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3.0 SUBSIDENCE ZONING • <br />3.1 ZONES OF MINOR SUBSIDENCE IMPACT <br />No subsidence impact is expected in those areas where there is a thick overburden <br />cover. ,4n overburden thickness in excess of 1,000 feet has been selected to define areas <br />of low subsidence impact, as shown on Map 27. No areas within the present Bowie #2 <br />mine plan lie under more than 1,500 feet of overburden. These areas are limited to the <br />central part of the permit area, where the ground surface is generally flat above the <br />northerly dipping D-seam. <br />The amount of subsidence arising from the mining of individual panels, will be small, <br />typically less than 6.0 feet. Maximum strains at surface resulting from mining will generally <br />be small, 3,500 u-in/in or less. The development of surface cracks due to subsidence is <br />not anticipated. Damage, if any, should therefore considered negligible to appreciable. <br />3.2 ZONES OF INTERMEDIATE SUBSIDENCE IMPACT • <br />Zones with between 500 and 1,000 feet of overburden cover above the D-seam are <br />designated as zones of intermediate subsidence impact on Map 27. Subsidence <br />magnitudes, 8.5 feet, and strains, 12,000 to 4,000 u-in/in, are predicted for these zones. <br />Appreciable to severe damage may result to surface structures, if present, due to <br />subsidence. There is a risk that subsidence fractures may occur at the ground surface <br />where the overburden within these zones is thinner and the geological and mining <br />conditions are unfavorable. <br />3.3 ZONES OF GREATEST SUBSIDENCE IMPACT <br />Zones of greatest subsidence impact are shown on Map 27 as those zones having <br />less than 500 feet of overburden cover. Large strains, 60,000 to 12,000 u-in/in, are <br />predicted within these zones. This subsidence could result in severe damage to any <br />surface structures located within these zones. There is a moderate risk of subsidence <br />cracks occurring anywhere within the zones. Experience at the Bowie #1 Mine show that <br />these high risk zones include areas where the overburden is less than 500 feet thick, and • <br />- 28 - 10/00 <br />P 2-a y <br />