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III.A.7.b Subsidence Control. BME intends to plan and control the surface subsidence by employing the latest technology of ground control. Subsidence is anticipated and its magnitude and extent have been predicted. The magnitude, extent and duration will be minimized to the greatest extent possible by an efficient mine layout, barrier and chain pillars, and a rapid and effective mining system and sequence. Longwall mining will produce the most subsidence, but the subsidence will be (relatively) short-lived, lasting less than 5 years after mining. Room and pillar mining will minimize initial surface subsidence during room advances and lower the surface an almost undetectable 0.025 feet or less. However, wherever chain pillars are not extracted upon the retreat from a panel the so-called "permanent' pillars 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 nearly 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 to 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. Permit Renewal #3 (Rev, 8199) III-10