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9.2 DESCRIPTIVE THEORIES 285 <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 pneumatic 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 seam depth. This <br /> seems to be true for the room-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 /> Width and Depth of Opening <br /> The width and depth of an underground opening are 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 ft <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 draw are drawn from P on both sides of a <br /> vertical dropped from P, they intersect the coal seam at points A and B. <br /> 8 <br /> 8 , <br /> « t <br /> o , • <br /> K , <br /> 4 , •� <br /> baIs <br /> • % <br /> 2 '� • a <br /> • • a <br /> At • • <br /> • <br /> 0 0.2 0.4 0.6 0.8 1.0 <br /> Subeldenct Factor, S/m <br /> 101g. 9.2.2 Subsidence factor versus depth of mining in Illinois coal fields. Different sym- <br /> bob indicate data from different mines. Data from Young(11). <br />