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fa otscatrrtve rrteoxtFs <br />~~ <br />u~ <br />c~waa a snu r~cxna <br />solo srow~aa <br />xa <br />wiory/osna <br />Pls. f.l.s Maximum possible subsidence verses tYPe of WrPP~ (It). Courtesy U.R. <br />Nuioaal l:oal Bond. <br />of roof materials to be caved or indirectly the magnitude of surface <br />subsidence. 'fhc typical figures fur maximum poasiblc subsidcnca for <br />caved, strip•packed and solid-stowed panels are 90%, 85 to 803'0, and 45 to <br />40% of seam thickness, respectively (Fig. 9.2.4 and Table 9.2.2). In <br />room-and-pillar mining, however, the maximum poasiblc subsidence is <br />proportional to the percentage of extraction above a threshold value, <br />which was found to be 40% in the central coal field (IO). <br />Staar fadinatlon <br />The outface trough subsidence shown in Fig. 9.2.1 is induced by excava- <br />tiona in horizontal coal seams. For inclined seams, the surface subsidence <br />trough is displaced toward the deeper edge of the opening and, depending <br />on the inclination a, may be located outside the dip edge of the opening <br />(Fig. 9.2.5). Subsidence is maximum at the point normal to the center of <br />the opening, rather than directly over the center of the opening as in the <br />horiwntu! seams. The angle of titres or limit angle is no longer constant, <br />but depends on the dip angle of the seam; it is smallest at the rise edge of <br />the opening and increases toward the dip edge. Figure 9.2.3 also shown <br />the variation of angle of draw with the dip angle a, as proposed by the <br />U.K. National Coal Board (l2). <br />Tlau <br />Field measurements have indicated that both instantaneous and dme- <br />dependent subsidence are associated with underground excavations, but <br />