NUMERICAL SIMULATIONS TO INVESTIGATE MOISTURE-RETENTION CHARACTERISTICS IN THE DESIGN OF OXYGEN-LIMITING COVERS FOR REACTIVE MINE TAILINGS

Acid generation in reactive mine tailings is an oxidation process that is dependent on availability of molecular oxygen. As a consequence of the diffusion coefficient of oxygen being several orders of magnitude higher in air than in water, influx of atmospheric oxygen into a material at depth can theoretically be minimized by maintaining a protective cover layer at high moisture content. Such oxygen-limiting covers are generally of finer texture than the material being protected. A numerical model was used to investigate the importance of moisture-retention characteristics in the transient drainage of such two-layer systems. The results show that the effectiveness of a material as a moisture-retaining cover is dependent on the magnitude of its air-entry value. The thickness of the cover maintained at full saturation after prolonged drainage also depends on the pressure head at which the underlying material approaches residual saturation.