Subsidence misconceptions and myths
- GAI Consultants, Inc., Monroeville, PA (United States)
Subsidence due to coal mining is poorly understood by non-specialists. This has led to numerous misconceptions and myths based on limited observations and lack of knowledge. The three most common are: (1) Mine maps are inaccurate, (2) Deep mines are not a problem, and (3) If no subsidence has occurred for many years after mining, there is no risk of future subsidence. Maps are important during mining and most are carefully prepared. Future use to evaluate conditions at mine level often includes drilling to confirm what the map shows. The idea of a safe depth from subsidence is often based on the false premise that mining results in sufficient breakup of the overlying rock strata that bulking compensates for the coal extracted. The safe depth idea first appeared in the literature about 1880 and remained prevalent well into this century. Sadly, it is still encountered. The modem understanding of fragmentation of the immediate mine roof with the overlying beds sagging down on the broken roof rock was first described in 1900. With full extraction mining, either longwall or retreat room and pillar, surface subsidence occurs regardless of the depth of the mine. Subsidence over longwall mines at depths of 2000 feet can be 90 percent of the mined seam thickness. Numerous studies of undermined sites conclude that mining occurred many years ago and since no subsidence has occurred, there is no risk of future movement. This is true if sufficient coal pillars have been left to support the overlying strate. However, every year subsidence occurs over mines that have been closed for 100 years or more. In a study of subsidera incidents over the Pittsburgh Coal, the senior authors found that 50 percent of the incidents occurred above mines that had been closed for at least 50 years and 10 percent over mines, closed for at least 80 years.
- OSTI ID:
- 405570
- Report Number(s):
- CONF-9608121-; TRN: 96:005886-0011
- Resource Relation:
- Conference: 15. International conference on ground control in mining, Golden, CO (United States), 13-15 Aug 1996; Other Information: PBD: 1996; Related Information: Is Part Of Proceedings of the 15th international conference on ground control in mining; Ozdemir, L.; Hanna, K.; Haramy, K.Y. [eds.] [and others]; PB: 783 p.
- Country of Publication:
- United States
- Language:
- English
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