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Subsidence Evaluation For <br />Exhibit 60 The Apache Rocks And The Box Canyon Mining Areas Page 7 <br />• maximum about midway between the point of inflection and the point of maximum vertical <br />displacement and decrease to zero again at the point of maximum vertical displacement. <br />4.3.1.1 Multiple Seam Mining <br />Subsidence parameters (vertical and horizontal displacement, tilt, and strain) increase in <br />proportion to the total thickness of coal mined from different seams where the mining panels, <br />chain pillars, and gate road entries are vertically superimposed. Thus, the subsidence parameters <br />determined for each coal seam extracted may be added. <br />In the westem panels of the Apache Rocks mining azea, both the E- and B-Seams are planned to <br />be extracted with the mining panels, chain pillars, and gate road entries vertically superimposed. <br />Under current plans, a maximum of 11 feet of coal will be extracted from the E-Seam, followed <br />by extraction of a maximum of 12 feet of coal in the B-Seam, after the overlying E-Seam mining <br />is complete. Maximum vertical displacement, tilt, curvature, and horizontal strain under this <br />plan aze shown in Table 2. <br />A descending sequence of coal seam extraction alleviates mining in coal and rocks that may have <br />been impacted by subsidence from mining coal seams in ascending sequence. Based on existing <br />geologic and hydrologic information, a descending sequence seems most appropriate in the <br />western panels of the Apache Rocks mining area. An ascending sequence of mining may be best <br />where lazge amounts of water and/or methane can accumulate in an overlying mined-out area if <br />coal were to be mined in a descending sequence. <br />4.3.1.2 Maximum Vertical Displacement <br />The following ranges of vertical displacements aze estimated for the Apache Rocks and the Box <br />Canyon mining azeas based on data obtained from subsidence measurements in the current <br />mining azea (Table I) and projected into the geologic and topographic environment of the <br />Apache Rocks and the Box Canyon mining areas (Tables 2 and 3, respectively). <br />Apache Rocks Minine Area. Overburden depth above projected eastern longwall panel centers <br />(B-Seam mining only) of the Apache Rocks mining area ranges from about 950 feet neaz the <br />head of Sylvester Gulch to about 2,250 feet near the south end of West Flatiron (see Map 14 for <br />details). Overburden depth above the westem panel centers ranges from about S00 feet to about <br />1,050 feet for the E-Seam and 700 feet to 1,250 feet for the B-Seam. <br />With a projected longwall panel width of 950 feet, and assuming that the chain pillar dimensions <br />will be similar to the current West Elk mining area, maximum subsidence (Sm=a~t) is predicted as <br />follows (Table 2): <br /> <br />831-032.181 Wright Water Engineers, Inc. <br />