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where room-and-pillar mining is planned, are considered to <br />be areas where significant subsidence will occur. <br />In this scheme, protective pillars such as safety <br />pillars or pillars along the mined-out panels were assumed <br />to act in a similar way to areas of entries of the mains and <br />submains where the room-and-pillar mining method with aloes <br />extraction ratio (usually lower than 50 %) will be applied. <br />These zones are assumed to be zones of limited subsidence. <br />The zone of the outccop barrier pillar along the <br />southern rim of the lease area has been classified as a <br />different unit. The problems of the outcrop barrier pillar <br />which will be left between the mine and the outcrop of the <br />coal seam on the southern rim of the Mesa are complicated <br />and they will be dealt with separately in Section 5. 7. <br />The subsidence analyses require the knowledge of the <br />mining height. The mining height was estimated with the help <br />of the D-seam thic}:Hess contours prepared by CWI. For <br />practical purposes, the mining heights of 9 and 10 feet were <br />assumed. The areas of these two basic heights are shown on <br />Plate 2. <br />• 9.4 EXISTING SUBSIDENCE MONITORING <br />CWI currently operates two monitoring systems on their <br />lease, as shown on Plates 2 and 10. <br />A widely spaced system covers the current mining area <br />on a uniform grid spaced on approximately 1,300 foot centers <br />east-west and 2,600 foot centers north-south. This system <br />will be extended to cover the entire lease as mining <br />progresses. <br />The monitoring stations in the vicinity of any mining <br />activity are surveyed quarterly. The system provides <br />valuable quantitative data on total subsidence but is too <br />widely spaced to provide specific data on draw angle. <br />A second monitoring system was installed to provide <br />detailed monitoring information during the mining of the 5 <br />north panel and associated development. This system was <br />developed in cooperation with and approved in principle by <br />F1LRD. Closely spaced stations on 125 foot centers located <br />along and perpendicular to the centerline of 5 north panel <br />are designed to enable accurate measurement of draw angle <br />and subsidence. The panel is t}~pical of pane] s in the <br />Grchard Valley Mine in that it is oriented apnro~:imately <br />• <br />- 24 - <br />