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App. II-8-1 <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 signficant 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 /> <br />