Laserfiche WebLink
Fall 2007 Subsidence and Geologic Field Observations <br />Box Canyon, Apache Rocks, and South of Divide Mining Areas <br />• <br />• <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 />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 (e.g., where no mining <br />has occurred in the area). In other areas they maybe difficult to distinguish, such as in areas that <br />have been mined and conditions are also favorable for construction, desiccation, or gravity- <br />induced tension cracks. Amore detailed discussion of each of these crack types is provided <br />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 rocks in <br />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 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 />831-032.780 <br />November 2007 <br />Wright Water Engineers, Inc. <br />Page 4 <br />