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He concludes that, <br />"Differential movement on the fault plane can be caused by the first workings <br />near the fault. If the coal is only taken from one side of the fault the angle of <br />draw can be greatly reduced, but with workings in more than one position the <br />normal angle of draw is likely to be observed. Once differential movement <br />has occurred, further workings in the area cause renewed movement which <br />is sometimes out of all proportion to the thickness and extent of the <br />extraction. The likelihood of movement is not influenced by the depth of the <br />seam but mainly by the position of the face." <br />He also observed that when the coal is mined out close to the fault at mine level, the <br />step on the surface will extend over the full width and not far past the extracted area. And <br />the prediction of the step height should only be done when the workings are underneath <br />the fault plane and on the upthrow side of the fault. <br />From this analysis it could be postulated that in the Bowie #2 Mine, regardless of the <br />depth, when a fault is encountered and coal is mined beneath the fault plane on the <br />upthrow side of the fault a step on the surface could result with a maximum height of <br />approximately (0.33) X 4.9'= 1.6' extending over and slightly past the retreated room and <br />pillar workings. One could also speculate that the rate of subsidence would be accelerated <br />due to the increased vertical pillar strain on development pillars. <br />The First North Mains mined into the fault in late 1997 or early 1998. The fault is <br />characterized as a shear fault as opposed to a roll fault and little or no goaf was <br />encountered. The fault trace was virtually dry. <br />The rock entries and raises should be supported to assure any debris within the fault <br />plane is supported in place and not allowed to cave or flow into the mine. Roof support <br />may include additional bolting, steel sets, steel sets with lagging and fill above the steel <br />sets to assure fault debris is supported and contained. <br />1.9 LONG TERM PILLAR STABILITY <br />The long term stability of the pillars in the bleeder entries in the Bowie No. 2 Mine was <br />evaluated using the Salamon and Munro pillar strength equation. To assure long term <br />PR -14 -20- 03/14 <br />