The ubiquitous, low fences around perimeters of construction sites are so common these days they don’t draw the attention they once did. Every construction site on which the earth must be excavated or otherwise disturbed has one or several such fences, but their function often is misunderstood.
From a distance the fencing material appears to be plastic sheeting, but it actually is a geotextile fabric that allows water to flow through. While the material does to some extent filter storm water runoff during heavy rains, the primary purpose of the fence is to cause rainwater to “pond” — collect in pool — so that sediment can settle and not run off site and pollute the nation’s surface waters.
To accomplish that, these erosion-control fences — commonly called silt fences — must be positioned in accordance with an erosion control plan that complies with guidelines of the U.S Environmental Protection Agency’s (EPA) National Pollutant Discharge Elimination System (NPDES) permit program, usually administered by the state in which the project is located. Local regulations may also apply.
Public utilities usually obtain the permits for water and sewer construction. Providers of electrical, natural gas and telecommunications services are responsible for permitting for their projects. Typically, erosion control plans are developed by the project’s consulting engineering firm which also handles permitting.
A silt fence is composed of the geotextile fabric, sometimes reinforced with a wire mesh backing, and attached to wooden or metal stakes. The fabric is buried several inches in the ground.
Every construction site is different, and an erosion-control plan must take into account the area’s topography. A variety of erosion-control measures may be employed, including contouring the surface, hydromulching, erosion-control blankets and planting vegetation to hold soil in place. However, silt fences are most common and certainly the most visible method of erosion control. Silt fences usually are located along perimeters of work sites, but also may be installed elsewhere within a site both as barriers and to direct flow of runoff water. A common configuration of fences is to install them in a “J-hook” pattern to create the ponding area.
Erosion control work may be performed by the general contractor or subcontractors. Fence construction can be categorized as underground construction because a critical element in correctly installing an erosion-control, or silt fence, is firmly securing the bottom portion of the geotextile fabric in the ground.
Oklahoma Erosion Control, Stillwater, OK, is a specialty contracting firm established in 2006 when owner Raygn Alexander recognized a niche market that wasn’t being adequately filled. The company installs silt fences and offers a service contract to maintain them during the course of a project.
“Equipment to install silt fences is highly specialized and not something a general contractor routinely has,” says Alexander. “For many general contractors, it makes sense to sub the silt fencing. And when a contractor has a big project far away from its home base, having the fence contractor maintain the fence may be the easier and most economical way to be sure fencing is in good repair for the life of the project.”
Hard to maintain
Maintenance of a silt fence is a problem that is obvious to even casual observers. Stakes and geotextile fabric laying on the ground, torn or loose fabric, gaps between the bottom of the fence material and ground make it evident the fence is not performing its intended function. Often the reason for failure can be attributed directly to improper fence installation.
The key to proper fence installation is securing the geotextile fencing material securely in the ground. There are two basic methods for doing that: trenching and static plowing or “slicing.” Recently vibratory plow silt fence attachments offer a third option.
Trenching is labor intensive and fabric not properly placed in the ground can result in failure of the fence, and it is rarely used for large fencing projects. Since the mid ’90s, slicing has been the primary method of installing silt fence fabric.
“We’ve never put in a fence by trenching,” says Alexander. “It’s too slow and costly with potential problems from failing to get fabric properly secured in the ground.”
Until recently, Oklahoma Erosion Control used a McCormick silt fence plow attachment pulled by a tractor. The attachment accommodates 3,000 foot rolls of 36-inch fence fabric and has a special blade with a chute through which the fabric passes. The blade is designed to keep fabric tight during burial in order to provide a fence without sags. An installation is made by lowering the blade/plow into the ground and moving the host vehicle forward with the plow attachment.
Fabric is placed into the ground to a depth of about 10 inches, leaving sufficient fabric for a 26-inch fence when fabric is attached to stakes, Alexander says. Wire mesh fencing also can be installed by the slicing process for projects that specify stronger fences because of fence length or stricter regulations.
“The narrow path of the blade does not cause much soil disturbance and leaves the fabric firmly held by the soil,” says Alexander. “Pulling hard on the fabric confirms that it is firmly in place. If compaction is needed, usually all that is required is to drive the wheels of the tractor over the path left by the blade.”
Alexander uses 48-inch wooden stakes, spaced about five-feet apart, and “pre-drives” them about two-inches deep. He then returns with a hammer drill and drives the stakes to a depth at least 12 inches. Fence material then is stapled to the stakes. Stakes, other supplies and a portable generator to power the hammer drill are kept in a compact utility vehicle.
“About 2,000 feet of fence are installed at a time,” Alexander explains.
In 2008, Alexander switched to a vibratory Ditch Witch plow fence-installing attachment mounted on a compact Ditch Witch skid-steer loader. The blade of the attachment is connected to a vibrating component, similar to those used for many years on equipment to install communications and power cable, and pipe for irrigation systems and other application.
Alexander is pleased with the package. “It does a good job installing fence, is lighter to haul which saves money on fuel, and also uses less fuel to operate,” he explains. “It is very easy to control in various depths. In most soils we work in it isn’t necessary to activate the vibrator. It plows through most conditions without it; but the vibrating capability is there if needed.”
When he was evaluating whether or not to enter the erosion control business, one factor that appealed to him was that with the right equipment, he would be able to effectively install silt fence without the assistance of another employee.
“The way I’m equipped,” says Alexander, “I can put in an average of 3,200 feet per day. The best I’ve done to date is 4,000 feet. About 50 percent of the fence I have installed, I have done myself without a helper.”
Alexander says fences still are installed by trenching. To anchor geotextile fabric in the ground, specifications for trenched projects usually require the fabric be placed in a “J” configuration with the bottom of the J containing fill, also a time-consuming task. Failure to secure the fabric can result in failure of the fence.
“We are paid per foot of installed fence,” says Alexander, “and it was clear from the start that it would be impossible to make any money trenching it. We evaluated available equipment, and chose the McCormick static plow, then shifted to the Ditch Witch skid-steer and fence attachment.”
With the decline of construction projects during the current recession, Oklahoma Erosion Control’s demand for silt fences has declined.
“We are going slow and maintaining,” Alexander observed.
FOR MORE INFORMATION:
Oklahoma Erosions Control (405) 334-8905
Ditch Witch, (800) 654-6481, www.ditchwitch.com
Silt Fence Vibratory Plow Installation
The newest method for installing silt fencing material is vibratory plowing, a proven technology that has been used for decades to install electrical and communications cable and small-diameter piping.
Vibratory plow silt fence attachments are available to fit tractors and various host vehicles, and Ditch Witch, a pioneer in the development of vibratory plowing equipment for utility applications, has adapted vibratory plowing technology for silt fence installation.
The Ditch Witch silt fence package includes a reel carrier to hold the supply of geotextile fabric, special plow blade and a guide wheel to position fabric at the required depth during installation. A small wheel is positioned behind the blade and rides on the ground’s surface to minimize surface disturbance.
Ditch Witch says the vibrating action of the system allows the plow blade to move through the soil with less horsepower than a static blade and cites other benefits such as speed of installation, effective placement of fabric in the ground and minimal soil disturbance. Blades are available for standard geotextile fabric and wire-backed fabric.
The Ditch Witch vibratory plow silt fence attachments can be installed at the factory or dealership on several of the company’s power units.
NPDES Permit Program
Water pollution degrades surface waters making them unsafe for drinking, fishing, swimming and other activities. As authorized by the Clean Water Act, the National Pollutant Discharge Elimination System (NPDES) permit program controls water pollution by regulating point sources that discharge pollutants into waters of the United States.
Point sources are discrete conveyances such as pipes or man-made ditches. Individual homes that are connected to a municipal system, use a septic system or do not have a surface discharge do not need an NPDES permit; however, industrial, municipal and other facilities must obtain permits if their discharges go directly to surface waters. In most cases, the NPDES permit program is administered by authorized states.
Since its introduction in 1972, the NPDES permit program is responsible for significant improvements to our nation’s water quality. (Summarized from information on the NPDES web site, http://cfpub.epa.gov/npdes.)