2013-11-13_PERMIT FILE - C1981018A (2)

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will deteriorate with time, and delayed subsidence incidents may result. Such unpredictable pillar failures have occurred over 100 years after mining. Longwall mining results in more subsidence than room and pillar mining principally because of greater extraction of the coal. Longwalling extracts Heady 100 percent of the coal within a longwall panel. Longwalling generally achieves an overall mine extraction in excess of 80 percent of the total resource. Room and pillar mining rarely recovers more than 55 percent of the total resource. Efficient pillar extraction, if feasible, may result in surface subsidence nearly equal in magnitude to that produced by longwall mining. Ninety-five to 98 percent of the subsidence resulting from longwall mining occurs during active mining. The remaining residual subsidence, 2 to 5 percent of the total, has been monitored for up to 4-1/2 years after cessation of mining. Long-term subsidence hazards are not expected with longwall mining since such hazards are all manifested in a fairly short time. Longwall mining at the Deserado Mine, as elsewhere, takes place where seam thicknesses are relatively uniform. Longwall equipment can adjust tp only a limited range of seam thicknesses. Room and pillar mining will be used where changes in seam thickness require more flexible mining equipment. • Subsidence is influenced by both the mining system and the geology. The lithology of the formation above a coal seam together with the bed thicknesses, bedding, cross-joint frequency and faults all control subsidence. Studies of the effect of faults on subsidence indicates that faults and fractures offer good sliding surfaces that can influence the angle of draw. A comprehensive geological investi- gation, including fault mapping and satellite imagery plotting, was conducted in the permit area (see geological section). This study failed to detect any faults or major fracture zones. The Staley Gordon Mine, abandoned in 1970, was mined using the room and pillar method. There is no detected subsidence above the abandoned mine although the overburden thickness is less than 200 feet and there are large caved areas in the abandoned mine. This indicates that the subsidence caused by small room and pillar sections will have minimal visual impact on the surface. However, the subsidence caused by the abandoned Staley Gordon Mine cannot be compared to the expected subsidence of the Deserado Mine because of its much larger scale of coal extraction. The highest recovery in caved pillar robbing areas of the Staley Gordon Mine was determined from the mine map to be approximately 68%. To calculate the maximum predicted surface subsidence and strain, a more conservative recovery of 75% was used. Using the actual depth of 300 feet at this location and a mining height of 6 feet, a maximum tensile strain of approximately 1700 micro-strains • Midterm Review (8/2002) III-l0