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Subsidence and Geologic Field Observations <br />Apache Rocks and Box Canyon Mining Areas <br />July 12-15, 2004 <br />2.0 FIELD RECOGNITION OF SUBSIDENCE AND NON-SUBSIDENCE <br />FEATURES IN THE WEST ELK MINING AREA <br />Four different types of cracks are 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 />(the gravity-induced tension cracks were called pseudo subsidence cracks in the 2002 report.). <br />Gravity-induced tension cracks can be distinguished easily in some cases-where, for example, <br />no mining has occurred in the azea. In other azeas they may be 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. This section, which was first discussed in the <br />2003 report, is presented-somewhat modified- again this yeaz in order to make this a stand- <br />alone report. <br />2.1 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 in rock mechanics terminology) occur (see Figures 5 to 7 on pages 14 and <br />15, Figures 13 and 13A on page 26, and Figures 16 and 17 on page 30 of the 2003 report for <br />examples). As discussed in Exhibit 60A, cracks close-and the underlying rocks become <br />compressive-below the neutral surface of rock behaving as a beam or plate. Therefore, any <br />water located in surface cracks is blocked from traveling downwazd into rocks in 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 />831-032.840 Wright Water Engineers, Inc. Page 4 <br />September 2004 <br />