Hydraulic test results for internal stability testing on coal discard

This study investigated the internal stability and filter compatibility of coal discard obtained from a tailings storage facility in Mpumalanga Province, South Africa, to assess its suitability as a cost-effective filter material in a coal co-disposal facility. The research addresses the dual challenges of managing vast coal discard stockpiles and ensuring the structural integrity of co-disposal facilities through effective seepage control. Specifically, the study evaluated the geotechnical properties of coal discard, such as its particle size distribution, moisture content, permeability, and void ratio. It also examined the hydraulic behaviour of the coal discard material under a controlled upward seepage test in a rigid wall permeameter.

Laboratory experiments were conducted on disturbed coal discard samples prepared at three different void ratios (0.5, 0.6, and 0.8) to simulate various degrees of compaction. Hydraulic tests were performed at different hydraulic gradients to determine the critical conditions at which internal erosion occurs, such as suffusion and particle migration. The experimental setup allowed for real-time pore pressure measurement, total head, and flow rate, while washed-out fines were collected periodically to quantify internal instability. Analytical methods, such as the Kenney and Lau criterion were used to assess the grading stability of the coal discard based on the particle size distribution and the onset of fine particle migration.

The particle size distribution curve of the coal discard material was classified as well-graded sandy gravel, with high permeability and a broad range of particle sizes that contribute to its inherent filter compatibility. However, the study also revealed that the internal stability of coal discard is sensitive to variations in void ratio and hydraulic gradient, with significant particle migration observed beyond certain threshold conditions. These findings imply that while coal discard may be suitable for use as a filter material in co-disposal applications, its performance is dependent upon maintaining grading characteristics like those tested. It is recommended that for coal discard materials with particle size distributions substantially different from those considered in this study, a dedicated set of seepage column experiments be performed to reassess internal stability as a function of void ratio.

This research contributes to both academic understanding and practical engineering by providing a basis for the sustainable reuse of coal discard in coal co-disposal facilities. The improved filter compatibility and internal stability assessment not only enable a reduction in environmental impact due to waste disposal but also offer a cost-effective alternative to traditional granular filter materials in coal co-disposal facilities.