The consequences of subsidence become more serious as increasing requirements for mineral products conflict with the needs of an expanding population for surface land area. As a means of controlling subsidence in urban undermined areas, the Bureau of Mines has developed the pumped-slurry process for backfilling underground mine workings that are inaccessible because of flooding, gas accumulation, or caving. In a mixing tank, fill materials are placed in suspension and conveyed in a closed system through a slurry pump, distribution pipeline, and injection borehole into subsurface mine voids. Materials used for fill include crushed waste rock left from mining and processing, fly ash, and sand. In many areas, the local mine pool provides adequate quantities of water. The objective of backfilling mine voids is to mitigate adverse environmental effects of underground mining. The potential for future subsidence is substantially diminished. If waste mine rock is used for fill material, unsightly waste piles are eliminated, restoring land for higher economic use. Removal of waste piles also eliminates bank fires caused by spontaneous combustion, noxious and poisonous gases, particulate matter, and sediment and acid drainage that may enter nearby streams and ponds.
Chap. 4 in Underground Mining Methods Handbook, Published by SME/AIME, 1982, PP. 210-226