Geotextiles, frequently used in erosion control blankets, turf reinforcement mats, and silt fencing, are put to use on construction sites as often as an old friend arriving early at a dinner party. Contractors have grown to equate their performance with a necessity for erosion prevention. Yet, there are many more everyday uses for geosynthetics that can frequently be overlooked: drainage control between layers of soil, geogrid as reinforcement for retaining walls, and a variety of cellular confinement systems that work to stabilize slopes.
Greening Washington
Often, geosynthetics can provide erosion control that’s essential for sites facing multiple challenges. In Renton, WA, a civil construction site that fits that description involved 2 miles of highway widening and improvements and the construction of a half-diamond interchange to a major city street, explains Ryan Andrews, field engineer for the project and a project manager with the Seattle-based Gary Merlino Construction Co. Begun in 2009 and on schedule for completion this year, the approximately $85 million project balances road improvements with environmental sensitivity. Local tribal treaty rights were to be respected during construction, and unforeseen conditions including an abandoned landfill and an abandoned coal mine at the site posed challenges when protecting water quality. The Green and Cedar River watersheds in the project area are home to fish protected by the federal Endangered Species Act. Water-quality tests had to be performed, and National Pollutant Discharge Elimination System (NPDES) requirements called for “all known, available and reasonable methods of prevention, control, and treatment” at the site.
With a requirement for 25 nephelometric turbidity units (NTUs) in water discharged from the site, crews needed to improve the site’s best management practices (BMPs) and revise the site’s stormwater pollution prevention plan, according to I-405 Corridor Design-Builders based in Renton, WA. Stream mitigation was also incorporated into the construction plan.
Washington weather patterns, with their notoriously consistent rainfall, also required sensitive planning with regard to water quality. “It rained 28 of 31 days in March 2011,” says Steve Mader, environmental compliance manager for I-405 Corridor Design-Builders. Workers faced large storms, but knowing the climate, adequate planning was made for the rain events.
“The overall schedule is dictated by weather. We have to take into consideration what the water is going to do when changing grades,” says Andrews, who added pumps to convey water as they were deemed necessary. “Until your permanent storm system is done and active, you’re managing the water the entire job. Probably the most important things are reducing exposure and knowing where the water’s going to go.
“The erosion control was extremely challenging on the project. The area presented challenges for detaining site water, steep slopes begged for stability, and the sheer volume of site water to manage was a constant struggle.”
Approximately $2.5 million of the project’s total budget was devoted to erosion and sediment control.
The project includes two precast girder bridges and 15 different walls-including cast in place, soil nail with cast-in-place fascia, block, reinforced slope, noise wall, and rockery, says Andrews. A Tensar International reinforced slope system was built on a particularly challenging section to support the new ramp and control erosion.
“The original concept for the west embankment of the new southbound I-405 off ramp to Talbot Road was retaining wall, about 1,000 feet long by up to about 30 feet in height. A retaining wall was recommended because the right of way is constrained,” says Mader. “The problem was that the underlying soil is alluvium and prone to liquefaction in a seismic event, which suggests that seismic criteria be incorporated into the wall design at considerable construction cost.
“The Tensar-reinforced soil slope is preferable because it can move in a seismic event without failure. Washington Department of Transportation standards allow reinforced soil slopes up to 70-degree steepness without constraint by wall seismicity specifications.”
Aesthetics were an issue onsite as well, because the new offramp was built on top of a 20,000-square-foot-embankment face, and, according to Andrews, designers sought a natural look.
“We used the Sierra Slope Retention System that Tensar offers, including a welded wire form [WWF]. However, we used a WWF for support as we were faced with an extremely steep slope,” he says. “Materials included 17,310-the slope face area-of Tensar Uniaxial Geogrid, Tensar Biaxial Geogrid, and WWF. It offered cost savings for materials, but took longer to build than other wall options. It basically holds the bridge approach.”
Another aspect that Andrews said made the Tensar system appealing is that no rock was required for use with the system, which provided cost savings at the site. “If you have a job where you have a lot of native soil material, you could use that material in the reinforced slope,” he says.
The construction process was completed in layers, beginning at the low point with the goal, Andrews explains, of reaching a long, flat run.
Not only did the project require a natural look, but it was designed also to be green. “Stakeholders wanted a green face to the reinforced soil slope for aesthetic considerations, which the Tensar system accommodates,” says Mader. “However, structural embankment constraints limited the amount of nonstructural topsoil that could be incorporated into the face of each lift to 6 inches horizontally, typically with common borrow as the structural component. An especially challenging segment of the reinforced soil slope relied on lightweight fill-pumice-as the structural element to avoid overloading a 48-inch water line.”
The solution for achieving a green slope face was a common one on construction sites, but it was used in an entirely innovative way.
“Sod was installed vertically on the face of the welded wire frames and backfilled with topsoil,” he says. “Installation of sod required regular watering for segments that were constructed in the dry season.”
The result is something workers and designers alike can take pride in, having accomplished the stability and aesthetic goals that were sought for the project.
“There’s an environmental performance incentive on the job from the Washington State Department of Transportation, and we received every penny of it,” says Andrews.
Phased Highway Construction
In Edmonton, AB, the Anthony Henday Drive project is another challenging highway construction project that incorporates unusual uses for geosynthetics with traditional methods.
“The goal of the project is to provide a ring road infrastructure around a city, to provide greater access to motorists to get to other parts of the city faster, safer, and without traffic lights,” explains Scott Falkenberg, an Edmonton-based employee of Layfield Geosynthetics, which is based in Richmond, BC. “We’re supplying the contractors with the product and the information needed to install it correctly. The budget for the total project is a little over $1.4 billion.
“There are 29 bridge structures and 21 kilometers and a six-lane divided freeway of new road. It’s all new construction. The project started the spring of 2009. It will be completed by November of this year.”
The southwest and southeast sections of the transportation corridor were the first two phases of the project. “This one is the northwest Anthony Henday Drive,” explains Falkenberg. “It’s set up in a way that it will divide some of the smaller cities around Edmonton. It’s a design/build contract.”
In July 2008, Northwest Connect General Partnership agreed to design, construct, operate, and maintain-through 2041-the Anthony Henday project. In addition to the bridge structures and freeway, there are eight interchanges that allow traffic to enter and exit the freeway, additional auxiliary and basic lanes, five flyovers, pregrading for future interchanges, and two rail crossings.
Several waterways onsite received careful consideration during planning and construction. “There are some ponds or small lakes that needed silt fence around them so it would not create too much turbidity,” says Falkenberg. “When you’re dealing with the erosion control, you’re dealing with engineers and project managers-how best to manage the slopes they’re dealing with, the expected rainfall, and products that can be put to use on the project.”
Layfield Products has provided road-building materials and geogrid, silt fence, erosion control blankets, turf reinforcement matting, silt fence, and spring berm. Falkenberg describes the latter: “It’s like a ditch check product. It’s a coiled spring wrapped in a geosynthetic material, and it’s used to dissipate water that flows down a channel or a ditch. What it allows you to do that other dissipaters wouldn’t is expand; it comes coiled up and it springs out to 6 feet wide. It’s installed with 8-inch staples into the ground. In a matter of seconds, it’s full height and full length.”
So far, 324 feet of spring berm has been purchased for use on the project, with more expected to be used prior to the project’s completion.
“It’s a massive undertaking. The wheels are in motion to bid the northeast section of the project,” says Falkenberg, adding that it should be open for bidding early spring to late winter next year. “This is not the final phase, but the largest section of ring road is the one we’re completing this year. The project is on schedule.”
That schedule is important, because weather plays a role in the revegetation set to begin this summer. The area received spring snow in 2011; the typical growing season stretches from April to September.
“It’s kind of a volatile climate,” says Falkenberg. “We’re coming out of 10-year below-average precipitation. This winter has seen particularly heavy snowfalls. They’d like to have this revegetated before it’s turned over to the city.”
Workers will begin applying groundcover, native grasses, and additional plantings as soon as weather permits.
In the Keystone State
In the mid-Atlantic United States, geosynthetics were used on another unusually challenging project. In the Skytop Ridge area of Bald Eagle Mountain in Centre County, PA, the state Department of Transportation (PennDOT) began construction of I-99 in July 2007. The road would play a pivotal role in the Keystone State’s web of highway systems, eventually connecting the Pennsylvania Turnpike with Route 80 and running directly through State College, PA, explains Greg Kramer of ACF Environmental. He notes that the project includes “a straight cut through the mountainside.”
Although PennDOT tested the rock at the site that was also used as fill and in bridge approaches and buttresses, “they didn’t know it was riddled with pyrite,” says Kramer. If the department had known, it would have saved the department considerable time and money.
As the Pittsburgh Post-Gazette documented around the time treatment -options were being explored in 2005, “It was PennDOT’s excavation at Skytop in 2002 and 2003 that first uncovered more than 1 million tons of highly acidic rock-sandstone threaded with the mineral pyrite which, when exposed to oxygen and water, produces sulfuric acid. The acidic runoff leaches into local residents’ groundwater wells and nearby trout streams, including Buffalo Run, a tributary of Spring Creek, which supports a naturally reproductive trout population and is designated as a “˜high-quality cold-water fishery.'”
Kramer says that after a variety of methods to repair the situation didn’t work, and where the rock could be -removed, it quickly was.
“They built an urban rock placement area-put 3 million yards, plus or minus, into that area, and capped that whole site off. Today if you were to go by there, you wouldn’t realize; now it just looks like a hillside,” he says, describing the area’s revegetation. “Two million yards that were exposed and had to be covered and bridge abutments that couldn’t be removed had to be capped off and sealed. The budget was estimated in excess of $100 million.”
Eventually, a solution was found using Appleton, WI-based Presto Products’ Geoweb cellular confinement system. Often used in road support and slope protection applications, Geoweb is also used for channel protection.
“They put some reinforcement and drainage through; any runoff that hits that impervious liner flows into drains that run underneath the road, into collector drains,” says Kramer. “They built a treatment plant. All the water gets piped down there.”
Throughout the project, PennDOT provided monthly water-quality information sheets to the public, which clearly show a reduction of sulfate levels in the unnamed tributary that leads into Buffalo Run and Buffalo Run itself.
Landfill Closure
Few projects face stricter regulations than landfills, with their phased closures.
At the LaSalle/Grant Landfill located in Jena, LA, the heavy clay soil with very low pH posed an erosion challenge-one that has finally been overcome in terms of long-term protection using geosynthetics.
“We just couldn’t sustain vegetation,” explains Delaney Lewis, district manager at IESI Corp. based in Jena. “We just could not sustain soil on the site. It called for 4 tons of lime per acre. That doesn’t include the fertilizer and the organics that would be required just trying to get a vegetation cover. We were called to do a landfill cap.”
As part of phase one, begun in 2008, state approval was sought for use of a ClosureTurf system at the site.
“You can’t have an exposed membrane,” he explains. “This system would meet all of the requirements.”
The system includes synthetic turf, eliminating the need to import soil and amendments in an attempt to establish vegetation. It consists of two layers of geotextile placed over a drainage geomembrane, AgruAmerica Super Gripnet, and infilled with sand. The geomembrane is the containment liner for the landfill. Rainfall penetrates the geotextile layers and sand onto the drainage liner.
“It allows for the erosion control,” says Lewis. “As the water hits the turf, it flows through onto Super Gripnet. The textile provides the ballast for the Super Gripnet and the UV protection.”
There are spikes on the underside of the Super Gripnet liner designed to increase stability on steep slopes. When heavy rain events are a common occurrence, this can be especially important. The LaSalle/Grant Landfill’s central Louisiana location has an average rainfall of approximately 60 inches per year, according to Lewis. In addition to erosion concerns, water quality on site is closely monitored.
“The whole thing acts like a filter block,” says Lewis. “It’s an artificial turf, similar to what’s on a football field. It won’t have any fading. The life of the turf is up to 50 years.”
Landfill gas migration poses another challenge for many closed landfills. “Once you get your vegetation up, landfill gas can seep through dirt cover,” explains Lewis. “This system gives an avenue to collect that gas.”
There are many requirements that must be met to cap a landfill. The Louisiana Department of Environmental Quality initially approved the system for use on 3 acres at the site as a test pilot study. It would be used instead of a traditional landfill cap.
“It met all requirements or exceeded them,” says Lewis.
The state then gave final approval for the system, which Lewis says was first used for this type of project in Louisiana but has since been approved for use in other states.
“We actually went on to do phase two and phase three, close to 10 acres, using the ClosureTurf system. We’ve not had to do anything to that area since we’ve put it down.”
