Failure mechanisms in large scale open pit slopes are more complex than could be considered through conventional slope design methods. Pit slope behaviour must be predicted accurately, because for very deep open pits, a small change of slope angle can have serious technical and economic consequences. Failure of hard rock slopes often involves both failure along naturally existing weakness planes and failure of intact rock. Without an advanced understanding of combined rock slope failure mechanisms, the validity of commonly applied methods of large scale slope analysis is questionable. The problem was investigated by means of a toolbox approach, in which a wide range of slope stability analysis methods were used and compared to address specific problems arising during slope design optimisation of the Cadia Hill Open Pit, NSW. In particular, numerical modelling is an advanced tool to obtain insight into potential failure mechanisms and to assist the slope design process. The distinct element method was employed to simulate complex rock slope failure, including fracture extension, progressive step-path failure and brittle failure propagation, which were previously often considered unimportant or too difficult to model. A new, failure-scale-dependent concept for the categorisation of slope failures with six categories ranging from 0 (stable) to 5 (overall slope failure) was suggested to assist risk-based slope design. Parametric slope modelling was conducted to determine the interrelationship between proposed categories and critical slope/discontinuity parameters. Initiation and progression of complex slope failure were simulated and described, which resulted in an advanced understanding of combined slope failure mechanisms and the important role of rock bridges in large scale slope stability. A graphical presentation of the suggested slope failure categories demonstrated their interrelationship to varied slope/discontinuity parameters. Although large scale slope analyses will always involve data-limited systems, this investigation shows that comprehensive, conceptual modelling of slope failure mechanisms can deliver a significantly improved insight into slope behaviour, so that associated slope failure risks can be judged with more confidence. The consideration of combined slope failure mechanisms in the analysis of large scale open pit slopes is essential if slope behaviour is to be realistically modelled.

Failure mechanisms in large scale open pit slopes are more complex than could be considered through conventional slope design methods. Pit slope behaviour must be predicted accurately, because for very deep open pits, a small change of slope angle can have serious technical and economic consequences. Failure of hard rock slopes often involves both failure along naturally existing weakness planes and failure of intact rock. Without an advanced understanding of combined rock slope failure mechanisms, the validity of commonly applied methods of large scale slope analysis is questionable. The problem was investigated by means of a toolbox approach, in which a wide range of slope stability analysis methods were used and compared to address specific problems arising during slope design optimisation of the Cadia Hill Open Pit, NSW. In particular, numerical modelling is an advanced tool to obtain insight into potential failure mechanisms and to assist the slope design process. The distinct element method was employed to simulate complex rock slope failure, including fracture extension, progressive step-path failure and brittle failure propagation, which were previously often considered unimportant or too difficult to model. A new, failure-scale-dependent concept for the categorisation of slope failures with six categories ranging from 0 (stable) to 5 (overall slope failure) was suggested to assist risk-based slope design. Parametric slope modelling was conducted to determine the interrelationship between proposed categories and critical slope/discontinuity parameters. Initiation and progression of complex slope failure were simulated and described, which resulted in an advanced understanding of combined slope failure mechanisms and the important role of rock bridges in large scale slope stability. A graphical presentation of the suggested slope failure categories demonstrated their interrelationship to varied slope/discontinuity parameters. Although large scale slope analyses will always involve data-limited systems, this investigation shows that comprehensive, conceptual modelling of slope failure mechanisms can deliver a significantly improved insight into slope behaviour, so that associated slope failure risks can be judged with more confidence. The consideration of combined slope failure mechanisms in the analysis of large scale open pit slopes is essential if slope behaviour is to be realistically modelled.