A familiar material–compost–and new application techniques helped revegetate a sensitive area before the summer rains on a difficult and time-sensitive project.

By Rod Tyler

The Asheville, NC, area, lying within the French Broad River basin, is a high-water-quality, pristine area. Local regulators want to keep it that way and are already paying attention to National Pollutant Discharge Elimination System (NPDES) Phase II regulations. Some corporations, such as a local company specializing in film manufacturing that needs pure water as a feedstock for producing high-quality film, have located their production facilities in the Asheville region for this reason. As Phase II implementation gets closer, we are seeing many watersheds clamp down faster in an effort to preserve this precious resource.

The Blue Ridge Parkway runs through the greater Asheville area, and other nearby national parks account for more than 50% of the land in the area. A recent project that had a potentially devastating effect on water quality brought together several different agencies and was performed under an extremely tight deadline. It involved a section of the Blue Ridge Parkway affected by a slide of rock and soil in late May, closing the parkway.

Because of the amount of parkland in the area, tourism is a big business, and that creates pressures whenever parks are closed–people make vacation plans to tour the area and become annoyed when they cannot. In this case, the project began with the goal of opening before the big Fourth of July weekend.

The parkway in the area of the slide was built in the 1960s. It was a ridgeline excavated and leveled to create the parkway road, creating steep banks above and below the road. To achieve this configuration, crews had cut off the toe of the slope. Over the years, the rest of the slope had pushed under the road and forced the pavement upward. Eventually, when the pressure was too much, the road ruptured and the rock behind it slid down the slope, covering the road.

Lloyd Middleton, a construction operations engineer who has worked for the Federal Highway Administration Eastern Federal Lands Highway Division for 45 years, was asked to oversee the demanding project. "We were asked to remove it in one month–20,000 cubic yards or 1,000 truckloads [triaxles] of soil and rock. You have to realize that working in the park is not your normal project. We needed to coordinate the concerns from the National Park Service, the Department of the Interior, and the Federal Highway Administration," explains Middleton. "Part of the slope is one-to-one, and part is one-half-to-one and dimensions were about 120 feet in elevation and 130 feet from the road. So getting to this slope is an issue, and that is why we considered choosing a new application and technology." The total site was a little more than 1 ac.

Middleton's role includes designing and building roads, including the Blue Ridge Parkway, for other federal agencies. He is familiar with conditions in a variety of areas; his current project and management duties take him east of Mississippi and sometimes include the Virgin Islands and Puerto Rico, but he has been focusing on North Carolina, South Carolina, Tennessee, and Georgia. "We are mandated to protect the environment, especially the water quality, and we normally want a green, vegetated slope to be the result when we are completed," says Middleton. "In this case, we had some other issues that included a late seed date and the need for quick vegetation establishment before the big summer rains came."

Middleton reviewed several technologies for slope stabilization and seeding, and after working in this role for more than 40 years, he had already encountered similar projects. "We had tried compost in limited areas before," he recalls. "We used wood chips with a nitrogen additive over 20 years ago, but we never had the application mechanism to get the stuff to the top of the slope without severe damage to the surrounding areas." Chipped and nitrogenized material used in the past did not allow immediate vegetation because the quality control was not as high as it is in today's commercially produced products.

Middleton's review of the recent compost products led him to wonder whether the compost would hold a 1:1 or even a 0.5:1 slope. Projects recently reported in various trade journals included a wide use of compost on 2:1 and shallower slopes. Middleton wanted to see if his hunch about compost simulating the natural forest floor was correct and, if so, if it would stay in place on such a steep slope. His review also included several options for hydroseeding and turf reinforcement mat (TRM) technology. These were similar in cost, ranging from $3/yd.³ to $5/yd.³

"One of the other main reasons we needed to consider using compost was because we do not want to introduce noxious weeds into the park," he says. "The park is very particular to this, and if we imported soil, straw, or hay, we would have to face those issues."

Opting to try a combination of compost blankets and netting on the slope, Middleton worked with supplier and installer Carolina Mulch Plus. Middleton explained that, because this was a new technology, it was important for him to witness it in a normal, heavy-rain situation. "We had the storm event we were waiting for about a week after installation, and it rained so hard it cleaned out the ditches," he notes. "But there was no compost moving or in the ditches."

The Project in Detail

Netting Slope

Project Site

This project was full of barriers and roadblocks that were obvious to all. First was the short time frame–the park wanted to open in less than a month, by the Fourth of July weekend. Second, the trucks used in the daily business for Carolina Mulch Plus, which are normally booked two to three weeks in advance, had to have their schedules rearranged to meet the short time frame. Other issues included working with the contractor on-site to time out the paving of the new road while application of compost was going on; using rappelling gear in steep-slope situations; product quality and selection, which included shipping products from outside the local area to meet project requirements; working with shear rock faces; identifying strategic areas where seeps and springs might cause problems in the future; getting around a late spring seeding under unpredictable climatic conditions; predicted heavy rains in the summer months; and, obviously, the challenges of working in the summer heat.

Middleton's choice, Filtrexx compost blankets in combination with LockDown netting (similar to the netting used in TRMs), was professionally installed by Toby and Theresa Hutchins from Carolina Mulch Plus. Compost blankets are simply a layer of compost loosely applied to the slope, achieving nearly 100% ground contact. Due to the undulating soil and slope conditions, the amount of rock and poor soil, and the steepness of slope, soil contact is very important in vegetation establishment. Compost blankets are pneumatically installed using a blower truck or equivalent device. The application consisted of layered compost, netting, and seed mixture. Besides soil contact, the main difference between this and other blanket technologies is that the seed is mixed into the entire 2-in. layer of growing medium. Most other technologies allow seeding over or under the matting tool, and when water flow moves the seed, occasional failure results in the micro area.

Final grade netting

Mountainside application

Getting the material to the slope required tools Middleton hadn't encountered in his earlier compost applications. "The blower truck hoses used here extended up to 350 feet, but nearly straight up, through the woods. If we had been blowing downslope, it would have been a lot easier," he chuckles. The trucks are capable of blowing up to 700 ft. on a flat surface, but the inclines on this project pushed the limits at 350 ft.

The other option was to use a motor grader and other larger equipment. One problem with large equipment is that the parkway pavement is narrow–20 ft. wide in most areas. Single 10-ft.-wide lanes are rare, and it is harder to do work with large commercial construction equipment. "Having the seed introduced into the compost is a good mechanism for delivery, and we do not need to mess with hassles about traffic control," claims Middleton.

A compost mixture of nutrient-grade and fibrous-grade compost was created by Toby Hutchins at his blending yard and delivered in bulk to the job site. "We thought we could save some freight by loading on-site, and the trucking situation in the parkway is not as friendly to semi-trailers as it is to our trucks or triaxles," he says. "Our plan is that the nutrient-grade compost supplies many of the necessary nutrients needed by young turf seedlings while maintaining a natural damp environment that drains well."

About three days after application, some of the grass had germinated, which helped hold the slope during subsequent rainfalls. Although it would have been better to complete the project during a better grass-growing time of year–either September or April–the demands of the holiday season and the traffic from tourism were important factors. For higher altitudes, other precautions or dates might have been required because of frost concerns.

Mickey Honeycutt from Carolina Mulch Plus applies a compost blanket to a severe slope.

Compost blankets have been becoming more common since the early 1990s, when a number of scientists showed that compost helped do two things that are critical in slope stabilization and vegetation establishment. First, compost slows down the water and creates less runoff, leading to higher infiltration rates. Most of the soils on any slope protected by any blanket are really poor. The rain that hits these soils infiltrates slowly or not at all, making vegetation establishment difficult. Second, compost can reportedly absorb nearly the entire first half-inch of rainfall. With the drought that is plaguing much of the country, this is important. For vegetation establishment, when a quick shower produces half an inch of rainfall, it is important to take advantage of as much of that as possible, especially in normally arid areas.

Preparing the slide area was extremely important, and Middleton's extensive experience was obvious. He coordinated grading with the local general contractor on-site. "We made a point not to ‘slick off' when doing the grading. It is important not to have vertical claw marks that channel water. The operator at Taylor & Murphy [the contractor] did a good job to make the teeth of the backhoe parallel with the slope, which filled up with loose dirt and did much better than bare slopes. The backhoe was basically used as a giant tamp to [create] indentations or imprints to catch loose material every few feet," Middleton explains. The operator used the backhoe as a punch to tamp and depress soil and rock in a pattern over the entire slope.

As the compost was applied, it covered the netting material that had been put down the week before by Hutchins and his crew. The netting served to increase shear strength in this case, long enough for the seed to germinate and begin to grow before the compost could slide down the slope. "This was no easy task," notes Hutchins. "We rappelled down this bank and tied off at the top of the slope in order to place the netting on the slope and pin it in. Unfortunately, due to the amount of rock on this project, we did not get as many pins in as we would have liked." The slide area is still predominantly rock, and despite the operator's efforts on the grade, many areas were only an inch or two above solid rock measuring 20-40 ft. across.

To break up the slope, mesh tubes filled with compost were installed at two locations across the entire slope to help reduce water speed and turn flows back into sheet flow, reducing the water energy and erosion potential. "Even when we were pounding in stakes for holding the FilterSocks [tubular mesh containers filled with locally manufactured, certified compost], we knew we had minimal holding power in the rock," says Hutchins. The tubes were filled on-site and were seeded like the blankets, using the blower-truck technology. Seed grows out of the tubes and into the slope, anchoring them to the slope.

Once the application was secure at the top of the slope, the project progressed quickly. Hutchins used an onsite high-reach loader as well as a boom lift, which allowed him to apply the materials directly to the slope from a much more comfortable position than standing on the slope itself.

"It is ironic how projects like this are clearly suited for us," says Theresa Hutchins. "Without [employees] Mickey and Fred being fully knowledgeable about climbing ropes, knots, and all the gear, we could not have accomplished this project as easily." Indeed, at least the top two-thirds of the project required the operators to be tied off on rappelling ropes, hanging from the top of the slope. Mickey and Fred are amateur rock-climbing enthusiasts and never thought they would use that skill for Carolina Mulch Plus installations.

Middleton also requested using compost in a blend with size-78 gravel for the road shoulder so it would be drivable and motorists would not leave ruts when they pull off. This blend of compost and gravel simulates structural soil formulas that have been used in other areas and is easy to apply.

For this project, Hutchins selected a turf seed mixture of about one-third annual rye and two-thirds fescue. Although it was probable that annual rye would not survive the heat, the goal was to achieve maximum stabilization in the shortest possible time. Because annual rye germinates in three to five days, it adds a three-dimensional holding power to the slopes. If the rye dies, the vertical holding power remains until the fescue can fill in. So far, the anticipated heat that is normal for this region has been less than forecasted.

To allow for protection at the top of the slope, a compost filter berm was installed to reduce on-flow water and turn it into sheet flow. Filter berms are an acceptable substitute for silt fence and are currently used in almost all 50 states. In this case, tying into existing vegetation was challenging. The roots, site debris, and existing vegetation made it hard to establish good ground contact with a perimeter tool such as silt fence, but compost filter berms are similar to compost blankets in that they have 100% ground contact and work well at the edges of slopes with existing vegetation.

Organic layers in the woods provide a natural erosion control. Next time you have a chance to look below your feet in the woods, you will notice very little erosion. Compost mimics this organic coating in a microscopic way while providing an excellent source in which seeds can germinate.

The Blue Ridge Parkway project so far has been a success. Depending on the impact from the seeps and springs, the team believes this slope will stay vegetated.

Rod Tyler is manager of Green Horizons and Filtrexx LLC in Grafton, OH.

SW - November/December 2002

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