Pillar design and coal strength.
Proceedings: New Technology for Ground Control in Retreat Mining, Pittsburgh, PA: National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 97-133, IC 9446, 1997 Mar; :49-59
A comprehensive data base was created that includes more than 4,000 individual uniaxial compressive strength test results from more than 60 coal seams. These data were compared with 100 case studies of in-mine pillar performance from the Analysis of Retreat Mining Pillar Stability (ARMPS) data base. Statistical analysis found no correlation between the ARMPS stability factor of failed pillars and coal specimen strength. Pillar design was much more reliable when a uniform coal strength of 6.2 MPa (900 psi) was used in all case histories. The conclusion is that laboratory testing should not be used to determine coal strength for ARMPS. Other analyses provided evidence of why laboratory strength does not correlate with pillar strength. The data showed clearly that the "size effect" observed in laboratory testing is related to coal structure. The widely used Gaddy formula, which predicts a significant strength reduction as the specimen size is increased, was found to apply only to "blocky" coals. For friable coals, the size effect was much less pronounced, or even nonexistent. Laboratory tests do not account for large-scale discontinuities, such as roof and floor interfaces, which apparently have more effect on pillar strength than small-scale structure.
Mining-industry; Underground-mining; Accident-prevention; Mathematical-models; Coal-mining; Laboratory-testing
Proceedings: New Technology for Ground Control in Retreat Mining