2014-10-10_PERMIT FILE - C1996083 (8)

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2.3 GROUNDWATER Total extraction (and, to a lesser extent, partial extraction) of coal seams causes collapse, fracturing, bed separation and bedding plane slip in the roof strata above the seam. All of these disturbances of the overlying strata can have a substantial impact on ground water if a major aquifer is within reach of the disruption. The height of the disturbed area depends on the thickness of the mined coal, the mining method, the rate of the mining face advancement, and on the geological characteristics of the overburden. The area of disturbance is generally divided into two or three zones, based upon the extent of fracturing, as follows: • zone of primary caving where the cave rock is completely disintegrated • zone of secondary caving where a network of horizontal and vertical cracks develops. • zone of bed separation where separation occurs primarily along pre- existing bedding planes. The total thickness of first and second zones, where the changes of permeability are substantial, typically reaches 3 to 3.5 times (Ropski and Lama, 1973) and rarely more than 10 times the height of the extracted seam (Wardell, 1970). The height of the third zone, or the total height where changes in permeability due to subsidence can occur, is described by various authors as 30t (where t is the fully extracted seam thickness), 58t (Gviroman, 1977), 33.7t (Williamson, 1978) and 30t (Wardell, 1976). The difference between indicated numbers is caused by variable geological conditions. A ratio between brittle (sandstone) and plastic (claystone) rocks in the overburden has an important impact on the fracture development. More plastic rock strata tend to bend without fracturing. The increase of permeability in the zones affected by subsidence can be substantial. The magnitude of increase varies from about 10 to 230 times the pre- mining values (Whittaker et al, 1979, Sikora, 1976, Neate and Whittaker, 1979). However, several studies indicated that this increase is only temporary and close behind the mining face there is an indication of increasing consolidation (40 meters behind the face of a longwall panel (Whittaker, 1979)). PR -14 -25- 03/14