Geosynthetics for erosion control

By Bill Tice

For most people driving through a major city or down an interstate highway, or crossing a mountain pass on a steep and winding rural road, geosynthetics are not at the forefront of their mind, yet these engineered solutions to erosion control challenges are probably all around them. The term geosynthetics covers a broad range of products that are commonly used in road building and other construction projects, or anywhere soil is disturbed or moved through manmade initiatives, and in areas where natural occurrences cause the unwanted movement of soil and other materials.

PHOTO: THUNDER MOUNTAIN ENTERPRISES

PHOTO: THUNDER MOUNTAIN ENTERPRISES

According to the Web site of the Geosynthetic Research Institute, which is part of Drexel University in Philadelphia, PA, the main categories of geosynthetics are geotextiles, geogrids, geomembranes, geonets, geocomposites, geosynthetic clay liners, and geopipes. Falling within these broad categories are more specific products such as geocellular confinement systems, which are fabric honeycomb-like grids that are placed on the ground and filled with soil and are often used to create a stable base for temporary or light traffic roadways; filter fabrics, which are used for drainage on parking lots and roadways and allow water to infiltrate while stabilizing the sub-base; geotextile tubes or "sausages," which are filled with sand and used to slow beachfront erosion and protect homes and other structures; erosion control blankets, which are temporary and designed to biodegrade or UV-degrade quickly; turf reinforcement mats, which are similar to erosion control blankets but are designed for more permanent applications; and silt fence, porous fabric that allows water to drain while catching soil and sediment.

For Beth Smiley, CPESC, technical manager at Thunder Mountain Enterprises in Sacramento, CA, the variety of geosynthetic materials available on the market today is a tremendous tool to help her problem-solve erosion control issues for her customers. Thunder Mountain provides comprehensive soil and water management services, including the implementation of best management practices and the installation of erosion and sediment control products. The company is also extensively involved in the preparation of stormwater pollution prevention plans, stormwater treatment, the installation of containment ponds, and the restoration of native habitats, landscapes, and streams.

Our aim is to cost-effectively meet the growing needs of landowners, developers, contractors, and industrial facility owners in managing water, soil, and onsite pollutants, and to help these clients understand and meet the requirements of today's complex environmental laws," explains Smiley, who is a co-owner of the company and handles technical sales and training activities. "We have developed an analytical approach to the business, which combines biotechnology, site hydrology, soil science, and advanced erosion control technology with construction expertise. Having a full range of geosynthetic products available helps us to provide tailored solutions to our customers' needs."

On a recent project for the California Department of Transportation (Caltrans), Thunder Mountain used Pyramat turf reinforcement mats from Synthetic Industries to permanently reinforce vegetation on a 150-foot-long slope. "The project was on California Highway 26, just south of Westpoint, California," explains Smiley. "The 0.5:1 slope sheds directly into the Mokelumne River via a drain outlet that is approximately 25 yards from the toe of the slope."

Approximately 100 feet of the slope had failed, meaning Caltrans had to act quickly to prevent sediment from reaching the river and to protect nearby homes. "They needed a fast and effective method to stabilize the slope," adds Smiley. "Prior to installing the Pyramat, Caltrans had used a synthetic netting on the slope with the intention of adding surface tensile strength at a low material and installation cost. This worked in the short term, but failed within one year of installation."

PHOTO: THUNDER MOUNTAIN ENTERPRISES

PHOTO: THUNDER MOUNTAIN ENTERPRISES

Caltrans considered the Westpoint site to be in "emergency status," and a maximum budget of $99,999.00 was allocated for the job that needed to be done. The entire project was 5,000 square feet with a width of 50 feet and a height of 100 feet. Site preparation was a key component of the project and included re-profiling and smoothing out the earthen surface to enable intimate fabric contact with the soil. Workers accomplished this by rappelling down the slope from the top and shaping the slope with hand tools and light power equipment.

Following these preparations, the slope was hydroseeded and then blanketed with the turf reinforcement mat (TRM). The TRM was secured at the top in a 1-foot by 1-foot anchor trench and held in place with 5,800 galvanized pins with a spacing of two-and-a-half pins per square inch. In addition, 650 3-foot-long mechanical anchoring devices were driven into the slope's underlying bedrock using a 40-pound jackhammer. The mechanical anchoring system worked with steel tendons, which, when tightened and secured with special locking washers, engaged the mechanical device. 

"Caltrans expressed complete satisfaction with the project and plans to use this method of slope stabilization on several similar projects in the future," adds Smiley. "We expect that by the spring of 2006, the vegetation on the Westpoint site will be well established, resulting in a completely stabilized slope."

Farther north in Washington state, ACF West is using geosynthetics for a number of large projects, including four detention ponds at a suburban Seattle housing development. The detention ponds, which range in size from 90,000 to 250,000 square feet, are part of Issaquah Highlands, a several-hundred-acre site located east of Seattle.

 "The ponds are lined with a 60-milliliter high-density polyethylene liner, and the slopes on the ponds range from a 2:1 to a 3:1 pitch," explains Ben Nelson with ACF West's Woodinville, WA, branch. "The site owners and the engineers were concerned about the integral shear between the cover soils and the membrane, and because these areas are accessible to the public who are buying the property around them, they were also concerned about aesthetic values."

PHOTO: ACF WEST

PHOTO: ACF WEST

PHOTO: ACF WEST

The solution implemented by ACF West was designed by Puget Sound–based Core Engineering and included the installation of a cellular confinement system from Envirogrid, which was augmented with polyester tendons placed on 20-inch centers and having a load capacity of 1,250 pounds per tendon. The cellular confinement system is placed on top of the liner, creating a honeycomb-type structure on the slope. This structure is then filled with soil and vegetated.

 "We have installed this system on all four of the ponds, from the original project almost four years ago to the latest one, which was completed in the summer of 2005," says Nelson. "They have been successfully backfilled and hydroseeded, and rocks have been strategically placed for architectural and aesthetic reasons. As for the public reaction, we have had nothing but praise, and we have not had a single failure."

The Woodinville office of ACF West used Envirogrid on another interesting project in the small town of Everson, WA, which is in the northwest corner of the state and close to the Canadian border. The Nooksack River, which runs though the area, is prone to flooding, especially in the spring when the Northwest experiences heavy rainfall and high runoff levels from Mount Baker.

 "The waste treatment plant for the town of Everson is located along the banks of the Nooksack and is protected by a levee system," explains Nelson. "There is limited public access to the river at an adjoining park, so the town wanted to improve that access while at the same time to maintain and improve flood protection for the municipal wastewater treatment plant."

Approximately 200 feet of the levee were rebuilt in the public access area using Envirogrid's EGA30, but in this case the tendons were not required as the length of the slope ranged from 20 feet to 26.5 feet, and the shear stress between the cover soil and levee did not require them. The cellular confinement system was secured to the 3:1 slope with No. 4 rebar in a J-hook configuration, while adjacent cells were adhered to each other with galvanized staples. This structure was backfilled and hydroseeded.

 "As the floodwaters rise, you now have an armored face to this levee," adds Nelson. "This prevents soil from being washed out and works in conjunction with the root system of the hydroseeded grasses. You end up with a positive result without using riprap or other hard armor. We feel this vegetated slope is more natural, safer, and offers improved habitat for small animals that are native to the area."

The project was completed in mid-August 2005, and although Nelson is expecting and predicting a positive outcome from the reconstructed slopes on the levee, he says the company will have to wait until after the rainy season to develop concrete data.

At the Newcastle Golf Course, a relatively new 36-hole championship-style course in the Seattle, WA, suburb of Newcastle, a total of five slopes failed during the winter of 1998. Those five slopes covered an area of approximately 12,000 square yards, which had to be reconstructed by replacing the slide material and covering the slopes with a geosynthetic material that would serve two purposes—the prevention of future surface erosion and the promotion of vegetation on the site.

PHOTO: MIRAFI

PHOTO: MIRAFI

PHOTO: MIRAFI

Northwest Linings & Geotextile Products Inc. in nearby Kent, WA, suggested a permanent TRM as a cost-effective solution to the problem. The TRM consists of 100% synthetic polyolefin fibers positioned between two high-strength, biaxially oriented nets and mechanically bound together by parallel stitching with polyolefin thread. Preparation work included removing the disturbed soils and replacing them with cohesive aggregate and clean pit run material. The areas were regraded to the original slope angle of approximately 2:1 and then hydroseeded.

The TRMs were installed from the top to the bottom using 6.5-foot-wide rolls with parallel seams overlapped by 4 inches and roll ends overlapped by 12 inches. The TRMs were anchored using 6-inch U-shaped staples that were secured every 3 feet on center. This helped create close contact between the TRM and the subgrade, preventing hydrostatic pressure. Additionally, to help secure the TRM, anchor trenches were dug at the top of each slide area and at 30-foot intervals across each slide area.

The solution provided by Northwest Linings proved to be economical, with the 12,000 square yards of TRM being installed by a four-man crew over a period of four eight-hour working days, or an average of 94 square yards installed per hour.

On the other side of the country, the Town of East Lyme, CT, was looking for a way to connect two separated public beaches by constructing an elevated pedestrian walkway. Parts of the beach, which is on Niantic Bay and part of Long Island Sound, were not accessible to the public due to an electrified Amtrak rail line.

 "The main railway line that runs from Boston to New York and Washington, DC, was built around the turn of the century and it runs down the coastline of Connecticut," explains Mike Egan, a professional engineer with the Rocky Hill, CT, office of CTE Engineers Inc., a national engineering firm headquartered in Chicago, IL. "When Amtrak electrified the rail line, access to the shore was severely cut off, including a main stretch of beach that was beyond the embankment that the rail line runs on."

To regain access to this cut-off section of beach, the town wanted to build 2,000 linear feet of elevated boardwalk and 3,000 linear feet of embankment walkway. CTE was contracted to do the engineering for the project. The construction could not compromise the rail line and had to be completed with minimal environmental impact due to the area's fish, clam, and lobster habitat. "What we had to do was build a new section of embankment into Niantic Bay by benching into the existing railway embankment, and we had to complete the project in a way that was as economical as possible," says Egan. He adds that in addition to money coming from the Town of East Lyme, funding for the project came from federal enhancement funds, which are part of the federal highway bill, and from the Connecticut Department of Environmental Protection.

The embankment was widened using a heavy riprap to armor the outer sloping layer, but for Egan, the biggest challenge was what to use for the inside of the embankment. "We had several suggestions, including using a modified riprap, but we had a requirement to ‘toe in,' which meant we had to dig a hole at the base of the embankment. We were removing a significant amount of granular material to meet this requirement, so we tried to reuse as much of this material as we could so that we didn't have to truck it offsite. This is where the geosynthetic material came into play."

The final design called for a wrapped face wall utilizing 4.5 feet of heavy riprap armoring, Filterweave 404 from TC Mirafi Construction Products, excavated granular fill, 8 inches of aggregate, 2 inches of stabilized stone dust, and a layer of Mirafi's Filterweave 700.

 "The contractor on the job, Baier Construction, found that as they excavated for the toe, they could place the excavated material ahead of them and armor the operation behind them," explains Egan. "This was a very productive and cost-effective way to do the project, and because the embankment and the face wall were wrapped, it was able to stand up against wave action during the interim construction period and before the riprap was placed."

The project, which was completed in June 2005, took a total of a year and a half to construct. "Everyone was very happy with the end result," adds Egan. "We needed to come up with a cost-effective way to handle a difficult construction challenge, and by using some innovative techniques and products, we were able to do just that."

Michael Dorsch has more than 17 years of experience in what he calls "value-added site solutions" for geotechnical and environmental applications. Dorsch is the owner of Woodstock, MD–based Geo Environmental Site Solutions LLC, a three-year-old company that he founded after working for a number of other companies within the industry, including manufacturers of geogrid products and lining systems. Today, Dorsch's company specializes in using new technology to find the right solution to clients' needs.

"Ninety percent of our sales comes from our consultation services," explains Dorsch. "We look at everything from planning to design and bidding to installation. Our ultimate goal is to find the best solution for our clients, and in doing so, we are always investigating new state-of-the-art practices and materials that add value to a job over traditional materials and methods."

Although Geo Environmental Site Solutions includes installation in its toolbox, Dorsch is quick to point out that in most cases, the company uses the services of other installation companies. "We distribute products, and we don't want to compete against some of the installation contractors that are also our customers, so we generally only take on the installation jobs that require special techniques, or the ones that other companies don't want to do or can't do," explains Dorsch.

Dorsch's company was recently involved with a project in Rosaryville State Park near Brandywine, MD. The 982-acre day-use park, which includes approximately 10 miles of trails that wind though areas of mixed hardwood forest and fields, is well suited for birding, horseback riding, hiking, and mountain biking. The park is also home to the historic Mount Airy Mansion, which was originally built in approximately 1660 as a hunting lodge by Charles Calvert, the Third Lord of Baltimore.

 "The park wanted to rehabilitate some of the trails and make them more accessible to the traffic using them," explains Dorsch. "These trails included a number of stream crossings, which were starting to erode due to the traffic. This was creating a danger for people using the park, so we worked with a contractor that specializes in trails, and came up with the idea of using a combination of a geotextile and a geocell. The trail was recut and regraded, and then at the stream fords, they laid down a nonwoven geotextile from Skaps Industries as a filter and separator. Envirogrid from GeoProducts was then employed over top and staked in place with minimal J-hooks, which had to be buried in the perforations in the grid and below the surface level to prevent any possibility of injuries to horses or people using the trails."

As the Envirogrid panels were installed, they were filled with a well-graded aggregate, which created a stable and permanent stream ford. "The stream ford now offers a firm passage with a stable base," adds Dorsch. "The water is able to run over top of the crossing, but the honeycomb-type cells prevent the soil and stone from being washed away by the erosive forces of the stream and the erosive forces of the continual traffic using the trails."

Dorsch says the solution was attractive to the park's administrators for several reasons. "We didn't need highly skilled labor for this project, so the contractor was able to use park volunteers for some of the installation, which was very cost-effective. The panels are also lightweight as they are made from polyethylene, and when they are collapsed, they are in long strips, so they are easy to handle. This allowed the job to be completed without any heavy-duty equipment."

In addition to the stream crossings, Dorsch's company supplied the park with coir logs from the Venture Fiber Co. The logs replaced silt fence that was to be used in parts of the project.

With the success of the first stream ford project, the park decided to use the same technique for another stream crossing project in November 2005. In addition, the contractor plans to use the same method and materials on similar projects in parks throughout the country.

 "After the first project, all of the stakeholders were extremely happy with the results," says Dorsch. "Everything was done in less than a day, it was cost-effective, and it was easy to install."

Author Bill Tice works in Blaine, WA.

EC - January/February 2006