BACKGROUND OF THE INVENTION

Soil disturbances during construction can result in sediment washing into waterways (e.g. streams, lakes, ponds, wetlands) and roadways during rain events. The sediment in the rainwater reduces clarity in the runoff, which negatively affects sunlight reaching aquatic plants and animals, while sediment on roadways poses safety hazards for drivers and can clog storm drains. Sediment is the number one pollutant of U.S. water resources even though sediment control is commonly required to comply with NPDES (National Pollution Discharge Elimination System) regulations. Sediment control is installed before soil disturbance begins. One type of sediment control is sediment-retention devices (SRDs), which include silt fence, silt socks, wattles, filter logs, compost and earthen berms, and storm inlet protectors. SRDs retain the soil on disturbed land until revegetation and permanent soil stabilization begin without the large area required by settlement ponds.

Silt fences are a popular SRD because the materials are relatively inexpensive and can pool the rainwater up to 24″ or more to allow the accumulated sediment to settle out while slowly filtering the rainwater. A single 100-foot run of silt fence can hold back 50 tons of sediment and water. A silt fence is composed of a permeable geotextile, such as woven, non-woven and mono-filament plastics, stretched between wooden or metal posts driven into the ground in regular intervals on the downhill side of the silt fence. The geotextile acts as a surface filter so the pores clog quickly to hold the rainwater back while the sediment settles out.

The bottom of the geotextile is trenched (e.g., 6″ wide by 6″-8″ deep on the uphill side) or static-sliced (e.g., 12″ deep slit) into the ground to prevent rainwater from running underneath. Both trenching and static slicing are affected by roots and rocks below the ground surface as well as terrain contours or property liners. After trenching and static slicing, the ground over the buried geotextile must be compacted to prevent rainwater from infiltrating into the air spaces and eroding into channels underneath the silt fence leading to sediment washout. A wire or chain-link fence, also held up by the posts, can be placed behind the geotextile to help distribute the hydraulic force on individual posts and reduce the stretching of the geotextile as has it holds back water and accumulated sedimentation. The wire or chain-link fence may double the cost of the silt fence installation and entails disposing of more material in a landfill when removed.

Regardless of the installation method, proper attachment of the geotextile to the posts is critical to combine the strength of the geotextile and posts into a unified structure. Silt fence failure, such as falling over and infiltration, caused by poor installation techniques is the major issue with this SRD. The silt fence must be inspected routinely after runoff events.

Another popular SRD to achieve 24″ of ponding is silt socks, which are tubes made of woven net fabric filled with compost, wood chips, or switch grass. The silt socks are held in place with wooden stakes driven through the top of the tube and into the around below. They filter the rainwater faster than silt fences due to larger pore size and depth filtration, which captures the sedimentation within the filler material instead of only on the surface. The issue with silt socks is that they are more expensive than silt fence. The compost and wood chip-filled versions are heavy to transport during installation and removal and weigh approximately 45 pounds per linear foot. The 24″ diameter socks are filled at the construction location, requiring numerous truckloads of compost or a source of nearby wood-chips. If the landowner approves of the pile of compost being left behind, compost socks can be cut open when no longer needed so that only the net fabric is sent to the landfill. Wood-chip socks are acidic and inhibit revegetation when cut open. The wood chips either need to be removed or neutralized into the soil. Switch grass socks are lighter weight but are still bulky to transport and are more easily displaced by rainwater, leading to sediment-laden rainwater escaping underneath. Independent of the type of filler in the sock, loose compost is typically blown against the bottom of the flow side to reduce channeling of sediment-laden rainwater under the sock where it does not conform to the surface.

An SRD that is resistant to poor installation techniques compared to a traditional silt fence, and that is less expensive to transport, install and remove than a silt sock is desired.

