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• Conversations with mine personnel did provide one event in No. I Mine <br />which can be used to gain an insight into the distribution of subsidence <br />effects within the overburden section. The particular event Involved <br />mining of the B-seam. which lies 40 ft. to 60 ft. above the A-seam. <br />sometime after the A-seam was mined. <br />Normal operating procedure was to mine out the B-seam first. prior to <br />mining out the lower A-seam. All mining utilized full extraction room <br />and pillar techniques. However, because of operational problems, there <br />was one panel where mining in the 6-seen was delayed. To avoid causing <br />delays in mining of the lower A-seam. management decided to 'write-off" <br />the B-seam panel. As a result. the A-seam panel was mined-out and the <br />• roof allowed to cave. <br />Sometime later. mining the 1n B-seen approached that particular panel <br />area where the A-seam had been mined out. Having encountered no opera- <br />tional problems, mining was advanced in to the panel area and the coal <br />was mined out with full extraction roam and pillar mining. It was indi- <br />cated that powder consumption 1n the particular B-seam panel was about <br />one-half of normal. <br />In this panel area the tnterburden between the A-seam and the B-seam was <br />approximately 50 ft. Therefore, this previous mining experience indi- <br />cates that the zone of caving and heavy fracturing due to caving will <br />• most probably not extend more than 50 ft. above the coal seam being <br />mined. <br />13 <br />