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

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the overburden. The caved zone is extremely permeable and if the caved zone breaches an aquifer the water will enter the mine workings as an unrestricted flow. 6.1.2 Fractured Zone Rocks in this zone undergo fracturing and fissurization both completely and partially across one or more rock layers and along bedding surfaces between layers. The bottom of the fracture zone is located where an individual bedding contact can be traced despite offsets and slight rotations between rock blocks. The fracturing decreases upward from open interconnected fractures and bedding surfaces to tight fissurization. Stream flow readings and water level fluctuations indicated by piezometers and packer tests in drill holes before, during and after longwall mining under and within the angle of draw outside panels have been used to determine the approximate upper boundary of the fracture zone (Bauer, et al, 1995: Mattson and Meggars, 1995a; Mattson and Meggars, 1995b; Peng, 1992). Whenever a monitoring well bottoms in what will be part of the fracture zone the water level and(or) pressure will initially rise slightly as the longwall face approaches, then drop significantly shortly after the longwall face passes and finally may recover somewhat over an extended period of time. Bauer, et al. (1995) reported that the water level returned to its pre-mining elevation within 2 years after mining ceased. Peng (1992, p. 143) indicates that the lower 2/3 of the fracture zone has increased hydraulic conductivity as the result of fracturing associated with subsidence. Peng states that the upper 1/3 of this zone has only minor, unconnected fractures and thus undergoes only a minor increase in water conductivity as the result of being subsided by longwall mining. Booth and Spande (1992) report an order of magnitude increase in water conductivity for an overlying sandstone as the result of subsiding in the fracture zone. According to Peng (1992, p. 6-8), the height of fracturing is a function of lithology and thickness of the stratigraphic layers. Table 7. Formulae for Predicting Fracture Zone Height (modified from Peng, 1992, p. 7), predicts the height of the fracture zone based on the competency of the overburden as indicated by the unconfined compression strength. The results of the application of this table to the planned 11-foot mining height at the Red Cliff Mine could result in the fracture zone extending 183 feet or 16.6 mining heights up into the overburden if that overburden were entirely "Hard and strong rock"; to a potential height of 123 feet or 11.2 mining heights if the overburden were "Medium hard rock"; to a potential height of 71.5 feet or 6.5 mining heights if that overburden were entirely "Soft and weak rock" overburden; to a potential height of 44.4 feet or 4 mining heights if that overburden were entirely "Weathered soft and weak rock" overburden. The Mesaverde Group overburden is a laterally and vertically variable mixture of sandstone, argillaceous shale/mudstone, siltstone and coal. Table 5. Lithologic Distributions for Dorchester Project Overburden, containing the lithologic logs from 13,880 feet of drilling for 19 drill holes at the nearby Dorchester Project site, indicates the probable considerable variability at the Red Cliff Mine Project Area. The probably dominant sandstone lithology, around 46%, could not be considered the "Hard and strong rock" with uniaxial compression strength greater than 5888 psi indicated on Table 7. The fact that it is a cliff former where present in canyon walls suggests it is locally probably in the range for "Medium hard rock". On the other end of the scale, the "Weathered soft and weak rock" does not fit the overburden, because it is not weathered. Therefore it is recommended that the maximum height of the fracture zone C-23 DBMS 315