In 2004, San Jose used 1,000 tons of shredded waste tires beneath 2,000 feet of light-rail track; the unusual layer reportedly reduced vibration and saved over $1 million

If you have ever travelled on a light rail line and felt a smoother ride than expected, shredded tires may have been part of the track bed. In 2004, the transit authority in Santa Clara Valley used approximately 1,000 tons of shredded scrap tires for 2,000ft of light rail on the extension of the Vasona Line.

According to CalRecycle, California’s state recycling agency, this not only reduced noise, but it also helped save the transit authority over $1 million compared to traditional methods of reducing vibrations. By using the shredded tires, they also prevented roughly 100,000 tires from being sent to landfill or causing a fire hazard. It seems too simple; however, there is some really complicated science behind it. It seems deceptively simple on the surface, but the solution rests on some genuinely complicated engineering science

The tire problem


Every year, millions of tires are turned into scrap tires within the US. Most people don’t really think about where they go. In 2018, Americans produced 9.2 million tons of rubber and leather waste, as calculated by the U.S. Environmental Protection Agency: still the most recent complete figures EPA has published in its Facts and Figures series as of this writing. This was only 3.1% of all municipal solid waste produced. Approximately 40% of scrap tire rubber waste specifically was actually recycled.

Scrap tires cannot be easily compacted, so they cause a buildup of gas and will ‘float’ to the surface in landfill sites. They are also infamous for causing fires in stockpiles and can burn for months, creating toxic smoke. The question “What do we do with old tires?” has been a long-standing issue in America.

How shredded rubber defied physics

Trains are massive, and the vibrations they generate transfer a lot of energy into the ground. This causes noise and vibration in surrounding buildings, which can be a real annoyance. To reduce this issue, transit authorities use specialist rubber ballast matting, elastic rails, or floating slab rail beds. All of these solutions work but can be costly. A cheaper alternative was found using Tire Derived Aggregate (TDA).

Researchers in a 2004 peer-reviewed study titled “Development of Tire Shred Underlayment to Reduce Groundborne Vibration from LRT Track,” published in the Geotechnical Engineering for Transportation Projects, looked at how effective shredded tires were as a base for ballast and tie rail tracks. They did this through a two-part field testing program: first measuring vibration attenuation without rail present, then installing a trial section of shredded tires under ballast and track to measure real-world performance. They found that using shredded tires does reduce ground-borne vibration and is a “practical and viable solution”. Plus, they also help reuse the scrap rubber. The success of the project in San Jose led to other transit systems in California also using this solution. These include BART’s Warm Springs and Berryessa extensions, and sections in L.A. Metro.

Why should this matter

It’s frustrating that a solution this straightforward to describe, even though validating it took real engineering science, remains so underused. We have a scrap tire surplus in the US, and our transit systems are struggling to get funding. However, despite this solution being available, tested, and recorded since the early 2000s, most major expansions across the US do not use it. This may not sit right with a generation that is becoming increasingly worried about climate change and seeing our public transport systems fall apart. It is not a lack of technology causing us to not use this solution, it is a lack of organisation. Next time a city is planning a new rail line and raising budget concerns, ask what is actually under the track.