Construction projects present varying levels of erosion control and seed germination challenges. This past summer the City of Windsor, ON, faced challenges of steep proportions in finding a solution for preventing sediment erosion and establishing turf growth on exit-ramp embankments that consisted of a 2:1 gradient ratio.

The roadway project in the southwest suburbs of Windsor, a city of 200,000 people that sits just west of Detroit, MI, across the Lake St. Clair channel, began in fall 2002. To provide a quicker and more direct route into the city for motorists, the city built exit ramps connecting Six Concession Rd. with Dougall Pkwy., a main thoroughfare that turns into Highway 401. The addition of the ramps was designed to accommodate residents living in new subdivisions that sit off of Six Concession Rd.

Six Concession Rd. runs north to south and crosses over Dougall Pkwy. The city built a westbound exit ramp on the north side of Dougall Pkwy. and an eastbound ramp on the south side, completing construction in December 2002. In addition, Six Concession Rd. was widened to make room for new turn lanes and sidewalks.

The project's builder had a less-than-desirable amount of land to work with because of the proximity of private property to Dougall Pkwy. As a result, the ramps' embankments were built at a steep angle.

This created an erosion control challenge for the city. An important facet of the roadway project included laying topsoil over the embankments' clay soil, seeding, and establishing healthy grass. But the embankments stood 23 ft. tall and featured inclines that were 2 ft. vertical for every 1 ft. horizontal.

"The surrounding landowners didn't want to give up much of their land, so we had to build the ramps in a really tight area," says Susan St. Louis, the project engineer from Dillon Consulting in Toronto, ON. "We were looking for an erosion control solution that would really hold up well on steep embankments."

Exploring Options
St. Louis turned to John Reynolds of Mulchit, a distributor in Putnam, ON, for consultation, having worked with him on previous projects. Reynolds discussed several erosion control options with St. Louis.

"This project was a big challenge," Reynolds says. "Erosion control was necessary to stabilize banks, establish seed growth, and prevent erosion of silt and soil into waterways."

Reynolds determined that erosion control blankets were not appropriate for use on steep slopes, such as those at Six Concession Rd. and Dougall Pkwy. Applying a blanket is more labor-intensive and requires stapling to the ground, Reynolds notes, which would be more tricky and dangerous working on and around steep slopes. In addition, the embankments' soil was coarse. Blankets can cause tenting when applied to coarse soil because they don't completely conform to the landscape, resulting in a lack of maximum soil coverage and protection.

Reynolds also ruled out conventional bonded fiber matrix (BFM) products. A BFM lacks the ability to efficiently grip and bond to the soil for an extended period, he believes, thus jeopardizing its effectiveness in erosion prevention and retaining moisture. In addition, rainfall limits the window of opportunity when BFM products can be applied.

Reynolds recommended using Profile Flexterra FGM (Flexible Growth Medium) from Profile Products LLC of Buffalo Grove, IL. This hydraulic mulch uses patented technology to combine both chemical and mechanical bonding. Mechanical bonding incorporates synthetic fibers into the matrix, giving it a stronger hold. This is an important characteristic in effectively controlling erosion under rainy and windy conditions and on steep slopes for an extended period of time.

"When applied, Flexterra forms like a blanket but clings to every inch of the ground," Reynolds explains. "It helps create a microenvironment for seed growth."

Putting Flexterra to the Test
St. Louis went with Reynolds's recommendation, and in June 2003, 4 in. of topsoil were applied to the native soil, a heavy clay common to Windsor that is not an adequate growing medium.

It was important to apply mulch as quickly as possible to prevent erosion of the topsoil. Using hydraulic equipment, Peter Palmari of Border City Landscaping in Tecumseh, ON, thoroughly covered the 11-ac. area with Flexterra, applying it at a rate of 3,200–4,000 lb./ac. Mixed with the mulch was a combination of seven grass seeds and fertilizers. Of course, the steep embankments were the most challenging areas in terms of efficiently applying the mulch.

"You really have to know how to spray steep slopes," Palmari says. "You have to adjust your spraying position, use the proper nozzle, and spray a very fine mist of mulch."

An additional challenge was the need to spray the same area from two opposite directions. Doing this, says Reynolds, prevents shadowing from occurring on the soil. Spraying from opposite directions ensures that every last square inch will be covered. Palmari completed the job in three and a half days, spending 10 hours each day.

Rainfall on one particular day did not slow him down. Although the threat of rain can hinder application of some erosion control products, Flexterra needs no cure time; it can be applied even if conditions are wet. As a result, there was no concern about the mulch washing away and no downtime having to wait for rain to pass.

"The combination of mechanical bonding plus a cross-linked hydrocolloid tackifier allows you to spray Flexterra despite the threat of rain," says Reynolds. "Other products, including standard bonded fiber matrices, require 24 to 48 hours before rain."

Windsor received little rainfall last summer, jeopardizing grass growth, but the mulch's interlocking matrix of synthetic and organic fibers creates more water-absorbing cavities to enhance seed germination. This matrix helped the mulch absorb the little amount of rain that did fall. Despite the steep slopes and dry summer, St. Louis was impressed with the end result of erosion control and seed germination. With healthy, consistent grass established on the embankments, the ramps were paved and opened by early 2004.

"The embankments look great," she says. "It's pretty awesome how it filled in. There were no signs of erosion but quite a bit of vegetation."

EC - March/April 2004