Laserfiche WebLink
Fall 2009 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 are observed in the West Elk Mine area: 1) subsidence cracks and <br />compression features, 2) construction cracks, 3) desiccation cracks, and 4) gravity-induced <br />tension cracks. Gravity-induced tension cracks can be distinguished easily in some cases (e.g., <br />where no mining has occurred in the area). In other areas they may be difficult to distinguish, <br />such as in areas that have been mined and conditions are also favorable for construction, <br />desiccation, or subsidence cracks and compression features. A more detailed discussion of each <br />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 />831-032.791 Wright Water Engineers, Inc. Page 5 <br />April 2010