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

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DRAFT 7.2 Maximum Horizontal Strain The maximum horizontal tensile strains are the most serious potential hazard with respect to anticipated subsidence impacts from longwall mining in the proposed Red Cliff Mine lease area. This involves protecting the public from larger open fractures, as shown on Figure 20. Tension Crack Over Starter Room, York Canyon Mine, when longwall mining at shallow depths (<500 feet). There is also the temporary potential for large boulders being dislodged from sandstone cliffs on the canyon walls by smaller tensile strains from deeper active longwall panels, as indicated on Figure 18. Ribside Tension Cracks in Road Fill and Cliff Face, York Canyon Mine. Table 10. Predicted Surface Fracture Widths Based on York Canyon Mine Measurements presents the relationship between predicted tensile strain and the measured width of selected open subsidence fractures above three longwall panels at the York Canyon Mine west of Raton New Mexico. The York Canyon Mine was mining coal in the Mesaverde Group, but the overburden lithology could well differ from that present at the Red Cliff Mine proposed Project Area. The horizontal tensile strain over the barrier pillars between panel groups will probably increase because the strain at the surface over the barrier pillar caused be each adjacent panel is additive. It is possible that the maximum horizontal tensile strain above the larger barrier pillars planned between panel groups could as much as double the tensile strain on the surface over the center of such a barrier pillar. This is possible because it depends on the panels on both sides being subcritical precisely enough to place the maximum tensile strain at the center of the barrier pillar. For example, using Figure 12. NCB Horizontal Strain Profile Graph in Affected Environment/Subsidence, the center of an 1000-foot wide panel at the depth of 2000 feet (Panel Width/Depth Ratio = 0.500) is 600 feet from the center of a 200-foot wide barrier pillar, 0.300 times the 2000-foot depth. The predicted tensile strain over the center of the barrier pillar from the first longwall panel to be completed on one side of the group barrier pillar is 95% of the predicted maximum horizontal tensile strain. If a longwall panel group with the same dimensions and depth is mined on the other side of the barrier pillar is completed it would add 95% of its maximum horizontal tensile strain at the center of the 200-foot barrier pillar, nearly doubling (approximately 1.9 times) the tensile strain at that location, and similarly increase the width of the predicted open fracture. The total additive tensile strain at other locations along the overlapping strain profiles could be conservatively predicted by superimposing the subsidence profiles from the two adjacent longwall panels. The rapidly changing overburden depths at the Red Cliff Mine Project Area could make estimating the total tensile strains across barrier pillars using the NCB method a time consuming process. 7.3 Maximum Tilt (G) The maximum slope or tilt change as the result of mining a longwall panel occurs at the inflection point between bending progressively more downward toward the center of the panel to bending progressively less downward closer to the center of the panel. On Figure 4. Plan View of Surface Subsidence Over a Longwall Panel in Affected Environment/Subsidence, this is the 0.50 Smax contour line. With the exception of subcritical panels, where the panel width is less than approximately 0.41 times the panel depth, the inflection line is within the Page 35 of 57