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January 2, 2020 Page 6-1 <br />Agapito Associates, Inc. <br />6 OPERATIONAL CONSIDERATIONS <br /> <br /> The design curves presented in the Appendices A through D provide Trapper with a <br />rational starting point for highwall web and barrier pillar layout. By using these design curves to <br />determine the minimum pillar width for each panel as mining progresses and adjusting that width <br />as conditions warrant, maximum resource recovery can be attained. Based on the observations and <br />analyses discussed in this report, the following comments related to implementation of the design <br />are made. <br /> <br /> Roof Stability and Dilution—The calculated standup times for the roofs of all HWM areas <br />indicate that the roofs should be sufficiently stable to allow HWM. However, using CMRR criteria, <br />the roofs are generally classified as weak and occasional roof falls may occur. The areas with the <br />least competent roof are predicted to be the H Seam in the L Pit, the F Seam in the Middle and <br />West I Pit, and the L Seam in the N Pit. If roof falls in these or other areas become problematic, <br />leaving a thin, 0.5- to 1-ft layer of top coal may improve stability. Floor conditions should also be <br />adequate, although where carbonaceous mudstone makes up the immediate floor, occasional floor <br />trafficability problems may occur. Again, leaving floor coal may solve the problem; however, <br />given the relatively thin seam thicknesses in some areas, frequent implementation of roof and/or <br />floor coal will have a relatively large impact on project recovery and economics. <br /> <br /> Mining Sequence <br /> <br />I and J Pits—From an HWM standpoint, the resource areas are independent of one <br />another. If the box cuts are mined before HWM, there is no preference regarding the sequencing <br />of HWM. However, the three I Pits partially overlie the G2 Seam HWM reserve and blasting in <br />the I Pits could adversely impact HWM panels in the G2 Seam. Therefore, the surface reserves of <br />the I Pit should be mined prior to HWM the G2 Seam. Decisions regarding which areas to mine <br />first (East, Middle, or West) should be based on operational preferences, as no known geotechnical <br />issues are involved. Given the side dip slightly to the west, mining from west to east would allow <br />the mining to progress away from any water accumulations in the pit. <br /> <br />N Pit—Stability of the southern highwall, updip from the box cut, needs to be preserved <br />as much as possible, in light of landslides that have occurred at Trapper related to pits oriented <br />along strike. To accomplish this, Trapper intends to subdivide the N Pit HWM into segments, to <br />keep the open cut length to a practical minimum. AAI has performed numerical modeling of the <br />HWM in N Pit that takes the seam dip and excavation sequence into account, and specific <br />recommendations for mining the south (updip) and north (downdip) HWM areas have been given <br />in the N Pit detailed report (AAI 2019a). Many tradeoffs exist, but in general, mining each segment <br />from the pit bottom up (i.e. mining the Q Seam first) is preferred, with the sequence of developing <br />and mining the pit segments from west to east. <br /> <br />Additionally, the following operational measures may be adopted to minimize the risk of <br />global instabilities, especially in the southern highwall: <br /> <br /> Blasting designs with longer delays, as currently implemented to overcome ground <br />conditions in the L Pit, should be appropriately applied to limit excessive damage of the <br />highwall rock mass and weak shale/mudstone layers.