Large volumes of fly ash are generated by the coal-fired power stations and is currently disposed onto waste dumps, with already limited space. Therefore, a need for an alternative ash disposal method arises. This study evaluates the feasibility of fly ash disposed as backfill into opencast coal mines. The change in the hydraulic properties of the ash under different conditions and over time play an important role in determining this feasibility. Leachate and tracer tests are conducted in the laboratory through Darcy column tests where;
(i) fly ash will be leached with acid mine water,
(ii) fly ash will be leached with saline mine water, and
(iii) fly ash will be leached with natural groundwater.
These experiments will be conducted with fly ash of different moisture content and ages (3 days, 28 days and 90 days old ash) to establish the change in hydraulic properties and porosity over time. Infiltration tests will also be conducted on the existing ash dumps in the field and results will be compared to that of the laboratory tests. Conceptual models will then be generated from a combination of the laboratory and field results. The study is still in progress, but the literature review suggests that the possible outcomes are: 1) hydraulic conductivity of the fly ash will be lower than that of the backfill spoils and is expected to further decrease over time, therefore acting as a barrier to the movement of groundwater, 2) general groundwater levels within the backfill are expected to rise; resulting in the decrease of the unsaturated zone and therefore limits oxygen exposure to backfill spoils, and (3) the alkaline nature of fly ash might potentially neutralize acidic levels of AMD. Fly ash, when disposed as backfill into opencast coal mines, might aid in the mitigation/prevention of AMD formation.