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Tonia Perkins <br />July 31, 2024 <br />Page 2 <br />Agapito Associates, Inc. <br />method is readily accepted by the Mine Safety and Health Administration (MSHA) as a design <br />basis for HWM pillars. Agapito specified a minimum allowable pillar width-to-height ratio of 0.8 <br />based on experience with similar HWM projects. In line with design criteria used in the study <br />conducted by Agapito in 2016, a minimum safety factor (SF) criterion of 1.6 was established for <br />web pillars in all the panels where the barrier pillar width-to-height ratio was anticipated to be less <br />than 4.0. The barrier pillar widths for each seam were calculated using the ARMPS-HWM method, <br />and their corresponding safety factors range from 3.5 to 4.5 for the range of conditions considered <br />in the N-Pit analysis. <br />Separate design charts were developed for web and barrier pillars in the M and Q seams, for a total <br />of four design sets. The design curves incorporate the assumption that all HWM panels in the N- <br />Pit will be comprised of 10 openings, which results in panel widths, calculated center-to-center on <br />barrier pillars, that may range from roughly 180 ft in the south M Seam to 270 ft in the North Q <br />Seam. <br />Design Charts <br />Design specifications are given in Figure 2 and Figure 3 for the M and Q seams, respectively. The <br />design tables for each seam are valid in the North and South portions of the N-Pit, as depths of <br />cover are provided to cover both areas. Each figure contains three separate components, labeled <br />(a), (b), and (c). The contents of each figure are arranged as follows: <br />•The (a) portion of each figure pertains to web pillars. The table provides web pillar widths <br />that are required for corresponding values of mining height and depth of cover. All values <br />are dimensioned in feet. The chart offers a graphical representation of the tabular data for <br />quick reference. The plotted lines flatten at lower depths of cover, which indicates that <br />pillar widths are limited by the minimum 0.8 width-to-height ratio criterion. <br />•The (b) portion of each figure pertains to barrier pillars. The table provides the required <br />barrier pillar widths for corresponding values of mining height and depth of cover. All <br />values are dimensioned in feet. The chart offers a graphical representation of the tabular <br />data for quick reference. <br />•The (c) portion of each figure pertains to estimated recovery in plan-view. The table <br />provides an estimated percentage of coal seam recovery for corresponding values of mining <br />height and depth of cover, which affect pillar geometry. The chart offers a graphical <br />representation of the tabular recovery data for quick reference.