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minimal subsidence of the overlying ground surface. Small pillars left for local roof support <br />will fail soon after the main pillar line retreats beyond them; they will not significantly delay <br />the rate at which surface subsidence occurs. For the case of partial extraction with yielding <br />permanent pillars, the rate of surface subsidence will correspond to the rate of yielding of <br />the pillars. <br />1.3 SUBSIDENCE MAGNITUDE AND PROFILE <br />The subsidence profile which results at ground surface from the mining of underlying <br />coal seams is a function of mining depth, the geometry of the extraction areas, the <br />character of the overlying rock mass, subsidence control measures taken and time. The <br />general nature of the subsidence profile and corresponding slopes of the profile are <br />illustrated on Figure 15 -1. The parameters used in subsidence prediction are also defined <br />on this figure. <br />Subsidence Magnitude <br />The maximum possible trough subsidence which can occur can be predicted in <br />areas of full extraction using the method developed by the British National Coal Board <br />(1975) based on the measurement of 187 subsidence profiles over 165 longwall coal <br />panels. The subsidence predictions for partially extracted room and pillar panels requires <br />modification of the NCB longwall method. <br />The predicted maximum subsidence value is a function of mining depth, mining <br />height, previously unsubsided ground and panel width. Because the previously mined King <br />Mine underlies a portion of the longwall panels the 0.9 virgin ground factor will not be used. <br />The following parameters exist; <br />D = Mining Depth = 1500 feet (max) <br />H = Mining Height = 9.5 feet (max) <br />W = Panel Width = 860 feet <br />VD = Virgin Ground Correction Factor = 0.9 <br />VB = Virgin Ground Correction Factor = 1.0 <br />PR -14 - 5 - 03/14 <br />