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2004 Geologic Hazard Field Observations <br />South of Divide Mining Area <br /> <br />5.0 GEOLOGIC FACTORS INFLUENCING SUBSIDENCE <br />The local geology may influence mining and subsidence in the following ways (this is a brief <br />summary; see Exhibit 60B, dated Apri12004, for further details). <br />Geologic structure: 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. In <br />relatively flat-lying, unfaulted coal seams, such as the South of Divide Permit Revision Area, the <br />most efficient longwall panel layout and extraction method can be utilized to recover a maximum <br />amount of the coal resource with a minimum amount of impact. <br />Strength and behavioral properties of the rocks: Strong, brittle rocks, such as sandstones and <br />siltstones commonly break and cave in lazge blocks-thus 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. Smaller caved fragments tend to decrease the <br />bulking factor and increase the height of caving <br />Stratigraphic sequence: The stratigraphic sequence of the rocks (distribution of rock types), for <br />example, controls the bulking factor and height of caving, as discussed in 2. In the South of <br />Divide Permit Revision Area, the first 200 to 300 feet of rock above the E Seam consists <br />primarily of siltstone, shale, claystone, locally lenticulaz sandstone, and coal seams. Therefore <br />the height of caving maybe increased slightly, compazed to the Apache Rocks and Box Canyon <br />mining areas. <br />Moisture content: Wet or saturated rocks in the mine roof and overburden tend to reduce the <br />r1 <br />L~ <br />bullring factor of the caved roof rocks, compazed to equivalent dry rocks. The amount of <br />subsidence may therefore be greater where the caned rocks aze saturated and less where they aze <br />~'S'• <br />831-032.621 Wright Water Engineers, Inc. Page 8 <br />November 2004 <br />