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West Elk Mine <br />In addition, the lithology of the overburden rock may control the subsidence factor. The subsidence <br />factor may be less where the overburden contains a greater proportion of thick, strong sandstones, <br />and greater where the overburden contains thin, weak shales. In the current mining area, a unit that <br />may reduce the subsidence factor is the locally thick Lower and Upper Marine Sandstones that <br />underlie the D and E Seams. These sandstones aze about 100 feet thick in the eastern panel azea and <br />the eastern part of the western panels of the Apache Rocks mining azea; they aze approximately 100 <br />to 125 feet thick in the Box Canyon mining azea and the northwestern part of the current West Elk <br />Mine area. In the South of Divide mining azea, the first 200 to 300 feet of rocks above the E <br />Seam consist primarily of siltstones, shales, claystones, local lenticulaz sands, and coal seams. <br />Moisture content <br />Wet or saturated conditions in the mine roof and overburden tend to reduce the bulking factor of the <br />caved roof rocks. Therefore, the subsidence factor commonly is greater under wet conditions than it <br />is in dry conditions. In general, the greater the saturation of the mine roof and overburden rocks, the <br />greater the subsidence factor. <br />Field Recognition of Subsidence and Non-subsidence Features in the West Elk Mine Area <br />There aze four different types of features that have been observed in the West Elk mining azea: <br />(1) Subsidence cracks and bulges, (2) Constmction cracks, (3) Desiccation cracks, and (4) <br />Gravity-induced tension cracks. They can be distinguished easily in some areas where, for <br />example, no mining has occurred in that azea. In other areas they may be difficult to distinguish, <br />• such as in azeas that have been mined, but where conditions are also favorable for construction, <br />desiccation, or gravity-induced tension cracks to occur. <br />Subsidence Cracks and Comnression Features <br />Subsidence cracks aze open cracks that most likely occur in azeas 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. As discussed in Exhibit 60B, cracks <br />close-and the underlying rocks become compressive-below the neutral surface (the boundary <br />between tensile and compressive strain) of the rocks downwarping as a single unit. Therefore, any <br />water located in cracks above the neutral surface is blocked from traveling downward into rocks in <br />compression below the neutral surface. <br />Cracks in the zone of maximum tension occur approximately perpendicular to the orientation of <br />the longwall mining faces (transverse cracks) and pazallel to the orientation of the longwall <br />mining panels (longitudinal cracks). The cracks commonly do not conform to a precise pattern <br />and as with other deformational processes in nature, crack orientation may be quite variable. <br />The transverse tension cracks that locally occur above the longwall mining face often have a <br />dynamic history. They open when the longwall face moves beneath a particulaz area, and they <br />close again when the longwall face moves out of the azea of mining influence. <br />Longitudinal cracks occur above, and roughly pazallel to the edges of the longwall mining panel <br />above the gate road pillars and the haulageway (or beltway) pillars. Longitudinal tension cracks <br />commonly remain open, particulazly in azeas above gate roads with arigid-pillaz configuration. <br />2.05-/11 RevisedJwie 2005 PR/0, Rev. March 1006; May 2006 PR/0 <br />