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9.2 DMRIPTIVB THWRI6.S 283 <br /> and vanishes at points c and f. The magnitude and extent of surface <br /> subsidence depend on the angle of draw, the depth and width of the <br /> underground opening,seam thickness,type of supports,seam inclination, <br /> and time. Some studies,notably those in the U.S. western coal field(5,6), <br /> showed that the amount of subsidence depends on surface topography in <br /> that the low area such as valley induce more subsidence than the <br /> neighboring hills. However, a similar trend has not been observed for the <br /> Eastern coal field (7). <br /> 9.2.2 Factors Influencing Subsidence <br /> Angle of Draw <br /> The angle of draw, denoted by;, is the angle between the vertical to the <br /> edge of the opening and a line connecting the edge of the opening and the <br /> point at the surface where subsidence diminishes to zero. Therefore, it <br /> defines the limit of subsidence with respect to a particular underground <br /> opening and is sometimes called the "limit angle." <br /> Angle of draw ranges from 8° to 45° depending on the coal field. Table <br /> 9.2.1 shows frequently cited angles of draw in different countries. There <br /> are indications that angle of draw may depend on depth, seam thickness, <br /> and local geology, especially major faults or fracture planes or the loca- <br /> tion of self-supporting strata above the coal seam. <br /> Sean Thickness <br /> The seam thickness of the excavated seam affects the magnitude of <br /> surface subsidence. Obviously, if everything else is equal, the thicker the <br /> 'fable 9.2.1 Commonly Cited And of <br /> Draw <br /> Angie of draw <br /> Country (deg) <br /> Netherlands 35-45 <br /> Germany 30-45 <br /> Northern France 35 <br /> Soviet Union 30 <br /> Great Britain 25-35 <br /> U.S.A. <br /> Eastern (9) 15-27 <br /> Central (10) 0-8.5 <br /> Western (5, 6) 12-16.0 <br />