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Rapid surface subsidence could occur over the mined <br />• area after abandonment if the workings caught fire and <br />permanent pillars burned. This should be prevented by <br />safeguarding against fires developing in the abandoned <br />workings. After abandonment, the workings should eventually <br />fill with water, which would preclude the possibility of <br />fire developing. <br />5.3 SUBSIDENCE MAGNITUDE AND PROFILE <br />The subsidence profile which results at ground surface <br />from the mining of underlying coal seams is a function of <br />mining depth, the geometry of the extraction areas, the <br />character of the overlying rock mass, subsidence control <br />measures taken, and time. The general nature of the <br />subsidence profile and corresponding slopes of the profile <br />are illustrated on Plate 11. The parameters used in <br />subsidence prediction are also defined on this plate. <br />Subsidence Magnitude <br />The maximum possible subsidence which can occur over an <br />extracted seam, Smax~ is given by the expression <br />• Smax = a m <br />where a = subsidence factor <br />m = seam thickness <br />The value of the subsidence factor depends on the nature of <br />the overlying rock and the method of packing (if any) after <br />mining. Since the subsidence factor is a measure of maximum <br />possible subsidence, it will apply for total extraction <br />where the extracted area is large relative to its depth <br />below surface. For small extraction areas or where the <br />extraction ratio is less than 100 0, such as where pillars <br />are left between panels, the amount of subsidence will be <br />reduced. The subsidence factor is usually assumed to be <br />independent of depth. <br />Brauner (1973) lists the values of subsidence factor <br />given in Table 1 for European coal fields. <br />- 30- <br />