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

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500 feet, still within the Existing Coal Lease area. The N 45° E direction of the Red Cliff Mine main entries indicates that the Main entries will probably turn half-right after passing under Buniger Canyon to drive east just south of the boundary between T. 7 S. and T. 8 S. Driving the mains in this direction would reach Big Salt Wash in approximately 8,200 feet at a depth of approximately 200 feet. It is anticipated that the main entries will split at Big Salt Wash with one branch continuing to the east, the 1st East Mains, and the other driven to the northeast under Big Salt Wash, the Northeast Mains. The East Mains would be the base for developing longwall panels as much as 14,000 feet to the south. If no longwall panels are driven to the north it could be possible to rob the barrier and main entry pillars on the retreat provided the retreat mining was protected by unmined coal on the north side of the 1st East Mains. This assumes that the individual longwall panels driven south off the 2nd East Mains are mined after the 2nd West Mains and that the 2nd East Mains longwall panels are sequenced from east to west and retreated from south to north following the retreat of the 1st East Mains pillars. Retreat mining the 2nd West Mains and the 2nd East Mains would probably require a third set of main entries driven from East Salt Creek or Munger Creek and across the north end of the Project Area. Mining beneath the perennial and intermittent stream courses will necessitate preventing water loss to the underlying workings. As discussed previously in section 6.1.2 Fractured Zone, water loss to the fracture zone is probable through 100 feet or less of overburden when longwall mining in the Red Cliff Mine Project Area. Big Salt Wash is particularly at risk because it also contains a road and has agricultural uses. Because there is no available depth of alluvium below any of the deeply incised canyons and the absence of any data on the potential fault control of the nearly trellis drainage pattern in the Project Area, conservatism must be used and a minimum of 200 feet of overburden required to positively prevent water loss from longwall mining under even intermittent stream courses. Table 10. Predicted Surface Fracture Widths Based on York Canyon Mine Measurements provides conservative estimates of fracture widths with respect to depth of overburden and panel width. 9.0 SURFACE SUBSIDENCE MONITORING Various governmental bodies may require a monitoring demonstration that the predicted subsidence effects are indeed conservative and not significantly exceeded. Specifically, a monitoring program over one of the initial longwall panels that will obtain subsidence data on the maximum vertical subsidence (Smax), tensile (+E) and compressive (-E) horizontal strains, angle of draw (a) and subsidence induced tilt (G) for this unique geologic environment. If room-and-pillar panels are mined it may be necessary to measure the same subsidence effects, or to demonstrate that sufficient pillars are left to prevent subsidence. The Surface Subsidence Monitoring Guidelines by Abel (1982) indicate one possible monitoring program that has been utilized to provide the data, when required. Figure 21. Subsidence Monitoring Program indicates the location of surface monuments for flat lying terrain. The rugged terrain and rapidly changing overburden depth in the Project Area will necessitate panel-by-panel monument spacing modifications in the field after the locations of the initial panels become available. Either monument spacing for the test C-41 DBMS 333