Compressibility of Fly Ash under Ko Loading

Coal combustion processes produce large volumes of fly ash. The compressibility of fly ash was studied using moist tamping and drypluviation methods to achieve different void ratios before subjecting the material to compressive oedometric Ko loading (i.e., zero lateral deformation throughout the test). The class F fly ash used in this research was produced at a Midwest U.S. power plant. The material was initially characterized with hydrometer and specific gravity tests, and maximum and minimum realizable void ratios were determined. The compressibility characteristics of the fly ash material were studied at four strain rates, 1%/h, 10%/h, 40%/h, and 100%/h, and three different initial void ratios. Changes in pore-water pressures of the fly ash were continuously measured when the material was subjected to different strains and loading rates, representing different on-site depositional or construction loading processes. Comparisons between incremental loading and constant rate of strain tests are presented in terms of procedure, test completion time, and compressibility characteristics. The Ko compressibility and consolidation behavior of this fly ash was rather insensitive to the tested strain rates, and very small values of pore-water pressures were generated regardless of the testing rates. The measured compressibility of the material was very similar under constant rate of strain and incremental loading tests. Constant rate of strain testing is recommended due to the short time for test completion and continuous measurement of excess pore-water pressure.