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January 2, 2020 Page 4-3 <br />Agapito Associates, Inc. <br />Table 4-2. ARMPS-HWM SF Criteria <br />Stability Factor Criteria <br />Overall SF <br />2.0 Applicable to all conditions <br />Web Pillar SF <br />1.6 When the panel width (excluding the barrier) exceeds 200 ft <br />1.3 When the panel width (excluding the barrier) is less than 200 ft <br />Barrier Pillar SF <br />2.0 When the barrier width-to-height ratio is less than 4.0 <br />1.5 When the barrier width-to-height ratio is greater than or equal to 4.0 <br /> <br /> <br />opening penetration provides support unaccounted for by tributary area loading. Following a <br />procedure suggested by Mark, Chase, and Campoli (1995), a less conservative approach would be <br />to calculate the design depth based on a weighted average as follows: <br /> <br /> Design depth = 0.75 (maximum depth) + 0.25 (minimum depth) (Eqn. 4-4) <br /> <br />In determining the minimum depth, some judgement should be exercised. AAI <br />recommends that the minimum depth beyond the crest of the highwall be used. The design process <br />is repeated for the design of subsequent panels as mining proceeds. <br /> <br />Trapper may elect to stack spoil material adjacent to some pit areas. The surcharge load <br />from spoil should be accounted for by adding the thickness of the spoil multiplied by the ratio of <br />the spoil to rock unit weight to the cover depth over the panel. For example, a 100-ft thick spoil <br />having a density of 100 pcf would have an equivalent rock height of about 62 ft (100 ft × <br />100 pcf/162 pcf). Therefore, the cover depth underneath the spoil should be increased by 62 ft <br />when determining the appropriate design depth for a given panel. By designing on a panel-by- <br />panel basis, changing mining heights and cover depths can be accommodated reasonably and <br />design performance can be continuously evaluated. <br /> <br />4.2 Pillar Design Tables and Charts <br /> <br />Required web and barrier pillar widths and associated recovery percentages for the <br />target seams were calculated using the procedure and ARMPS-HWM criteria discussed above. <br />Design tables and charts for the normalized in-situ coal strengths from Table 4-1 are given in the <br />Appendices A through D for each Pit and target seam. In cases where the planned HWM panel <br />width is less than 200 ft, and therefore the ARMPS-HWM SF criterion is reduced, separate charts <br />are provided. These charts apply primarily to pit endwall areas. The tables and charts are used to <br />determine pillar widths and panel recoveries for specific values of mining height and cover depth. <br />The figures designated as “a” include a table for determining the required web pillar width given <br />in inches, and a chart showing the width expressed both in inches and feet. The “b” figures give <br />the required barrier pillar width expressed in feet in both the tables and charts. The charts <br />sometimes show a flat area at low cover depths, corresponding to pillar widths being limited by <br />the minimum 0.8 width-to-height ratio criterion (these widths are printed in italics in the design <br />tables). Some barrier pillar charts also show an area where the curves spread apart, corresponding <br />to the change of SF when the barrier pillar width-to-height ratio exceeds 4. The recovery tables