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

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the center of a panel; compressive strain (-e) is the unit shortening between any two points on the surface moving closer together as the ground surface deflects downward towards the center of a panel. The maximum values are for the unique conditions present at an individual panel. Subsidence Trough: Atrough-like depression (downwarped area) that occurs directly above and somewhat outside the panel where coal is being extracted; the trough is caused by differential vertical displacement of the ground surface. Coal Extraction Thickness (m): The thickness of coal being mined; this value may be less than the actual seam thickness, because some coal of low quality may not be mined, some coal may be left in the roof ("top coal") for roof stability, or the seam may be too thick to be mined completely. Overburden Depth (d): The vertical distance between the top of the coal seam being mined and the ground surface above it. Critical Panel Width: The minimum mining panel width necessary to cause maximum subsidence on the ground surface, generally along a line over the center of a panel. The length of the mining panel must also be equal to, or exceed this critical width. Critical width varies from 1.0 to 1.4 times the mining depth (overburden thickness). Critical Panel Length: The length of the mining panel (length of coal area extracted) necessary to cause maximum vertical displacement (1.0 to 1.4 times the overburden depth). Supercritical Panel Length and Width: A mining panel with a length and width that is greater than the critical mining width. Super Panel: Two or more mining panels that behave like one large panel because the gateroad pillars have crushed; the overlying subsidence profile looks roughly like a very wide single panel. Angle of Draw (a): The angle (from a vertical reference) of a straight line projected from the edge of the mining panel to the limit of measurable subsidence outside the edge of a panel at the ground surface. Break Angle (R): The angle (from a vertical reference) between a straight line projected vertically upward from the edge of the mining panel to the point of maximum extension (maximum tensile strain - +E) at the surface above the panel. Bedrock: Rock that was originally formed under natural conditions, in contrast to unconsolidated material (colluvium, alluvium, and soil) derived from bedrock. Cleat: A system of planar cross-bedding fractures in coal; there commonly are two cleat sets that are nearly perpendicular to each other. Lineament: A linear topographic feature, which can be observed on-site and on aerial photographs, that often indicates a fault or an extensive fracture or fracture system that may more readily erode, frequently controlling the drainage pattern. C-2 DBMS 294