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How can backfilling mined-out areas reduce surface subsidence?

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coal mining Schematic diagram of an underground coal mine, showing surface facilities, access shafts, and room-and-pillar and longwall mining methods.

In coal mining, surface subsidence occurs when the collapse of overlying strata during extraction travels to the surface. Backfilling is a technique used to mitigate this effect by filling the void created by mining with materials such as sand, fly ash, or waste from coal-preparation plants.12

By filling these voids, the backfill material provides structural support that prevents the overlying strata from collapsing into the mined-out area. In the context of thick-seam mining, where a seam is extracted in multiple slices, backfill can also serve as an artificial floor or roof for subsequent slices, further stabilizing the area. While this method is effective for controlling subsidence and managing roof conditions, it is generally more expensive than allowing the roof to cave, which is the preferred practice in many operations where surface impact is not a primary concern.23