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

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able to push uphill against gravity after the face passes. If the longwall panel subsequently advances under the ridge, that side of the ridge will displace down slope on that side of the ridge. In the course of extracting the underlying coal, a ridge with steep slopes in adjacent valleys will subside more than would be the case in flat terrain. Parts B and C of Figure 9. Localized Mining Induced Slope Angle Changes indicates how this will occur. The potentially additive subsidence on ridges will increase the tensile strain and the width of open surface cracking. Higher compression ridges, but negligible tensile fractures, are likely to occur in narrow valley bottoms, because the overburden on both sides will try to move toward the bottom of the valley as the subsidence trough approaches and then passes the valley bottom. Consequently subsidence impacts are likely to be greater on narrow ridges and lesser in narrow valley bottoms than they would be in more subdued terrain. • Strains and displacements on steep slopes with thin alluvial cover, particularly cliffs, may cause surface fractures on the order of a several inches to more than two feet wide and possibly 25 feet deep, compared to a fraction of an inch to a few inches wide and a few feet deep in valley bottoms at the same overburden depth. When the relief is subdued and terrain gentle, the surface fractures will be consistent in width and depth and generally follow a smoothed ovaloid around the panel perimeter. See Figure 4. Plan View of Typical Subsidence Over a Longwall Panel in Affected Environment/Subsidence. Cracks will tend to be widest (approaching 20 inches wide) and deepest (possibly 50 feet) along prominent joints and fractures on the steepest slopes and cliffs, which, in turn may become less stable and more susceptible to landslides and rockfalls. • Landslides and rockfalls will be most likely to occur where mining approaches the outcrop, and the overburden depth is decreasing. Tilting and tensile strain elongation of the ground surface is greatest where the overburden is the least. The greatest subsidence impact is likely to occur in geologic hazard areas where either of the following two conditions occur: 1. The subsidence-induced tilt direction, which is towards the longwall panel, parallels the slope direction, which temporarily increases the slope of the valley wall, but the progressively greater depth progressively decreases the surface tensile strain. See Figure 9. Localized Mining Induced Slope Angle Changes in part C. 2. The direction of longwall face advance is in the same direction as the slope inclination, which opens progressively wider surface fractures, i.e. as the longwall face moves from deeper towards shallower overburden progressively increases the surface tensile strain, but temporarily decreases the slope of the valley wall. See part B of Figure 9. Localized Mining Induced Slope Angle Changes. 5.2 Variable Overburden Thickness For any mining panel width and coal extraction thickness, the maximum subsidence, tilt, and strain at the ground surface should decrease with increasing overburden depth. A C-19 DBMS 311