No abstract
Confidence in stability assessments of large rock slopes may be improved by greater understanding the persistence of adverse discontinuities, and the proportion and location of intact rock bridge content within the slope. This paper presents a discussion of the challenges and uncertainty in characterising discontinuity persistence and intact rock bridges, with reference to results from field investigations of open pit slopes at three mines using digital photogrammetry, ground-based LiDAR, and modified 2D window mapping methods. A conceptual numerical model is then devised, where a distinct element numerical code was applied to investigate the influence of rock bridges on brittle rock mass failure and dilation in a model large open pit slope. Distinction between co-planar or out-of-plane intact rock bridges, and larger 'rock mass bridges' between more persistent discontinuities is considered necessary and the authors suggest that a fracture network engineering approach tailored to large open pits may be helpful for their characterisation. With modified trace mapping procedures, intact rock bridges may be quantified in terms of an intensity parameter R 21 that describes the total length of inferred rock bridge traces per unit area within a mapping window. An analogous blast-induced damage intensity factor B 21 is also introduced, that describes the total length of blast-induced fracture traces per unit area in a mapping window. For numerical models, a damage intensity parameter D 21 is applied, which quantifies the intensity of fracturing that develops inside a modelled slope. Large rock slope failures rarely occur entirely along completely continuous, pre-existing basal sliding surfaces. Even if major pre-existing structures exist, deformation and failure of large slopes in hard rock is more likely to involve a combination of shearing and dilation of pre-existing discontinuities such as joints, with a degree of stress-induced brittle fracturing of intact rock (Sjöberg, 1999). The process of brittle crack initiation, propagation and coalescence is progressive (Eberhardt et al., 2004), and may be characterised by a time-dependent degradation of rock mass strength in localised zones of stress concentration, that may eventually lead to (1) the formation of a continuous sliding surface and (2) the development of kinematic freedom and finally slope failure (Stead et al., 2006). The potential complexity of slope failure mechanisms increases with the scale of the slope. In open pit mines, inter-ramp or overall slope failure surfaces may have irregular or step-path geometry, involving rupture through several structural domains with different shear strength properties and different local failure mechanisms. McMahon (1979) introduced the step-path simulation method during investigations for the Bougainville open pit mine in Papua New Guinea with the probabilistic STEPSIM code. Later, the probabilistic step-path simulation approach was further developed for a slope optimisation study at Ok Tedi mine, resulting in the STEPSIM4...
Surface mining operations are increasingly transitioning to underground mass mining operations like block caving to extend the life of the mine. The presence of the open pit above the developing block cave will lead to complex stress-strain interactions, pit slope movements and ground deformations that may extend beyond the periphery of the pit, potentially having an adverse effect on mine infrastructure and operations within the zone of disturbance. These can be further complicated by major fault zones that crosscut the mine property. Experiences at the Palabora Copper Mine in South Africa provide one such example of a transition to block cave mining where the interactions that developed between the underground workings and overlying steep rock slopes evolved into a massive 800 m high pit slope failure. To properly assess the rock mass response to these interactions for the management of pit slope stability, surface subsidence and setback distances for mine infrastructure outside the pit rim, as well as for planning future underground development, understanding the controlling influence of geological structures on the ground response is crucial. Results are presented here from a series of 3D continuum and discontinuum numerical models that investigate the interactions between the open pit, block cave mine and large scale geological features present at Palabora. These results are compared to a detailed review of pit slope monitoring data to help provide understanding of the complex slope displacement patterns recorded and apparent pit slope kinematics.
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