Coal Combustion Residuals’ Geotechnical Behavior under Dewatering Activities

Sluiced Coal Combustion Residual (CCR) materials have inherent vulnerabilities due to their tendency to have low unit weights and their state of deposition, and they can potentially exhibit highly contractive or dilative behavior during shearing. Flow failure and slope instability of surrounding dikes (typically built with CCRs or clays) due to dewatering are typically considered as potential failure modes (PFMs). The objective of this paper is to review the undrained behavior of CCR material during shearing due to dewatering, evaluate the potential consequences of dewatering (i.e., removal of free water) existing facilities, and provide general recommendations to mitigate PFMs associated with dewatering activities. Changes in shear stresses and strains, pore water pressure, and slope stability are evaluated for a representative containment dike as a result of dewatering activities. For the slope evaluated, the results show that the variation in shear stresses and pore water pressures due to dewatering are expected to be small and, therefore, the expected influence on the stability of the slopes to be small. However, known incidents of instability at CCR impoundments with gentle slopes have occurred across the industry and warrants careful examination of the loading mechanisms and material behavior during loading. Based upon this evaluation, this paper includes specific recommendations to reduce risks during construction dewatering.