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2004 Geologic HazaN Field Observations <br />South of Divide Mining Area <br />• <br />6.0 FIELD RECOGNITION OF SUBSibENCE AND NON~UBSIDENCE <br />FEATURES IN THE WEST ELK MINING AREA <br />Four different types of cracks are observed in the West Elk mining area: 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. 1n other areas, they maybe difficult to distinguish, such as in <br />aeeas that have been mined but where conditions are also favorable for constmction, 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 occtn- in areas 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 downward into rocks in <br />compressron. <br /> <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 />wmmonly 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 particular crack area, and then close <br />again when the longwafl face moves out of the area of mining influence in this area. <br />Longitudinal cracks occur above, and roughly parallel to the edges of the longwafl mining panel <br />above the gate road pillars. Longitudinal tension cracks commonly remain open, particularly in <br />831-032.621 Wright Water Engineer:, Inc. Page 9 <br />November 2004 <br />