A quick and useful explanation of longwall mining and associated subsidence is to be found in two four-page information sheets put out by BHP Billiton. Seems the Illawarra Coal mine (Australia) will involve longwall mining and this may induce subsidence for apparently no backfilling of the 3.5-m high workings is proposed. I quote: “Surface subsidence commences once the coal is extracted from beneath and develops progressively as a wavefront as longwall mining proceeds. Surface subsidence from longwall mining does not occur as a sudden collapse. Approximately 90 percent of subsidence occurs over a period of 6 weeks. Generally, all significant subsidence from the mining of a longwall panel is complete within 12 months of mining.” To see more download these two sheets: