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West Elk Mine •• 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 azea (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 azeas. No significant similaz 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 similaz thickness. For a similaz 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 fast east-trending side canyon of Sylvester Gulch. Fracture-Controlled Drainages Based on mapping by Mr. Dunrud 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, particulazly Mt. Gunnison. Section 2.04.6 (Geology Description) includes additional discussion and references relating to the nature and continuity of fractures. The conservative approach may be to assume that the drainage system is fracture controlled. But even if fractures control the present drainage system, they may not extend downwazd 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 in the South of Divide permit revision azea aze fracture controlled, it is extremely unlikely that they extend downwazd to the E Seam through multiple shale and siltstone units. Using this conservative evaluation, it is now important to evaluate the potential impact that subsidence may have on any pre-mining fractures. Evaluation of subsidence due to downwarping of laterally-constrained strata shows that rock strata with different deformation and strength chazacteristics deform as discrete units. For example, strata of shale and siltsone behave as units discrete from sandstone. Above the fractured zone and within 2.05-116 Revised November 1004 PRI G