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

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DRAFT There are approximately four overburden zones to consider and analyze in the trough subsidence process over a longwall panel. Figure 5 Conceptual Representation of Subsidence Deformation Zones in Affected Environment/Subsidence presents one such representation (Peng, 1992). There a four generally agreed zones of overburden response to longwall mining. They are (1) the caved or collapsed zone , (2) the fractured zone, (3) the continuous deformation zone and (4) the near-surface zones. These zones are really transitional from one to another, and not sharply bounded. 6.1.1 Caved Zone After the removal of the coat under the roof of a longwall panel, the immediate roof collapses and caves upward to fill up the mined void. Piggott and Eynon (1977) calculated the height of the collapse zone over a longwall panel in coal measure rocks as 2 to 3.3 times the mining height based on a typical range of percent swell of 30 to 50%, see Figure 10. Potential Collapse Heights Above Different Mine Opening Geometries. The collapsed rock is a jumbled mass of rubble that will be partially reconsolidated by the overburden load. The collapsed rock no longer gives the appearance of having been part of a bedded or stratified sedimentary formation. H.F. Schulte (1957) reported that the height of the rubble zone exposed in a winze excavated down into the center of a worked area was 2.4 times the mining height above the seam floor. P. Kenny (1959) reported observing and measuring the active height of caving into the original roof above a longwall panel to range from two to four times the mining height, depending on the angle of repose, fragmentation, bed thickness and swell of the immediate roof rocks. S. Peng (1992) reported the height of the caved zone is normally 2 to 8 times the mining height, depending on the properties of the immediate roof and 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 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 Page 25 of 57