2008-02-22_APPLICATION CORRESPONDENCE - C2008086 (43)

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(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 cover and, therefore, 1-foot wide surface fractures opening when undermined by a longwall panel at the shallower depths, under approximately 500 feet. The conductivity of the valley fill alluvium in the valley bottoms will potentially increase when longwall mining is performed under the valleys. No loss of surface or groundwater into the mine should occur, provided the fracture zone is not intersected. 7.0 PREDICTED SUBSIDENCE OVER THE RED CLIFF MINE PROJECT AREA The NCB subsidence effects prediction method was used to estimate worst-case maximum vertical subsidence (Smax), maximum tensile (+E) and compressive (-E) strains and maximum slope change or tilt (Gmax) as the result of longwall mining 11 feet of coal at depths of 200 feet, 500 feet, 1000 feet, 1500 feet and 2000 feet employing the potential 800-foot wide, 900-foot wide, 1000-foot wide, 1100-foot and 1200-foot wide longwall panels. In addition, the location of maximum vertical subsidence, maximum tensile strain, maximum compressive strain and maximum slope change with respect to the centerline of the panel conditions described above were calculated. Prediction of the maximum surface fracture widths were made using fracture measurements collected at the York Canyon Mine and NCB calculated tensile strains for the fracture measurement locations relative to the underlying mined longwall panels. 7.1 Maximum Vertical Subsidence (Smax) By itself, simply vertically lowering the ground surface would not be a problem. However, the ground surface is only lowered over and near a longwall panel as the coal between the panel headgate and tailgate pillars is progressively extracted and the longwall face is advanced. The surface subsidence trough advances with the longwall face and all sides of the longwall panel deflect downward toward the center of the panel, where the vertical subsidence is maximum. The bending of the overburden develops as the longwall panel progresses and forms a stable semi-permanent trough after the panel is completely mined. The maximum vertical subsidence over a panel is of major importance because it contributes to the magnitude of extension, compression and tilting. These subsidence effects can potentially damage surface and underground structures, infrastructure improvements and hydrologic features as well as potentially adversely impacting nearby C-27 DBMS 319