TDA Will Cushion New Light Rail Line
The Bay Area Rapid Transit District (BART) intends to incorporate “green” construction in a planned route extension by using recycled tires underneath the new rail lines. The decision is applauded by the Department of Resources Recycling and Recovery (CalRecycle), which promotes the use of tire-derived aggregate (TDA) in engineering projects.
Construction crews plan to use at least 250,000 scrap tires which have been processed into 3-inch tire-derived aggregate and laid under large sections of the tracks, to act as shock absorbers, reducing vibration and noise along the route as BART is built from Fremont to San Jose’s Berryessa neighborhood in the next seven years.
TDA is shredded tire rubber used as a replacement for naturally mined materials such as gravel, but has numerous additional advantages. It is safe, reliable, and above all, a cost-effective option that keeps waste tires out of landfills. When used under rail tracks, TDA acts to reduce noise and vibration that can affect nearby buildings and residences.
“Tires are a valuable resource. They are almost indestructible,” Stacey Patenaude, materials recycling engineer with the state Department of Resources Recycling and Recovery in Sacramento said.
Under the BART plan, crews will dig down about two feet and lay an 18-inch layer of TDA, then encase them in fabric. The shredded tire rubber will be compacted and covered in gravel, with the tracks built on top. TDA costs about $150 a foot, said Patenaude. A more traditional method of reducing train vibration, by building the tracks on top of rubber mats and concrete slabs, costs about $250 a foot, she said.
The most expensive technique can cost up to $900 a foot. In that method, called a floating slab, crews build concrete trenches and sit the tracks on top of large rubber discs, which act like shock absorbers.
“Everybody wants mass transit,” said Patenaude. “If you can save money and still accomplish what you need to do, that’s a win-win.”
CalRecycle experts have worked for more than a decade to promote the use of TDA. Their efforts received a big boost after the Federal Transportation Authority (FTA), a major funding source for light rail systems around the country, accepted the use of TDA in the BART Silicon Valley Extension Project.
The plan to bring BART to San Jose has three main phases. The first is a five-mile section that will extend from Fremont south to Warm Springs. Crews are already working on that stretch, and the line is scheduled to open to the public in 2014.
Shredded tires will be used in sections of the line, although exact amounts won’t be known until the final construction contracts are awarded, according to BART.
The next section will run 10 miles from Warm Springs to the Berryessa neighborhood in North San Jose at Las Plumas Avenue. Construction will start in 2012. That section will use 7,800 feet of shredded tires and 7,500 feet of the floating slab method to cut vibration in areas where buildings are closest to the track or the most sensitive structures, like hospitals, are located, Valley Transportation Authority (VTA) officials said.
The final six miles, from Berryessa to Santa Clara, involve digging tunnels under downtown San Jose. No firm dates are set yet because funding has not been secured, although some estimates place completion at 2025.
The idea of placing tire derived aggregate under railroad tracks began more than ten years ago when CalRecycle commissioned the first field test of TDA as a vibration dampening material. The results of that test led the firm to recommend that the Valley Transportation Authority use TDA for its Vasona Line light rail extension in Santa Clara County.
Before proceeding, VTA built a test section of the TDA vibration track in its San Jose railyard. CalRecycle supplied the TDA, and monitored construction and testing of this 2001 pilot project. Ultimately, VTA decided to proceed with construction using TDA, and CalRecycle agreed to follow up with post-construction analysis after the route opened in 2005. Use of TDA as part of the Vasona Line extension resulted in a savings of $1 million and kept 100,000 old tires out of the state’s landfills.
The Federal Transportation Authority, which is helping to finance the BART extension, wanted further proof that TDA would continue to perform well. CalRecycle again funded tests of the existing Vasona Line, and the results showed that TDA continued to perform well. The FTA was also convinced the product is cost-effective, high-performing and a common-sense approach to vibration dampening.
In addition to its noise-dampening attributes, TDA acts as a stable lightweight fill that can be used as a retaining wall backfill and to construct embankments. It also has superior drainage properties when used in landfill drainage systems.
About 660,000 shredded tires were used to build the Dixon Landing Road onramp at Interstate 880 in Milpitas a decade ago.
“It saved Caltrans $250,000 that they would have spent buying crushed volcanic rock from Oregon,” Patenaude said.
tate officials have paid for tests to see whether the tires, which contain petroleum products and metals, leach toxics into groundwater. Although small amounts of iron and manganese were detected, they are in low levels below health concerns, said Patenaude. Tests also were done in which tiny shrimp were placed in the runoff water and suffered no health consequences, she said.
CalRecycle is currently promoting the use of recycled tires through its Green Roads campaign.
As part of the campaign, CalRecycle experts are available to speak with local and regional agencies, as well as members of the media, about TDA’s engineering, cost-saving and environmental benefits. Grants and technical assistance are available for communities interested in using tire-derived aggregate in civil engineering projects, as well as using rubberized asphalt concrete for road paving projects.
© Scrap Tire News, December 2010