A challenging site and adverse weather at a New Jersey Department of Transportation highway construction project demonstrated the ability of the FGM to control erosion and promote establishment of vegetation on long, steep slopes.
Weighing the choices
Phase II of the EPA's National Pollutant Discharge Elimination System (NPDES), which went into effect in March 2003, requires a Storm Water Pollution Prevention Plan (SWPPP) for all construction sites as small as one acre. This plan must include steps for limiting the amount of sediment that may wash off the site.
Traditionally, covering a disturbed slope with 2-3 tons per acre of relatively inexpensive straw has been used in certain situations to control erosion. However, this practice requires labor to handle the smaller bales and special equipment to handle big bales as well as straw blowing machines to apply the straw. Seeds in the straw can also introduce weeds and other unwanted species to the site.
Rolled erosion control blankets are among other options. Usually, they require smoothing the surface of the slope prior to installation.
Otherwise, rocks, clods, debris and other surface irregularities can create gaps between the blankets and the soil, leaving much of the underlying soil exposed. Water can flow into these openings to create unseen rill erosion. The labor costs to prepare the slopes and staple the blankets in place can eat up budgets and cut into profits quickly, especially on larger projects. Also, this approach may not be practical if the slope is too steep to work on safely or can't be reached on foot.
A hydraulically applied bonded fiber matrix requires much less time and labor to install. The material conforms closely to uneven slope surfaces to prevent erosion and can be sprayed on inaccessible sites. However, to be effective, it should not be applied on saturated soils and may require up to 48 hours without rainfall to fully dry and cured.
A fit for NJ DOT
The New Jersey DOT had learned about hydraulically applied bonded fiber matrix during a training seminar on hydraulically seeding practices.
The New Jersey DOT site provided an ideal opportunity for a test. It involved 4.4 acres of bare, fill slopes near Sparta. The 60-ft-high slopes, with gradients of 2.5H:1V and steeper, were constructed as part of a project to widen state route 15 from two to four lanes.
Aspen Landscaping Contracting seeded the slopes in July of 2003 with one of the DOT's standard mixes - tall fescue, Kentucky bluegrass, Chewings fescue and perennial ryegrass - and applied 300 lb per acre of 10-12-10 fertilizer. The crew then applied the hydraulically applied bonded fiber matrix on about half the site at the manufacturer's recommended rate of 3,500 lb per acre. As a demonstration, this rate was increased to 4,000 lb per acre in some swales. For comparison, the rest of the site was covered with 2 in. (about 1,000 lb per acre) of straw, a common erosion control practice on the DOT's projects.
During the grow-in phase after application in mid-July, several storms drenched the site with as much as 2 to 3 in. of rainfall at a time. Despite the heavy rain and stormwater runoff, the hydraulically applied bonded fiber matrix performed well, according to the project participants.
