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Sutuidence Evaluation For Exhibit 608 South of Divide Mining Area Pace 11 • In Cerrains with slopes less than about 30 percent, the depth of the neutral surface can be estimated by dividing the maximum horizontal strain values by those of maximum curvature at a given location. The calculated depth of the tension zone to the neutral surface~he boundary between tension above and compression below-ranges from 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 because most of the monitoring network was located on slopes exceeding 30 percent. An unpublished study for the U. S. Bureau of Mines (Engineers International) indicated that surface crack depth rarely is greater than 50 feet.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 mining area is estimated to be 1) 5 to 15 feet in terrain sloping less than, or equal to, 30 percent, 2) 10 to 35 feet in terrain sloping more than 30 percent, and 3) 40 to 50 feet in thick, brittle sandstones in ridges (Table 2). Crack depth will 1>7tely be at a maximum value above massive coal homers. Crack depth may therefore be greatest above the 700-foot-wide protective barrier system projected between longwall panels E4 and ES (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 (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 (Figure 2) will likely close once equilibrium conditions occur. However, any cracks present above rigid chain pillars, homer 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 ten years of annual observations in the West Elk mining area by the author (from 1996 to 2004), particulazly Ute 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 longwall panels. I~lo cracks were observed in the alluvium and colluvium of Sylvester Gulch and Deep Creek (estimated thickness range is 25 to 150 feet) during periodic field observations in the Apache Rocks and Box Canyon mining areas. The neaz-surface alluvial materiai consisted of primazily 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 the Dry Fork and Lick Creek drainages (estimated thickness range is 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 the South of Divide mining area, even considering the shallow overburden. The probable reason for the lack of cracking in alluvial and colluvial deposits is that the fine sand- to clay-sized material and overlying soil can yield without cracking or bulging as it deforms as a discrete unit or units during the subsidence process. The alluvium observed by the 831-032.690 WrigMWaterEngineers, Inc.