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When is backfill used in thick-seam coal mining?

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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 thick-seam coal mining, backfilling is utilized when specific geological or safety conditions prevent the use of caving, which is otherwise the preferred practice.1

Specifically, backfill is employed when:
* Roof conditions require that there be no caving of the overlying strata.1
* The seam has a liability for spontaneous combustion, necessitating the elimination of voids.1
* Surface subsidence is not allowable, requiring the void to be filled with materials such as sand, fly ash, or waste from coal-preparation plants.2

When used in the extraction of thick seams divided into multiple slices, the backfill material serves a functional purpose by acting as an artificial floor or roof for the subsequent slice. While backfilling is practiced in many countries for technical and environmental reasons, it is generally more costly than caving methods.12