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Probable Hydrologic Consequences New Elk Mine 26 <br />Table 16. Predicted Subsidence for Mining in the Allen and Apache Coal Seams <br />Maximum Maximum Horizontal Strain Maximum Maximum Curvature <br />Scenario Subsidence millistraiu Slope 1 /ft <br />(ft) Tensile Po <br />Compressive ( %1 sitive Nepative <br />Case 1 - 450 feet of overburden <br />One mined panel <br />1 2.6 <br />1 6.89 <br />1 6.88 <br />1 1.29 <br />1 0.99 <br />1 0.99 <br />Two mined panels <br />1 2.6 <br />1 6.88 <br />1 6.89 <br />1 1.29 <br />1 0.99 <br />1 0.99 <br />Case 2 - 1,200 feet of overburden <br />One mined panel <br />1 2.0 <br />1 2.55 <br />1 4.91 <br />1 0.47 <br />1 0.26 <br />0.14 <br />Two mined panels <br />1 2.6 <br />2.58 <br />1 2.59 <br />1 0.49 <br />1 0.14 <br />1 0.14 <br />Subsidence over the mined panels would be manifested at the surface as gentle, almost imperceptible <br />troughs with shallow tension cracks at the edges and in other areas of local extension. The maximum <br />potential subsidence associated with mining adjacent panels in either the Allen or Apache would be 2.6 <br />feet. Subsidence would be most likely to occur in areas with thin overburden. The calculated subsidence <br />over one panel at a depth of 1,200 feet is 2.0 feet. Retreat mining and subsidence below the Purgatory <br />River is not planned, and a subsidence buffer zone will be left in place around the alluvial valley floor. <br />Published research indicates that subsidence from mining has predictable effects on surface water and <br />groundwater hydrology. Kendorski (1993), recognized five zones above subsided panels that have <br />different hydraulic responses and characteristics. These zones are summarized in Table 17. <br />Table 17. Hydrologic Effects of Subsidence above Mining Panels <br />Zone <br />Effect <br />Extent <br />Characteristics <br />Increased Transmissivitv <br />Zone of complete stratigraphic disruption. Extends two to eight times the mining <br />Caved Zone <br />8 to 52 feet above <br />height above the seam. Roof strata collapse into the opening and form rotated blocks <br />intersected groundwater <br />quickly drained into mine <br />mined seam <br />with increased volume that limits the upward progression of caving. Groundwater <br />intersected by the caved zone is quickly drained into the mine workings. <br />Increased Transmissivitv <br />From top of caved <br />Zone of vertically transmissive fractures without rotation. Bedding retains original <br />Potential for localized <br />zone to a maximum of <br />attitude and does not fall or detach. Fracture density and permeability increase <br />Fractured Zone <br />interconnection of <br />about 195 feet above <br />downward. The combined height of the fractured and caved zones can be 20 to 30 <br />groundwater in the Allen, <br />mined seam <br />times the mining height. Increased vertical and horizontal permeability allow for <br />Apache and Maxwell seams <br />drainage of intersected water into the mine workings. <br />Zone of beam deformation in which strata sag but do not form vertically connected <br />Increased Storage <br />from top of fractured <br />fractures. Bed dilation increases groundwater storage but does not allow for vertical <br />Potential for temporary <br />zone to a maximum of <br />transmission of water into underlying zones. Generally extends from about 24 to 60 <br />Dilated Zone <br />decrease in water levels as <br />about 390 feet above <br />times the mining height above the seam. Wells and surface water intersected by the <br />the additional storage <br />mined seam <br />dilated zone will experience temporary reductions in level /volume as water goes into <br />volume is filled. <br />storage. Water levels typically recover to pre- mining levels after the increased <br />storage volume has been filled. <br />No Change <br />top of the dilated zone <br />Zone of no significant effect on transmissivity and storage. Strata are affected by <br />Continuous <br />Constrained <br />Potential for Hydrologic <br />to 50 feet below <br />subsidence but are not sufficiently deformed or strained to dilate and significantly <br />Zone <br />Zone <br />impacts is minimized. <br />ground surface <br />increase storage. Overall extensile strains remain less than 0.1 %, the point at which <br />rock masses are not sufficiently disrupted to increase permeability. <br />Surface <br />Increased Transmissivitv <br />Surface zone with shallow tension fractures caused by subsidence. Fractures <br />Potential for localized, <br />Fracture <br />0 to 50 feet below <br />generally extend less than 50 feet below ground surface and are spatially related to <br />Zone <br />short - duration changes in <br />ground surface <br />panel /trough edges and areas of local extension. Changes in hydraulic regimes are <br />groundwater levels. <br />short lived because fractures are localized, shallow, and quickly filled by sediment. <br />rvote: tone tmcKnesses are calculatea hasea on a mining height of 6.5 feet. <br />4164A.110421 <br />TA G,< <br />Tvl+�l�it Q[Al Dn 1A7 <br />Whetstone Associates <br />A /7n1 1 <br />