Investigation of coal bumps in the Pocahontas No. 3 seam, Buchanan County, Virginia

Attempts to recover reserves of coal situated within barrier pillars in the Beatrice Mine using the room and pillar method have thus far been largely frustrated due to magnitude, in or near the working places shortly after commencement of retreat mining in a given area. Since pillar mining offers the opportunity for achieving relatively high production at a low cost per ton the development of methods for the detection and distressing of areas of high bump probability in advance of mining would significantly improve the competitive position of the deep mines operating in the Pocahontas No. 3 seam in Buchanan County, Virginia.

Unstable ground movements occurring within a panel of pillars removed using continuous mining equipment were detected using the microseismic monitoring system developed by the U. S. Bureau of Mines. Areas of high stress occurring within the panel were detected by a series of convergence points installed by the mine operator and by small diameter auger holes drilled into individual pillars. The highest levels of seismic activity were recorded during the mining of pillars which, though themselves suffering convergence to some extent, were situated inby areas in which greater amounts of closure had occurred. The presence of anomalous geologic structures in the roof strata outby, as well as the presence of pillars of irregular geometry, in close proximity to areas of high seismic occurrence was also noted.

Of the pillars probed using auger drilling it was determined, using criteria developed in Poland for evaluation of bump prone pillars, that two pillars in the next panel to be mined presented an extreme hazard, while several others were of intermediate hazard potential. The practice of injecting water into pillars through holes drilled well in advance of mining was found to be largely ineffective, though injection carried out immediately following drilling of the hole in a highly stressed pillar did induce rapid convergence for a period of approximately two weeks.