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

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DRAFT 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 overlying and underlying coal seams. All such features have a limited tolerance for these potentially adverse effects. The magnitude of the potentially adverse impacts is directly related to the maximum subsidence, i.e. the greater the subsidence the greater the magnitude of the impact, provided the depth and panel dimensions do not change. The magnitude of the potentially adverse surface impacts is inversely related to the mining depth, i.e. the magnitude of potentially adverse impacts decrease as the mining depth increases. Great Britain has lead the world in researching these relationships because every major metropolitan area, except London, was underlain by multiple mineable coal seams. It is possible to somewhat mitigate the adverse impacts by varying panel width, by designing gateroad pillars between panels to yield when the first of two adjacent panels is mined and crush after the face of the second panel is mined past and by positioning longwall panels with respect to a particularly important surface feature. The conservative NCB maximum vertical subsidence prediction for supercritical longwall panel widths is 0.9 times the mining height (m) for overburden has been previously subsided. The NCB method specifies that the previously subsided maximum vertical subsidence prediction be multiplied by 0.9 for ground that has not been previously subsided. The adjustment for previously unmined ground is referred to as the "virgin" ground correction in Great Britain. Subsidence over the proposed Red Cliff Mine Project Area was analyzed as virgin ground because none of the proposed lease area appears to have been previously mined. The overall supercritical subsidence factor for virgin ground is 0.81 times the mining height. Page 30 of 57