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West Elk Mine <br />Block glide due to MCC mining of the B-Seam will not occur in bedrock beneath the coal zone <br />mined at the Oliver No. 2 Mine. This is based on two factors: (1) these rocks occur beneath the <br />North Fork valley and are, therefore, laterally constrained; and (2) the B Seam in this area has a dip <br />angle of 2.6 percent which is less than that of the E/DO Seam. <br />Effects ofRugged Topography on Subsidence and Mine Stresses <br />The subsidence factor (a) reportedly can vary significantly in draws and on ridges in rugged <br />topography. Gentry and Abel (1978, p. 203-204) report that vertical displacement was 25 to 30 <br />percent greater on a ridge than it was in an adjacent draw in the York Canyon (Raton, New <br />Mexico) longwall mining azea (see Exhibit 608, Figure 4). Based on this information, the <br />subsidence factor is projected to be closer to 0.6 in deep draws and closer to 0.8 on isolated <br />ridges in the current and South of Divide mining areas. No significant similaz influence is <br />expected in this mining area because there are few, if any, isolated ridges. <br />Based on observations by Mr. Dunrud in the Somerset Mine in the mid-1970s, stresses tended to be <br />significantly higher beneath isolated ridges than they were beneath more uniform overburden of <br />similar thickness. For a similar mine geometry, roof falls, bumps (rock bursts), and floor heaving <br />were noticeably greater beneath the ridges than they were beneath more uniform overburden of <br />similaz thickness, because there is little or no lateral constraint to distribute the weight of the <br />isolated load of the ridge. <br />The rugged topography on the north, west, and south flanks of West Flatiron may cause stresses to <br />be concentrated beneath isolated ridges. Overburden thickness will increase by 500 to 1,000 feet in <br />horizontal distances of 1,500 feet similar to the isolated ridge north of the first east-trending side <br />canyon of Sylvester Gulch. <br />Fracture-Controlled Drainages <br />Based on mapping by Mr. Dunrud in the Somerset azea and on recent field work, Mr. Dunrud <br />believes that there is reasonably good, but certainly not conclusive, evidence that some drainages <br />are controlled by fractures and/or joints. The Dry Fork of Minnesota Creek and some of its <br />tributaries exhibit lineaz trends on satellite images and on high-altitude photographs that indicate, or <br />at least suggest, fracture control (Dunrud, 1976, p. 14-15). These fractures have been caused in part <br />by stresses generated by the West Elk Mountain intrusive bodies, particularly Mt. Gunnison. <br />Section 2.04.6 (Geology Description) includes additional discussion and references relating to the <br />nature and continuity of fractures. <br />The conservative approach may be to assume that the drainage system is fracture controlled. But <br />even if fractures control the present drainage system, they may not extend downward as continuous <br />joints of fractures to the E Seam located several hundreds of feet below. Even if the fractures were <br />present in the more brittle sandstone units, it would be very unlikely that these fractures would <br />occur in the softer siltstone and shale units. Even under the conservative approach that the <br />drainages of Sylvester Gulch (Pane125} and in the South of Divide permit revision area aze fracture <br />controlled, it is extremely unlikely that they extend downward to the E Seam through multiple shale <br />and siltstone units. Using this conservative evaluation, it is ndw important to evaluate the potential <br />impact that subsidence may have on any pre-mining fractures. <br />2.05-140 Revised June 2005 PRIG, Rev. March 2006; May 2006 PRIG, Nov. 2006TR107 <br />