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<br />F~ibit 60C <br />Subsidence Evaluation and 2004 Geologic Hazard Field Observations <br />West Flatiron Lease Area <br />5.0 GEOLOGIC FACTORS INFLUENCING SUBSIDENCE <br />The local geology may influence mining and subsidence in the following ways (see F_xhibit 60 <br />for more details): <br />Geologic stractnre: The attitude (dip and strike) of the bedrock, faulting, jointing, and coal <br />cleat directions often control the general layout of the mine and the method of mining. <br />Strength and behavioral properties of the rocks: Strong, brittle rocks, such as sandstones and <br />sdtstones commonly break and cave in large blocks- fihus increasing the bulking factor and <br />decreasing the height of caving behind the longwall. Conversely weak, soft rocks, such as shale <br />and claystone, often cave in smaller fragments, which decreases the bulking factor and increases <br />the height of caving. <br />Stratigraphic sequence: The sttatigtaphic sequence of the rocks (distribution of rock types), for <br />example, controls the bulking factor and height of caving, as discussed above. <br />iVloistnre content: Wet or saturated rocks in the mine roof and overburden tend to reduce the <br />bulking factor of the caved roof rocks, compared to equivalent dry rocks. <br /> <br />831-032.650 Wright Water Engineers, Inc. Page 6 <br />Novrvnber20od <br />