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2.05.6(6) <br />The subsidence implications of mining through the fault are summarized in <br />Exhibit 15. Inadequate roof support could result in a conical chimney <br />collapse which could breach 285 feet of overburden. Additionally, regardless <br />of the depth, when a fault is encountered and coal is mined beneath the fault <br />plane on the upthrow side of the fault, a step on the surface could result. As <br />explained in section 2.05.3(1), the Operator is committed to adequately <br />support the openings developed through the fault trace to avoid the <br />subsidence implications explained above. <br />Also when, during development of the panels near the outcrop, low quality <br />coal is encountered, one row of development pillars will be left in place to <br />prevent the potential crushing, sliding, or other failure near the outcrop. See <br />Exhibit 15, Section 1.7. Zones Along Outcrop Barrier Pillar. <br />As explained in Exhibit 15, Section 1.6, data from the Bowie No. 1 Mine and <br />field measurements of subsidence cracks in the Mesaverde Formation by <br />Dunrud (1976), indicate subsidence cracks may develop through overburden <br />• thicknesses of up to 800 feet under unfavorable conditions. While <br />unfavorable conditions can not be defined exactly, they may include zones <br />of weathered coal and overburden. Under normal conditions,. subsidence <br />cracks do not appear likely to propagate through more than 500 feet of <br />overburden. <br />The permit area has been segregated into three zones of expected <br />subsidence impact. The zone of greatest subsidence impact is in areas <br />where the overburden is between 110 and 500 feet. The zone of <br />intermediate subsidence impact is in areas where the overburden is between <br />500 and 1,000 feet. The zone of minor subsidence impact is in areas where <br />the overburden is over 1,000 feet. Map 27, Subsidence, presents the <br />locations of these three zones. The discussion in Volume III, Exhibit 15 <br />Subsidence concludes that under normal conditions, subsidence cracks do <br />not appear likely to propagate through more than 500 feet of overburden. <br />Overburden thicknesses over 800 feet have been classified as having a <br />negligible risk of surface fracturing developing. This is a conservative upper <br />limit under normal conditions. <br />• <br />TR-031 2.05 - 106 - 02104 <br />~ FpRovED <br />S ~u/ny <br />