Dewatering Strategies at Clay- Lined Tailings Impoundments to Reduce Environmental Risk

Abstract Only - Environmental risk increases when the design life of a clay-lined tailings impoundment is exceeded. The design life is based on the permeation rate of the liner, a function of average hydraulic head. hydraulic conductivity and thickness. Closure activities typically include dewatering of tailings to reduce the hydraulic head prior to full permeation of the liner, near the end-of facility life. In preparation for closure activities at a 120-acre tailings impoundment. Condor evaluated four methods of dewatering saturated tailings: installation of deep drainage ditches, electrokenesis, wick drains and well point extraction. Tailings are approximately 25 feet deep and consist of saturated silt- sized particles of clay- shale with interbedded layers of porous crystalline evaporates. Deep narrow drainage ditches could have been installed with long-armed trenchers or using slurry- filled techniques, but required significant investment without confidence that trenches or drains would stay open in the soft tailings. Gravel fill was rejected due to desire for processing of ore grade tailings. Alternative wide ditch excavation would have required a costly mining, effort. Electrokenesis, the movement of water in an electric field, required excessive power costs. Wick- enhanced drainage required loading by solids or water, which would have complicated later recovery of ore grade tailings. Pilot wells of small diameter (4") were installed with low-ground pressure track augers and were surprisingly productive. Numerical modeling of large diameter wells indicated that little advantage would be gained; tripling the well diameter would only increase yield by 15%. Installation of larger diameter wells would be much more difficult, requiring the construction of roads onto the tailings. The physical stratigraphy and chemistry of the tailings dictate special extraction techniques, with well screens in the liner sand blanket, automated pump-data monitoring and hot water back flush. A phased dewatering program ensures steady progress without financial risk on uncertain methods.