The effect of slope curvature in rock mass shear strength derivations for stability modeling of foliated rock masses

Abstract

In open pits the instability mechanisms are multifaceted involving, shearing along or over the asperities of the defect surface and or block rotation at very low normal stresses and shearing along or through asperities and block rotation including rock mass failure due to intact rock breakage at somewhat higher degrees of confinement. In an open pit environment it is common to have pit designs with varying profiles. This difference in wall geometry would result in varying magnitudes of effective shear strength being mobilised. The contribution of the lateral confinement to the degree of shear strength mobilised is often overlooked in these strength models. In this paper a case study will be presented, which compares the different slope performance outcomes within similar geotechnical conditions with, the only variant being the slope geometry and the influence of lateral confinement. An algorithm will be developed, based on observations from this site case study, linking the concavity of a slope face to the degree of (extra) shear strength mobilised. It is the authors intent that this algorithm can be used in a similar manner to which highway engineers use 'curve-speed models' to determine safe approach speeds for corners given a particular road curvature, (in our case pit slope concavity) and approach speed (rock mass characteristics). This relationship has been successfully implemented for forward analyses at the mine in question, and has lead to the successful optimisation of slope geometry. Copyright 2013 ARMA, American Rock Mechanics Association.