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298 SURFACE:SUBSIDENCE <br /> ORINGINAL SURFACE LEVEL <br /> 7', : <br /> g f=V V <br /> GOAF SEAM <br /> LEGEND: <br /> PSUBSIDENCE +:EXTENSION <br /> B+SUBSIDENCE AT CONTRACTION <br /> BOTTOM ¢;LIMIT ANGLE <br /> V:DISPLACEMENT <br /> Fls. 9.4.1 Horizontal and vertical components of ground movement (8). Courtesy U.K. <br /> National Coal Board. <br /> A graphical method of predicting the complete subsidence profile was <br /> developed by the U.K. National Coal Board(12)based on a large number <br /> of field observations. For example, the subsidence profile for a longwall <br /> face w meters wide in a scam in meters thick at a depth ul'h meters below <br /> the surface can be determined from Fig. 9.4.2 as follows: <br /> For w - 160, h - 400, and m = 1.4, <br /> Shn - 0.37 <br /> S = 0.37m <br /> - 0.37 x 1.4 <br /> - 0.518 (m) <br /> where S is the maximum subsidence, directly above the center of the <br /> opening.The determination of the subsidence profile is best carried out in <br /> a table like Table 9.4.1. Row 1 in the table lists steps of ratio of local <br /> subsidence to maximum subsidence (s/.Y) between 0 for the subsidence <br /> edge and I for the center point. The number and interval of steps are <br /> arbitrary. Multiply each step in row I by.S = 0.518 to obtain now 2;locate <br /> distance value d/h (where d is distance from the center of the panel) for <br /> w/h =0.4 from Table 9.4.2 for row 3;then multiply values in row 3 by h = <br /> 400 to obtain the actual distance from the center of the panel for row 4. <br /> Row 2 lists actual subsidences for points listed in corresponding columns <br /> of row 4. The final subsidence profile is plotted in Fig. 9.4.3. The subsi- <br /> dence profile predicted by this method usually comes within ±10'%of the <br /> actual field measurements. <br />