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App. II-B-1 <br /> dikes also cross the main slopes and are thought to <br /> be the feathering remains of the thick (30-foot) dike <br /> crossed in the 2 and 3 south sections of the No. 1 <br /> mine and the main slopes of the No. 4 mine. <br /> A fault with a displacement of 8 to 12 feet up to the <br /> southwest, increasing to the southeast, was <br /> encountered at the bottom of the Main Slopes (No. <br /> 1) ... a significant roll-down in the coal bed preceded <br /> the fault. <br /> In 1986, a zone of small faults was encountered <br /> crossing the main slopes of#2 ....some roof control <br /> difficulties resulted, and an associated down-roll of <br /> the coal bed, with an effective dip increasing from 10 <br /> degrees to in excess of 15 degrees. <br /> An igneous dike was encountered in #2. It had <br /> bulged out in the coal substantially, measuring <br /> about 30 feet in the three entries that have crossed <br /> it. Coal marginal to the dike was highly stressed, <br /> and significant bouncing accompanied mining. <br /> The depth of the seams being mined, the nature of the surrounding rock, stress <br /> zones and methane are also significant in determining the progress of mining in <br /> Coal Basin. <br /> Depth of the mine ranges from 1,700 to 3000', making it the deepest coal mine in <br /> North America. The weight of the overlieing rock creates significant pressure and <br /> stress on the area being mined. Depth of cover is the most important factor which <br /> determines the type of mining method which can be utilized. In fact, only the <br /> introduction of longwall mining, which allows coal recovery at greater depths than is <br /> possible with other methods, such as room-and-pillar, allows for recovery of the <br /> coal reserves being mined today. <br /> The coal itself is very soft in both seams, having a Hardgrove grinding index of over <br /> 100,and a compressive strength in the range of 600 to 1100 psi. At the same time, <br /> the rock strata above and below the seams is very strong. The rock intervening <br /> between seams has a compressive strengths ranging from 8,500 to 24,900 psi. <br /> Pressures manifest in the weakest rock, namely the coal. <br /> The coal is also gas bearing, with an average methane content of about 570 cubic <br /> of methane per ton of coal in place, and non-permeable, so the gas is liberated <br /> only when the structure of the coal itself is broken, as occurs in mining, or when the <br /> coal itself fails. There is methane in the surrounding rock strata as well. <br /> 2 <br />