In the 1990s, a highway project in Oregon triggered a landslide. Engineers removed soil and buried 580,000 old tires to fix it

The United States generates hundreds of millions of discarded tires every year, creating a persistent challenge for waste management, recycling infrastructure and finding useful markets for the material. But in the early 1990s, engineers in Oregon found an unusual way to turn thousands of unwanted tires into part of a highway repair project.

Instead of sending the tires to a landfill or leaving them in stockpiles, officials shredded approximately 580,000 waste tires and used them as lightweight fill beneath a section of U.S. Highway 42 in southern Oregon. The unusual material was selected for a very specific engineering reason: the highway project had triggered movement in an old landslide, and replacing heavy soil with much lighter shredded tires could reduce the load on the unstable ground.

The project would eventually become an early example of how tire-derived material could be used in road construction and geotechnical engineering, rather than being treated simply as waste.

The highway project had a landslide problem

The repair was part of a larger improvement project on U.S. Highway 42 in southwest Oregon. Engineers were widening an existing highway embankment by about 6.1 metres (20 feet) and raising it by approximately 1.2 metres (4 feet).

The existing embankment was about 3.3 metres (11 feet) deep. Adding more material increased the weight on the ground beneath it, and that additional load reactivated an old landslide. The slide began moving progressively downhill, perpendicular to the highway.

A geotechnical investigation found that the movement could be controlled by changing how the weight was distributed. Engineers proposed reducing the load on the unstable portion of the embankment while adding a counterbalance farther downslope.

That created an unusual construction problem: what could replace conventional soil without adding as much weight?

Engineers considered sawdust and shredded tires

Two lightweight materials were considered for the replacement fill: sawdust and shredded waste tires. Sawdust had one obvious advantage. It was lightweight and readily available. But engineers were concerned about its long-term deterioration underground. Shredded tires offered another possibility: they were lightweight, relatively durable and, importantly, Oregon had an incentive for their beneficial use.

The design therefore called for some of the existing embankment soil to be replaced with shredded tires. The soil that was removed could then be used to construct the downslope counterbalance needed to help stabilize the landslide.

The repair also incorporated a rock blanket and a trench-drain system designed to keep groundwater below the shredded tire material. In other words, the tires were not chosen simply because officials wanted to dispose of old rubber. Their low weight was the key engineering advantage.

580,000 tires travelled hundreds of kilometres

Approximately 580,000 waste tires were shredded and transported to the highway project from four different sources. Some of the material travelled between roughly 240 and 440 kilometres (150 to 275 miles) to reach the construction site. Once delivered, the shredded tires were placed and compacted using a bulldozer.

The resulting tire fill was then covered with approximately 0.9 metres (3 feet) of soil. Above that went a conventional pavement structure consisting of about 58 centimetres (23 inches) of aggregate base and 20 centimetres (8 inches) of asphalt pavement.

The engineers monitored what happened underground

The unusual construction did not simply disappear beneath the highway once the asphalt was laid. The shredded tire fill was instrumented and monitored for a year after installation. Engineers used equipment including inclinometers, piezometers, settlement plates and survey hubs to track movement, groundwater conditions and settlement. They also conducted falling-weight deflectometer tests to examine pavement performance.

The monitoring showed that the shredded tire material compressed as additional loads from the soil cap and pavement were applied. The amount of compression appeared to be related partly to the thickness of the shredded tire layer. There was also evidence of continued creep or compression during the monitoring period under traffic loading.

Yet the results did not mean that the highway could not function. The pavement deflections were considered within acceptable limits after the 20.3-centimetre (8-inch) asphalt layer was installed. The compacted density of the shredded tires varied from approximately 730 to 845 kilograms per cubic metre, depending on the stage of compaction and loading.

Why old tires worked as road fill

Traditional soil is comparatively heavy. If a highway embankment is placed over or near an unstable slope, adding more soil can increase the forces acting on the ground and potentially contribute to movement. Shredded tires, by contrast, have a much lower density than conventional soil. Replacing part of a heavy embankment with lightweight tire material can therefore reduce the load imposed on the underlying ground.

That made the Oregon project particularly interesting from an engineering perspective. The tires were not merely being recycled into something useful; their physical properties were helping solve the problem created by the highway itself.

The approach also demonstrated that waste materials can sometimes have engineering characteristics that make them useful in applications very different from their original purpose.

Cost was another factor in the decision. The in-place shredded tire fill was reported to cost approximately $16.82 per cubic metre ($12.87 per cubic yard). That figure included a significant rebate from the Oregon Department of Environmental Quality for beneficial use of the waste material.

Without the rebate, the reported cost would have been about $35.16 per cubic metre ($26.91 per cubic yard).

The Oregon experiment therefore brought together three separate problems: a highway embankment that had become unstable, a large quantity of unwanted tires and the need for an economical lightweight construction material.