Laserfiche WebLink
area depends on the thickness of the mined coal, the mining method, the rate of the mining <br />face advancement, and on the geological characteristics of the overburden. <br />The area of disturbance is generally divided into two or three zones, based upon the <br />extent of fracturing, as follows: <br />o zone of primary caving where the cave rock is <br />completely disintegrated <br />o zone of secondary caving where a network of <br />horizontal and vertical cracks develops. <br />o zone of bed separation where separation occurs <br />primarily along pre-existing bedding planes. <br />The total thickness of first and second zones, where the changes of permeability are <br />substantial, typically reaches 3 to 3.5 times (Ropski and Lama, 1973) and rarely more than <br />10 times the height of the extracted seam (Wardell, 1970). <br />The height of the third zone, or the total height where changes in permeability due <br />to subsidence can occur, is described by various authors as 30t (where t is the fully <br />extracted seam thickness), 58t (Gviroman, 1977), 33.7t (Williamson, 1978) and 30t <br />(Wardell, 1976). <br />The difference between indicated numbers is caused by variable geological <br />conditions. A ratio between brittle (sandstone) and plastic (claystone) rocks in the <br />overburden has an important impact on the fracture development. More plastic rock strata <br />tend to bend without fracturing. <br />The increase of permeability in the zones affected by subsidence can be <br />substantial. The magnitude of increase varies from about 10 to 230 times the pre-mining <br />values (Whittaker et al, 1979, Sikora, 1976, Neate and W hittaker, 1979). However, several <br />studies indicated that this increase is only temporary and close behind the mining face <br />there is an indication of increasing consolidation (40 meters behind the face of a longwall <br />panel (Whittaker, 1979)). <br />• <br />- 22 - 10/00 <br />'" PR ay <br />