Synthetic turf system shows benefits beyond efficient erosion control.
By Alexandria Hayes
After four years as a district landfill manager for IESI Corp., based in Forth Worth, Texas, Delaney Lewis was discouraged. Although one of the Louisiana landfills for which he was responsible exhibited ideal geology, the soil of the LaSalle-Grant Landfill in central Louisiana was highly erodible. As a result, Lewis spent every spring repairing the slopes, amending the soil with lime, seeding, and hydromulching, only to watch his hard work end up as sediment at the bottom of the landfill. It became evident that erosion-control success would require an unconventional approach.
Lewis and IESI’s south region engineer, Mike Friesen, asked their contacts if anyone had a potential solution. Industry veteran Juene Franklin, from the Houston, Texas, engineering firm Riley, Park, Hayden and Associates, directed them to a new product developed to reduce greenhouse gas emissions and lower post-closure liabilities at landfills. Franklin thought the product, a synthetic turf system, could mitigate slope failures such as those at the LaSalle-Grant facility.
A new approach
The LaSalle-Grant Landfill sits on 232 acres about 50 miles northeast of Alexandria, La. Opened in 1991 and permitted as a Type I and Type II facility, it is owned by LaSalle Parish and operated by IESI. The 65-acre landfill accepts 500 tons of combined municipal solid waste and industrial waste per day.
In October 2008, installation of the new synthetic turf system commenced over 2.5 acres of the landfill. It consists of three primary components: two layers of woven geotextiles with tufted UV-resistant polyethylene grass laid over a 50-mil LLDPE structured drainage geomembrane, which is infilled with sand.
The geomembrane layer serves as the containment liner atop the landfill’s intermediate soil cover. Integral 3.6mm studs on the top surface facilitate drainage, while integral 4.4mm spikes on the undersurface provide friction. The turf’s grass blades are interlocked with 3/4–1 inch of sand ballast that, combined with the liner’s surface studs, provide sufficient interface friction so the structured geomembrane and turf layers do not require anchoring for stability. They are anchored for termination purposes only at the toe or on the outside of a perimeter swale, depending on the site design.
Rainfall penetrates through the sand and into the high-transmissivity drain liner below, which can handle rainfall of more than 4 inches per hour. Erosion energy resides in the structured geomembrane and not in the sand surface.
When installed the turf looked “just beautiful,” Lewis said, but he wondered whether a system so simple to implement could fix such an intractable problem. However, the IESI team was persuaded enough by the initial results that in February 2009, the company moved on to Phase 2 of the project, covering another 3 acres.
When Lewis and his colleagues saw the cover in action during the spring 2009 runoffs they were convinced of the turf system’s ability to provide long-term erosion control. Since the initial installations, the covered area has endured 73.5 inches of rainfall, including some in excess of 4 inches per hour. Three months after Phase 1 was installed, a tornado spinning across the front of the landfill generated 70 mph shear winds. Then a levee situated above the turf area broke, releasing 5 acres of water to wash across the turf. None of these events affected the turf system.
“The grass looks great, the sand didn’t move and there was no erosion,” Lewis said. “After we put the turf down, we didn’t have to do anything to it again,” he said.
“We [believe that] this cover meets or exceeds the intent of the EPA’s Subtitle D landfill closure regulations,” said Friesen. “The life of the grass is 50 or more years,” Friesen said. And if the grass color begins to fade in 55 years, he noted, it’s simple and relatively cost-effective to replace the grass. That would not affect the integrity of the LLDPE structured geomembrane, which will never need to be replaced.
As the IESI team members realized how effective the turf system was at preventing side-slope erosion, they also discovered other ways it was proving beneficial.
Speed. It took a crew about four days to install the first 2.5 acres of the system, with no delay on cap performance while waiting for grass to grow.
Impact. Installation had little impact on ongoing landfill operations, since there was no need for heavy equipment to traverse the property to deliver vegetative support soil. With soil-poor locations where dirt needs to be transported significant distances, the turf system eliminates the destruction of borrow locations as well as the cost of both the top 2 feet of borrow soil and its transportation, potentially a significant cost savings.
Maintenance. Once the turf system was installed, there was no need to rebuild slopes, fertilize, plant seed, or mow grass. Perimeter roads remain clear of silt, water runoff is clear, and paper blows across the surface and is collected at litter fences.
Compliance. Because the underlying structured geomembrane is impermeable, the turf system cap meets or exceeds EPA Subtitle D regulations. The product’s strength and durability provide protection from leachate, while eliminating gas emissions by containing 100 percent of the methane.
Gas control. The turf system precludes the need for gas wells and piping. Pulling a vacuum on the structured geomembrane allows all gas to be vented for flaring or alternative energy generation. Under the turf system, the gas rises to the surface due to positive pressure and generates little condensate to be caught and managed. Because it is economically feasible to close as little as an acre at a time, overall site emissions can be reduced in a working landfill by closing smaller areas. As soon as an area is closed, all emissions are controlled. Also, the structured geomembrane protects against oxygen infiltration, eliminating that as a fire pathway.
Environmental benefits. By reducing borrow soil locations, the turf system prevents additional land destruction. Capturing 100 percent of methane provides options for carbon credits and for potential energy conversion. The turf minimizes leachate, as the LLDPE structured geomembrane keeps water out of the landfill, and prevents siltation, as water runs cleanly off the synthetic surface. It also results in reduced carbon emissions since heavy equipment is no longer required to prepare a vegetative soil cover on top of the geomembrane liner.
Both Friesen and Lewis agreed that IESI’s cost savings have been significant. Estimated maintenance and soil cover savings range from $18,000 to $44,000 per acre per year, according to the turf system’s manufacturer. Savings will vary, but all sites will enjoy an increase in vertical landfill area gained by reducing the layer above the geomembrane liner.
Another area of savings is in capital spending and bond requirements. Friesen speculates that the millions of dollars set aside for future gas-system development and post-closure cap maintenance may be dramatically curtailed with the turf system.
“Our landfill gas system costs have been reduced by 85 percent and post-closure costs have been reduced as well,” Friesen said. “Add to that the cap’s ability to act as a gas system, and then future revenue from carbon credits and energy projects—it’s the icing on top of the cake.”