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DRAFT <br />Strains and displacements on steep slopes with thin alluvial cover, particularly cliffs, <br />may cause surface fractures on the order of a several inches to more than two feet <br />wide and possibly 25 feet deep, compared to a fraction of an inch to a few inches <br />wide and a few feet deep in valley bottoms at the same overburden depth. When the <br />relief is subdued and terrain gentle, the surface fractures will be consistent in width <br />and depth and generally follow a smoothed ovaloid around the panel perimeter. See <br />Figure 4. Plan View of Typical Subsidence Over a Longwall Panel in Affected <br />Environment/Subsidence. Cracks will tend to be widest (approaching 20 inches <br />wide) and deepest (possibly 50 feet) along prominent joints and fractures on the <br />steepest slopes and cliffs, which, in turn may become less stable and more <br />susceptible to landslides and rockfalls. <br />• Landslides and rockfalls will be most likely to occur where mining approaches the <br />outcrop, and the overburden depth is decreasing. Tilting and tensile strain elongation <br />of the ground surface is greatest where the overburden is the least. The greatest <br />subsidence impact is likely to occur in geologic hazard areas where either of the <br />following two conditions occur: <br />1. The subsidence-induced tilt direction, which is towards the longwall panel, <br />parallels the slope direction, which temporarily increases the slope of the valley <br />wall, but the progressively greater depth progressively decreases the surface <br />tensile strain. See Figure 9. Localized Mining Induced Slope Angle Changes <br />in part C. <br />2. The direction of longwall face advance is in the same direction as the slope <br />inclination, which opens progressively wider surface fractures, i.e. as the longwall <br />face moves from deeper towards shallower overburden progressively increases <br />the surface tensile strain, but temporarily decreases the slope of the valley wall. <br />See part B of Figure 9. Localized Mining Induced Slope Angle Changes. <br />5.2 Variable Overburden Thickness <br />For any mining panel width and coal extraction thickness, the maximum subsidence, tilt, and <br />strain at the ground surface should decrease with increasing overburden depth. A single panel <br />may range from supercritical under shallow overburden to subcritical under deeper overburden. <br />• Gate road yield pillars will tend to yield more with increasing overburden depth, such <br />that two or more adjacent panels begin to approach the theoretical behavior of a <br />single super-panel at overburden depths greater than 1,000 to 1,500 feet. At these <br />depths, gateroad yield pillars may be loaded beyond the minimum loading and will <br />begin to crush. Even yield pillars are extremely unlikely to yield to the level of the <br />adjacent caved, broken and compacted gob behind the shield canopies at the face of <br />the longwall panel. Figure 3 Estimated Gateroad Pillar Loads From Mining First <br />Adjacent Panel indicates the minimum load the planned 30-foot by 80-foot gateroad <br />yield pillar must support. The 80-foot by 180-foot gateroad pillars are designed to <br />support the load arched from over the gob when the first adjacent panel passes as <br />the result of the yielding of the 30-foot by 80-foot pillar. Figure 3 also shows the <br />estimated maximum rigid pillar load transferred onto the 80-foot by 180-foot gateroad <br />pillar after the first adjacent panel has passed. <br />Page 22 of 57 <br />