2022-09-29_PERMIT FILE - C1980007 (5)

Home Browse Search

Subsidence Evaluation for the Exhibit 60E Southern Panels, Apache Rocks West, & Sunset Trail Mining Areas Page 20 831-032.923 Wright Water Engineers, Inc. December 2021 Cracks that occur above the mined longwall panel area also tend to close, once longwall 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, barrier pillars, or the outer limit of mined/unmined coal may remain open where permanent tensile stresses remain after mining is completed due to the convex curvature of the subsidence profile. During the past 25 years of annual, or semi-annual, observations in the area of the West Elk Mine area by WWE, 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 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. Longwall mining has already occurred in the E-seam under Dry Fork, where overburden thickness reaches a minimum of less than 400 feet. No cracks were observed in the alluvium of Dry Fork following longwall mining; therefore, no significant cracking in alluvial and colluvial deposits is anticipated with proposed mining in the Southern Panels mining area. The Sunset Trail mining area will include South Prong, where conditions are expected to mirror mining under Dry Fork, where alluvial and colluvial deposits are present. Reaches of South Prong underlain by bedrock will be more prone to surface cracking, with projected depths of 5 to 15 feet. 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 in the Southern Panels and Sunset Trail mining areas is estimated to vary in thickness from about 25 feet to 75 feet. This same reasoning also applies to the colluvium in the area. Although subsidence cracks were locally observed in colluvium less than a foot to a few feet thick, no cracks were observed in colluvium more than about 10 feet thick. 5.4 Angle of Draw The draw, or limit, angle (φ, from a vertical reference) in the Somerset area ranges from about 8 to 21 degrees. The angle of draw measured for F-seam room-and-pillar mining at West Elk Mine, which has overburden rock lithology similar to the-E-seam, ranged from 11.3 to 16.1 degrees and averaged 14.4 degrees. The angle of draw for B-seam longwall mining at West Elk ranges from about 15 to 17 degrees after accounting for F-seam mining influence (Table 1). MCC collected survey data from the subsidence monuments following mining of E-seam longwall panel E1. WWE’s analysis of that data indicates that the mean angle of draw is approximately 16°, with a range of 14° to 19° predicted for the Southern Panels (Table 2) and Apache Rocks West (Table 3) mining areas. For the Sunset Trail mining area, the angle of draw is projected to range from 14º to 19º (Table 4).