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Fall 2018 Subsidence and Geologic Field Observations <br /> Southern Panels Mining Areas <br /> 4. Continuous annual observations find substantial weathering of previously-observed <br /> subsidence cracks with edges rounding, widths reducing, and depths filling with eroded <br /> material. The only exception to these observations has been those cracks in thick,exposed, <br /> brittle sandstone units above previously mined B-seam panels (i.e., Apache Rocks) where <br /> rounding of edges and filling of cracks have occurred over time but widths have remained <br /> relatively constant. <br /> 5. The length of time that tension cracks are expected to be visible before the effects of erosion <br /> and deposition, mass wasting, infilling, and revegetation obliterate them (duration of <br /> cracks), is a function of their location with respect to the mine geometry and type of <br /> material in which the cracks formed. Crack duration in zones of permanent tensile stress, <br /> such as above mine boundaries and unmined pillars between longwall panels, commonly <br /> last: 1) from one to three years in colluvium, 2) from three to six years in soft, friable <br /> sandstone, and 3) many decades in hard, durable sandstone. However, cracks that form in <br /> the zone of temporary tensile stress, such as above moving longwall faces,commonly close <br /> again when the longwall moves out of their area of influence. <br /> 6. Observed mine-induced subsidence effects have been less in the Southern Panels Mining <br /> Area than were observed annually in the Box Canyon and Apache Rocks B-seam mining <br /> areas dating back to 1996. The more subdued topography and the fewer cliffs and ledges <br /> of the Southern Panel Mining Area reduce the potential for rockfall/landslide areas where <br /> E-seam mining has been underway since December 2008. <br /> 7. Field visits have revealed the healing and sealing capacity of cracks in surficial material by <br /> weathering, mass wasting, and crack infilling over time. This is particularly true in the <br /> colluvium that covers much of the surface of the Southern Panels Mining Area. The <br /> healing and sealing capacity of these materials causes softening and rounding of the crack <br /> edges as well as reduction of crack continuity and depth to a point of being nearly <br /> imperceptible within a year or two. <br /> 8. Mining activities within the Southern Panels Mining Area have caused no reported impacts <br /> on surface flow or induced inflows to the underground mine workings even while mining <br /> 831-032.900 Wright Water Engineers, Inc. Page 23 <br /> April 2019 <br />