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fs uttacttltmve rttgowrs sss <br />the method of filling the gob. In solid stowing the gob is completely filled <br />with rock fragments. This can be accomplished by either the hydraulic or <br />the pnwmatic method. Strip packing is a method of partial stowing in <br />which the filling material is packed in strips with a minimum width of 3 ft. <br />In caving, the gob is left unfilled and the roof is free to cave. There are <br />some indications that the subsidence factor varies with xam depth. This <br />sccros to be lruc fur the nwm•and-pillar mining in the Illinois coal fields <br />(Fig. 9.2.2), for example. But in most cases the factor is treated as <br />independent of depth. <br />wrdur and o~pv, of opanins <br />The width and depth of an underground opening ere intimately related as <br />far as surface subsidence is concerned, because together they determine <br />the "critical area" or "area of influence." Consider a coal seam lying h tt <br />below the surface and a point P on the surface (Fig. 9.2.3). If lines inclined <br />at an angle equal to the angle of drew arc drawn from P on both sides of • <br />vertical dropped from P, they intersect the coal seam at points A and B. <br />o <br />M <br />4 <br />e <br />n <br />0 <br />0 <br />Wg. f.].t Sutwidenu factor renur dep~b or mining in Illinar coal IkWs. DiRuaol agar <br />bob ladieats Jars fmm dlQetent nuns. DWa from Young (II). <br /> <br />8ubaldanea Faetar, &m <br />