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October 11, 2011 Page 5 <br />3.0 SUBSIDENCE PREDICTION ANALYSIS AND RESULTS <br />AAI used the subsidence prediction program SDPS (Surface Deformation Prediction <br />System)2 to develop representative numerical models of surface deformation due to retreat <br />mining. The "Influence Function" module of SDPS was applied. This module assigns a <br />mathematical expression (in this case, the bell- shaped Gaussian function) to predict the <br />subsidence distribution induced by excavation of a unit area. The influence function method has <br />the ability to superpose the influences from multiple and irregular mine geometries. In addition <br />to vertical subsidence, SDPS estimates other subsidence indices such as strain, slope, and <br />curvature. <br />Three subsidence prediction models were developed, representing possible multi -seam <br />mining scenarios involving the Blue, and Apache Seams. The selection of the Apache Seam <br />over the Allen Seam was guided by the fact that the Apache Seam has a lesser depth of cover as <br />compared to the Allen Seam, which is an influential factor on surface deformations. The <br />sequential extraction of several adjacent panels in both the Blue and Apache seams was <br />simulated using a mining height of 5 ft in each seam. The models assumed overburden depths of <br />200 ft for the Blue Seam, and 450 ft for the Apache Seam. These depths were chosen to <br />represent the shallowest multi -seam mining scenario within the permit boundary that may impact <br />the surface structures through subsidence. For each of the seams, the extraction thickness was <br />assumed to be constant at 5 ft within the models. <br />The three models were developed with a varying number of mined -out retreat panels in <br />the two seams. In all three models, one edge of the mined -out panels in the twoseams was fixed <br />vertically and the widths of the mined -out workings were varied with respect to the fixed edge. <br />Single panels were assumed to be 540 ft wide and 3,000 ft long. Model 1 had single panels <br />mined out in both the seams. Model 2 had four panels mined out in the upper seam (Blue) and <br />two panels mined out in the lower seam (Apache). Model 3 had four panels mined out in both <br />the Blue Seam and the Apache Seam. Even though the actual mine plan may have the multi - <br />seam panels from the two seams oriented at different angles from each other, the panels were <br />assumed to be parallel in the models for simplicity, and the parallel geometry should result in a <br />worst -case subsidence impact prediction. <br />The input parameters used in the numerical analysis were assumed based on guidelines <br />provided in the SDPS manual and on AAI's experience and judgment. The review of limited <br />borehole lithology in the projected multi -seam workings area indicated that the percentage of <br />hard rock in the overburden was estimated to be 51 % for the Blue Seam and 61 % for the Apache <br />Seam. Consequently, based on SDPS recommendations for western United States (U.S.) <br />conditions, the maximum subsidence factor (the ratio of maximum possible subsidence -to- <br />extraction thickness) was assumed to be 0.33 for the Blue Seam and 0.26 for the Apache Seam. <br />Another required input parameter is the influence angle, which is the angle between the <br />projection of the inflection point (the zero curvature point on the surface subsidence profile) to <br />seam level, and the limit of measurable subsidence, as measured from horizontal. The influence <br />angle is approximately equal to the complement of the angle of draw. The angle of draw is the <br />2Agioutantis, Z. and M. Karmis (2002), "SDPS for Windows: An Application for Subsidence Prediction, Optimum <br />Mine Design and Environmental Control," SWEMP, Cagliari, Italy, 6 p. <br />Agapito Associates, Inc. <br />