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<br />case 80 ft. The minimum 1200-ft depth precludes the possibility of chimney <br />subsidence of the surface. <br />SUBSIDENCE CONTROLS <br />The magnitude of surface subsidence effects over longwall panels is <br />controlled by the mining height, the panel width, the depth, whether rigid <br />or yielding gateroad pillars are to be used and whether the ground has been <br />previously subsided. In the case of room and pillar mining the minimum <br />width of the pillars and the percent extraction are additional controlling <br />factors. <br />The dip of a coal seam can effect subsidence. However, seam dip does <br />• not effect the magnitude of subsidence effects, only their location. The <br />dip of the Cameo B seam is insignificant, less than 5°. <br />The slope of the ground surface effects the magnitude of surface strains. <br />The strain adjustment necessary in mountainous terrain (NCB, 1980, p. 29-31) <br />in a function of the maximum subsidence induced tilt. Since the maximum <br />mining induced tilt of the surface in the affected area does not exceed 1° <br />this correction is minor. The strain correction (x) is; <br />x = COS °~ 1 - 1 <br />COS oc2 <br />°Cl - initial ground slope <br />oC2 - subsided ground slope <br />(1) <br />• <br />