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Superposition of subsidence from adjacent panels will <br />increase the total subsidence at any point so that it <br />exceeds that which would occur above a single sub-critical <br />panel. Flowever, the barrier pillars which remain between the <br />panels will prevent subsidence from ever reaching the <br />maximum possible subsidence. <br />The maximum subsidence which will occur when panel <br />widths are sub-critical will be a complex function of <br />overburden depths and pillar strengths. This makes it <br />difficult to predict maximum subsidences at intermediate <br />overburden depths. At large overburden depths, superposition <br />of subsidence profiles from adjacent panels and the failure <br />of barrier pillars between panels will result in a <br />relatively flat bottom of the subsidence trough. <br />A conservative (upper bound) estimate of the maximum <br />subsidence can be obtained by assuming that subsidence is <br />reduced from the maximum possible subsidence in proportion <br />to the amount of coal left in the barrier pillars. The <br />extraction system of 900 foot panels separated by 50 foot <br />barrier pillars at the proposed mine results in 13 0 of the <br />coal being left permanently .in place. Assuming a subsidence <br />factor of 0.7 for the total extraction case, the maximum <br />subsidence which is to be expected for .he proposed mine <br />• extraction system is 0.7 x 0.87 = 0.51 times the extracted <br />thickness. This results in subsidence of 4.9 feet for an 8 <br />foot extraction height and 6.1 feet for a 10 foot extraction <br />height. This maximum subsidence can be expected to occur <br />over sub-critical panel widths at about the overburden depth <br />where the barrier pillars between panels begin to fail. The <br />rate at which the maximum subsidence is achieved will de_oend <br />on the time taken for the pillars to fail. <br />A lower bound (uncor.servative) estimate of the maximum <br />subsidence can be obtained using the data presented by Abel <br />and Lee (1980). Their data is principally from measured <br />subsidence above room-and-pillar mines in w:,ich the pillars <br />failed, and cannot be expected to accurately predict <br />subsidence over wide panels separated by barrier pillars. In <br />room-and-pillar~oorkings, the pillars will be relatively <br />close together. When pillars fail, they will crush and <br />flatten until their (horizontal) cross-sectional area <br />increases sufficiently to increase their load carrying <br />capacity. This will generally prevent the roof and floor <br />from coming into contact, and so will reduce subsidence <br />compared to subsidence over wide panels. <br />• - 33 - <br />