Maine buried 20,000 old tires beneath a road and they helped against frost damage. Now engineers are exploring them for walls, highways and more

Engineers in Maine tried an unusual solution to two stubborn problems at once: roads that can fall apart during spring thaw and more than a million discarded tires generated in the state each year.

Instead of sending the tires to a landfill, stockpile or furnace, they shredded about 20,000 of them and buried the pieces beneath a 600-foot stretch of Dingley Road in Richmond.

The experiment was designed to answer a simple question: Could old tires actually make roads more resilient in a place where freezing and thawing can wreak havoc on the ground?


The early results were surprisingly encouraging.

A road built with 20,000 discarded tires

The experiment was developed by University of Maine civil engineer Dana Humphrey, who was interested in whether shredded rubber could do something conventional gravel could not.

The tire pieces were cut into chunks roughly 2 to 3 inches across, with their steel belts still inside. They were then spread across a 600-foot section of road, creating a layer between 6 and 12 inches thick.

Workers didn't simply replace the road with tires. The rubber chips went underneath the conventional gravel, with successive layers of ordinary road material placed above them.

That distinction mattered: the tires weren't meant to become the surface vehicles drove on. They were being used as a kind of engineered foundation layer.

More than 100 temperature sensors were installed at different depths so researchers could watch what happened inside the road throughout the winter.

And when spring arrived, the difference was noticeable.

The rubber seemed to keep the frost at bay

Maine's roads have to endure a particularly punishing cycle.

During winter, water in the soil freezes. When temperatures rise, that frozen ground begins to thaw, leaving the road foundation softer and less stable. Traffic can then turn gravel surfaces into a mess of ruts, potholes and damaged sections.

But the tire-chip section behaved differently.

Researchers found that frost penetrated to only about half the depth observed in nearby gravel-only sections. The neighboring conventional sections also deteriorated considerably during the spring thaw, while the experimental stretch held up better.

The explanation was hiding inside the tires themselves.

Shredded rubber contains plenty of air spaces, and those spaces give the material an insulating quality. Instead of allowing heat to move through the road structure as readily as conventional mineral fill, the tire layer helped separate the road surface from the deeply frozen ground below.

In other words, the waste material wasn't merely sitting underneath the road.

It was changing the way heat and water moved through it.

Tires had another advantage: they were incredibly light

The experiment revealed another potentially useful property.

Gravel weighs roughly 125 pounds per cubic foot, while the tire material used in the experiments weighed around 40 pounds per cubic foot.

That's a huge difference when you're moving thousands of cubic feet of material.

A lighter fill puts less weight on the soil underneath, making shredded tires potentially useful in places where conventional earth or gravel would place too much pressure on weak ground.

The irregular pieces also create spaces that allow water to move through the material.

That gave tire chips three useful characteristics: Low weight, insulation, and drainage. Those properties meant the material could potentially be useful for more than frost protection.

Maine had a very good reason to experiment

There was another problem hiding behind the road experiment: what to do with all the tires people throw away.

Maine was estimated to generate around 1.2 million unwanted tires every year.

Some were stored in enormous piles. Others were burned for energy or processed into other products. But finding economical uses for old tires remained difficult.

Large tire stockpiles aren't harmless, either. They can collect rainwater and become breeding grounds for mosquitoes, while massive tire fires can burn for extended periods and produce difficult-to-control smoke.

So if road builders could use whole mountains of discarded tires as construction material, the idea could potentially solve two problems simultaneously.

Instead of asking "How do we get rid of these tires?", engineers could start asking:

"Where can these tires do useful work?"

It wasn't the same as rubberized asphalt

There was already a different way of putting old tires into roads: grinding them into crumb rubber and incorporating that material into asphalt.

But that required considerably more processing.

One mile of interstate resurfacing in Maine could consume roughly 6,000 tires, but the rubber reportedly cost more than $20 per tire.

The tire-chip approach was different.

Rather than grinding the rubber into a fine material, engineers could cut tires into relatively large pieces and use enormous quantities of them as lightweight fill.

That potentially meant using far more waste tires without paying for the same degree of processing.

The experiment didn't stop at one road

Humphrey and graduate students at the University of Maine also explored another application: using tire chips as lightweight fill behind a retaining wall.

The planned wall was about 15 feet high.

The logic was straightforward. If conventional soil or gravel could be replaced with something dramatically lighter, it could reduce the forces pushing against a retaining structure or a slope.

That opened the door to possible applications in other challenging environments, including steep roads and areas with weak or unstable ground.

Researchers also wanted to know whether the material could work beneath conventional pavement rather than simply beneath a gravel road. A later test planned for North Yarmouth would put tire chips beneath an asphalt-topped highway.

Earlier testing had suggested that once compacted with ordinary road-building equipment, the material could support loads from heavy trucks.

An idea born from two waste streams

The fascinating part of the experiment wasn't simply that engineers discovered tires could be put beneath a road.

It was that the characteristics that make tires difficult to dispose of can become useful engineering properties.

Rubber is lightweight. It doesn't transfer heat as readily as conventional mineral fill. And shredded pieces create voids that can help with drainage.

Those same characteristics that make a discarded tire awkward waste can make it valuable as a specialized construction material.

Maine's experiment therefore pointed toward a broader idea: sometimes the solution to an infrastructure problem doesn't come from producing a better conventional material.

It can come from looking at a waste product differently.

And beneath one Maine road, thousands of tires got a second job — not as tires, but as part of the road itself.