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• Upper bound and lower bound estimates of subsidence for <br />eight foot and ten foot extraction heights are plotted on <br />Plate 13. Calculations based on Wilson's (1972) yielding <br />pillar design method indicate that failure of the barrier <br />pillars between mining panels will occur when the overburden <br />height reaches about 500 feet. At overburden heights of less <br />than 500 feet, subsidence would be better estimated by <br />superposition of subsidence profiles from adjacent panels. <br />In practice, the bottom of the subsidence trough will <br />be uneven to some extent, with humps present in the profile <br />immediately above pillars. The actual profile could be <br />estimated by superposition of subsidence profiles of <br />individual panels if the pillars remained stable. However, <br />this method cannot be used where the pillars are likely to <br />col lapse (SEH, 1975, p. 22). <br />Actual subsidence at the proposed mine will probably <br />lie considerably closer to the conservative estimate than to <br />that predicted by the method of Abel and Lee because of the <br />method of extraction. For this reason, and to insure that <br />subsidence is not underestimated, the conservative values <br />have been used for all quantitative subsidence estimates. <br />• Subsidence Profile <br />The shape and size of the subsidence profile which <br />typically forms above an extracted seam depends on the <br />relationship between the width of the mined zone (w) and its <br />depth (h). All extras*_ion areas having the same ratio of <br />w/h result in the same maximum subsidence, S. Maximum sub- <br />sidence generally occurs over the center of the extracted <br />zone and will equal the maximum possible subsidence (Sm~x) <br />only if the extraction area exceeds a certain critical size <br />termed the critical area. For sub-critical extraction areas, <br />the maximum subsidence will be less than the maximum <br />possible subsidence. For super-critical extraction areas, <br />the subsidence equals the maximum possible subsidence over <br />the central portion of the subsidence profile for a width <br />equal to the amount by which the extraction width exceeds <br />the critical width. <br />The accuracy with which the subsidence profile can be <br />predicted is limited in mining regions where there has been <br />no previous measurement of subsidence profiles. The SEH <br />method can be used to obtain the general form of the curve, <br />but the profile obtained will be in error by an unknown <br />amount. <br />• - 34 - <br />