IntroductionAssessing the stability of rock slope cuttings and benches in real-time, as excavations progress and ground conditions become apparent, using analytical approaches such as kinematics, limit equilibrium or finite and discrete element models is practically impossible in both civil and mining engineering projects. The rate of excavation is too fast for this. The same limitation usually applies to tunneling, although large underground openings (e.g. caverns) are sufficiently stationary for thorough and more necessary analysis, and the same applies to high rock slopes.Several empirical methods for assisting rock engineering design have been developed in the last 50 years and are used for a variety of applications by rock engineers and engineering geologists, primarily for tunneling and support of underground excavations. In the case of rock slopes, some empirical methods predict support, reinforcement and performance of excavated slopes. However, aside from Q-slope, no empirical rock engineering methods provide guidance in relation to appropriate, long-term stable slope angles in which reinforcement and support is deliberately absent. Such slopes actually dominate the demand by a huge margin.
Newcrest Mining Limited open pit operations at Telfer (Western Australia) have excavated steep slopes. Current and future planned mine designs also intend to implement steep slopes in order to maintain profitable ore to waste strip ratios. When not adequately considered in the design process, rock falls can present a significant hazard in open pit mines. The management of rock fall hazards becomes particularly vital for steep slopes. Numerical models are often used to assess the effectiveness of benched slope designs or rock fall barriers to minimise risk to personnel or equipment. Commonly used numerical modelling software and simulation impact theories include: • 'RocFall'-two-dimensional lumped-mass impact model (2DLM). • 'Trajec3D'-three-dimensional rigid body impact model (3DRB). Numerical models use coefficients of restitution to characterise the amount of energy lost due to the inelastic deformation during the collision of a rock with the slope or bench. The input parameters are vastly different for 2DLM and 3DRB and they are seldom calibrated with any site-specific rock fall case studies or field test data during project feasibility studies, and often remain uncalibrated through the operating life of the mine. In order to best manage rock fall hazards for steep slopes through design, a series of rock fall trajectory field tests were carried out to facilitate the development of calibrated 2DLM and 3DRB numerical models. The calibrated models were then utilised to assess the effectiveness of various slope design geometries. The influence of model selection was found to have a significant impact upon the results. This paper compares rock fall trajectory predictions obtained from calibrated 2DLM and 3DRB models for steep slope designs in hard rock.
Ground failure on natural and engineered rock slopes is a geological hazard with potentially fatal consequences to the public or personnel in the mining industry. Aerial reconnaissance with the use of unmanned aerial vehicles (UAV) is rapidly becoming standard practice for geotechnical and engineering geological site investigations, enabling faster and safer data collection on slopes, which are often difficult to access on foot. Data obtained from aerial reconnaissance alongside conventional field investigations assist in the development of an engineering geological model that can form the basis of various stability analyses including kinematic, limit equilibrium and finite element analyses, and even rock fall simulations. This paper presents two case studies in which remote reconnaissance is used as an initial method of site investigation to classify natural and engineered rock slopes. The case studies from San Leo in Italy and an open pit mine in the Caribbean are used to demonstrate the effectiveness of these techniques for developing a preliminary engineering geological model from which stability analyses can be derived to predict future ground behaviour to assist in managing risks associated with the geological hazard.
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