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2006 Subsidence and Geologic Field Observations <br />Box Canyon, West Flatiron, Apache Rocks, and South of Divide Mining Areas <br />2. The season for observing much of the area in detail is only about six months long given <br />winter access limitations. <br />2.0 FIELD RECOGNITION OF SUBSIDENCE AND NON-SUBSIDENCE <br />FEATURES IN THE WEST ELK MINE AREA <br />Four different types of cracks are observed in the West Elk Mine area: 1) subsidence cracks and <br />bulges, 2) construction cracks, 3) desiccation cracks, and 4) gravity-induced tension cracks. <br />Gravity-induced tension cracks can be distinguished easily in some cases-where, for example, <br />no mining has occurred in the area. In other areas they maybe difficult to distinguish, such as in <br />areas that have been mined, but where conditions are also Favorable for construction, desiccation, <br />or gravity-induced tension cracks to also occur. A more detailed discussion of each of these <br />crack types is provided below. <br />' 2.1 Subsidence Cracks and Compression Features <br /> <br />Subsidence cracks are open cracks that most likely occur in areas where the ground surface has <br />undergone extension during subsidence processes. Cracks as much as 4 inches wide, for <br />example, have been observed in sandstone outcrops at Apache Rocks where zones of maximum <br />extension (or tension in rock mechanics terminology) occur. Cracks close-and the underlying <br />rocks become compressive-below the neutral surface of rock behaving as a beam or plate. <br />Therefore, any water located in surface cracks is blocked from traveling downward into rocks in <br />' compression. <br />Cracks in the zone of maximum tension aze located roughly perpendicular to (transverse cracks) <br />and parallel to (longitudinal cracks) the orientation of the longwall mining panels. The cracks <br />commonly do not conform to such a precise pattern. As with other deformational processes in <br />nature, crack orientation maybe quite variable. <br />831-032.750 Wright WaterEngineers, Inc. Page 3 <br />December 2006 <br />