Laserfiche WebLink
2004 Geologic Hazard Field Observations <br />South of Divide Mining Area <br /> <br />6.0 FIELD RECOGNITION OF SUBSIDENCE AND NON-SUBSIDENCE <br />FEATURES IN THE WEST ELK MINING AREA <br />Four different types of cracks aze 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 />Gravity-induced tension cracks can be distinguished easily in some cases-where, for example, <br />no mining has occurred in the azea. In other aeeas, they maybe difficult to distinguish, such as in <br />areas that have been mined but where conditions aze also favorable for construction, desiccation, <br />or gravity-induced tension cracks to occur. <br />6.1 Subsidence Cracks and Compression Features <br />Subsidence cracks are open cracks that most likely occur in aeeas 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. 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 downwazd into rocks in <br />compression. <br />n <br />-~.I <br />Cracks in the zone of maximum tension aze 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 maybe 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 particulaz crack azea, 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 pillazs. Longitudinal tension cracks commonly remain open, particularly in <br />831-032.621 Wright Water Engineers, Inc. Page 9 <br />November 2004 <br />