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Exhibit 60C <br />Subsidence Evaluation and 2004 Geologic Hazard Field Observations <br />West Flatiron Lease Area <br />r 1 <br />LJ <br />6.0 FIELD RECOGNITION OF SUBSIDENCE AND NON-SUBSIDENCE <br />FEATURES IN THE WEST ELK MINE AREA <br />Four different types of cracks aze observed in the West Elk mining azea: 1) subsidence cracks <br />and 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 may be difficult to distinguish, such as in <br />areas that have been mined, but where conditions aze also favorable for construction, desiccation, <br />or gravity-induced tension cracks to also occur. <br />6.1 Subsidence Cracks and Compression Features <br />Subsidence cracks aze open cracks that most likely occur in azeas where the ground surface has <br />undergone extension during subsidence processes. Cracks as much as 3.5 inches wide, for <br />• example, have been observed in sandstone outcrops at Apache Rocks where zones of maximum <br />extension (or tension) occur. As discussed in Section 5.3.2 of Exhibit 60, cracks close-and the <br />underlying rocks become compressive-below the neutral surface of rock behaving as a beam or <br />plate. Therefore, any water located in surface cracks is blocked from traveling downward into <br />rocks in compression. <br />Cracks in the zone of maximum tension are 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 may be quite variable. <br />The transverse tension cracks that are located above the longwall mining face have a dynamic <br />history. They open when the longwall face moves beneath a particular crack area, and then close <br />again when the longwall face moves out of the area of mining influence in this area. <br />Longitudinal cracks occur above, and roughly parallel to the edges of the longwall mining panel <br />• above the gate road pillars. Longitudinal tension cracks commonly remain open, particulazly in <br />831-032.650 Wright Water Engineers, Inc. Page 7 <br />November 2004 <br />