improperly installed and must be correctly placed and the soil compacted or the fence will fail.

After construction on the site is complete, stakes and fencing material are removed.

MAINTENANCE

After a silt fence is installed, it must be carefully maintained. Breaks in fencing material, downed stakes, and other damage must be repaired immediately. Sediment and debris that accumulates on the uphill side of the fence must be removed to prevent overflows during rain events. Project owners are subject to fines for failure to properly maintain silt fences and other erosion barriers. Some specialist contractors who install silt fencing also provide cleaning and maintenance services.

SUMMARY

The EPA has documented that water pollution degrades surface waters, making them unsafe for drinking, fishing, swimming, and other activities. As authorized by the Clean Water Act, the NPDES permit program controls water pollution by regulating point sources that discharge pollutants into U.S. waters.

An important part of the NPDES program is controlling storm water runoff from construction sites. Of the various methods for accomplishing this, the most common is the use of sediment control fences, commonly identified as silt fences.

Geotextile fabric for silt fencing is available with and without wire mesh backing. Wire backing strengthens fences, but disposal is more time-consuming when fences are removed after a project is completed.

To do its job properly, a silt fence's fabric must be fixed securely in the ground. There are two basic methods for accomplishing this: trenching and plowing.

Trenching is slower, more labor-intensive, and if not carefully done with fill soil properly compacted, the fabric may come out of the ground, causing the fence to malfunction.

Plowing methods include static plowing, also known as slicing, and vibratory plowing.

A static plowing attachment mounted on a tractor, skid-steer loader, or other vehicle, includes a reel carrier and blade. Fencing material feeds through a chute in the blade and into the ground as the host vehicle moves forward. A vibratory plow component mounted on the host vehicle differs from static equipment by incorporation of a vibrating mechanism that rapidly oscillates vertically, allowing the plow blade to more easily pass through the earth. Fabric mounted on a reel carrier feeds through the blade's chute and is positioned to the correct depth by a rotating disc. A roller mounted behind the blade presses down on the slit to facilitate restoration. Because vibratory plowing makes more efficient use of available horsepower, smaller host vehicles may be used and vibratory plowing usually is faster than static plowing.

Since its introduction in 1972, the NPDES permit program is responsible for significant improvements to our nation's water quality. The NPDES website—http://cfpub.epa.gov/npdes/—provides detailed information about the storm water program and permitting, and has links to access individual state requirements.