An appealing feature of geosynthetic materials is their ability to help nature help itself. Described by the Erosion Control Technology Council in St. Paul, MN, as products that are “manufactured from polymeric material used with soil, rock, earth, or other geotechnical engineering material” as a central element of a manmade structure, system, or project, geosynthetic materials help save time and money in projects around the world. Many geosynthetic materials including erosion control blankets and geotextile tubes keep erosion at bay while maintaining an organic aesthetic, a valuable attribute in today’s green design climate.
Stability, resistance to biodegradation, and immunity to chemical reactions also describe the allure of geosynthetic materials for contractors. Geotextiles and geomembranes, turf reinforcement mats (TRMs), or silt fencing—the use of all these geosynthetic materials for erosion control in innovative ways is a market generating heat like the UV rays some of the products work to protect.
Preserving a Piece of Baltimore History
Federal Hill, a park located in Baltimore across from the city’s Inner Harbor, is a historic hill where erosion has been a problem for the last 200 years. Dave Snyder, a certified professional in erosion and sediment control (CPESC) and president of Webtec, a manufacturer of geosynthetic materials for the construction industry based in Charlotte, NC, describes the high-profile project that was designed to stabilize the area for the future.
“The existing soil was highly erosive. Finally what was decided was to remove the soil and place 5 to 7 feet of crushed stone on the 2:1 to 2.5:1 slope,” he explains. “On top of that was placed a nonwoven geotextile. Placed on top of that was a cellular confinement system.” Topsoil was then placed within the cellular confinement system.
“The drainage stone was there for stability of the slope and for drainage. The fabric was there to serve as a separator. The cellular confinement system was made up of TerraCell geocell. Along with that geocell was the use of polyester tendons,” says Snyder of the project that began in 1999 and was completed in 2000. “The polyester tendons were run through the TerraCell into an anchor trench. There was a sidewalk placed halfway up the slope. It basically changed a very long slope into two smaller slopes. That was a very good design idea. From the midpoint of the slope up over the slope of Federal Hill, the tendons were tied to a plastic pipe tied to an anchor trench.”
Because it was located within a city, access to the site was also a challenge. Geotextiles, conveniently, bring with them simple application methods.
“It was a city project. They had a limited space to work. They had a stone wall at the bottom of the slope that could not be disturbed. The cellular confinement system was needed to hold that soil at that degree of slope,” says Snyder. “Two of the biggest challenges are the length of the slope and the degree of slope. A temporary erosion control blanket was placed on top of the whole slope.” Grass makes up the top of the slope.
“The pipe in the bottom of the anchor trench is very similar to a curtain rod,” he says. “We’re hanging that veneer of soil on that slope. We’re hanging it by the geocell containing the soil. That veneer is held up by the tendons that are attached to the plastic pipe at the midpoint under the sidewalk and then halfway up the slope.”
The project’s success has protected a respected American landmark.
Erosion Applications on a Different Shore
When you think of shoreline applications, you may picture an inlet, river, or ocean, but erosion affects shorelines as varied as the earth’s waterways themselves. At the Chicago Botanical Gardens, the saturated soil in a small lake was being displaced from the bank into the pond, explains Daniel Senf, a registered professional engineer and director of business development for Presto Products Co., based in Appleton, WI. “When we get into erosion of shorelines we’re talking not only about wave action, but the fact that saturated soils many times cannot support their own weight and start to flow. We can prevent that movement by confining the soils within the cells of a cellular confinement system,” says Senf. “[The topsoil at the gardens] wasn’t pure sand, wasn’t pure clay. There is a lot of wetland-type vegetation that is displayed. The shoreline was becoming destroyed. It was rebuilding the embankment.”
The Chicago Botanical Gardens, he explains, had not needed to add an extensive shoreline protection treatment such as this in the past. Shoreline stability in particular posed a challenge for project workers.
“The lake had to be partially drained and the shoreline regarded and then stabilized before being rebuilt,” says Senf, adding that Geoweb, a cellular confinement system, was used to structurally hold the soil in place as part of a larger system of erosion control applications used to rebuild the shoreline in a permanent solution.
Carex comosa, Thalia dealbata, Decodon verticillatus, Juncus effuses, and Colocasia ‘black magic’ are some of the water plants that were installed to help anchor the system.
“A team of Chicago Botanical staff, civil engineers, landscape architects, and ecologists worked on this project,” notes Patricia Stelter of Presto Products, adding that the project at the Chicago Botanical Gardens was similar to work the company performed at Morton Arboretum in Lisle, IL. There, she says, “They created shallow ‘aquatic shelves’ from benching a variety of materials: underwater sheet pile, Geoweb cellular confinement system, wet perennials, emergent vegetation, geotextile, large boulders, and angular granite.”
These days, Senf says that only about 1% to 2% of the solutions created by Presto Products consist of synthetic materials; the other 99% are natural materials.
“The problem was an unstable shoreline. The solution is a stable shoreline that will have a sustainable vegetative medium,” he says. “Geoweb is a container, and a container without content has no value. The problem is making the content work. The content becomes part of the system that we’re creating.”
Combining Silt Fence With BMPs for Added Protection
When the Minnesota Department of Transportation (Mn/DOT) embarked on building a major bridge across the Mississippi River in Newport, MN, team leaders incorporated miles of silt fencing into their erosion control plan. Silt fence was used as part of the staging and phasing of the bridge and ramp work for the job, known as the Wakota project, according to Dwayne Stenlund, a CPESC working in an erosion control engineering unit within Mn/DOT. Silt fencing, he explains, was used “to force stormwater back into sheet flow to protect the ramp to westbound 494 from northbound TH61 in Newport.
“You’re using silt fence for insurance rather than relying on it only for prevention,” says Stenlund, who prefers using silt fence in combination with other best management practices (BMPs). “We’re overusing silt fence when we don’t combine it with erosion control. We combine erosion control with sediment control because without that combination, there is no hope. We do use silt fence as part of a staging program, particularly when dealing with alternative storm pond protection. It is easy to put up quickly. We now have super-duty silt fence details and pay items that use a concrete barrier as the support of silt fence geotextile. Sediment never rotates this fence type, and it never seems to be driven over as in the past.”
Mechanized installation of silt fence means that as a job changes, controlling erosion and sedimentation with a silt fence can help meet the project’s altered needs. In the Wakota project,which is estimated to cost well in excess of $100 million, two lines of silt fence were installed and filter fabrics were used “making steps in ditches to trap small amounts of sediment,” according to Stenlund.
Mn/DOT often makes cages out of silt fence and continues to find new uses for the sediment control product.
“Geotextiles used without understanding the performance limits always results in some form of failure to public safety or to waters of the state,” says Stenlund. “There is a performance limit to all BMPs. Silt fence is the most abused of all current BMPs, and is used in areas that cannot succeed. For example, geotextile silt fence rings must be adjusted to prevent flooding, but may then compromise sediment control. Sediment control and erosion control is a delicate balance between safety and performance.
“I believe silt fence needs to change with the different stages or phases. It’s the right tool for many [stages and many applications]. We’re using it in many different design areas. We’re no longer using it just to ring a project.”
In the future, he predicts, people will be “using silt fence smarter” as part of value engineering, and will probably be using less of the product but using it more effectively.
Geotextiles in a Race Against the Clock
Timing and coordination were essential to the success of an erosion control project in November 2005 at a creek crossing in Camas, WA. Erosion control needed to be established at a creek crossing that served as an access road to a development with more than 120 lots. Complex environmental challenges meant the work had to be completed within a set number of hours, not days.
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Photo: CSI Geosynthetics |
| Work on a creek crossing in Washington state |
“It was a really steep site with a creek. It discharges into a local protective lake where they happen to have the local fishing derby for kids,” says Alex Zimmerman, a CPESC working in erosion prevention and sediment control services at CSI Geosynthetics in Vancouver, WA. The work included adding scour protection, slope restoration, and energy dissipation. “The road had to cross the creek. Due to the nature of the creek diversion and the potential stream disruption, the City of Camus required that the project be completed in one day.”
Many hours were spent selecting the appropriate equipment and materials that could be used on the project, particularly because of the slope and the time constraints.
“The coordination of the crews was key, with everyone coming together as a team,” says Zimmerman, who worked on the project. “Preplanning was very important.”
The project operated on a budget of approximately $50,000 for work completed in and around the stream.
“The first step was diverting the creek. We blocked it off using sandbags, some plastic sheeting, and sheets of plywood to block off the flow,” explains Zimmerman. “Then we used a 6-inch electric submersible pump to divert the flow. We also used four smaller pumps to move the muddy water from construction activities to settling tanks prior to discharge into the vegetative area.”
Workers had to excavate the soil to make room for the placement of a geotextile fabric as well as riprap, he says, before placing energy dissipation pads, moving back over the slopes before they were regraded and covered with erosion control blankets and Earth Saver straw wattles.
“We did use the biodegradable netting,” says Zimmerman. “They didn’t want plastic netting that could trap animals. We used both coconut fiber blankets and coconut straw blankets.”
The creek was then returned to its original channel. Geotextiles provided the instant soil protection needed for the site, according to Zimmerman.
“We were late in the season. We get moisture retention from the blankets. We get added warmth,” he says. “You get the instant soil protection and the increased vegetation establishment.”
In another example, it was the effects of time that caused one community group to become concerned when it appeared a future investment was literally slowly disappearing before their eyes.
When erosion damaged the sideslopes of a channel in Corona, CA, and valuable land was being washed away, Sukut Construction worked with Riverside County Flood Control from October 2004 through June 2005 to curb the problem through the use of TenCate’s Mirafi 1100N, a heavyweight, nonwoven geotextile.
The geotextile was placed on recently graded banks before aggregate and riprap were installed, according to Chip McCallum of TenCate Mirafi, based in Pendergrass, GA. The material was selected for its high puncture strength as well as its ability to hold back fine-grained sediment with its fine openings and its high permittivity. While the role of the geotextile was to protect the soil located behind the rock placement, says McCallum, there was also a need for swift dissipation of water preventing pressure buildup behind the fabric if the channel’s water elevation dropped quickly.
Two years later, the project is considered a success and the property adjacent to the area is now developed.
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Photo: Webtec LLC |
| Federal Hill site after installation of EC blanket |
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Photo: GDOT |
| Coir mat with vegetation growing through |
Environmentally Sensitive Blanket Protection
Erosion control blankets can be especially useful in sensitive wildlife areas. In Georgia’s Whitfield and Murray Counties, for example, the Department of Transportation embarked on an erosion control project at a bridge crossing the Conasauga River. “GDOT’s standard practice is to place Type I riprap underneath our bridges on the riverbanks to protect the banks from scour and erosion,” says Leigh Priestley, assistant manager of GDOT’s Environmental Compliance Bureau in Atlanta. “This project had multiple endangered species within the project corridor, which required the Department to comply with Section 7 of the Endangered Species Act. The Fish & Wildlife Service did not want riprap to be placed on the riverbanks because it could be fatal to both the small endangered fish and the mussels. Their experience had shown that too often these small aquatic organisms were getting caught in the pockets or voids between the rocks, particularly if the water rose and fell abruptly, and then the fish or mussels would die.
“The challenge here was to also prevent scour and erosion, which would increase sediment loads in the area and could also be detrimental to the endangered organisms.” In place of the riprap, a coir mat was selected to provide bank stabilization.“This mat was constructed with coconut fibers and contained no synthetic webbing, which could also cause small organisms to become fatally trapped. The coir matting has an open weave, which allows for excellent revegetation through the mat while providing protection from scour and erosion caused by storm flows,” says Priestley. “The riverbanks were graded, grassed, and mulched, and then the coir mat was installed on both sides of the river underneath the bridge.”
Approximately 3,000 square feet of material were used to stabilize the riverbanks. “We also stabilized a small tributary that fed into the river with the same mat, and we used approximately 1,000 square feet of material for this tributary,” she notes.
There are many reasons erosion control blankets are considered an affordable choice, according to Priestley, when selecting an erosion control method for a project. “There are a variety of types of blankets on the market to meet a variety of stabilization needs, from environmentally sensitive areas, to cut slopes, to stormwater ditches,” she says. “The products are usually readily available and easy to install, as opposed to the cost and installation of rock in all areas of the state. Another long-term benefit for the environment is mats allow vegetation to grow, which protects the soil from erosion and allows for infiltration of water back into the soil to recharge the groundwater in the area.”
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Photo: TenCate Mirati Geosynthetics |
| Geotextile underlies aggregate and riprap on the bank. |
The project proved a success for GDOT. “The blanket allowed the Department to meet its obligation to the Fish & Wildlife Service to protect the endangered species and to prevent erosion and sedimentation of the riverbanks,” says Priestley.“We needed a long-term, effective method that could withstand the rise and fall of the river during and after storm events. The coir mat gave us that strength while providing an environment that supported vegetation establishment.”
With the appeal of their natural appearance and myriad uses including soil protection, turf reinforcement mats are another option for a variety of earthen structures.
Can turf reinforcement mats be used to protect water quality? The City of Durham, NC, approved a project that would repair the area near a water treatment plant, where water from Lake Michie is used to provide 22 million gallons of drinking water for the city daily. The city wanted to make sure soil from the back slope, with a 3:1 pitch in the plant area, would not erode, which could weaken an important dam as well as pollute the drinking-water supply.
Instead of opting for a hard-armor solution, the project’s engineers from Hazen & Sawyer PC selected a “green” geotextile. They protected the slope using Colbond’s Enkamat II, a composite TRM used to foster vegetative growth while protecting the soil from erosion. Eight thousand square feet of Enkamat II were delivered to the project site.
“This particular project had raw slopes that needed to be stabilized due to dam repairs. As an alternative to costly and labor-intensive concrete, Enkamat II not only offered short- and long-term erosion protection, but also offered an aesthetically pleasing aspect to the surrounding area by incorporating lush vegetation to the site,” says Jason Willis, CPESC, of Colbond in Lawndale, NC. “Once the site was at finish grade, the contractor was able to seed, fertilize, and lime the site and then install the mat.”
As with many applications of geosynthetic materials for erosion control, the challenge was met and results were seen fairly quickly. According to Willis, the lush, healthy grasses that established themselves at the site with the help of the three-dimensional, netless, biodegradable TRM have since stabilized the slope.
Applied correctly, geosynthetics can bring permanence to seemingly temporary solutions, and a natural grace to the disturbed landscapes they help strengthen. Their use may have grown only in the last few decades, but the new and innovative applications for geosynthetic materials will carry them as an erosion control solution well into the future.