Steepening Pit Walls- Geotechnical and Economic Implications

Abstract

Current approach to pit-wall analysis tends to separate designs into distinct categories. The first consideration involves analysis of slope in which discontinuities actively participate in the failure mobilization. The second involves design analysis of non-structural controls which could results in a wide range of failure mechanisms. The third and the least sought for approach is the economic assessment of the slope design which could render optimized pit slope and the bench face angle scenarios economic or otherwise. The sensitivity of the third consideration has become imperative in the face of depleting pit reserves as any positive turn over in the economic analysis could revive distress mine operations. A typical operating pit sited west of Ghana’s metalogenic province which was near collapse due to ore depletion was re-designed through steepening of the slopes to expose more resource. Geotechnical, window and photogrammetry mappings were carried to augment the historical data available. Several numerical analytical tools were also employed to model the pitwall angles. Four steeper slopes were designed from the current BFA of 65° to 70°, 75°, 80° and 85^o. This was done to assist in stability, reduce stripping ratio and improve cash flow. The results indicated that discontinuities steeply oriented towards east direction could make the slope highly susceptible to planer and toppling failures The studies concluded that it may not be feasible to steepen BFA from the current base angle due to the predicted steeper angles as a results of overall slope instabilities even though the potential economic benefit is substantial. If steepening would be considered, major modifications of slope geometry; rock improvement, extensive slope monitoring and a good groundwater control management mechanisms may be required.

Keywords: Birimian , Auriferous, Pitwalll, Instability, Failure, Economic Analysis, Batter face angle,.