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West Elk Mine <br />Geologic Factors Influencing Subsidence - 2.05.6 (6)(e)(i)(B) <br />Subsidence is influenced by the local geology in the following ways: <br />Geologic Structure <br />Attitude of the bedrock, faulting, and jointing may control mine layout and mining method. In <br />steeply dipping, faulted coal beds, for example, a certain mine layout and method, such as room - <br />and -pillar or limited panel -pillar may be required. Joints often control the way in which the roof <br />rocks break, cave, and fracture, both underground and at the surface during mining and subsidence. <br />In relatively flat -lying, unfaulted coal seams like the South of Divide and current mining area, <br />there is latitude to develop the most efficient layout and method to recover a maximum amount of <br />the coal resource with a minimum of impact. <br />Strength and behavioral properties of the rocks <br />These properties may control the amount and rate of subsidence. Strong, brittle sandstones and <br />siltstones tend to break and cave in large blocks on the mine floor. The bulking factor is greater for <br />strong rocks than it is for soft, weak rocks. The greater bulking factor of strong, caved material <br />commonly reduces the height of caving and the subsidence factor over soft, weak rocks. <br />Conversely, the height of fracturing often is greater for strong, brittle rocks than it is for soft, weak <br />rocks. <br />Stradwaphic sequence <br />The stratigraphic distribution of rock units (stratigraphic sequence) influences the effects of mining <br />and subsidence. For example, strong and brittle sandstones in the mine roof, as discussed above, <br />can reduce the height of caving compared to shales, whereas sandstones in the fractured zone above <br />the caved zone may increase the height of fracturing compared to shales. <br />2.05-120 Rev. 06/05- PR10, 03/06- PR10, 05/06- PRIO, 11/060- TR107, 04/07- 7R108, 09/07- PR12, 02/08- PR12; 11110-TR124 <br />