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<br />allows the mine workings to be oriented without rib spelling problems arising. <br />i <br />In addition, the lack of ground water seepage allows the workings to be layed <br />without regard to drainage slopes. <br />The main structural consideration of the mine layout is the width of <br />the panels (number multiple entries) and the width of the entries, iA panel <br />width of 250 to 300 feet with five entries have proved satisfactory for devel- <br />opment and pillar extraction. This width allows for room development upon <br />retreat from the panel during pillar extraction. Stresses due to ovirburden <br />thickness are minimal compared to deeper operating coal mines. Maximum over- <br />I <br />burden cover does not exceed 300 feet at any point, and the stress field seems <br />to act in a vertical direction. No ground forces other than vertical hive been <br />observed by inspection during mining operations. The shallow overburden cover <br />would accommodate an increase in panel width or the number of of multiple <br />entries without undue stress problems. No major faults or slip planes criss- <br />cross the mine workings, thus allowing for an even greater degree of optional <br />orientation of the workings. <br />The width of the openings have been limited to twenty feet for control <br />of the immediate roof (the thickness of the roof to the top of the resin <br />bolting horizon). Thinly laminated shale and sandstone bedding comprise the <br />I <br />immediate roof which must be supported with bolts or timber post. Failure of <br />the rock occurs as thin slabs dropping away when the roof is undercut. This <br />failure can best be controlled by limiting the advance of depth of cut and by <br />maintaining entry widths to eighteen feet or less to a minimum of fourteen <br />feet. When roof conditions dictate fourteen feet of width, the pillar shape <br />must be changed to a parallelogram shape to allow restricted machinery cutting <br />cross-cuts. <br />SURFACE SUBSIDENCE <br /> Since all coal extraction is underground, the only possible surface <br /> disturbances due to mining result from ground subsidence. Visual inspection of <br /> surface may subside uniformally above a mined-out area without noticeable <br /> visual effects that can only be determined by a precision survey of the sur- <br /> face. Subsidence may be of the order on only fractions of an inch.. Maximum <br />surface subsidence cannot exceed the thickness of the coal removed. <br />