More than five decades ago, civil engineers in northern Illinois carried out a major engineering experiment by using 246,000 cubic yards of coal fly ash beneath 7,500 feet of a major expressway embankment. The pioneering project, completed in 1972 near Waukegan along the US Route 41 corridor, was one of the earliest large-scale examples of recycling industrial waste for American highway construction. Detailed technical studies of the project were later published by the National Cooperative Highway Research Program in NCHRP Synthesis 199, titled Recycling and Use of Waste Materials and By-Products in Highway Construction.
Long-term monitoring showed that the alternative fill performed very well, with no signs of structural settlement, frost heave or problems caused by rising groundwater. The Federal Highway Administration also included the project in its detailed user guidelines. The findings helped change how state transportation departments across North America viewed industrial byproducts. They showed that fine ash collected from coal-fired power plants could replace traditional soil and aggregate used as backfill.
Published under the Transportation Research Board, inspections after construction found no detectable uneven settlement across the 7,500-foot section, according to NCHRP Synthesis 199. Research records kept by the Federal Highway Administration in document FHWA-RD-97-148 noted that the Waukegan project became an important model for state transportation departments in Delaware, Michigan, Minnesota, Pennsylvania and Wisconsin. Monitoring over several years showed that the fly ash core performed as well as, or better than, traditional soil embankments. The ash also had pozzolanic properties, meaning it could react with moisture and free lime over time to form cement-like bonds. This helped increase its strength as the embankment aged. Monitoring teams also found no signs of frost heave during severe winter freezes. They did not detect capillary water movement that could damage or weaken the asphalt pavement above. These states later created formal standards for using fly ash in structural fills during the late 1970s and 1980s.