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• <br />Spring 2011 Subsidence and Geologic Field Observations <br />Box Canyon, Apache Rocks, and South of Divide Mining Areas <br />2.0 FIELD RECOGNITION OF SUBSIDENCE AND NON - SUBSIDENCE <br />FEATURES IN THE WEST ELK MINE AREA <br />Four different types of cracks have been observed in the West Elk Mine area: 1) subsidence <br />cracks and compression features, 2) construction cracks, 3) desiccation cracks, and 4) gravity - <br />induced tension cracks. Gravity- induced tension cracks can be distinguished easily in some cases <br />(e.g., where no mining has occurred in the area). In other areas they may be difficult to <br />distinguish, such as in areas that have been mined and conditions are also favorable for <br />construction, desiccation, or subsidence cracks and compression features. A more detailed <br />discussion of each of these crack types is provided below. <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 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 the <br />compression zone. <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 cracks that are located above the longwall mining face have a dynamic history. <br />They open when the longwall face moves beneath a particular crack area, and then close again <br />when the longwall face moves out of the area of mining influence. <br />Longitudinal cracks occur above, and roughly parallel to, the edges of the longwall mining panel <br />• above the gate road pillars (Figure 1). Longitudinal cracks commonly remain open, particularly <br />831 - 032.793 <br />September 2011 <br />Wright Water Engineers, Inc. Page 4 <br />