California buried about 270,000 waste tyres beneath a highway near Confusion Hill; the rubber became lightweight fill for two new bridges

A California highway project found an unusual use for thousands of discarded tyres. In 2009, the California Department of Transportation completed a road repair project at Confusion Hill on US 101 in Mendocino County, where longstanding rockslides had repeatedly threatened the highway. The new route bypassed the unstable area using two bridges, but engineers faced another problem, adding conventional fill would have placed too much weight on an underground drainage structure beneath one bridge embankment. Instead, they turned to tyre-derived aggregate (TDA), made from shredded waste tyres, as a lightweight fill material. The project ultimately used about 270,000 waste tyres, giving the discarded rubber a structural role beneath the road.Confusion Hill's rockslides had threatened US 101 for yearsThe Confusion Hill project addressed a longstanding problem along US 101 near Piercy in Mendocino County. According to CalRecycle, rockslides had occurred in the area for thousands of years and had affected the highway for at least 17 years before the project. Roadway slip-outs had become more frequent, while falling debris and slope movement caused repeated closures and maintenance problems. In 2001 alone, roadway stabilisation work cost $3.2 million. Engineers needed a more reliable route through the slide area that would keep US 101 open to traffic. The solution was to bypass the unstable section with two new bridges rather than continue relying on repeated repairs to the existing route.Engineers needed a fill material that would not overload the groundThe bridge route created a separate engineering challenge. CalRecycle says the north bridge embankment sat above an existing underground drainage structure that could not handle the additional load created by raising the embankment. Engineers therefore needed a material that could provide the necessary fill while adding less weight than conventional soil or rock. Tire-derived aggregate was selected for the job because it is lightweight and cost-effective. TDA is produced by processing waste tyres into pieces of rubber and is used in civil engineering applications including road repairs, embankments and slope stabilisation. TDA can weigh about one-third as much as soil and other similar construction materials, making it useful where excessive loading could cause problems.About 270,000 waste tyres went beneath the highwayFor the Confusion Hill project, the existing fill material over the underground structures was removed and replaced with lightweight tire-derived aggregate. CalRecycle also records that the project used approximately 270,000 waste tyres and was completed in 2009. The tyres were not simply buried whole; they were processed into TDA and used as engineered fill within the road project. The work was a lightweight-fill project for rockslide and road repair, with the TDA incorporated into the embankment stabilisation. This turned a large quantity of waste tyres into a construction material while addressing the weight limitations around the drainage structure beneath the bridge embankment.The project showed how old tyres can become infrastructure materialConfusion Hill became one example of California's wider use of tire-derived aggregate in civil engineering. TDA is a material that can be used for lightweight fill behind retaining walls, road repairs and embankment construction, as well as for drainage and vibration mitigation. Its lightweight and free-draining properties can make it useful where conventional fill would create excessive loads or other engineering difficulties. The state's project profiles include other applications involving hundreds of thousands of waste tyres. Rather than sending every discarded tyre to disposal, projects such as Confusion Hill demonstrate how processed rubber can become part of infrastructure. CalRecycle continues to support TDA as a recycled material for engineering projects across California.You use AI every day. Now get your AI Quotient. Take the AIQ test.