2008-01-16_APPLICATION CORRESPONDENCE - C2008086 (3)

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DRAFT 3) The weathering of the in-place bedrock progressively decreases with depth until it transitions into fresh bedrock. In addition, many of the bedding cross joints become discontinuous as the weathered bedrock transitions into fresh bedrock. Fresh bedrock has a tensile strength, albeit normally more than an order of magnitude less that its compressive strength. The upper soil-like materials in this zone are generally quite weak and cannot sustain any subsidence induced tensile strain without rupturing. These fragmented materials are stretched as the bedrock they rest on bends downward toward the center of the subsidence trough and then compressed as they reverse the bend as they approach closer to the center of the trough. See Figure 7. Critical Panel Width for Maximum Subsidence in Affected Environment/Subsidence. The in situ horizontal stress in the soil layer will be the active soil pressure, approximately one-third the gravitational stress at that depth. Longwall mining under weakly-bonded alluvium at similar depths, from 240 to 440 feet, will probably subject the area toward the center of a panel to subsidence induced compressive stress. The compressive stress is commonly evidenced by mounds, as shown on Figure 15. Cross Panel Compression Ridge in Alluvium, York Canyon Mine. In general, when fragmented materials like alluvium once deform in compression the easier it appears to continue deforming at the same location. Figure 16. Cross Panel Tension Cracks in Alluvium, York Canyon Mine shows a series of sub-parallel tension cracks in fragmental soil-like alluvium. The presence of one tensile crack in alluvium does not necessarily release the tensile strain over any significant distance. The underlying weathered bedrock materials range from extremely weak in tension and compression immediately under the fragmented soil zone layer to much weaker in tension than in compression in fresh bedrock. The in situ horizontal stress in bedrock is the remaining residual stress within the rock layers in coal bearing formations, such as the Mesaverde Group, present in the swamp deposits at the time of solidification when buried under generally thick shallow sea sediments. The original solidification stress field was probably very close to hydrostatic, equal in all directions. Uplift and erosion has progressively reduced the overburden confining pressure, but not the is situ horizontal pressure. Large shear stress can develop between the vertical and horizontal stresses when uplift and erosion is rapid, and thrust faulting or even major overthrusts may occur when the horizontal stress is released in a short period of geologic time. When uplift is gradual, the shear stress can be released gradually by long-term creep and yielding of the rock toward the lower vertical stress. The time necessary for different rock types to deform (yield) significantly to release the higher horizontal stress was discussed in detail by S. Warren Carey (1954). The stronger and more competent the rock type, the longer it takes for the shear stress to dissipate. However, the horizontal stress decreases as it approaches the ground surface. The lower the horizontal stress the more readily can the natural bedding cross joints open when the upper part of a layer (bed) is subjected to subsidence induced tensile bending stress. Large single fractures open at the ground surface when there is only a thin layer of fragmented material above weathered bedrock as was the case for Figure 17. Ribside Tension Crack On Steep Slope, York Canyon Mine. This open fracture follows the offset pattern of two joint sets in the underlying bedrock. Figure 18. Ribside Tension Cracks in Road Fill and Cliff Face, York Canyon Mine shows a sequence of small tension cracks in the road fill that disappear in the bedrock exposed in a cliff face. The tensile strain was sufficient to open joint blocks and(or) tilt a few of the outer sandstone blocks at the cliff face and topple them onto the roadway. It should be anticipated that longwall mining under the canyon walls will present a similar hazard for rock to roll out from undermined sandstone outcrops. The slopes of the canyon walls are certainly steep enough within the Red Cliff Mine Project Area to result in thin fragmented soil Page 29 of 57