Unsaturated Properties of Mixtures of Waste Rock and Tailings: Fid-Bau Portal

Unsaturated Properties of Mixtures of Waste Rock and Tailings

Wickland, B. E. / Wilson, G. W. / Fredlund, D. G. / Wijewickreme, D.

Mixtures of mine waste rock and tailings, tailings alone, and waste rock alone were examined for soil-water characteristic curves (SWCC's) along the drying path. Laboratory data were compared with matric suction measurements and observations from a two year meso-scale column study of self-weight consolidation under free drainage. Waste rock alone had a near zero air-entry value and reached residual water content at low matric suctions. The mixture remained saturated relative to waste rock alone, a finding that was attributed to the presence of a fine-grained tailings matrix. The tailings had a high air-entry value, but also had significant volume change with increasing matric suction. The mixture had less total volume change than tailings for a given change in matric suction. The lower total volume change of mixtures was attributed to the presence of an internal "waste rock skeleton." Both the mixture, and tailings alone had SWCC's with a change in slope that was dominated by changes in volume, rather than a distinct air entry value (AEV), and a volume change correction was required to accurately determine the SWCC. Under prolonged drainage and/or high matric suctions, the mixture developed cracks within the tailings matrix. In general, the results indicate that mixtures can remain tension saturated for long periods of time relative to waste rock alone. Mixing for disposal therefore has the potential to limit the rate of acid rock drainage, particularly for wet climates.