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9.2 DESCRIPTIVE THEORIES 287 <br /> 1.0 <br /> 0 CAVING Oa STRIP PACKING <br /> 0.s <br /> .�E <br /> 0.4 <br /> SOLID STOWING <br /> 0.2 <br /> 0 0.2 0.4 0.6 0.6 1.0 1.2 1.4 1.6 1.6 2A <br /> WIDTH/DEPTH <br /> I78. 9.2A Maximum possible subsidence versus type of support (12). Courtesy U.K. <br /> National Coal Board. <br /> of roof materials to be caved or indirectly the magnitude of surface <br /> subsidence. The typical figures fur maximum possible subsidence for <br /> caved,strip-packed and solid-stowed panels are 9M,85 to 809b,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 possible subsidence is <br /> proportional to the percentage of extraction above a threshold value, <br /> which was found to be 4001b in the central coal field (10). <br /> Seam /neJination <br /> The surface trough subsidence shown in Fig. 9.2.1 is induced by excava- <br /> tions 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 or, 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 /> horizontal seams. The angle of draw 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 9.2.5 also shows <br /> the variation of angle of draw with the dip angle a, as proposed by the <br /> U.K. National Coal Board (12). <br /> Tim <br /> Field measurements have indicated that both instantaneous and time- <br /> dependent subsidence are associated with underground excavations, but <br />