Rock Mechanics Study of Shortwall Mining. Final Technical Report, 30 April 1979

摘要

This report describes a rock mechanics study performed in conjunction with a demonstration of the shortwall mining method at the Bethlehem Mining Corporation Hendrix Mine No. 22. There were three general areas of investigation: (1) determination of geologic structure and physical properties of the rocks; (2) measurement of rock stresses, including horizontal as well as vertical rock stresses; and (3) measurement of ground movement including surface and subsurface movements. In addition, response tests were performed on flat borehole pressure cells (BPC's) and on vibrating wire stressmeters which were used to determine changes in rock stress. In situ rock stresses were measured by the borehole deformation method. Ground movement was monitored by measurements of convergence, floor heave, and differential roof sag in development openings; by surface subsidence measurements; and by measurement of subsurface subsidence with multiple position borehole extensometers placed in boreholes drilled from the surface. Observations and conclusions were as follows: (1)the roof rocks, as well as the floor rocks, were very competent; (2) there was very good correspondence between beam theory and cracked-beam theory with the observed failure of the main and immediate roofs; (3) the main shale roof caved 90 feet up to the bottom of a thick, competent, sandstone bed, which deflected but did not cave; (4) the chain pillars between panels crushed during the mining of the third panel, and this factor, plus the presence of the overlying sandstone bed, resulted in a flat-bottomed, subsidence trough on the surface, which did not reflect the presence of the underlying chain pillars; and (5) the failed chain pillars acted as efficient strip packs, which limited subsidence to less than 45 percent of the coal thickness and took most of the overburden load. (ERA citation 05:002441)