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By Dan Rafter
The project is a major one, and an expensive one.
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| The taxiway portion of the LAX project
is expected to be completed in 2008. |
The Los Angeles International Airport is in the middle of spending $333 million to build a new runway and center taxiway at its south airfield. The project, which started in July 2006, will make the airport’s runways safer and allow larger aircraft, such as the heavily publicized Airbus A380 and Boeing 787 Dreamliner, to land on them. (See “Preparing for the Airbus” in the September 2007 issue of Stormwater for more details of the project.)
With all the money and manpower involved in the costly South Airfield Improvement Project, it’s easy to overlook one of the most important facets of the construction job: the installation of more than 438,000 square feet of an underground porous paving system that will allow maintenance roads to act as infiltration swales between the airfield’s two south runways.
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| What most attracted
engineers to Grasspave2
was its permeability. |
The paving system, Grasspave2 from Invisible Structures of Golden, CO, will allow construction vehicles to drive on the maintenance roads without killing the grass atop it. This works because the pressure from the vehicles, instead of pressing down on the grass, is transferred to the base of the Grasspave2 paving system. The grass, then, is unharmed. At the same time, the porous paving system allows stormwater to percolate back into the ground and prevents water and sediments from running offsite.
Engineers working on the Los Angeles airport project aren’t the first to turn to Grasspave2 for this type of project, says Kate Wright, staff engineer with Invisible Structures. State and federal regulatory agencies are placing more pressure on contractors to monitor the amount of runoff from their work sites. Because of this, products such as porous pavement, which make it easier for potential runoff to soak back into the ground, have become more valuable, she notes.
“The EPA is really focusing on this,” Wright says. “The agency is finding that any surface material that is impervious, materials such as asphalt and sidewalks, are adding up to more problems farther down the river. Taking care of the runoff directly on the site is the best way to go. Grasspave allows you to do that. And you can still drive on it, which is important when you’re dealing with maintenance roads like we are in Los Angeles. The stormwater is soaking into the ground. It’s not going anywhere.”
Officials with HNTB, the engineering design firm handling the Los Angeles airport project, ordered construction crews to install Grasspave2 on a 20-foot-wide maintenance road that runs the length of the project’s runways and taxiways.
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| Another advantage of porous pavement is its load-bearing strength. |
HNTB drew up the design documents for both a new south airfield runway and a center taxiway that will run parallel between Los Angeles International’s two south runways. Construction crews completed the new runway, known as Runway 25 Left/7 Right, in late March of this year. Crews are now building the center taxiway and additional taxiways linking the two runways to the newly built center taxiway. Engineers estimate that crews will complete the taxiway portion of the project in mid-June 2008.
Ryan Damery, project manager with HNTB, says his company chose Grasspave2 because the porous pavers would allow water to reach the series of infiltration cells located directly under the paver units.
“We needed something that would allow the maintenance road to be used,” Damery says. “We have beach sand at one end of the project. If you can imagine trying to build a service road on beach sand, it’s not easy. You do need the pavers to provide some stability for your vehicles. But we also needed the infiltration of any stormwater. This gave us that.”
The porous paving system allows users to park, drive, walk, or ride on a grass surface. The structure provides load-bearing strength and protects vegetation root systems from compaction. The large amount of void spaces within the Grasspave2 cross-section provides space into which rainfall can soak. Stormwater, then, is slowed as it moves through and across Grasspave2 surfaces. This allows the stormwater to deposit suspended sediments along the way. Active soil bacteria consume suspended pollutants and even moderate amounts of engine oils.
The Grasspave2 system itself is made up of a sandy gravel base course, a polymer-fertilizer mixture, a ring-and-grid structure, sharp concrete sand, and grass seed or sod. It is available in rolls of either 1 square meter or quarter-meter units.
The product’s permeability is what most attracted the engineers with HNTB. Grasspave2’s porous pavement collects the runoff from the concrete runways and taxiways, filters out major pollutants, and either delivers the stormwater to an underground detention system or lets it naturally infiltrate the ground.
Engineers have graded one section of the Grasspave2 system as an infiltration swale that delivers stormwater to inlet grates connected to an underground detention system.
Damery says that Grasspave2 pavement sits on 12 inches of plastic media blasting material. Construction crews also removed 18 inches of soil from outside the units to provide sand filtration for the runoff, enough to filter out bacteria from the stormwater before it infiltrates an underground unit or reaches groundwater supplies.
The project also includes cyclonic filters to handle runoff that exceeds the infiltration rates that the existing soil can handle. The filters—which collect debris, pollutants, sediments, oils, and grease—treat the runoff water before it reaches the city storm drain system. HNTB is using these filters mainly at the east side of the airport where clay soil conditions do not allow for infiltration rates that are high enough to satisfy Standard Urban Stormwater Mitigation Plan requirements with the infiltration swale alone.
This installation, of course, is just one small part of the overall improvement project at Los Angeles International Airport. The new runway and taxiways are designed to improve safety at the airport’s south airfield by reducing the number and severity of runway incursions that occur there.
A runway incursion, according to the Federal Aviation Administration (FAA), happens when an aircraft, vehicle, or object on the ground creates a collision hazard or ends up too near an aircraft taking off, landing, or intending to land. From 2000 through 2003, Los Angeles International suffered the highest number of runway incursions of any US commercial airport. In 2006, the airport had eight runway incursions. The FAA classified two of these as having had the potential to result in accidents. Fortunately, no accidents stemmed from the incursions.
The opening last April of the new runway came eight months after demolition work started on the former Runway 25L. The new runway is located 55 feet south of its older counterpart and has the same dimensions—11,095 feet long and 200 feet wide. The new version, though, is situated so that the airport can better handle the needs of larger new aircraft such as the Airbus A380.
Dan Rafter is a technical writer based in Illinois.
SW October 2007
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