Nearly 300 tonnes of unwanted glass were headed for the landfill. Idaho used them to build a highway instead and solved a local landfill crisis

Discarded glasses often get dumped in the landfills but engineers in Idaho once found an innovative use of them, solving two problems at once. In 2002, nearly 300 tonnes of discarded glass were headed toward Idaho's waste stream. Then, engineers found a surprising second life for the material — using it as the foundation beneath a highway near Ketchum.

Along a 200-foot stretch of Highway 75, transportation officials placed an eight-to-12-inch layer of crushed glass cullet beneath the road surface. The unusual pilot project turned material that had little value to recycling programs into a structural component of the highway.

The idea was championed by the late Idaho State Senator Clint Stennett, who viewed the project as an example of how government, businesses and local communities could work together to reuse materials that would otherwise become waste.


Rather than sending bottles from the Ohio Gulch Recycling Center to distant processing facilities, the glass was crushed locally into small granular pieces called glass cullet and incorporated into the highway's base.

And crushed glass isn't quite as fragile as it sounds.

When processed to the appropriate size, glass cullet can behave much like conventional sand or crushed aggregate. Federal Highway Administration guidance has identified processed glass as a potential substitute for conventional materials in applications including road embankments, structural fill and pavement base layers.

Its engineering properties are part of what made the Idaho experiment possible. Properly processed cullet can compact effectively, allow water to drain through it and remain relatively resistant to moisture-related changes. Those characteristics are particularly useful beneath roads, where trapped water can weaken pavement structures over time.

There was another potential advantage for central Idaho: temperature.

Crushed glass can have lower thermal conductivity than some natural stone materials. In a region where freezing temperatures and repeated freeze-thaw cycles can damage roads, that characteristic could help reduce some of the stresses affecting the underlying roadbed.

The project also tackled a less obvious problem with recycling: transportation costs.

Glass is heavy, but its value can be relatively low. For rural recycling programs, shipping large quantities of discarded glass to distant processing facilities can therefore become expensive enough to undermine the economics of recycling it.

Using the material close to where it was collected changed that equation.

Instead of paying to move waste glass long distances, Idaho could process it locally and put it to work in nearby infrastructure. At the same time, the approach reduced the amount of material entering landfills and potentially reduced demand for newly quarried aggregate.

Safety and environmental testing were also important. Before crushed glass could be incorporated into a road, engineers needed to ensure it wouldn't create handling hazards or introduce contaminants into surrounding soil and groundwater. Proper processing reduces the size and sharpness of the glass fragments, while testing can verify that the material is suitable for use in construction.

The Idaho experiment wasn't an isolated idea, either. Federal research has examined the use of recycled glass in several highway applications, including granular subbase, structural fill, asphalt mixtures and utility trench backfill. Some states have also established specifications allowing processed glass to be incorporated into certain pavement materials.

The bigger idea is surprisingly simple: what looks like garbage can sometimes become a construction material with a second life.

For communities struggling with landfill capacity, expensive recycling logistics and the rising demand for construction materials, projects like Highway 75 point toward a potential circular solution. Instead of treating discarded glass as something that must simply be hauled away, it can be processed and put back into the infrastructure around the community.

And beneath this particular stretch of Idaho highway, nearly 300 tonnes of unwanted glass are doing exactly that.