Measurement of sub-surface strata behaviour in bord and pillar mining: a case study

Ranigunj coalfield (India) has been facing significant problems due to subsidence as a result of underground mining of coal seams. The occurrence of the thick seams in close proximity, shallow depth, great variation in the nature of superincumbent strata from site to site, followed by the slow rate of extraction, has aggravated subsidence problems. In the recent past, many such workings have collapsed damaging a large number of surface structures.

In view of the aforesaid need, a comprehensive project financed by the Ministry of Energy (Coal), was undertaken by the Department of Mining Engineering, Indian Institute of Technology, Kharagpur, at a Colliery in Ranigunj Coalfield.

This investigation revealed that the caving horizon occurred at 4 times the extracted seam height, which differs from the findings in the U.K., U.S.A. and Australia. Also, it appears possible to estimate strata displacements from geological and physicomechanical information from the overlying rocks using the bending moment principle. Further, it was observed that the shallow workings could give rise to a “sink hole” type of subsidence, which is dangerous and could adversely affect the environment, safety of the public and properties.