Abstract
Stope wall failures result in ground stability problems, ore dilution, revenue loss, etc. This work assesses mined primary transverse open stope (32S 204) in AngloGold Ashanti-Obuasi Mine to address stability problems mine-wide. The stope (17 m strike length, 30 m wide, 27 m high and dipping 68°NW) has the Obuasi Fissure as its hanging wall boundary. Back analysis of the performance showed that major geological structures –that is, Obuasi Fissure, Main Reef Shear, etc.-filled with graphitic materials have low shear strength. These structures significantly influence stope hanging wall stability. Undercutting of Obuasi Fissure on 32 level with crosscut and reef drive “Tee” compromised the fissure stability destroying rockmass integrity within the stope. Thus hanging wall overbreak of 2036 tonnes of wall rock diluted mined ore. An underbreak of in-situ ore accounted for 5439 tonnes ore loss. These reduced planned stope grade of 8.94 to 7.97 g/t (11% loss) on completion of mining activities. Limited geotechnical data prevented conventional rockmass classification, estimation of design rockmass strength and rockmass rating for the stope. However, stope height (30 m), dipping 68° is unlikely to present serious rockmass stability and/or overbreak conditions in mined out walls. Blast holes (rings) deviations were minimal, reducing overbreak during blasting.
Key words: Stope performance, cavity monitoring system, dilution, overbreak, underbreak, tonnage, grade.
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