UC Davis

The failure of impounded fly ash at the Kingston Fossil Plant in 2008 led to severe economic and environmental consequences from the flow of ash for almost one kilometer downstream. With over 700 active surface impoundments and federal disposal regulations enacted in 2015, the coal-power industry is required to properly address the risk and possible consequences associated with impounded fly ash. The Electric Power Research Institute and the University of California Davis, Center for Geotechnical Modeling collaborated to advance the understanding of possible flow failures of impounded ash, including conditions that lead to its triggering, methods to identify its potential in the field, and the potential impacts of dewatering. A centrifuge testing program began in 2018 to explore the influence of initial density and dewatering of impounded ash, with a focus on its runout behavior after a model impoundment is subjected to a loss of lateral confinement. This study presents the results from the one centrifuge test performed at 60 g and three 1 g tests conducted in 2021, with comparisons made to four previous 60 g tests. Fly ash was deposited into a novel centrifuge container, with a pair of gates opened in flight to induce an idealized containment structure failure. The strength and stiffness of the ash were measured prior to the failure using a miniature CPT penetrometer and bender elements for the 60 g test and a miniature T-bar penetrometer for the 1 g tests. Pore pressures were measured throughout the entire 60 g test using tensiometers, and water contents were measured using moisture probes for all tests during specimen preparation and testing. Various cameras were used to track the runout after the opening of the gates. Failure mechanisms of the ash ranged from slope instability to a drastic flow failure in the 60 g tests, and flow failures to a slumping-like failure in the 1 g tests Although the runout results between the 60 g and the 1 g tests agree in the effect of ash density, the failure mechanisms were not found to be comparable due to differences in confining stress between the two test types. Ash strength and stiffness and water table height were found to impact the runout length in all tests. The results from this testing program and comparison to the previous centrifuge tests demonstrate that dewatering can improve the stability of fly ash impoundments and that looser deposits are more prone to flow failures associated with large runout distances.