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<br />i~ <br />2.0 <br />Subsidence Field Observations <br />West Elk Mine <br />July 22-24, 2003 <br />FIELD DIFFERENTIATION OF CRACKS IN COAL MINING AREAS <br />There are four different types of cracks that can be observed in the West Elk Mining area: <br />subsidence cracks, construction cracks, desiccation cracks, and gravity-induced tension cracks. <br />(Note: The gravity-induced tension cracks were called pseudo subsidence cracks in the 2002 <br />report.) They can be distinguished quite easily in some cases; for example, where no mining has <br />occurred in the area. Conversely, cracks can be difficult to distinguish in areas that have been <br />mined, but where conditions aze also favorable for construction, desiccation, or gravity-induced <br />tension cracks to possibly be operative. <br />2.1 Subsidence Cracks <br />Subsidence cracks are open cracks that most likely occur in areas where the ground surface has <br />undergone extension during subsidence processes. Cracks up to 3.5 inches wide have been <br />observed in sandstone outcrops at Apache Rocks where zones of maximum extension (or tension <br />in rock mechanics terminology) occur (see Location 11 in this report for details). <br />Cracks in the zone of maximum tension occur approximately 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 is often quite variable. <br />The transverse tension cracks located above the longwall mining face have a dynamic history. <br />They open where [he longwall face moves beneath a particulaz azea and close again when the <br />longwall face moves out of the area of mining influence of this area. <br />Longitudinal cracks occur above, and roughly pazallel to the edges of the longwall mining panel <br />above the gate road pillars. Longitudinal tension cracks commonly remain open, particulazly in <br />areas above gate roads with arigid-pillaz configuration. The cracks may stay open or close in <br />831-032.610 Wright Water Engineers, Inc. Page 3 <br />September 2003 <br />