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Subsidence Evaluation For Exhibit 60 The Apache Rocks And The Box Canyon Mining Areas Page 19 5. State Geologists from most coal-producing states were contacted regarding the known occurrence of any block glide in such gently-dipping rocks as in the Oliver No. 2 Mine azea. Not one could recall an occurrence of block glide, including those from eastern states where coal seams typically overly a sofr underclay making block glide more likely (John Rold, written communication, November I5, 1996). 6.2.2 Block Glide Potential Due to Mining in the 8-Seam in the Oliver No. 2 Mine Area Block glide due to MCC mining of the B-Seam will not occur in bedrock beneath the coal zone mined at the Oliver No. 2 Mine. This is based on two factors: (1) these rocks occur beneath the North Fork valley and are, therefore, laterally constrained; and (2) the B-Seam in this area has a dip angle of 2.6 percent which is less than that of the E/DO-Seam (Figure 9). 6.3 Effects of Rugged Topography on Subsidence and Mine Stresses The subsidence factor (a) reportedly can vary significantly in draws and on ridges in rugged topography. Gentry and Abel (1978, p. 203-204) report that vertical displacement was 25 to 30 percent greater on a ridge than it was in an adjacent draw in the York Canyon (Raton, New Mexico) longwall mining area (Figure 3). Based on this information, the subsidence factor is projected to be closer to 0.6 in deep draws and closer to 0.8 on isolated ridges in the Box Canyon • mining azea. No significant similaz influence is expected in the Apache Rocks mining area because there are few, if any, isolated ridges. Based on observations by the author in the Somerset Mine in the mid-1970s, stresses tended to be significantly higher beneath isolated ridges than they were beneath more uniform overburden of similar thickness. For a similar mine geometry, rooffalls, bumps (rock bursts), and floor heaving were noticeably greater beneath the ridges than they were beneath more uniform overburden of similar thickness, because there is little or no lateral constraint to distribute the weight of the isolated load of the ridge. The rugged topography on the north, west, and south flanks of West Flatiron may cause stresses to be concentrated beneath isolated ridges. Overburden thickness will increase by 500 to 1,000 feet in horizontal distances of 1,500 feet similaz to the isolated ridge north of the first east- trending side canyon of Sylvester Gulch. 7.0 FRACTURE-CONTROLLED DRAINAGES Based on mapping by the author in the Somerset area and on recent tield work, the author believes that there is reasonably good, but certainly not conclusive, evidence that the drainages (Sylvester Gulch, an unnamed drainage west of Box Canyon, and Box Canyon) in the Box Canyon permit revision area are controlled by north-trending fractures and/or joints. These 831-032.181 Wright Water Engineers, Inc.