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EXECUTIVE SUMMARY • <br />At the request of Mr. John Rusnak, Director Geology, Peabody Energy, NSA <br />Engineering, Inc., performed evaluations of the geotechnical design requirements and <br />related operational considerations for hiQhwal] mining at the Seneca Mine near Hayden, <br />CO. Highwall mining with a Peabody Energy-owned Superior Highwall Miner (PM) <br />will be conducted, with potential penetration up to 1,000 ft into the highwall. Two <br />different azeas were analyzed: the Seneca IIW Area, and the Yoast Area. Within the <br />Seneca IIW area, three seams will be targeted. From the lowest elevation upward, they <br />aze the Wolf Creek, Sage Creek, and \\~adge Seams. In the Yoast area, only the Wolf <br />Creek and Wadge Seams are tazgeted. <br />This report addresses the followine specific issues relative to highwall mining <br />with the Superior Highwall Miner (SHM) at the Seneca Mine: y <br />• What specific geotechnicaVgeological constraints exist within the identified coal <br />seams and mining areas that may limit highwall mining applications? <br />• For each seam to be mined, what minimum web pillar dimension (as a function <br />of depth, mining height, and material properties) will ensure that highwall <br />stability is unaffected by mining operations? <br />• What is the potential for "cascading pillar failure," and if required, "'hat • <br />minimum dimension of barrier pillar will be required to sufficiently isolate <br />extraction panels within and bet.yeen major mining regions along the existing <br />highwall? <br />• For each coal seam, what is the potential for out-of-seam dilution of the mined <br />product, taking into account roof stability for 11.5-ft•~+ide rectangular openings <br />and anticipated roof/floor conditions? <br />• \~Vhere the \i'olf Creek and Sage Creek Seams are mined in close proximity, to <br />what degree will seam interaction dictate special operational or design <br />considerations? <br />• \\'hat operational measures can be taken to increase high~~ all mining efficiency <br />and safety? <br />To best address these questions, 1~'SA's evaluation ~a~as conducted by completing <br />three Tasks. including (1) site characterization (review of site conditions and current <br />highwall stability, core recovered from the Yoast Area, physical properly testing, etc.): <br />(2) engineering design (empirical and numerical modeling evaluation of optima] mining <br />plans for the Seneca IIW and Yoast Areas); and (3) operational considerations <br />(identification of relative risks to mining by pit, production efficiency. potential for out- <br />of-seam product dilution etc.). • <br />