Pitwall Stability Analysis-Case Studies*

Abstract

The importance of pitwall stability in design and economy of open pits has been a subject of considerable research in rock mechanics for some time now. Imperceptible slow mass movement has been a secret agent in inducing major pitwall failure. The problem is location specific and is associated with stress development, shear strength, discontinuity orientations and groundwater regimes. This paper therefore sets out to present pitwwall stability analysis in some selected pits within the Birimian environment in Ghana. Several attitude measurements were taken particularly on the footwall and along the slope face of the oxide, transition and fresh (sulphide) rock materials from some selected pits. The geologic data was subjected to the Romana’s Slope Mass Rating (SMR) analytical procedure and the pitwalls monitored through installed prisms. The oxide/ transition zones gave an SMR quantitative value of 43.1 in the footwall and 44 within the hangingwall, indicating a partial stability at 50o. The fresh rock (sulphide) gave a value of 64 in the footwall showing stability at 55o. As usual, the fresh rock was less problematic as indicated by the SMR assessment compared to the oxide and the transition materials. Creep movement was observed to predominate, being nearly 250 mm/day within the oxidised zone but lesser in the transition and fresh rocks. Induced pitwall failure pattern within the Birimian was more of composite failure (ie.,planar and toppling) capable of destabilising several pits if unchecked.