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Subsidence Evaluation for the <br />Exhibit 60E South of Divide and Dry Fork Mining Areas Page 15 <br />• monitoring network at West Elk Mine. Crack depth may be much less than this J ro'ected 50 to <br />P <br />100 foot range of maximum values because most of the monitoring network was located on <br />slopes exceeding 30 percent. An unpublished study for the U. S. Bureau of Mines (Engineers <br />International) indicated that surface crack depth rarely is greater than 50 feet. Cracks will also <br />be less extensive or terminate where shale and claystone layers occur. Based on annual field <br />subsidence observations, maximum crack depth in bedrock in the South of Divide and Dry Fork <br />mining areas is estimated to be 1) 5 to 15 feet in terrain sloping less than, or equal to, 30 percent, <br />2) 10 to 35 feet in terrain sloping more than 30 percent, and 3) 40 to 50 feet in thick, brittle <br />sandstones in ridges (Tables 2 and 3). <br />Crack depth will likely be at a maximum value above massive coal barriers. Crack depth may <br />therefore be greatest above the 700 -foot -wide protective barrier system projected between <br />longwall panels E4 and E5 (Figure 1). The crack depth is projected to be less (probably 10 to 20 <br />percent less) above the panel chain pillars, where even the rigid pillars are predicted to yield 10 <br />to 30 percent of the coal extraction thickness (Tables 2 and 3). <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 during <br />active subsidence between rocks of different strengths (Figure 2) will likely close once <br />equilibrium conditions occur. However, any cracks present above rigid chain pillars, barrier <br />pillars, or the outer limit of mined/unmined coal may remain open where permanent tensile <br />stresses remain after mining is completed due to the convex curvature of the subsidence profile. <br />During the past ten years of annual observations in the West Elk mining area by Dunrud, <br />particularly the Apache Rocks mining area, no cracks were observed above mined -out longwall <br />panels in colluvium more than an estimated ten feet thick. No cracks have been observed in <br />alluvium above mined -out longwall panels. No cracks were observed in the alluvium and <br />colluvium of Sylvester Gulch and Deep Creek (estimated thickness range is 25 to 150 feet) <br />during periodic field observations in the Apache Rocks and Box Canyon mining areas. The <br />near - surface alluvial material consisted of primarily sand, silt, clay, and soil in the two areas <br />mentioned, and was located above rigid pillars and panel boundaries where the overburden depth <br />ranges from 800 to 1,050 feet. The alluvium and colluvium in the Dry Fork and Lick Creek <br />drainages (estimated thickness range is 25 to 75 feet), on the average, contains more clay than <br />does the Deep Creek alluvium. Therefore, it is very unlikely that cracks will occur in colluvium <br />and alluvium in the stream valleys of the South of Divide mining area, even considering the <br />shallow overburden. This should also be the case for the stream valleys in the Dry Fork area, <br />where the overburden is 800 to 1,800 feet thick. <br />The probable reason for the lack of cracking in alluvial and colluvial deposits is that the fine <br />sand- to clay -sized material and overlying soil can yield without cracking or bulging as it <br />deforms as a discrete unit or units during the subsidence process. The alluvium in the South of <br />Divide and Dry Fork mining areas also varies in thickness from about 25 feet to 75 feet. This <br />same reasoning also applies to the colluvium in the area. Although subsidence cracks were <br />locally observed in colluvium less than foot to a few feet thick, no cracks were observed in <br />colluvium more than about 10 feet thick. <br />0 <br />831- 032.810 Wright Water Engineers, Inc. <br />