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DRAFT <br />from Peng, 1992, p. 7), predicts the height of the fracture zone based on the competency of the <br />overburden as indicated by the unconfined compression strength. The results of the application <br />of this table to the planned 11-foot mining height at the Red Cliff Mine could result in the fracture <br />zone extending 183 feet or 16.6 mining heights up into the overburden if that overburden were <br />entirely "Hard and strong rock"; to a potential height of 123 feet or 11.2 mining heights if the <br />overburden were "Medium hard rock"; to a potential height of 71.5 feet or 6.5 mining heights if <br />that overburden were entirely "Soft and weak rock" overburden; to a potential height of 44.4 feet <br />or 4 mining heights if that overburden were entirely "Weathered soft and weak rock" overburden. <br />The Mesaverde Group overburden is a laterally and vertically variable mixture of sandstone, <br />argillaceous shale/mudstone, siltstone and coal. Table 5. Lithologic Distributions for <br />Dorchester Project Overburden, containing the lithologic logs from 13,880 feet of drilling for <br />19 drill holes at the nearby Dorchester Project site, indicates the probable considerable <br />variability at the Red Cliff Mine Project Area. The probably dominant sandstone lithology, <br />around 46%, could not be considered the Hard and strong rock" with uniaxial compression <br />strength greater than 5888 psi indicated on Table 7. The fact that it is a cliff former where <br />present in canyon walls suggests it is locally probably in the range for "Medium hard rock". On <br />the other end of the scale, the "Weathered soft and weak rock" does not fit the overburden, <br />because it is not weathered. Therefore it is recommended that the maximum height of the <br />fracture zone for planning should be a weighted average of 46% "Medium hard rock" and 54% <br />"Soft and weak rock", for a worst case estimate for the fractured zone height as 95 feet. <br />0.46 times 123 feet plus 0.54 times 71.5 feet =,95 feet <br />The potential for draining surface water into the Red Cliff Mine is low, but probably precludes <br />longwall mining under stream courses and water impoundments when the bedrock overburden <br />thickness is less than 95 feet. Consideration should be given to geophysically measuring the <br />thickness of alluvium beneath valley where the total overburden thickness above the Cameo <br />Seam is 200 feet or less, as shown on Figure 13. Overburden and Outcrop Map for the <br />Project Area. <br />Page 26 of 57 <br />