2008-04-30_REPORT - M1973021 (3)

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~ ' TETRA TE~1~ • • The geotechnical investigations for the North Quarry all describe localized areas of poor condition rock. The features described in two cases (the shear zone in the grey rock area and the clay laden • seam) cross through multiple mined benches in the quarry. Therefore, the rock quality was a concern as quarry slopes developed. • Additionally, foliation and joint orientations were a safety concern for the North Quarry. The Central • Quarry (which the slope design was based on) is oriented north to south with the most exposed rock on the west (favorable foliation) and east (unfavorable foliation) faces. The North Quarry is oriented west to east with the most exposed rock on the north and south faces. The foliation generally dips • west but is also folded in a manner resulting in localized dips out of the north and south faces. The foliation pattern provides the potential for sliding and wedge failures with the highest potential being on the east faces, followed by the north and south faces. Due to the higher safety concerns associated with the rock mass in the North Quarry as compared to the Central Quarry, highwalls were developed with significantly shorter spans below elevation 6,455 (above elevation 6,455 the highwalls were planned to be 40 feet). The change in highwall span to 40 feet resulted in a bottom elevation of the quarry at 6260. The changes in mining affected the design of the north reservoir. The original high water line between elevations 6020 and 6080 was moved to elevation 6408. The original bottom elevation of the quarry at 5770 was moved to approximately elevation 6260 (Figure 1). CONCLUSION The original mine plan for the North Quarry was not achievable due to safety and operational concerns. AI utilized reasonable, accepted mining practices and incorporated additional guidance from the DRMS for the day to day development of the North Quarry. Changes to the annual mine plan have resulted in a bottom depth of 6260. Compared to an original estimated bottom depth of 5770. These revisions reduced the volume of the excavation from about 2500 AF to 1400 AF. REFERENCES Lyman-Henn, 2001, "Geotechnical Addendum-2001 Annual Report, Morrison Quarry, Morrison, Colorado" Lyman-Henn, 2002, "Geotechnical Addendum-2002 Annual Report, Morrison Quarry, Morrison, Colorado" Lyman-Henn, 2003, "Geotechnical Addendum-2003 Annual Report, Morrison Quarry, Morrison, Colorado" Lyman-Henn, 2004, "Geotechnical Addendum-2004 Annual Report, Momson Quarry, Morrison, Colorado" Lyman-Henn, 2005, "Geotechnical Addendum-2005 Annual Report, Morrison Quarry, Morrison, Colorado" -5-