2021-02-05_PERMIT FILE - C1980007A (5)

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Subsidence Evaluation for the Exhibit 60E Southern Panels, Apache Rocks West, & Sunset Trail Mining Areas Page 11 In areas of two-seam mining, it is possible for the heights of the fractured zones to become cumulative as the height of fracturing from the underlying B-seam could extend up to the E-seam. This potential exists in the Apache Rocks West and Southern Panels mining areas. However, this phenomenon would not increase the height of fracturing above the E-seam, which would govern the potential for near-surface impacts. Also,with increasing height in this zone, and as lateral and vertical constraints increase, fracturing that could impact water bearing zones will tend to occur more in zones of convex upward curvature, along separated bedding planes toward the center of the panel, and along local cracks in zones of convex downward curvature(Figure 2). Fracturing within the expected zone of fracture may cease completely where soft shales and claystones occur as alternating sequences with sandstones. Drainage into the fractured formations,however,may cease after mining is complete and any water bearing zones present may be restored. This is particularly likely in the upper part of the fractured zone in shale sequences between sandstone layers, once subsidence is completed and the separated beds re-compress and close in response to overburden load (Figure 2). Although very few water bearing zones have been encountered, evidence of restored water levels has been measured and reported in some wells in the West Elk Mine subsidence monitoring area after B-seam mining and subsidence were complete. 5.3 Continuous Deformation Zone and Near Surface Zone These two zones are discussed together because the ground surface is where nearly all measurements are made that monitor subsidence processes active in the zone of continuous deformation. The near surface zone, which typically consists of weathered bedrock, colluvium, alluvium, and soil a few feet to a few tens of feet thick, may deform differently than the underlying bedrock (Figure 2). Field studies by Dunrud indicate that near-surface colluvium and alluvium, which consist of predominantly clay and silt, can undergo significantly more extension without rupturing than can the underlying material. In both the Somerset, Colorado and Sheridan, Wyoming field study areas, colluvium and alluvium 5 to 10 feet thick were observed to cover cracks as much as 10 to 14 inches wide so that there was no indication of the underlying fractures. The zone of continuous deformation, which is transitional to the overlying near-surface zone and also to the underlying zone of fracturing, undergoes differential vertical lowering and flexure as laterally-constrained plates (in three dimensions) or beams (in two dimensions). With flexure, shear occurs at the boundaries of rock units with different strength and stiffiiess, characteristics, such as sandstones and shales. Zones of tension above the neutral surfaces of a rock unit, for example, become compressive above the boundary with another rock unit and below its neutral surface (Figure 2, Enlargement 2). Any cracks, therefore, which occur in the tension zone of a rock unit, terminate at the neutral surface,because the unit is in compression below this point. 831-032.912 Wright Water Engineers, Inc. December 2020