Approximately 59,000 m3 (77,000 cu yd) of F-class fly ash were used to construct a portion of the subbase for a 3.4-km- (2.1- mile-) long stretch of county highway truck route in west-central Illinois. The route consisted of a 7.3-m- (24-ft-) wide county road, and portions were built on a zero to 1.8-m (zero to 6-ft-) thick compacted F-fly ash subbase. Laboratory testing indicated the fly ash would serve adequately as a road subbase, conforming to IDOT standard specifications. Construction began in early June 2002 and was finished by early September 2002. No unexpected construction delays were encountered and the performance of the fly ash fill exceeded expectations.