2008-03-21_PERMIT FILE - C1980007A (5)

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West Elk Mine • coal seams typically overly a soft underclay making block glide more likely (John Rold, Written Communication, November 15, 1996). Block Glide Potential Due to Mining in the B 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. Effects of Ruafed 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 (see Exhibit 60B, Figure 4). 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 current and South of Divide mining areas. No significant similar influence is expected in this mining area because there are few, if any, isolated ridges. Based on observations by Mr. Dunrud 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, roof falls, 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 similar to the isolated ridge north of the first east-trending side canyon of Sylvester Gulch. Fracture-Controlled Drainages Based on mapping by Mr. Duiuud in the Somerset area and on recent field work, Mr. Dunrud believes that there is reasonably good, but certainly not conclusive, evidence that some drainages are controlled by fractures and/or joints. The Dry Fork of Minnesota Creek and some of its tributaries exhibit linear trends on satellite images and on high-altitude photographs that indicate, or at least suggest, fracture control (Dunrud, 1976, p. 14-15). These fractures have been caused in part by stresses generated by the West Elk Mountain intrusive bodies, particularly Mt. Gunnison. Section 2.04.6 (Geology yDescription) includes additional discussion and references relating to the nature and continuity of fractures. The conserv'ati%;e az,nroacl. be to assume that the drajilage system is fra_.?re _ =_trGided. But ?r r even if fractures control the present drainage system, they may not extend downward as continuous joints of fractures to the E Seam located several hundreds of feet below. Even if the fractures were present in the more brittle sandstone units, it would be very unlikely that these fractures would • occur in the softer siltstone and shale units. Even under the conservative approach that the drainages of Sylvester Gulch (Panel 25) and in the South of Divide and Dry Fork permit revision 2.05-140 Revised Ane 2005 PRIG, Rev. March 2006; Alav 2006 PRIO, Nov. 2006TRIO7.April 2007TRIO8; Sep. 2007 PR12; Feb. 2008 PR-12