In 2003 and 2006, California used 224,700 waste tires behind two freeway walls; the lighter fill reduced pressure on the structures by about 30%

In California, engineers managed to use recycled tires for various purposes, including construction of freeway walls. In 2003 and 2006, California used 224,700 waste tires as part of two freeway retaining wall projects. Instead of using only traditional soil behind the structures, engineers filled the space with tire-derived aggregate (TDA), a lightweight material made from shredded tires. The goal was to reduce the pressure placed on the walls while creating a useful destination for old tires. CalRecycle explains that the projects demonstrated how TDA could reduce pressure on retaining walls by about 30 percent when used as lightweight backfill material.

When old tires became construction material

Discarded tires are tough to manage as they are meant to last longer. Those same characteristics that help tires last for thousands of miles on the road also help them resist natural weathering once they are thrown away. Therefore, engineers began exploring whether those properties could be beneficial. The recycling of tires resulted in a new product: tire-derived aggregate (TDA), which is lighter than conventional soil and can be applied to various civil engineering projects that involve the use of soil, such as road repair, embankments, drainage, and retaining walls. The idea was based on a simple engineering principle: reducing the weight of material behind a wall reduces the force pushing against that structure.

The freeway walls that became a testing ground

The experimental approach was directed to two retaining walls built as part of freeway improvement projects in Riverside County in California. The first project, known as Wall 119, was built in the Route 91/60/215 freeway improvement area. Engineers placed approximately 837 tons of TDA (the equivalent of approximately 83,700 waste tires) behind around 300 feet of retaining wall. A second project, Wall 207, was completed in 2006 just east of Interstate 215 and near Chicago Avenue. That wall used approximately 1,410 tons of TDA, representing about 141,000 waste tires. Together, the two projects reused 224,700 tires. The shredded tire debris was located behind concrete retaining walls, and the debris was confined from surrounding soil with the use of geotextile fabric. The engineers also placed monitoring instruments to determine the response of the walls compared to the traditional soil-filled structures.

Retaining walls constantly resist pressure from the material placed behind them. Traditional soil creates significant force because of its weight, especially when large amounts of fill are needed. Tire-derived aggregate changes the calculation because it weighs much less than conventional fill. This enables engineers to lessen the load on the structure and, in some designs, build less concrete and steel.

Waste becomes infrastructure

The freeway wall projects were not only about finding a use for waste tires, but also about participating in a broader movement to re-imagine the potential of recycled materials for infrastructure. Traditional construction depends heavily on natural resources such as soil, gravel, and stone. Using recycled materials like TDA can reduce demand for some raw materials while solving disposal problems at the same time. Recycling materials such as TDA can reduce demand for some raw materials while also addressing disposal issues. The use of Tire-Derived Aggregate has continued to be researched in California for other engineering applications. CalRecycle says the properties of TDA, such as its low weight and water movement, make it useful as a lightweight material for fills, repairing roads, vibration dampening, and drainage systems.

The future of recycled materials in infrastructure

Experts have monitored the behavior of recycled materials over several decades, particularly in projects exposed to changing weather, traffic, and environmental conditions. California has also developed guidance and research programs to help engineers safely design projects using tire-derived aggregate. Such measures show how recycled materials may contribute to more efficient infrastructure designs by making better use of existing resources.