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<br />6dribd 60C <br />Subsidence Evaluation and 2004 Geologic Hazard Field Otrservations <br />West Flatiron Lease Ala <br />areas above gate roads with arigid-pillaz configuration. The cracks may stay open or close in <br />areas above gate roads with a combination rigid-pillaz/yield-pillaz configuration <br />Compression features (bulges and warps) also occur above the longwall mining panels in areas <br />where the ground surface undergoes compression in the subsidence process. The compression <br />features, which occur toward the center of the mining panel in zones of maximum compression, <br />are usually more difficult to recognize. They often are masked, or absorbed, by soil and <br />colluvium, or are hidden in the brash and grass. They also may be indistinguishable from natural <br />humps and mounds in the soil and colluviurn. <br />6.2 Desiccation Cracks <br />Desiccation cracks tend to occur in claystones and siltstones of the Mesaverde and Wasatch <br />Fomrations in the West Elk mining area where the rocks are weathered to clays and silts. The <br />process of desiccation involves the shrinking of the clays and silts after a dry period that follows <br />a wet period, when the material swells (i.e., the shrink/swell process). <br />• <br />Desiccation cracks can often be recognized by their irregular, branching and diverging pattern- <br />less regular than typical subsidence cracks. Some of the largest desiccation cracks in the West <br />Elk mining area were observed in clays of the Barren Member of the Mesaverde Formation in the <br />Horse Gulch-Minnesota Reservoir area where there had been no mining. The largey more <br />regulaz desiccation eracks and construction cracks may be confused with subsidence cracks in <br />areas where mining has oceun~ed. However, transverse and longitudinal tension cracks caused by <br />subsidence have a defutite spatial relationship to the longwall mining panel causing the cracks. <br />6.3 Pseudo Subsidence Cracks (Gravity-Induced Tension Cracks] <br />Cracks have been observed on high, steep ridges and neaz cliffs, and in landslides, in the Box <br />Canyon and Apache Rocks mining areas. The cracks look very much like subsidence cracks, but <br />they cannot be, because no mining had been done in the area. The extensive crack on a narrow <br />ridge on West Flatiron, which was as much as 3.5 in wide and 150 feet long on August 27, 2002, <br />831-032.650 Wrght WaOer Engineers, Inc Page 8 <br />November 2004 <br />