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Subsidence predictions have been approached from a conservative standpoint and actual <br />measured subsidence has always been less than predicted. Depth increases as the mine <br />progresses to the north and as depth increases, subsidence is decreased. As acknowledged by <br />DMG and OSM, subsidence of archaeological sites over LW-2 was found to not cause significant <br />impacts. The sites in lease COC-51551 were predicted to undergo less subsidence than the sites <br />over LW-2 and therefore are not expected to be significantly impacted. However, longwall mining <br />under these sites did not occur since this area became inaccessible when part of the D Seam was <br />sealed due to the fire. <br />III.A.8 Mine Ventilation A permanent fan is installed at the D-Portal ventilation slope. This fan <br />operates in a blowing configuration pushing fresh air into the mine from the Ventilation Entries and <br />through the working area. It operates continuously with the exception of scheduled maintenance <br />periods. MSHA approved the detailed plans for the ventilation of the underground working. Two <br />ventilation shafts were constructed in the B-Seam to exhaust return air. Details of the shaft <br />construction are given on Maps 151 and 155. The first raised bored shaft is about 210-feet deep <br />and 12-feet inside diameter. The second is 965-feet deep and 14-feet inside diameter. The shaft <br />openings are fenced off and elevated to control unauthorized access to MSHA standards. The <br />shaft sites are approximately 100'x200'. Alight use road about 2410 feet long was constructed <br />from the existing mine haul road to the first shaft site (see Map 151). A 1400 foot long light use <br />road extension from RDH-3 services the second shaft site. <br />Return shafts RS-1 and RS-2 were constructed to the B-Seam to allow return air to exhaust. RS-1 <br />is about 300 feet deep and RS-2 is about 308 feet deep. Both RS-1 and RS-2 are steel lined and <br />five foot diameter. RS-3 steel lined, six feet in diameter, and 310 feet deep. The upper and lower <br />20 feet of the annulus between the steel liner and the shafts are grout sealed with concrete. The <br />middle zones of the annulus are gravel filled with weep holes drilled in the liner to allow drainage. <br />A light duty access road, approximately 8500 feet long, was constructed to RS-1 and RS-2. RS-3 <br />requires a 1550 foot light use road. A 3-foot diameter, 362-foot deep ventilation shaft was drilled <br />to the West Mains in the D-Seam. Map 153 shows the details of RS-1 which are typical of the <br />other return shafts and roads. Locations are shown on Map 147. <br />During early 1987, gassy wnditions were encountered at the Deserado Mine. To relieve those <br />conditions, BME drilled a 12-1/4 inch hole (LW1-1) and six (6) 10-5/8 inch holes (LW1-2 to LW1-7) <br />into the gob created behind the protective shield of the longwall 1. In September, 1988, a vent <br />hole (LW1-8) was drilled to relieve methane gas in the sealed area of the longwall panel #1 to <br />improve ventilation in other working areas of the mine. In January 1990, an additional ten (10) <br />10-5/8 inch holes (LW2-1 to LW2-8, LW2-46 and LW2-6B) were drilled in the longwall #2 and <br />Technical Revision #&4 (4/2007) 111-26 <br />