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Subsidence Evaluation For Exhibit 608 South of Divide Mining Area Page 11 • measurements, therefore, provides a means of calculating the depth of the neutral surface, and hence the maximum depth of tension cracks from the surface. The neutral surface is the boundary between tensile and compressive strain 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 those of maximum curvature at a given location. The calculated depth of the tension zone to the neutral surface-the boundary between tension above and compression below-ranges from 50 to 100 feet in the 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 An unpublished study for the U. S. Bureau of Mines (Engineers 7ntemational) indicated that surface crack depth razely is greater than 50 feet.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 in the Apache Rocks mining azea by the author, maximum crack depth in bedrock is estimated to be 1) 5 to 15 feet in terrain sloping less than, or equal to, 30 percent, and 2) 10 to 35 feet in terrain sloping more than 30 percent (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 ES (Figure 1). The crack depth is projected to be less (probably 10 to 20 percent less) above the panel chain pillazs, where even the rigid pillazs aze 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 sepazations during active subsidence between rocks of different strengths (Figure 2) will likely close once equilibrium conditions occur. However, any cracks present above rigid chain pillazs, barrier pillazs, 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 azea by the author (from 1996 to 2004), particulazly 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 longwall panels. No 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 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 in the Dry Fork and Lick Creek drainages (estimated thickness range is 25 to 150 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 azea. C~ 831-032.620 Wright Water Engineers, Inc.