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Finding an environmentally friendly erosion control solution can be particularly difficult when the earth itself seems to be working against you.

The choice of slope protection techniques can be influenced by budget, the soil, and the steepness of the slope itself. But developers are finding innovative ways to battle the age-old problem of stabilizing steep slopes in challenging situations, while softening the edges of their work, either through revegetation or with products that display a strong, natural aesthetic.

Whether they are intended to prevent sediment from sliding into a waterway or protect motorists on a busy tourist highway from falling rock, the projects highlighted here all employ environmentally sensitive, long-term slope stabilization practices. Though they are located in various parts of the country, the goal for each project was the same: create or employ a system that will stabilize the site for many years to come, while making the most of the site’s natural attributes.

Highway Development Challenges
Stabilizing slopes beside highways can be challenging. Traffic looms nearby, and even though people will be passing the site at high speeds, aesthetics must be balanced with function because the end result will be so visible. Add the potential for falling rock into the mix, and what would be an everyday challenge for an erosion control company suddenly becomes more intense.

In scenic Oak Creek Canyon, located between Sedona and Flagstaff, AZ, one such a highway project faced challenges from the beginning, in August 2007, posed by falling rock from above as well as the traffic below.                     

Jim Monnett, a project supervisor consultant for the Arizona Department of Transportation and a senior project supervisor and materials specialist for AMEC, explains that the site featuring a conglomerate of boulders and sandy material is based in a telluvian slope with about a 35-degree angle.

“The rocks from the volcanic layers up above come down into the road. It was a hazard to traffic and hikers,” he says. The project had plenty of challenges: “It was in live traffic. Half the road was shut down. There was always the potential of boulders coming down. We had to build a rock-retaining fence. It flooded a couple times.”

Set within the Coconino National Forest, Oak Creek Canyon is a magnet for tourists who come to fish, camp, hike, and enjoy a scenic drive through the area’s natural beauty. Any product added to the site needed to blend in with the red rock surroundings, while completely exposed to the big sky that frames the area.

The prime contractor, DBM, had worked with San Juan Capistrano, CA–based Boulderscape in the past. The company offered a product with the natural aesthetic desired for the site.

“The Forest Service wanted to drive by and not really see it,” says Monnett.

Boulderscape installs soil-nail walls faced with a structural shotcrete material. The micropile was made of high-strength steel structural and vertical beams and the anchors used were 130 feet long. While challenging, this type of an intense, stable system is common for Boulderscape, a company that is often called in to make stabilization systems appear so natural that passersby don’t even notice any such improvements have been made.

Photo: Arizona DOT

Construction crew working on the anchors of the Oak Creek Canyon walls

Working with a $7.5 million budget for two walls, the project, a long-term instillation solution, was created in a way that allowed maintenance to approach the site and clear up boulders.

“The wall was built upside down. You couldn’t remove all the material or the slope would [crash down],” says Monnett. Material was removed as each section was built. “We stabilized it in lifts from the top down. Each lift included a beam with reinforcement and anchors tied together with that. The cables in the anchors were stressed to over 100 kip loading and locked off to maintain that pressure.”

The Arizona Game and Fish Department approved a mix of native seed for the site suited to the ecologically sensitive area.

“Any area that was disturbed will be revegetated, other than the face of the wall itself,” explains Monnett. “We had to be finished by April 1 because of the spotted owls returning to their nests. They believe that the spotted owl is nesting in the canyon.” The spotted owl is a federally listed threatened species.

No longer just loosely scattered to protect disturbed soil on construction sites, straw can also be used as part of the process of long-term slope stabilization.

The California Department of Transportation (Caltrans) hired Selby’s Soil Erosion Control Co. Inc. in June 2007 to control erosion on a 2:1 slope approximately two and a half miles long on Pigeon Pass over Highway 84 from Pleasanton to Livermore, CA, where a new highway was being developed.

“The steepness definitely was a challenge,” says Dustin LaMantain of Selby’s, which is based in New Castle, CA, with a satellite office in San Francisco. “The size of the slope was a challenge as well.”

More than 50 rows of Earth Saver fiber roll wattles were placed on the hard, sandy, silty soil on the approximately 1,000-foot slope.

“We dig a trench for the wattle, then set the wattle into the trench,” says LaMantain, noting that biologists for the Caltrans District 4 region specified all native seed used. “After vegetation has made its home on the slope, the wattles are biodegradable.”

The wattles will naturally reduce the rate of flow, as well as absorb water and filter runoff. They will also help prevent any pollutants from the runoff from reaching nearby surface waters.

Forty percent of the ongoing project will be completed this year. It will take between two to three years for the material in the roll to completely break down into the slope, he says, and by then it will protect the soil enough for the vegetation to take hold.

“We don’t anticipate going back,” says LaMantain, referring to adding any additional long-term soil protection. “On some of these areas what was [specified] was seed. Hydrostraw was another application—mixed with seed, fertilizers, and compost and sprayed on the slope.”

LaMantain says Earth Saver’s recycled, naturally weed-free California rice straw wattles were selected because they are the “top wattle in the industry.”

“They’ve gotten competitive in their pricing,” he says, “but they take the quality to a different level.”

Adjusting Instillation Techniques at a Commercial Site in the South
A developer based in Nashville, TN, sought to evacuate a hillside in an attempt to gain enough flat land to create a new building for a national retail chain’s store, says Frank Amend, a project engineer and southeastern United States regional manager for Geobrugg North America, based in Rocky Mount, NC.

“The idea was to blast and remove material to leave a competent high wall,” says Amend. “During the blasting process, over-blasting occurred that resulted in severely fragmented vertical walls along a V-cut that already were sloughing significant-sized rocks.”

While mechanically stabilized earth walls and backfill were considered a solution, they proved too expensive for the approximately $850,000 project that began in January 2007 and lasted for five weeks. The developer also considered shotcreting the entire face, but, Amend explains, this option was also expensive, and a local forestry service agent sought a “greener” solution for the site, which has native clay and shale soils. When Geobrugg North America was contacted, company representatives suggested using a TECCO rockfall drape beside one slope of the V-cut and a TECCO slope stabilizing system along the opposite side’s high wall.

“The upper TECCO soil nailed slope was 0.5 to 1 vertical, up to 23 feet tall,” says Amend. “Rock slopes were up to 55 feet vertical.”

Accessing the site was difficult, particularly during a significant portion of crane-basket-only drilling and during the
instillation, says Amend.

R. Michael Bivens, a professional engineer for the site’s contractors Rembco Geotechnical Contractors of Knoxville, TN, says a crane basket was specially built for the job. A drill had to be directly mounted on the basket itself.

“We were able to put the drill basket in a few place we weren’t able to reach from the ground,” says Bivens.

The system was then hydroseeded, and the site now drains naturally.

“Our solutions yield much more visually pleasing results than the shotcreted area,” says Amend, noting that the owner requested an area of shotcrete fill to solve the problem of fragmented rocks on the high walls. “And they are less expensive to install.”

Wave Action Spurs Need for Slope Protection Beside Shoreline
Few elements can erode soil like water. Global warming is a hot topic when discussing the causes of high tides, but there are a surprising number of causes of shoreline erosion, and not all of them have natural causes such as heavy rainfall or flooding. Sometimes it is manmade sources that cause the greatest risk to lake shorelines.

Large motorized boats caused huge wave action on the shoreline of David Gigure’s property on Lake Viola in Webster, WI.

“It started to cause shoreline erosion on our property,” he says. “We have a little shelf that comes up from the water. It was being undercut 6 feet back. It was just a matter of time before that whole thing collapsed.”

When you live beside a protected lake, there are some restrictions on the erosion control methods that you can use. Gigure says that under the Wisconsin Department of Natural Resources requirements, he wasn’t permitted to use rock to solve the erosion problem.

“It had to be a natural material that would decompose and eventually disappear,” he says. “This is one of the few lakes in northern Wisconsin that’s very clear. They are trying to keep it as natural as possible.”

Gigure says sugar sand made up the nearly 20% slope that needed protection. In October 2005, when the lake water was very high, he used ShoreSox to prevent further erosion on the site. The product packages cornstalk fiber inside biodegradable, lined burlap, combined with photodegradable mesh. The manufacturer uses cornstalks that would otherwise be plowed under the Midwestern farmland, in effect creating a second market for one crop.

Paul Cook, a conservation specialist at the Burnett County Land and Water Conservation Department based in Siren, WI, is familiar with the work done at the site where he has monitored the shoreline periodically. Because the product is biodegradable, he explained, no county permit was required.

“The rectangular shape of the modules themselves on this particular slope almost created a terrace effect. That’s going to slow the water down some,” says Cook, noting that in this respect ShoreSox placed on the bank of the hill helped slow water velocity as well as filter the runoff. “It’s kind of an unintended advantage of the product.”

Cook compared ShoreSox against some other available products that could have been used at the site.

“It’s much cheaper than rock. It’s so much easier to install. You don’t have nearly the amount of labor,” he says. “The product has a very good staking system. Coconut or coir fiber logs have been used for years on similar sites; often times they are staked in with duck anchors.”

Seasonal factors can also affect erosion control product choice.

“Our winter seasons are long. We get a lot of ice push,” says Dan Harrington of the Department of Natural Resources based in Spooner, noting the ShoreSox seem to be holding up well. “A lot of folks put rock or riprap along the shorelines.”

At many of those sites, he says, the riprap must be replaced over time.

It’s important that a product’s anchoring system not be loose, which can happen with some methods of erosion control if the product is poorly anchored or not installed properly. ShoreSox uses a loop of photodegradable burlap through which a stake is placed, anchoring the product to the ground and making it resistant to wind and wave action.

“The whole system will move a little bit, but it always seems to be where it’s supposed to be,” says Cook. “I think the staking system is very good. These products are local products. We don’t grow coconuts in Wisconsin.”

The modular system can follow the curves and uneven angles of a shoreline.

While people can insert plantings in material such as coir immediately, Cook advises people wait before revegetating because the shredded cornstalks within the ShoreSox begin decomposing as soon as they are installed.

“It’s like a big bag of enclosed compost,” he says. “It generates a lot of heat, just like any good compost pile would, particularly if it’s in a sun-exposed spot. The shredded cornstalks need a little time.”

There has been no further erosion at the site, Gigure says, since the project was completed.

“It also allowed the hillside to start growing natural plants,” says Gigure. “A lot of the vegetation is coming back now, which will stabilize the shore and the hill.”

Sedge and irises were later planted at the site, completing the effect of a soft, natural appearance beside the lake.

Subtle Stabilization in a Residential Area
A slope located near a creek bed at the Winding Walk subdivision, a Shea Homes Community in Charlotte, NC, had been seeded and strawed twice, both times unsuccessfully, in an effort to stabilize the slope. The summer heat, threatening to challenge revegetation attempts, made a speedy solution for sediment protection essential.

In June 2007, the developer hired erosion control company Eco-FX, an affiliate of SpreadRite Organics of the Carolinas based in Charlotte, to install a compost blanket using the company’s pneumatic blower truck system. Shea Homes wanted seed planted on the site to germinate quickly on the 3:1 slope in the mostly clay to clay-loam soil. This would protect off site sediment from sliding into the creek below.

Photo: Arizona DOT

Installing shotcrete facing over the soil-nail wall

“The compost application encourages significantly improved seed germination by amending the soil profile with important nutrients to the growing zone. The compost blanket was chosen due to the rapid germination factor of the product,” says J. R. Stewart of Eco-FX. “Compost is a recycled product that adds organic value to the seeding application and helps increase water infiltration for rapid germination. The compost also acts as an erosion control product prior to germination, offering slope stabilization on the day of installation.”

Germination at the site took place within seven to 10 days following installation. “This is exceptional considering the compost blanket was installed mid-summer, with daily temperatures in the 80s to 90s,” says Stewart.

Excessive heat didn’t slow the revegetation process, which was successful within the development during the remainder of the long summer days.

Rodents Threaten Slope Stability Near the Big Easy
New York City may be famous for combating its rat population, but one Louisiana parish has its own ongoing battle with nutria. The large aquatic rodents have become more than an annoying pest in Jefferson Parish, a suburb of New Orleans, as they have evolved into an erosion control problem. The animals burrow into the canals and eat the roots of the vegetation that supports the banks.

“They multiply like rabbits. They adapt very well. Their only natural predator is alligators,” explained Bill Duplaisir, assistant director of the Jefferson Parish Drainage Department. “We have an open canal system, 340 miles of open channel,” he says, noting that the rains cause the water in the canal to rise and fall. “We get about 100 inches of rain a year on average.”

The department tries to prevent the nutria from attacking the canals and from tunneling under roadways. The situation has gotten so desperate that the sheriff’s office has a joint program with the Jefferson Parish Drainage Department in which they organize people to actually go out and try to shoot the nutria. In March 2007, the Jefferson Parish Drainage Department, open to ideas to prevent damage to the sloping canal banks, tried a new approach.

Workers installed about 250 feet of Global Material Technologies Xcluder GEO between two culverts along a canal as a test to determine if the product could help protect the banks. The stainless-steel-fiber-based geotextile is designed to prevent animals from burrowing into the landscape. The material can be left in place indefinitely, will not rust, and, if necessary, can be easily removed. Jefferson Parish workers scraped the canal bank and installed the product, beginning at the waterline and then proceeding up the bank’s approximately 1.5:1 slope.

“It took a couple days. We did it with our in-house forces and equipment,” says Duplaisir. A thin layer of soil was then placed over the Xcluder GEO, and the area was seeded with Bermuda grass.

“It grows about 12 inches,” says Duplaisir of the choice. “We cut it before it gets to that height.”

The site was monitored for both nutria activity and grass re-growth. Jefferson Parish officials noted that the grass successfully grew again in the problem area and the bank was restored to the way it appeared prior to the geotextile instillation. No nutria activity has been observed at the site since the Xcluder GEO instillation.

“The material worked,” says Duplaisir, adding, however, that the nutria moved further down the bank and the battle will continue. “It would work for us [again] in specific problem areas, such as in between two culverts.”

Natural-looking erosion control methods like this one are becoming more and more common on slope stabilization sites across the country. This is because products that work with nature and products that encourage the growth of native revegetation can help reign in the costs of slope stabilization over the long run, as well as enhance a site’s aesthetic appeal.