Abstract
Pit slope optimisation is a complex process, as appropriate designs must be provided for different slope elements at batter, inter-ramp and overall slope scales. Maximum achievable parameters at just one scale do not always satisfy design stability criteria for the other slope scales. Therefore, the pit slope optimisation process often requires several iterations to reach an optimum result. However, a simple analytical approach can be useful for slope design for early stage studies or to develop a basis for initial pit shell design upon which further detailed pit slope optimisation analyses can be conducted.
Providing recommendations for preliminary slope angles requires understanding of which scale of instability is the most critical one controlling the overall slope angle. In strong rock masses, achievable overall angle is usually controlled by limitations in batter-berm geometry. In weak rock masses, assessment of overall slope stability is a key factor to identify optimum slope angle, and for medium strength rock masses both scenarios may be applicable. This paper explains a simple analytical approach for identifying overall slope angle, which is based on a method for identifying collective shear strength properties within the rock mass constituting the overall slope and then using established design charts to determine appropriate slope angle. The approach has a key limitation: it requires that there should be no strong influence of structural fabric on rock mass shear strength in a directional manner (i.e. the rock mass must be isotropic); however, heterogeneous rock masses can be assessed if the properties of the lithologies or domains are not vastly dissimilar.
Some case studies are presented to illustrate the preliminary slope angle recommendations based on the simple analytical approach. The appropriateness of the resulting slope designs has been assessed using stability analyses.
