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areas of large overburden cover, where barrier pillars <br />between panels can be expected to fail completely. At <br />intermediate overburden heights, where only partial yielding <br />of these pillars occurs, the amount of subsidence will be <br />reduced depending on the amount of pillar yielding, and <br />areas of reduced subsidence immediately above the pillars <br />will become more pronounced. At low overburden heights, <br />distinct troughs will occur over individual panels. The <br />maximum subsidence over individual panels at low overburden <br />heights will vary depending on the panel width, panel <br />length, and overburden height, but may approach the maximum <br />possible subsidence (Sm x = 7 feet for a 10 foot mining <br />height, 5.5 fzet for an ~ foot mining height) if the panel <br />width approaches the critical width. Subsidence over barrier <br />pillars between panels may become quite small at small <br />overburden heights. These resulting troughs and ridges have <br />been neglected in the development of the generalized plot of <br />subsidence contours. <br />There is same evidence that the subsidence factor may <br />decrease at low overburden heights. Available evidence in <br />this regard is contradictory, and no applicable quantitative <br />data are available. The possibility of this effect has not <br />been accounted for in the subsidence contour map. This may <br />result in some overestimation of subsidence at low <br />• overburdens. <br />Subsidence contours are based on subsidence profiles <br />developed using SEH methods. Experience with western U.S. <br />coals suggests that, in practice, the subsidence profile may <br />be somewhat sharper than that predicted. This would result <br />in subsidence contours being crowded more closely together <br />around the perimeters of extraction zones. The limit of draw <br />shown on Plate 14 is taken from our estimates of draw angle <br />as presented in Section 5.4 rather than that implied by the <br />SEH subsidence profiles. <br />The subsidence contours have been developed assuming <br />that the pillars within the mains and submains will be left <br />permanently in place. This has resulted in little subsidence <br />being predicted over these areas. Zf the coal from these <br />areas can bz fully recovered, the overlying ground surface <br />can he expected to subside by an amount similar to the <br />maximum subsidence which occurs in adjacent extraction <br />areas. <br />• - 36- <br />