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

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for planning should be a weighted average of 46% "Medium hard rock" and 54% "Soft and weak rock", for a worst case estimate for the fractured zone height as 95 feet. 0.46 times 123 feet plus 0.54 times 71.5 feet = 95 feet The potential for draining surface water into the Red Cliff Mine is low, but probably precludes longwall mining under stream courses and water impoundments when the bedrock overburden thickness is less than 95 feet. Consideration should be given to geophysically measuring the thickness of alluvium beneath valley where the total overburden thickness above the Cameo Seam is 200 feet or less, as shown on Figure 13. Overburden and Outcrop Map for the Project Area. 6.1.3 Continuous Deformation Zone This zone, which is transitional to the underlying fracture zone, is from the upper limit of the fractured zone to the near-surface weathered bedrock and soil zone. See Figure 5 Conceptual Representation of Subsidence Deformation Zones in Affected Environment/Subsidence. This zone contains subsidence induced fractures, but the fractures in this zone do not persist from bed to bed and generally not across even a single bed. Pre-mining cross bedding joints remain tight through the subsidence induced downward deflection that moves with the underlying and advancing longwall face. Obviously, the continuous deformation zone can have considerable thickness, potentially hundreds of feet thick, when the overburden depth to the mining horizon is a 1,000 feet or more and the fracture zone is on the order of 100 feet. The downward deflection of the beds during subsidence above the fracture zone as the overburden beds bend toward the void left by the longwall mining operation. The deflecting beds approximate psuedo-elastic plates. The upper part of each plate-like bed undergoes subsidence induced tensile strains which may open bedding cross joints. These tensile strains are in the area from the limit of subsidence outside the panel and the inflection point between downward bending and upward bending slightly inside the active panel from the gateroad pillars. There is a similar inflection point slightly inside the active panel from the starter room. when it bends down toward the void left by the longwall mining operation. The lower part of each plate-like bed undergoes subsidence induced compressive stress that balances the tension. In the part of the trough-like subsidence curve where the bed is bent back to its original inclination the stresses are reversed in each bed, compression in the upper part and tension in the lower part. Strain relief overcoring has demonstrated that there are 3-dimensional compressive stresses in the rock below the ground surface. The horizontal compression appears to prevent the opening of pre-mining cross bedding joints in the tensile stress zone associated with the downward bending in the continuous deformation zone. After the longwall is completed, the bending pattern will be repeated over the recovery room pillars. 6.1.4 Near-Surface Zone Most subsidence measurements are made at the top, ground, surface of this zone. From top to bottom, the near-surface zone typically consists of: (a) A relatively thin layer, generally a few feet at most, of either fragmented residual soil, weathered from the underlying rock, or colluvium that has C-25 DBMS 317