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Subsidence Evaluation For Exhibit 80 The Apache Rocks And The Box Canyon Mining Areas Page 13 Cracks were also observed south of Lone Pine Gulch and north of the Mautz cabin. After extensive analysis by former Colorado State Geologist, John Rold, and the author, it was concluded that the cracks were the result of both landslide movement and mining activities in the F- and B-Seams. During field investigations by the author and WWE personnel, it became evident to the author that longwall mining has caused less surface cracking than did room-and-pillar mining in the nearby Somerset area. Cracks in the overburden more than about 20 coal-extraction thicknesses above the longwall mining panels should extend no further downwazd from the ground surface than the depth to the neutral surface of the upper downwarped surface (about 80 to 100 feet in [he current mine monitoring azea and perhaps 50 feet to as much as 200 feet in the West Flatiron area and to perhaps as much as 200 feet in the western panels of the Apache Rocks mining area, where both the E- and B-Seams are to be mined). Cracks that occur within the panel area also tend to close, once mining Faces move out of the surface azea of influence (DeGraff and Romesburg 1981). Any local bed separations during active subsidence between rocks of different strengths (Figure 1) will likely close once equilibrium conditions occur. However, any cracks present above rigid chain pillazs or mine • boundaries may remain open tivhere permanent tensile stresses remain after mining is completed due to the convex curvature of the subsidence profile. Maximum horizontal tensile and compressive strain is significantly greater above rigid chain pillazs and lease boundaries between two mines than it is above longwall mining faces. This is because tensile strains caused by mining the two adjacent panels are additive above the common chain pillars or unyielding mine boundary pillars. Cracks tend to be wider and deeper above barrier pillars or lease boundary pillars than chain pillars because of their greater rigidity (for example, the large tension crack on the north side of Lone Pine Gulch). 4.4 Angle of Draw The draw, or limit, angle (~, 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 ranged from l 1.3 to 16.1 degrees and averaged 1d.4 degrees. The angle of draw for B-Seam longwall mining at West Elk ranges tiom about 15 to 17 degrees after accounting for F-Seam mining influence (Table 1). Based on this information, the anticipated draw angle will likely be closer to the 1 ~ to 17 degree range than the upper limit of 21 degrees for longwall mining in both the Apache Rocks and the Box Canyon mining areas. In the western panel area of the Apache Rocks mining area, the area affected by mining should be determined by projecting the draw line from the lowest • seam mined (i.e., the B-Seam when it is mined). 831-032.181 Wright Water Engineers, Inc.