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Volume change and permeability of mixtures of waste rock and fine tailings Wickland, Benjamin Eli

Abstract

Current methods of mine waste disposal have resulted in environmental problems on a global scale. In an effort to circumvent problems inherent to segregated disposal of waste rock and tailings, this thesis examines an alternative mine waste disposal technique where waste rock and tailings are combined as a homogenous mixture. One type of waste rock and one type of tailings were examined individually and as mixtures for volume change, permeability, and soil-water characteristics. Laboratory investigations included compressibility testing in a large diameter consolidation cell, hydraulic conductivity by constant head test alternated with static loading, and soil-water characteristic curve by pressure plate test. Tailings rheology was examined in a cup and bob rotational viscometer. A two-year meso-scale column study of self-weight consolidation was also conducted. The behaviour of mixtures of waste rock and tailings is dependent on mixture design, and mixtures may therefore be designed for specific geotechnical properties. Design variables examined included mixture ratio and initial tailings solids content. Mixture ratio was found to have an optimum value where the tailings slurry just fills the voids between rock particles. Tailings solids content was found to have limits for mixing defined by rheological yield stress. Mixtures had compressive strains similar to waste rock alone and much lower than tailings alone. The finding was attributed to the presence of a load bearing waste rock skeleton. The hydraulic conductivity of mixtures was similar to tailings and much lower than waste rock. The finding was attributed to the presence of a fine-grained tailings matrix. While waste rock alone was observed to remain unsaturated, mixtures remained saturated for long periods of time under conditions of free drainage without access to water. Mixture particle structure was conceptualized through the use of a particle model and described quantitatively by waste rock skeleton void ratio and tailings matrix void ratio. Methods for relating mixture compressibility and hydraulic conductivity to structure are proposed. While exhaustive testing of the range of possible mixtures and behaviours was not possible, this thesis has provided a fundamental theoretical basis for understanding mixture design and behaviour with respect to mixture particle structure.

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