U.S. highwall operators have been seeking a method to maintain an optimum coal rib thickness throughout the entire depth of the entry, particularly in mines where the depth of penetration can be up to 400 m. In Appalachian coal mines, the typical coal rib thickness is 1 to 2 m, depending on the mechanical properties of the particular coal seam being mined. If the rib becomes too thin, the hazard of ground fall increases and with it the resulting risk of burying the mining equipment. Freeing a buried machine is an expensive process, places workers at risk, and produces no coal. Conversely, if the rib becomes too thick, the excess coal left behind is permanently unrecoverable and makes the rib thin for the next hold. Although the current typical alignment procedures utilize precision surveys, geological and mechanical forces during mining produce uncertainties in position nearly as large as the rib thickness itself at depths of 300 to 400 m despite the accuracy of the initial alignment.

   http://holmessafety.org/1998/OCT98.pdf (PDF, 1855 KB)

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