Shear Strength Parameters of Sand Fly Ash Cement Mixtures

According to a 2012 American Coal Ash Association Coal production Survey Report, US coal fired power plants produced more than 109 million tons of waste that year. Approximately half of this waste is the valuable by-product fly ash. There are three classes of fly ash: cementitious class C and non-cementitious classes F and N. Over half of the fly ash produced is used in the geotechnical/construction industries. Most geotechnical soil stabilization studies using fly ash are focused on controlling shrink-swell potential of clays. This study utilized the less desirable class F fly ash to assess the improvement of shear strength parameters of granular soils. Two mix designs were developed and tested using consolidated undrained, unconfined compression, and triaxial testing. Mix designs consisted of 15% fly ash with 0.5 or 1% cement, and poorly graded Ottawa sand compacted using a standard effort at 10 percent moisture content. Consolidated undrained testing on Mix 1, which included flushing and saturating the specimens, produced higher shear strength parameters than for the sand alone. However, the results were inconsistent with respect to the increase in shear strength parameters with time. Unconfined compression testing was then conducted on both Mix 1 and Mix 2 to assess strength gain with time. Results showed both mixes gained appreciable strength with time but doubling the cement did not double the unconfined compressive strength. Triaxial testing was then conducted on Mix 1 using specimens that were not flushed or saturated. This testing was used to determine if flushing destroyed the specimen soil fabric. The shear strength parameters from the triaxial testing were very similar to those determined from consolidated undrained testing. This demonstrated that flushing did not affect the shear strength parameters. Inconsistent triaxial test results from fly ash-cement-sand mixes have been previously reported in the literature