Pulverized Coal Ash as Structural Fill

Untreated pulverized coal ash, consisting of fly ash with varying percentages of bottom ash, was used as a structural fill material to support the foundation of a new precipitator for a power generating station in Indianapolis, Indiana. Geotechnical studies for this fill included evaluation of index and chemical properties, strength, compresibility, compaction and aging characteristics. Field density and plate load tests were conducted during fill construction. Standard and static cone penetration tests were used to evaluate the consistency of the compacted fill and total settlements of the foundation were measured. For this project, compacted coal ash was used successfully as a structural fill material. Although the ash was inherently variable, it could be compacted satisfactorily if the moisture content was maintained below the optimum value and if the percentage of fines was below about 60%. The load-settlement relation for the foundation on compacted ash could not be inferred from standard or static cone penetration tests, largely because these tests do not adequately sense the effects of prestressing due to compaction. Plate load tests proved to be a reliable indicator of the observed settlements.