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West Elk Mrne <br />• The stratigraphic distribution of rock units (stratigraphic sequence) influences the effects of mining <br />and subsidence. For example, strong and brittle sandstones in the mine roof, as discussed above, <br />can reduce the height of caving compazed to shales, whereas sandstones in the fractured zone above <br />the caved zone may increase the height of fracturing compazed to shales. <br />In addition, the lithology of the overburden rock may control the subsidence factor. The <br />subsidence factor may be less where the overburden contains a greater proportion of thick, strong <br />sandstones, and greater where the overburden contains thin, weak shales. In the current <br />mining area, a unit that may reduce the subsidence factor is the locally thick Lower and <br />Upper Marine Sandstones that underlie the D and E Seams. These sandstones are about 100 <br />feet thick in the eastern panel area and the eastern part of the western panels of the Apache <br />Rocks mining area; they are approximately 100 to 125 feet thick in the Box Canyon mining <br />area and the northwestern part of the current West Elk Mine area. In the South of Divide <br />mining area, the first 200 to 300 feet of rocks above the E Seam consist primarily of <br />siltstones, shales, claystones, local lenticular sands, and coal seams. <br />Moisture content <br />Wet or saturated conditions in the mine roof and overburden tend to reduce the bulking <br />factor of the caved roof rocks. Therefore, the subsidence factor commonly is greater under <br />wet conditions than it is in dry conditions. In general, the greater the saturation of the mine <br />roof and overburden rocks, the 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 area: <br />(1) Subsidence cracks and bulges, (2) Construction cracks, (3) Desiccation cracks, and (4) <br />Gravity-induced tension cracks. They can be distinguished easily in some azeas 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 Compression Features <br />Subsidence cracks are open cracks that most likely occur in areas where the ground <br />surface has undergone extension during subsidence processes. Cracks as much as 3.5 <br />inches wide, for example, have been observed in sandstone outcrops at Apache Rocks <br />where zones of maximum extension (or tension in rock mechanics terminology) occur. As <br />discussed in Exhibit 60B, cracks close-and the underlying rocks become compressive- <br />below the neutral surface (the boundary between tensile and compressive strain) of the rocks <br />downwarping as a single unit. Therefore, any water located in cracks above the neutral <br />surface is blocked from traveling downward into rocks in compression below the neutral <br />surface. <br />Cracks in the zone of maximum tension occur approximately perpendicular to the <br />• orientation of the longwall mining faces (transverse cracks) and parallel to the orientation <br />of the longwall mining panels (longitudinal cracks). The cracks commonly do not conform <br />1.05-107 RevrsedJune 1005 PRt 0 <br />