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West Elk Mine <br />hence the maximum depth of tension cracks from the surface. The neutral surface is the <br />boundary between tensile and compressive strain. <br />In terrains with slopes less than about 30 percent, the depth of the neutral surface can be <br />estimated by dividing the maximum horizontal strain values by the maximum curvature values at <br />a given location. The calculated depth of the possible tensile zone to the neutral surface, ie: <br />the boundary between tension above and compression below-ranges from about 50 to 100 <br />feet in the subsidence monitoring network at West Elk Mine. Crack depth may be much less <br />than this projected 50 to 100 foot range of maximum values. An unpublished study for the <br />U.S. Bureau of Mines (Engineers International) indicated that surface crack depth razely is <br />greater than about 50 feet. Cracks will also be less extensive or terminate where shale and <br />claystone layers occur. <br />Based on annual field subsidence observations in the Apache Rocks mining area by Mr. <br />Dunrud, maximum crack depth in bedrock is estimated to be (1) 5 to 15 feet in terrain <br />sloping less than, or equal to, 30 percent and (2) 10 to 35 feet in terrain sloping more than <br />30 percent (Exhibit 60B, Table 2). <br />Crack depth will likely be at a maximum value above massive coal barriers, Crack depth <br />may therefore be greatest above the 700-foot-wide protective barrier system projected <br />between longwall panels E4 and ES (Exhibit 60B, Figure 1). The crack depth is projected to <br />be less (probably 10 to 20 percent less) above the panel chain pillars, where even the rigid <br />pillars are predicted to yield 10 to 30 percent of the coal extraction thickness (Exhibit 60B, <br />Table 2). <br />Cracks that occur above the mine panel area also tend to close, once mining faces move out of <br />the surface area of influence (DeGraff and Romesburg 1981). Any local bed separations <br />during active subsidence between rocks of different strengths (Exhibit 60B, Figure 1) will <br />likely close once equilibrium conditions occur. However, any cracks present above rigid chain <br />pillars, barrier pillars, or mine boundaries may remain open where permanent tensile stresses <br />remain after mining is completed due to the convex curvature of the subsidence profile. <br />During the past nine years of annual observations in the West Elk mining area by Mr. <br />Dunrud (from 1996 to 2004), particularly in the Apache Rocks mining area, no cracks were <br />observed above mined-out longwall panels in colluvium more than an estimated ten feet <br />thick. No cracks have been observed in alluvium above mined-out longwall panels. <br />No cracks were observed in the alluvium and colluvium of Sylvester Gulch and Deep Creek <br />(estimated thickness range: 25 to 150 feet) during periodic field observations in the Apache <br />Rocks and Box Canyon mining areas. The near-surface alluvial material consisted of <br />primarily sand, silt, clay, and soil in the two areas mentioned, and was located above rigid <br />pillars and panel boundaries where the overburden depth ranges from 800 to 1,050 feet. <br />The alluvium in the Dry Fork and Lick Creek drainages (estimated thickness range: 25 to <br />150 feet), on the average, contains more clay than does the Deep Creek alluvium. <br />Therefore, it is very unlikely that cracks will occur in colluvium and alluvium in the stream <br />valleys of the South of Divide mining area. <br />2.05-119 Revised November 2004 PRIO <br />