HYDROGEOLOGY OF AN UNDERGROUND COAL MINE: EVIDENCE OF POST-FLOODING PILLAR FAILURE

After closure, underground coal mines beneath regional drainage will eventually fill with water which must be treated before discharging to the surface. The high average hydraulic conductivity typical of coal mines results in small changes in hydraulic head across individual mines. This study examines water-level hydrographs for a coal mine near Morgantown, WV USA which closed in 1995. Operation of a transfer (injection) pump (up to 7 x 10⁵ m³/month) into this mine results in spikes in hydraulic head in the deep portion of the mine reaching heads of > 50 m higher than the shallow portion of the mine where water levels are controlled by pumping. This suggests the presence of an intramine blockage which restricts water flow within the mine. This is despite a set of 10 parallel main entries each measuring 5 m wide and 2 m tall. Main entries are supported by large pillars of unmined coal and are not expected to fail. The blockage is proposed to be the result of pillar punching or floor heave rather than a roof collapse due to the observed low permeability of the blockage. A probable location for the blockage was identified with a large scale mine map and supported with a groundwater flow model (MODFLOW). Such blockages restricting flow across closed mines are potentially problematic for water management after mine closure.