2008-03-21_PERMIT FILE - C1980007A (5)

Home Browse Search

West Elk Mine In terrains with slopes less than about 30 percent, the depth of the neutral surface can be estimated by dividing the maximum horizontal strain values by the maximum curvature values at. a given location. The calculated depth of the possible tensile zone to the neutral surface, ie: the boundary between tension above and compression below-ranges from about 50 to 100 feet in the subsidence monitoring network at West Elk Mine. Crack depth may be much less than this projected 50 to 100 foot range of maximum values. An unpublished study for the U.S. Bureau of Mines (Engineers International) indicated that surface crack depth rarely is greater than about 50 feet. Cracks will also be less extensive or terminate where shale and claystone layers occur. Based on annual field subsidence observations, maximum crack depth in bedrock in the South of Divide muting area is estimated to be (1) 5 to 15 feet in terrain sloping less than, or equal to, 30 percent (2) 10 to 35 in terrain sloping more than 30 percent, and (3) 40 to 50 feet in thick, brittle sandstones in ridges (Exhibit 60B, Table 2). Crack depth will likely be at a maximum value above massive coal barriers. Crack depth may therefore be greatest above the 700-foot-wide protective barrier system projected between longwall panels E4 and E5 (Exhibit 60B, Figure 1). The crack depth is projected to be less (probably 10 to 20 percent less) above the panel chain pillars, where even the rigid pillars are predicted to yield 10 to 30 percent of the coal extraction thickness (Exhibit 60B, Table 2). Cracks that occur above the mine panel area also tend to close, once mining faces move out of the surface area of influence (DeGraff and Romesburg 1981). Any local bed separations during active subsidence between rocks of different strengths (Exhibit 60B, Figure 1) will likely close once equilibrium conditions occur. However, any cracks present above rigid chain pillars, barrier pillars, or mine boundaries may remain open where permanent tensile stresses remain after mining is completed due to the convex curvature of the subsidence profile. During the past nine years of annual observations in the West Elk mining area by Mr. Dunrud (from 1996 to 2004), particularly in the Apache Rocks mining area, no cracks were observed above mined-out longwall panels in colluvium more than an estimated ten feet thick. No cracks have been observed in alluvium above mined-out long`vall panels. No cracks were observed in the alluvium and colluvium of Sylvester Gulch and Deep Creek (estimated thickness range: 25 to 150 feet) during periodic field observations in the Apache Rocks and Box Canyon mining areas. The near-surface alluvial material consisted of primarily sand, silt, clay, and soil in the two areas mentioned, and was located above rigid pillars and panel boundaries-where the overburden depth ranges from 800 to 1,050 feet. The alluvium and colluvium in Sylvester Gulch, Dry Fork, and Lick Creek drainages (estimated thickness range: 25 to 75 feet), on the average, contains more clay than does the Deep Creek alluvium. Therefore, it is very unlikely that cracks will occur in colluvium and alluvium in the stream valleys of Sylvester Gulch (Panel 25) or the South of Divide mining area even considering the shallow overburden. The probable reason for the lack cracking in alluvi,.tm is that the fine sand- to clay-sized material and overlying soil can yield without cracking or ,ul?gir?g as it deforms as a discrete --it, or units during in the subsidence process. The alluvium observed by Mr. Dun rud during geologic mapping activities also varies in thickness from more than ten feet to many tens of feet in the West Elk mining area, including the South of Divide mining area. This same reasoning also applies to the colluvium in the area. Although subsidence cracks were locally observed in 2.05-133 Revised June 2003 PR10, Rev. Alarch 2006; Alay 1006 PR10, A'ov. 2006TR107..4p7-i12007TR108; Sep. 2007 PR12; Feb. 2008 PR-12