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• conservative, estimate of subsidence. <br />The maximum possible subsidence which will occur in any <br />extraction zone, Sma is determined by multiplying the <br />mining height by tie subsidence factor. Assuming a <br />subsidence factor of 0.7, the maximum possible subsidence is <br />5.6 feet for an B foot extraction height and 7.0 feet for a <br />10 foot extraction height. Subsidence of this magnitude is <br />predicted only when the panel width exceeds the critical <br />width, the total length of the panel exceeds 1.9 times the <br />overburden depth, and there is total extraction with no <br />pillars left. <br />The maximum subsidence occurring over a single panel, <br />S, after all pillars have been extracted, can be predicted <br />using the relationship of subsidence to width and depth <br />presented in SEH. For 900 foot (122 m) wide panels, the <br />maximum subsidence predicted for varying overburden depths <br />is presented on Plate 12. The maximum subsidence over a <br />single panel (S) equals the maximum possible subsidence <br />(Smax) only when the width of the panel is super critical. <br />Maximum Subsidence Over Mined Areas <br />• The maximum subsidence over mined areas (S) would equal <br />the maximum possible subsidence (Smax - ~.0 feet) if the <br />total width of the extracted zone exceeded the critical <br />width, the length of the extracted zone exceeded 1.9 times <br />the overburden height, and no pillars were left between <br />adjacent panels. Such a case may not arise at the proposed <br />mine. <br />In practice, barrier pillars averaging 60 feet wide <br />will be left between all panels. The effect these have on <br />the magnitude of the maximum subsidence and on the <br />subsidence profile will depend on the overburden depth. <br />At shallow overburden depths, these pillars will <br />probably not fail and will cause a distinctly cyclical <br />subsidence profile to develop. The maximum subsidence <br />between the pillars will equal the maximum possible <br />subsidence if the width of t}ie extracted panels exceeds the <br />critical width. <br />At greater overburden depths, compression and failure <br />of the barrier pillars will become significant, while <br />concurrently the ratio of the panel width to the overburden <br />height decreases. As a resul=, the subsidence profile will <br />become morn uniform with increasing overburden depths. <br />-3''- <br />