The Effect of Surface Topography on the Stability of Coal Mine Openings

MOLINDA, GREGORY M., U.S. Bureau of Mines, Pittsburgh, PA

Coal mine roof quality has long been observed to appeared to deteriorate when mining progressed beneath surface valleys. A survey of seven Appalachian coal mines shows a clear correlation between mining beneath stream valleys and hazardous roof conditions. Of all bad-top occurrences in the surveyed mines, 52% of them happened directly beneath the bottom-most part of a valley.

This relationship can be due to a number of associations ranging from structural control of drainage to stress concentrations beneath the valley apex. BuMines research has recently produced evidence of valley stress relief as a significant mechanism for the production of poor rock mass quality beneath stream valleys. Bedding plane slips and thrust faults, due to horizontal compression perpendicular to the valley trend, have been observed beneath a number of valleys. This "valley effect" may be due to transfer of load from the adjacent hillsides or relief of regional stress in the valley due to low confinement. Horizontal stress measurements in a coal mine roof beneath a valley with poor roof conditions confirms partial stress relief.

Additionally, numerical modeling of a number of valleys over a single mine property calculates the "valley effect" on the stress field. The model also demonstrates the influence of depth of cover and orientation of the valley to the direction of principal stress on the likelihood of shear failure in the roof.

AAPG Search and Discovery Article #91005 © 1991 Eastern Section Meeting, Pittsburgh, Pennsylvania, September 8-10, 1991 (2009)