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October 11, 2011 Page 6 <br />angle from vertical between the edge of the panel at seam level and the limit of measurable <br />subsidence. The default influence angle in SDPS for Colorado, 66.2 °, was applied. Another <br />input parameter for the analysis, edge offset, was estimated to be 50 ft for the Blue Seam and <br />112.5 ft for the Apache Seam. Edge offset is defined as the horizontal distance between the <br />inflection point (the transition point between zones under tension and compression) on the <br />surface and the edge of the mined -out panel. The edge offset was estimated using guidelines <br />given in SDPS, based on the width of the panel and depth of the workings. A schematic <br />representation of the relevant input and output parameters used in this analysis are presented in <br />the Appendix. <br />Subsidence profiles across the lesser dimension of mined -out panels for all the four <br />models are shown in Figure 1. The figure presents the progression of the subsidence trough in <br />the lateral direction, in the aftermath of panels mined in each of the two seams. The subsidence <br />profile in Model 1 is a symmetrically deformed trough, as the panels in both the seams are <br />aligned in this case and the ultimate subsidence is a sum of subsidence induced by equally <br />dimensioned panels in the two seams. In Model 2, a total of four panels are excavated in the <br />upper Blue Seam and two panels in the lower Apache Seam, which is reflected in the combined <br />deeper subsidence trough from both the seams to the left and the lateral extension of the <br />subsidence basin to the right due to the two additional mined -out panels in the upper seam. The <br />predicted subsidence trough is extended all the way to the right in Model 3, when four panels are <br />mined in both the seams. It may be inferred from the results that the ultimate shape of the <br />surface depression from multi -seam retreat mining will be a function of the final effective width <br />of mined -out workings. <br />Maximum horizontal strain plots for the three models are presented in Figure 2. Figure 2 <br />depicts the cumulative effects of horizontal strains emanating from mined -out workings in both <br />the seams. As in the subsidence profile plot, the maximum horizontal strain plot for Model 1 <br />shows symmetric peaks at both edges of the equally dimensioned mined -out panels in both the <br />seams, representing high tensile strains. As the width of the mined -out panels are extended to <br />the right in Models 2 and 3, the tensile strain peaks resulting from the individual panel edges in <br />the two seams may be seen to have been isolated. For example, the two tensile strain peaks on <br />the right of the Model 2 plot represent maximum tensile strains anticipated from retreat mining <br />of Blue and Apache Seam workings, from right to left, respectively. The peak tensile strain on <br />the left edge, which is fixed for all the panels, is the sum of the aforementioned two peaks for <br />Model 2. The same observation can be made for the other models as well, meaning that the <br />predicted horizontal strain at any given point is the arithmetic sum of horizontal strains resulting <br />from mined -out workings in each seam. <br />The predicted plots of maximum slopes and curvatures for all the models are presented in <br />Figures 3 and 4. These surface deformation indices also follow a trend similar to the subsidence <br />and horizontal strain plots. Horizontal limits of the panels used in the four models are indicated <br />in all the subsidence plots. <br />Agapito Associates, Inc. <br />