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f.3 oescwtmVg TH6ARIBS 3tl <br />and vanishes et 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 />•l underground opening, seam thickness, type of supports, seam inclination, <br />and time. Some studies, notably those in the U.S. western coal field (3, 6), <br />slwwal that the amount of subsidence depends on surt'ace topogrrphy 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 Faeton LsBuencing Subcldence <br />Atrg4 ~ Drotr <br />The angle of draw, denoted by (, it 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 43° depending on the coal field. Table <br />9.2.1 shows frequently cited angles of draw in different countries. There <br />arc indication that angle of draw may depend on depth, seam thickness, <br />end local geology, especially mr4jor faults or fracture planes or the loca- <br />tion of self-supporting strata above the coal seam. <br />Ssas Thicknru <br />The stun thickness of the excavated seam affects the magnitude of <br />surface subsidence. Obviously, if overything else is equal, the thicker the <br />Trbk f.3.1 Commonly Clted Males d <br />Draw <br /> <br />Country Angle of draw <br />(deg) <br />Nctherlandr 35-43 <br />Germany 30-5 <br />Northam France 3S <br />Sovia Union 30 <br />Oren Britain 23-33 <br />U.S.A. <br />Eastern (9) IS-27 <br />Central (10) 0-83 <br />Western (3, 61 12-16.0 <br />