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2011-02-14_PERMIT FILE - C1980007A (3)
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2011-02-14_PERMIT FILE - C1980007A (3)
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Last modified
12/13/2018 7:01:03 AM
Creation date
4/14/2011 9:10:05 AM
Metadata
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Template:
DRMS Permit Index
Permit No
C1980007A
IBM Index Class Name
Permit File
Doc Date
2/14/2011
Doc Name
SUBSIDENCE EVALUATION
Section_Exhibit Name
Exhibit 60E Subsidence Evaluation for the South of Divide and Dry Fork Mining Areas
Media Type
D
Archive
Yes
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Subsidence Evaluation for the <br />Exhibit 60E South of Divide and Dry Fork Mining Areas Page 10 <br />Also, with increasing height in this zone, and as lateral and vertical constraints increase, <br />fracturing that could impact water bearing zones will tend to occur more in zones of convex <br />upward curvature, along separated bedding planes toward the center of the panel, and along local <br />cracks in zones of convex downward curvature (Figure 2). Fracturing within the expected zone <br />of fracture may cease completely where soft shales and claystones occur as alternating sequences <br />with sandstones. <br />Drainage into the fractured formations, however, may cease after mining is complete and any <br />water bearing zones present may be restored. This is particularly likely in the upper part of the <br />fractured zone in shale sequences between sandstone layers, once subsidence is completed and <br />the separated beds re- compress and close in response to overburden load (Figure 2). Evidence of <br />restored water levels has been measured and reported in some wells in the West Elk Mine <br />subsidence monitoring area after B -seam mining and subsidence were complete. <br />5.3 Continuous Deformation Zone and Near Surface Zone <br />These two zones are discussed together because the ground surface is where nearly all <br />measurements are made, that monitor subsidence processes active in the zone of continuous <br />deformation. <br />The near surface zone, which typically consists of weathered bedrock, colluvium, alluvium, and <br />soil a few feet to a few tens of feet thick, may deform differently than the underlying bedrock <br />(Figure 2). Field studies by Dunrud indicate that near- surface colluvium and alluvium, which <br />i consist of predominantly clay and silt, can undergo significantly more extension without <br />rupturing than can the underlying material. In both the Somerset, Colorado and Sheridan, <br />Wyoming, areas colluvium and alluvium 5 to 10 feet thick were observed to cover cracks as <br />much as 10 to 14 inches wide so that there was no indication of the underlying fractures. <br />The zone of continuous deformation, which is transitional to the overlying near- surface zone and <br />also to the underlying zone of fracturing, undergoes differential vertical lowering and flexure as <br />laterally- constrained plates (in three dimensions) or beams (in two dimensions). With flexure, <br />shear occurs at the boundaries of rock units with different strength and stiffness, characteristics, <br />such as sandstones and shales. Zones of tension above the neutral surfaces of a rock unit, for <br />example, become compressive above the boundary with another rock unit and below its neutral <br />surface (Figure 2, Enlargement 2). Any cracks, therefore, which occur in the tension zone of a <br />rock unit, terminate at the neutral surface, because the unit is in compression below this point. <br />5.3.1 Vertical and Horizontal Displacement, Tilt, and Horizontal Strain <br />Differential vertical lowering of the continuous deformation and near surface zones causes <br />vertical displacement (S), horizontal displacement (Sh), tilt (M), and horizontal strain (E). In flat <br />or gently sloping terrain (slopes less than about 30 percent), surface profiles of subsidence <br />depressions are similar to flexure of fixed -end, laterally constrained beams. Tensile stresses are <br />present in areas of positive curvature, which become zero downward at the neutral surface, then <br />reverse to compressive stresses below the neutral surface. <br />831 - 032.810 <br />Wright Water Engineers, Inc. <br />Irm <br />
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