PERMFILE103730 - Laserfiche WebLink

Home Browse Search

West Elk Mine strong rocks than it is for soft, weak rocks. The greater bulking factor of strong, caved material commonly reduces the height of caving and the subsidence factor over soft, weak rocks. Conversely, the height of fracturing often is greater for strong, brittle rocks than it is for soft, weak rocks. Stratigraphic sequence The stratigraphic distribution of rock units (stratigraphic sequence) influences the effects of mining and subsidence. For example, strong and brittle sandstones in the mine roof, as discussed above, can reduce the height of caving compazed to shales, whereas sandstones in the fractured zone above the caved zone may increase the height of fracturing compared to shales. In addition, the lithology of the overburden rock may control the subsidence factor. The subsidence factor may be less where the overburden contains a greater proportion of thick, strong sandstones, and greater where the overburden contains thin, weak shales. In the current mining azea, a unit that may reduce the subsidence factor is the locally thick Lower and Upper Marine Sandstones that underlie the D and E Seams. These sandstones are about 100 feet thick in the eastern panel azea and the eastern part of the western panels of the Apache Rocks mining azea; they aze approximately 100 to 125 feet thick in the Box Canyon mining azea and the northwestern part of the current West Elk Mine area. In the South of Divide mining area, the first 200 to 300 feet of rocks above the E Seam consist primarily of siltstones, shales, claystones, local lenticular sands, and coal • seams. Moisture content Wet or saturated conditions in the mine roof and overburden tend to reduce the bulking factor of the caved roof rocks. Therefore, the subsidence factor commonly is greater under wet conditions than it is in dry conditions. In general, the greater the saturation of the mine roof and overburden rocks, the greater the subsidence factor. Field Recognition of Subsidence and Non-subsidence Features in the West Elk Mine Area There are four different types of features that have been observed in the West Elk mining area: (1) Subsidence cracks and bulges, (2) Construction cracks, (3) Desiccation cracks, and (4) Gravity-induced tension cracks. They can be distinguished easily in some azeas where, for example, no mining has occurred in that azea. In other areas they may be difficult to distinguish, such as in azeas that have been mined, but where conditions aze also favorable for construction, desiccation, or gravity-induced tension cracks to occur. Subsidence Cracks and Comuression Features Subsidence cracks are open cracks that most likely occur in areas where the ground surface has undergone extension during subsidence processes. Cracks as much as 3.5 • inches wide, for example, have been observed in sandstone outcrops at Apache Rocks 2.05-107 Revised November 1004 PRIO