![]() When used in structural fills or embankments, fly ash offers several advantages over natural soil or rock. List of fly ash embankment or structural backfill projectsconstructed in the United States. Neither of these two projects showed any signs of undue settlement or adverse environmental impacts over the 3-year monitoring period. (3,4) The monitoring consisted of sampling and analyzing ash physical and engineering characteristics, evaluating ash placement and compaction behavior, collecting and analyzing samples from groundwater monitoring wells, and periodically taking settlement readings from selected locations within each embankment. Two of the ash embankment projects listed in Table 5-4 (one in Delaware and one in Pennsylvania) were monitored for construction and postconstruction performance over a 3-year time period. This list, which may not be complete, encompasses at least 21 highway construction projects (2), involving an estimated 2 million or more cubic yards of fly ash, in some cases mixed with bottom ash.Īlthough very few of these projects have been monitored for long-term performance, none of the states in which these projects have been constructed have indicated any dissatisfaction or concern with the embankments or backfills constructed using fly ash in their states. Table 5-4 provides a list of the number, estimated size, location, and year of construction of the known fly ash embankment or structural backfill projects that have been built in each of these states. At least nine states have a specification for the use of fly ash as an embankment material. States that have thus far used fly ash as an embankment or structural backfill material include Arizona, Delaware, Illinois, Maryland, Massachusetts, Michigan, Minnesota, Missouri, Ohio, Oklahoma, Pennsylvania, Virginia, West Virginia, and Wisconsin. (1) Fly ash has also been used as a structural backfill material behind retaining walls and bridge abutments. It has been reported that since 1970 at least 14 states have used fly ash to construct or repair embankments. Fly ash use as a structural fill or embankment material was pioneered during the 1950’s in Great Britain, where it is still bid as an alternate borrow material on roadway fill projects in areas where it is available. Lignite or subbituminous fly ashes, which are usually self-cementing, can harden prematurely when moisture is added, resulting in potential handling problems and inability to achieve the required degree of compaction. Nearly all of the fly ash used for embankment construction is anthracite or bituminous coal fly ash. But, as with most fine-grained soils, fly ash can be easily handled and compacted at more intermediate moisture contents, and does exhibit some cohesion. When saturated, fly ash becomes an unmanageable mess. When dry, fly ash is cohesionless and considered by many as a dusty nuisance. Compared with conventional soils used to build embankments, fly ash is somewhat of a unique engineering material. Coal fly ash has been successfully used as a structural fill or embankment material for highway construction projects in a number of different locations throughout the United States.
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