Geosynthetics at Work

May 1, 2010

Geosynthetics are amazing materials. They drain, contain and filter. They separate as well as hold together. They work underground and underwater, on mountainsides and on rooftops.

According to the International Geosynthetics Society, the words geotextiles and geomembraneswere coined only about 30 years ago, at the first International Conference on Geotextiles, in Paris. Since then, we have added geogrids, geonets, geosynthetic clay liners, geofoam, and geocomposites. These materials are made of polypropylene, polyester, high-density polyethylene (HDPE), linear low-density polyethylene (LLPDE), and polyvinyl chloride (PVC), to name a few. All are flexible and durable.

Geotextiles are one of the largest groups of geosynthetics. Their fibers are either woven, knitted, needle-punched, or matted together into fabrics. They’re porous to varying degrees and are used for separation, reinforcement, filtration, and drainage. Geomembranes are another large group of geosynthetics. These are relatively thin, impervious sheets of polymers. They’re often used for linings and covers to contain liquids or vapors in landfills, reservoirs, and canals.

Geogrids are stretched sheets of polymers that form open grids. Uniaxial geogrids stretch in one direction and have rectangular apertures. Biaxial geogrids stretch in both directions and have virtually square apertures. They can be filled with soil, aggregate, or concrete and almost always are used for reinforcement and to improve load-bearing capacity.

Geonets or geospacers are made of two sets of strands of either HDPE or LLDPE. They form a net with large channels that water can flow through and are often used for drainage.

Two different geosynthetics are often used together on projects that need two applications, such as a liner to separate subsoil from fill and a geogrid for soil reinforcement. Some of them can be bonded together to form geocomposites-for example, a geotextile and a geonet in landfills. A liquid enters through the geotextile and travels along the geonet until it exits.

The following three projects-one on detention ponds, one on a steep slope, and one on a roof-employed a variety of geosynthetics.

Detention Ponds in Seattle
The I-405 Corridor was built to relieve congestion on the I-5 in Seattle, WA, but because of the population growth in the area, it’s been needing relief itself. The 30-mile section of the I-405 east of Lake Washington is one of the most congested in the state, and traffic is expected to increase.

A project to widen the freeway has been under way since 2003. It includes construction of best management practices (BMPs) to contain stormwater runoff from the road before the water flows into Lake Washington. Finding the best way to handle the runoff was a challenge, says Kitt Hawkins, project manager for Northwest Linings & Geotextile Products Inc. (NWL), in Kent, WA. He managed the installation of two detention ponds near the expanded I-405 in Renton, just south of Lake Washington.

“There isn’t a lot of space,” Hawkins says. The lake runs practically the entire length of the corridor, urban, and suburban development cluster along both sides of it, and high and long glacial ridges line the area. “We needed to take the water somewhere, and there wasn’t a lot of room.”

The solution was to build two shallow ponds near the freeway, but one of the spaces, a former district supply yard, posed a big problem.

“We had to maximize the space, which was very little,” he says, “so we had to use steep slopes. Then we needed a geocell so we could grow something on it and make it look like a nice wetland.” The company used PRS Neoweb, a new honeycomb cellular confinement system by Geogrid, over a liner made by Agru America Inc.

The design/build team included engineers with the Washington State Department of Transportation, Kleinfelder, Tri-State Construction, and NWL. NWL is the leading supplier of geotextiles to the Northwest market. It’s also the largest installer of geomembranes, geotextiles, geocomposites, and geogrids on the West Coast, including Alaska, Hawaii, and the Pacific Rim. For this project, the company handled the submittals and got approvals for the project.

“Because it was a design/build project with the DOT, there were lots of meetings at the site, walking the DOT engineers through the process,” Hawkins says. “But once it got going, it was a very simple project.”

The smallest space was also the most critical, so Tri-State built it first. Crews excavated to a depth of about 6 feet, then graded the area and compacted the subgrade. They maximized the space to 105 feet by 185 feet by cutting the slopes from between 2:1 to 1:1. The clay subsoil was compact and unyielding, and the stones were smaller than three-eighths of an inch, so they didn’t have to use a geotextile under the liner to stabilize the subsoil or to protect the liner.

They put down a 40-ml HDPE liner that has been accepted by the state’s Department of Ecology for stormwater storage. Because of the high groundwater table in the area, regulations require a lining system to ensure stormwater does not come in contact with present groundwater. The HDPE-lined pond also allows turbidity to settle before entering the local stormwater system. If accidental spills occur on the highway, the pond will also give WSDOT crews a better chance at cleaning up spilled materials before they can further pollute local tributaries, says Paul Gilmore, a project manager with NWL.

Crews installed Neoweb on top of the liner around the entire circumference of the pond so vegetation could grow on the steep slopes. According to Geogrid literature, Neoweb is made of a polymer alloy that “combines the fatigue resistance and low temperature flexibility of HDPE with the dimensional stability and creep resistance of engineering thermospastics.” Neoweb’s three-dimensional cells are perforated, which improves drainage. The perforations also enhance plant growth by allowing water, nutrients and soil organisms to pass through the cells, and help stabilize the soil by allowing plant roots to interlock.

“It’s a neat project,” Hawkins says. “It’s all accordianed tight to make a neat package, but it was time-consuming. You can’t anchor the geocell in the traditional way, because of the liner underneath. You have to use a “˜dead man’-a pipe in the anchor trench-and tie high-strength tendons to hold the cells in place. Once the geocell has been anchored and stretched, filling with the soils can begin.” Tri-State used a conveyor truck to place the sand in the cells and on the bottom of the stormwater pond.

The larger pond, just off SR 167, is in a former general contractor’s yard. It’s long and narrow, approximately 300 by 85 feet. It was filled with construction debris and nonsuitable soils. Tri-State overexcavated to remove much of the loose fill and debris. Then sand was hauled in and rolled to create a new, stable subgrade, on top of which crews installed the liner.

“There was plenty of room on this one,” he says. “They could do gradual slopes, 3:1 or 4:1. They only did small portion of geocell there, just on the steeper slope.”

Tri-State Construction hydroseeded the slopes of both ponds with clean organic material and native seeds. The fill is smooth to the top of the geocell.

Hawkins doesn’t expect the ponds to need any maintenance. “When you backfill with an HDPE liner, it will probably last 500 years,” he says. “And once the Neoweb is filled and locked in place, there’s no problem with vehicles driving on it. The ramp into the pond is lined with geocell. Geocell was originally designed by the military for just this type of application.”

NWL charged $84,000 for both projects. The liner portion for both ponds took about two days to install, and the geocell portion took about three days, he says.

“This project went really smooth. We have crews that have been with the company for a long time. They go in and it’s like clockwork.”

Casagrande House
When Tara and Jerry Casagrande added two levels to their single-family home in Alexandria, VA, they built two rooftop gardens among the trees.

“The homeowners are very conscious of their ecological footprint, and with their new addition wanted to have a building that featured a variety of green building practices and materials,” says Gregory Long, the landscape architect who designed the greenroofs. Long is the owner of Capitol Greenroofs LLC in Arlington, VA, which installed the two vegetated roofs.

Controlling erosion is tricky on greenroofs, especially on sloping ones, as in this project. Vegetation and the growing medium composition are the most important factors when selecting the system, says Jeremy Bodkin, who provides engineering and technical support on projects for Cell-Tek Geocellular Confinement Systems, in Crofton, MD, which supplied the system used on the project. Stormwater goes through the plant medium on the roof, where some of it is stored, Bodkin says. The medium slows the rest down and filters it, so there’s less erosion and fewer sediments going to storm drains.

For this project, Long chose Colbond’s Enkadrain 3611, a waterproofing mat that also holds water, and Cell-Tek’s SG 200 Stabilizer Grid system. Cell-Tek’s system helps hold the plant medium in place and allows stormwater to flow evenly to the lower ends of the roofs.

“Designing for lateral drainage is critical on a sloped roof because you need to be sure that hydrostatic pressure is minimized,” Long says “Cell-Tek has holes to let the water pass through but still holds the soil in place. The product is perfect for sloped applications as well, because it allows me to maintain a uniform depth of soil.”

Some cities, including Alexandria, encourage the use of greenroofs as a low-impact development (LID) BMP. In addition to slowing down and filtering stormwater, the roofs also cool it, which is important, Bodkin says.

“On hot summer days, asphalt and dark-colored roofs absorb heat. Rainwater hitting the roofs is heated up, and it’s still hot when it gets to storm drains and the bay. The superheated water adversely affects all plant and aquatic life in the bay.”

The project was a challenge, Long says. “It was something that we were interested in completing because about 95% of our greenroof projects in the past have been installed on projects with a slope less than a 1:12 pitch.” Both greenroofs at the Casagrande home are arched, with approximately a 3:12 pitch at their steepest points, near the edges. Together they cover roughly 740 square feet.

The roofs had to be able to drain very quickly in the event of an extreme downpour, Long says. He designed them to drain at 23 gallons per minute per square foot. That rainfall intensity equates to about a 500-year storm for the Washington DC metropolitan area.

The balance between drainage and absorption is delicate but critical. If the roof drains too fast, the soil dries out too quickly. If it drains too slowly, the plant roots can rot and the weight of the water can be too much for the roof to withstand.

The support structure was designed to support at least 30 pounds per square foot of dead loads and 20 pounds per square foot of live loads. The dead load for this greenroof assembly is approximately 20 pounds per square foot and includes the weight of the membrane, drainage mat, Cell-Tek, soil, and vegetation. Live loads fluctuate, for example, with the weight of stormwater, snow loads, and people who are up on the roof to maintain it.

A 5-inch wooden curb was built around the perimeter of both roofs to keep the system from sliding. The waterproofing consists of a rubber EPDM membrane that was adhered to the plywood deck. Capitol Greenroofs then rolled out the 1.5-inch Enkadrain mat, which allows water to flow easily toward the low ends of the roof. It holds approximately 0.10 to 0.15 gallons per square foot, which is about one-quarter inch of rainfall.

Crews installed the 2-inch-thick Cell-Tek Stabilizer Grid on top of the mat. This system is made with ultrasonically welded, 100% recycled HDPE and can be filled with gravel, soil, or aggregate. It can be used for applications such as greenroofs, erosion control, slope protection, channel revetment, and load support. To prevent wind lift and sliding, the system was attached to the lower roof by a stainless steel cable that runs through the middle of the cells from the roof ridge to the low end. The higher roof didn’t require cabling.

On greenroofs, keeping the soil evenly moist is also a challenge, Bodkin says. “In the air, the soil is more susceptible to winds, and it’s shallower than on the ground, so it tends to dry out faster.” And on sloped roofs, conditions become drier at the top while water collects at the bottom.

Capitol Greenroofs filled the cells with a mix that contains soil, gravel, and vermiculite, which is lightweight and holds moisture, and covered the mixture with a 1-inch-thick, prevegetated coconut-fiber mat. Most rain events in this area are less than 1 inch, and the plant medium on this roof absorbs about 55% to 60% of the annual rainfall, Long says. The rest passes through the medium to one of four drains on the low ends of both roofs. The drains are covered with washed river-rock ballast.

Most of the plants are sedums, which are especially well-suited to greenroofs. They store water in their tissues, and some can last 60 days without watering. In addition, their fibrous root system allows them to grow well in shallow soils. Their roots also grow through Cell-Tek’s lateral holes into the mat, which helps secure the system.

Maintenance during the first year includes fertilizing in the spring and light weeding and an application of fungicide for disease control, if necessary, in the summer. In the fall, leaf litter, branches, and seedpods might need raking. As the vegetation matures, there is greater coverage and less weed pressure, and the system will begin to require less maintenance.

This project cost approximately $22.50 per square foot for the entire roof package, minus the metal coping and the aluminum downspouts, which were installed by others, Long says. The project normally would have taken about one week to complete, but crews did the waterproofing in the fall of 2008 and installed the vegetated coverings in the spring of 2009. They used a platform hoist and ladders to gain access to the two roofs and to bring up the building materials.

“The clients love it,” he says. “CellTek’s system helps so the greenroof doesn’t shift and the soil doesn’t slide. It’s pretty slick.”

Yeager Airport
What do you do when an airport is on a mountaintop and you have to extend the runway? You extend the mountaintop.

Yeager Airport, three miles east of Charleston, WV, sits on a hilltop that drops off sharply on all sides, more than 300 feet above the Elk and Kanawha rivers. It covers 767 acres and has two runways. To meet new Federal Aviation Administration standards, the main runway, Runway 5, had to be extended to create an emergency stopping apron for airplanes.

Designers from Triad Engineering Inc., a multidisciplinary consulting firm with offices along the mid-Atlantic region, had a number of options, including building bridge structures, retaining walls, and reinforced slopes. The firm chose to build a reinforced slope largely because it was a more economical solution and involved less construction than the others.

“It was basically a design for a reinforced slope that made the original slope steeper,” says Dane Ryan, civil engineer services manager with Triad Engineering, who assisted in the design and selection of materials for the project.

Crews infilled along the face of the slope and fastened the infill to the original slope with two geosynthetics from TenCate, Miramesh, and Miragrid. The infill allowed the southwest end of the runway to extend 500 feet. A taxiway and the runway’s safety overrun area were also extended. The team chose TenCate’s Mirafi products for drainage. All three products were chosen for their strength, availability, and cost-effectiveness.

The slope measures 242 feet in height, making it the tallest reinforced 1:1 slope in North America. The project won the Award of Excellence in the geosynthetics projects category in the 2007 IFAI (Industrial Fabrics Association International) International Achievements Awards.

This isn’t the first time the airport has moved mountains. When it was built in the mid-1940s, the tops of seven hills were sheared off and the valleys between them were filled with the removed soil in order to create enough flat land. The Kanawha Airport, as it was known then, became the Yeager Airport in 1985 to honor then-Brigadier General Chuck Yeager, a native of nearby Lincoln County.

The steep terrain was the biggest challenge. “The original hillside was probably about 1.5:1 [horizontal to vertical],” Ryan says. “Now it’s 1:1.”

The contractor, Cast & Baker Corp. of Canonsburg, PA, began at the bottom of the slope and worked up in 1.5- and 3-foot vertical intervals. Crews installed crushed sandstone as backfill in conjunction with Miramesh GR and Miragrid 20XT and 10XT geogrids to reinforce the slope.

According to TenCate Geosynthetics, Miramesh is made of green high-tenacity monofilament polypropylene yarns that are woven together to produce an open-mesh geotextile. It is designed for secondary reinforcement and face erosion protection for reinforced slope stabilization and mechanically stabilized earth wall applications, as well as for quick germination of vegetation, which also provides erosion protection and improves stability. Crews embedded the Miramesh 3 feet into the slope face and 2.5 feet down the face and embedded the geogrid at the same time. The Miragrid 20XT and 10XT used as the primary reinforcement are made of high-tenacity polyester yarns coated with PVC.  Miragrid geogrids are extremely durable and were ideal for the harsh fill used on this project. The geogrid is made of woven polyester fibers coated with polymer and provides horizontal reinforcement for the slope. The geogrids are 175 feet long, Ryan says, and this is where crews ran into another challenge.

“The geology of the hills is rock-shales and sandstones,” he says. “In order to accommodate the grid lengths in some areas of the fill slope, they had to excavate back.”

They also installed Mirafi G200N along the back of the excavation to intercept and drain seepage water from the existing mountainside away from the reinforced slope. The material is a bonded, nonwoven filter fabric that can collect large quantities of subgrade water and conduct it to a discharge pipe or collection system. It’s ideal against excavation cuts of retaining walls or slopes and similar applications.

Crews next revegetated the slope to blend in with the surrounding hills.

The project cost approximately $25.5 million. The airport secured funding through the FAA. The slope construction started in August 2005 and was completed in April 2007.

“It was a meticulous process, but it did go well,” Ryan says.

About the Author

Janet Aird

Janet Aird is a writer specializing in agricultural and landscaping topics.