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• 2.05.6(6) <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 maydevelop 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 />A table at the end of this section presents a listing of springs ponds and <br />water rights. The table presents the elevation of each feature and the <br />estimated depth each lies above the coal seam to be mined. Generally no <br />subsidence protection will be provided for springs unless they are a decreed <br />water right and only then if the spring has an overburden depth of less than <br />800 feet. Most of the springs and ponds within the permit boundary and <br />adjacent area have overburden depths which exceed 800 feet. <br />A limited extraction zone as referenced below is defined as first mining only <br />with no pillar slabbing. The proposed location of the limited extraction zones <br />are presented on Map 27, Subsidence. <br /> <br />PR-04 2.05 - 107 - 01/01 <br />