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West EIk Mine 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/D0. Effects of Rugged Topography on Subsidence and Mine Stresses The subsidence factor 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. Based on this information, the subsidence factor is projected to be closer to 0.6 in deep draws and closer to 0.8 on isolatai ridges in the Box Canyon mining area. No significant similar influence is expected in the Apache Rocks 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 tray cause stresses to be concetttrated beneath isolated ridges. Overburden thiclmess 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. Fracture-Controlled Drainages Based on mapping by Mr. Dunrud in the Somerset azea and on recent field work, Mr. Dunrud 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 aze controlled by north-trending fractures and/or joints. These fractures have been caused in part by stresses generated by the West Elk Mountain intrusive bodies - particuiazly Mt. Gunnison. Section 2.04.6 (Geology Description) irtcludes additional discussion and references relating to the nature and continuity of fractures. The conservative approach tray be to assume that the drainage system is fracture controlled. But even if fractures control the present drainage system, they may not extend downward as continuous joints of fractures to the B-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 in the Box Canyon permit revision area aze fracture controlled, it is 2.05-126 RevisedJurt. 1495 PRQ6; l/96 RN03; AevisedJan. 1998 PR08