2008-01-16_APPLICATION CORRESPONDENCE - C2008086 (3)

Home Browse Search

DRAFT over 25°. Angles of draw were predicted at 25° at two coal mines in Colorado mining in the Mesaverde Group based on drillhole lithology. Later survey measurements indicated angles of draw of 21 ° and 22°. A 19° to 22° angle of draw is on the low end of the range of values reported for the countries listed on Table 6. Angles of Draw for Coal Mining in the United States and Europe. The British National Coal Board's (NCB) conservative 35° angle of draw has, however, been measured in Pennsylvania (Auchmuty, 1931). The larger NCB angle of draw estimate will be used because it should overestimate the area outside a longwall panel potentially affected by mining. In addition, the NCB maximum subsidence value (SmaX) calculated from the flatter English terrain measurements was 17% to 21 % greater than what was measured for ridge tops over three longwall panels in Mesaverde Group rocks and mountainous terrain at the York Canyon Mine west of Raton, New Mexico. NCB predicted subsidence in topographic lows were 55% greater than measured at the York Canyon Mine. This implies that the maximum tensile strain, compressive strain and tilt estimated using the NCB method may be similarly greater than what will be measured in the Project Area because the strains and tilt are directly proportional to the maximum panel subsidence (Sm~) value. 5.0 TOPOGRAPHIC FACTORS AFFECTING SUBSIDENCE 5.1 Rugged Terrain The Red Cliff Mine Project Area is located in canyon-ridge topography. As shown on Table 3, overall slope angles range from 21° to 41° (38% to 87%) for canyon walls ranging from 400 feet to 920 feet high. Cliff sections are present on some canyon walls where thicker sandstones outcrop. Because of this rugged terrain, subsidence related surface impacts may change several times as the overburden depth changes along the roughly 7,300-foot to 13,500-foot lengths of the longwall panels. Subsidence, strain and tilt predictions will be less certain than would be the case in more gentle and flatter terrain. For example, vertical displacement may be as much as 30 percent greater over narrow ridge tops. The overburden ahead of a moving longwall face will move down slope as the subsidence trough ahead of the longwall face approaches but will not be able to push uphill against gravity after the face passes. If the longwall panel subsequently advances under the ridge, that side of the ridge will displace down slope on that side of the ridge. In the course of extracting the underlying coal, a ridge with steep slopes in adjacent valleys will subside more than would be the case in flat terrain. Parts B and C of Figure 9. Localized Mining Induced Slope Angle Changes indicates how this will occur. The potentially additive subsidence on ridges will increase the tensile strain and the width of open surface cracking. Higher compression ridges, but negligible tensile fractures, are likely to occur in narrow valley bottoms, because the overburden on both sides will try to move toward the bottom of the valley as the subsidence trough approaches and then passes the valley bottom. Consequently subsidence impacts are likely to be greater on narrow ridges and lesser in narrow valley bottoms than they would be in more subdued terrain. Page 20 of 57