In 1972, Illinois built a highway embankment with 246,000 cubic yards of coal ash. It’s still holding up even after 50 years

More than five decades ago, engineers in northern Illinois tried something that was unusual for highway construction at the time: instead of using hundreds of thousands of cubic yards of traditional soil and aggregate, they built the core of a major road embankment with 246,000 cubic yards of coal fly ash.

The experiment took place near Waukegan in 1972, beneath a roughly 7,500-foot section of expressway. More than 50 years later, the project has become an important example of how an industrial waste product could be transformed into a useful construction material.

And the surprising part is how well the massive embankment performed.

An Unusual Solution to Two Expensive Problems

The project tackled two growing problems at once.

Highway construction required enormous quantities of soil, which had to be excavated and transported to construction sites. At the same time, coal-fired power plants across the Midwest were producing huge amounts of fly ash as a byproduct of burning pulverized coal.

Fly ash consists of extremely fine mineral particles collected from power-plant exhaust systems. Because the particles are lightweight and mostly spherical, engineers began investigating whether the material could replace conventional fill.

In Illinois, transportation authorities decided to test that idea on a much larger scale.

During construction, contractors placed the fly ash in carefully controlled layers and compacted it to form the main core of the highway embankment. A thick layer of natural soil was then placed over the ash along the slopes and road base, helping protect the material from erosion and wind.

Engineers also installed instruments throughout the site to monitor how the unusual embankment responded to traffic, moisture and northern Illinois' repeated freeze-thaw cycles.

The 50-Year-Old Experiment Had a Remarkable Result

The monitoring produced encouraging results.

Inspections found no detectable uneven settlement across the 7,500-foot section. Engineers also reported no significant frost heave and no evidence of capillary water movement that could threaten the pavement above.

The fly ash had another useful property: its pozzolanic behavior.

When certain fly ashes react with moisture and available lime, they can develop cement-like bonds over time. That reaction can increase the strength of the material as it ages, making the embankment more stable rather than simply relying on the original compaction.

The project therefore demonstrated something engineers had been trying to establish: industrial waste from power plants could potentially serve as structural fill in large transportation projects.

It Became a Model for Other States

The Waukegan project did not remain an isolated experiment.

Later technical studies documented the project and its performance. The National Cooperative Highway Research Program's NCHRP Synthesis 199, which examined the recycling and use of waste materials and byproducts in highway construction, cited the project and reported that inspections found no detectable uneven settlement across the highway section.

Federal Highway Administration research also identified the Waukegan project as an important example for transportation agencies.

States including Delaware, Michigan, Minnesota, Pennsylvania and Wisconsin subsequently developed standards for using fly ash in structural fills during the late 1970s and 1980s.

What began as an unconventional highway experiment was helping establish a new way of thinking about industrial byproducts.

But Using Coal Ash Today Is More Complicated

The engineering success of the 1972 project does not mean coal ash can simply be placed beneath a modern highway without precautions.

Environmental concerns surrounding coal combustion waste have led to much stricter requirements over the decades. Fly ash can contain trace elements, including heavy metals, that can pose risks if they migrate into groundwater.

Modern projects therefore require considerably more environmental planning, including groundwater assessments, protective layers and monitoring designed to prevent contaminants from reaching surrounding water sources.

That distinction is important: the Waukegan project demonstrated the structural potential of fly ash, while modern regulations place much greater emphasis on controlling its environmental risks.

A Highway Experiment That Changed How Waste Was Viewed

The 1972 Illinois project was ultimately about more than one highway embankment.

It showed that a material once regarded primarily as industrial waste could be engineered into a useful component of major infrastructure. The 246,000 cubic yards of fly ash replaced a substantial amount of conventional fill while surviving decades of traffic and harsh Midwestern winters without the settlement and frost problems engineers were watching for.

More than 50 years later, the experiment remains an intriguing example of turning industrial waste into infrastructure—and of how an unconventional idea on one Illinois highway helped influence the way transportation agencies approached recycled construction materials.