Synthetic turf system solves landfill erosion challenge and saves money
Delaney Lewis, landfill district manager for ISEI Corp., faced a dilemma recently. The soil of the LaSalle/Grant landfill in Louisiana was highly erodible, exhibited a high plasticity index, and had a natural pH of 4.0.
Consequently, Lewis had spent every spring repairing the slopes, amending the soil with lime, seeding and hydro mulching, only to watch his hard work end up at the bottom of the landfill in sediment. He had tried everything he knew how to do, but had failed at every attempt to rectify the problem. It became evident that success would require an unconventional approach.
Lewis and IESI South Region Engineer Mike Friesen asked industry veteran Juene Franklin of Houston engineering firm Riley, Park, Hayden and Associates Inc. if he knew of a solution. He directed them towards a recently patented product developed to reduce greenhouse gas emissions and lower post-closure liabilities for landfill owners. Franklin thought the product, a synthetic turf system, could mitigate slope failures such as those IESI had been experiencing.
The synthetic turf system, AgruAmerica’s ClosureTurf, consists of three major components: two layers of woven geotextiles with tufted UV-resistant polyethylene grass laid over a 50-mil linear low-density polyethylene (LLDPE) structured drainage geomembrane and 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 underside provide friction.
The turf’s grass blades are interlocked with 0.75 to 1 inch of sand ballast, which combines with the liner’s surface studs to provide sufficient interface friction that the layers don’t require anchoring for stability. They are anchored for termination purposes at the toe or on the outside of a perimeter swale depending on design. Rainfall penetrates quickly through the sand into the drainage liner below.
Two phases prove benefits
The LaSalle/Grant landfill sits on 232 acres near Alexandria, Louisiana. It accepts 500 tons of municipal solid waste and industrial waste a day, and has slopes that range from 3H:1V to 4H:1V. The turf system’s 43-degree interface friction looked promising as an effective side-slope stabilizer on that facility. “I felt we didn’t have any other options, so why not just try it,” Lewis says.
In October 2008, Lewis and Friesen oversaw installation of the turf system over 2.5 acres of landfill. It was fast and easy, and looked “just beautiful,” Lewis says. But he wondered whether a system so simple could fix such an intractable problem. Intrigued, IESI moved into phase 2, covering another three acres.
When spring runoff began, Lewis and his colleagues were convinced of the turf’s ability to provide long-term erosion control.
In its first year, the covered area endured 73.5 inches of rainfall, some in excess of 3 inches per hour. Three months after phase 1 was installed, a tornado spinning across the front of the landfill a quarter-mile from the turf cap generated 70 mph shear winds. A levee above the turf area broke, sending five acres of water across the turf. None of the events affected the turf.
“The grass looks great; the sand didn’t move, and there was no erosion,” Lewis says. Recent tests subjected a 3:1 slope to the equivalent of 7 inches per hour of rainfall with virtually no erosion of infoll. Wind tunnel tests showed the material withstanding winds higher than 120 mph without subsurface anchoring.
“We’d been killing ourselves working and reworking these slopes and now it appeared we had a really good answer,” says Lewis. “After we put the turf down, we didn’t have to do anything to it again.”
The turf system required no mowing, reduced leachate, emitted no fugitive gas, and stayed in place under extreme weather conditions.
Efficiencies found
It took a work crew about four days to install the first 2.5 acres of turf system. Even in wet seasons, turf can be installed in a few clear days. There is no delay on cap performance while waiting for grass to grow. Installation has little impact on the landfill, as there is no need for the use of heavy equipment.
Turf-covered areas require no maintenance and water runoff is clear. Durability is high; the grass component maintains its strength in the long term, with a 50-year-plus lifetime. The 50-mil LLDPE structured drainage geomembrane lasts indefinitely if installed and maintained as per the manufacturer’s instructions.
Maintenance and soil cover savings range from $18,000 to $44,000 (US) per acre per year, depending on costs of soil, labour and supplies, the manufacturer says.