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January 2, 2020 Page 6-3 <br />Agapito Associates, Inc. <br />of the Q Seam. Therefore, designs for each seam are independent of one another, and the pillars in <br />adjacent seams do not need to be columnized. The same is true for the K and Q Seams in the L Pit. <br /> <br /> Contingency Planning—Because the web and barrier pillar designs for each seam are <br />independent, should poor mining conditions be encountered in one seam or area of the pit, designs <br />for the remainder of the seams or pit should not be impacted. <br /> <br /> Highwall Stability—The design curves provided in this report are intended to provide for <br />pillar stability during active mining operations and therefore, preserve the integrity of the highwall. <br />If any situations not contemplated during the underlying HWM studies (AAI 2018, 2019a, 2019b) <br />are considered by Trapper, these should be analyzed separately. <br /> <br />Nearby Blasting—It is common practice to limit blasting to within 1,500 ft of HWM <br />operations. If exceptions to this procedure are necessary, HWM operations should cease until the <br />blast is complete and the highwall, pillars, and openings have been re-examined for any damage <br />caused by the blast. <br /> <br /> Required Pit Width—The standard ADDCAR launch vehicle requires a minimum pit <br />width of 150 to 200 ft. This includes a stand-off distance of 25 ft between the launch vehicle and <br />the highwall. At the pit bottom, a spoil catch berm is also required. <br />Hole Closure—Leaving several holes open adjacent to the active hole is recommended to <br />allow observation of the web pillars and possible signs of movement. AAI recommends that a <br />maximum of eight holes be left open for this purpose. To keep the open holes at or below this <br />number, spoil or other suitable material should be placed over the mouth of the entries to close <br />them and prevent personnel entry, which is prohibited per 30 CFR 77.1502 (US Department of <br />Labor 2019). <br /> <br />Long-Term Subsidence Potential—The pillar designs included in this report provide for <br />an acceptable safety margin against pillar failure during active mining. Trapper should be aware <br />that, historically, subsidence has occurred over some HWM panels and that there is a risk of <br />subsidence associated with any form of underground mining, including HWM. Because this risk <br />increases with time, every effort should be made to accomplish the planned HWM in a timely <br />manner. Use of the design curves based on normalized coal strengths given in this report will <br />reduce the likelihood of long-term subsidence but will not eliminate the possibility. <br /> <br />Spontaneous Combustion—Should web pillar failure occur, the risk of spontaneous <br />combustion is increased. This is due to coal crushing and possible air circulation through the <br />highwall miner openings to the surface, through subsidence cracks. Again, while the design curves <br />provided in this report are intended to reduce the likelihood of pillar failure, Trapper should be <br />aware of the consequences should failure occur. <br /> <br />