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2005 Subsidence and Geologic Field Observations <br />Box Canyon, West Flatiron, Apache Rocks, <br />and Sauth of Divide Mining Areas <br />2.0 RECOGNITION OF SUBSIDENCE AND NON-SUBSIDENCE FEATURES <br />Four different types of cracks are observed in the West Elk Mining area: 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. They may be difficult to distinguish in other areas, such as <br />those that have been mined, but where conditions are also favorable for construction, desiccation, <br />or gravity-induced tension cracks also occur. <br />2.1 Subsidence Cracks and Compression Features <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 closes-and the underlying <br />rocks become compressive-below the neutral surface of rock behaving as a beam or pta[e. <br />' Therefore, any water located in surface cracks is blocked from traveling downwazd into rocks in <br />compression. <br />' Cracks in the zone of maximum tension are located roughly perpendiculaz 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 pazallel to the edges of the longwall mining panel <br />above the gate road pillars. Longitudinal tension cracks commonly remain open, particularly in <br />a31-032.710 Wright Water Engineers, Inc. Page 4 <br />November 2005 <br />