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Spring 2018 Subsidence and Geologic Field Observations <br />South of Divide and Dry Fork Mining Areas <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 SOD and Dry Fork mining <br />areas 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 SOD and Dry Fork mining areas reduce the potential for rockfall/landslide areas <br />where 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 SOD and Dry Fork mining areas. The <br />healing and sealing capacity of these materials cause softening and rounding of the crack <br />edges as well as reduction of crack continuity and depth to a point of imperception within <br />a year or two. <br />8. Mining activities within the SOD and Dry Fork mining areas have caused no reported <br />impacts on surface flow or induced inflows to the underground mine workings even while <br />mining directly beneath Dry Fork. Of similar significance is the lack of observed cracks <br />in alluvial materials. <br />831-032.900 Wright Water Engineers, Inc. Page 21 <br />August 2018 <br />