Authors: Speakman, H

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Speakman, H 2022, 'A mixed-method approach for major excavation ground support evaluation', in Y Potvin (ed.), Caving 2022: Fifth International Conference on Block and Sublevel Caving, Australian Centre for Geomechanics, Perth, pp. 279-288,

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The short and long-term stability of major excavations is crucial to the successful development and long-term performance of both the excavations and their mining function. Detailed geotechnical assessments and analyses are pivotal to understanding the causes of instability when planning a major excavation and are essential to performance forecasting and applying engineering and mitigation controls. This paper describes a mixed-method approach, which not only evaluates the suitability of a range of ground support regimes but provides a good practice framework that can be applied to future major excavation assessments during a feasibility study. This approach includes the collection of geological and geotechnical data, rock mass classification, the specification of strength and stress parameters, preliminary identification of support requirements using empirical methods, such as the Q, RMR, and GSI systems, kinematic joint and wedge analysis, and 2D and 3D numerical modelling. These methods take into consideration the local rock conditions and structural geology and the ground support response based on cover hole data to evaluate best the suitability of an underground excavation and related support elements before construction begins. The mixed-method approach was tested by assessing the construction of a new major excavation for an unnamed block cave operation. Detailed geotechnical data was obtained from two cover holes. The results identified the stress environment, the likely modes of failure, mean joint sets, potential wedge formations and key influences of joint properties, the likely failure zone, and the potential impact of the void created by caving. In addition, the approach allows a wide range of suitable support regimes to be assessed. The result was a support evaluation unique to the operation and local conditions. This framework helps to successfully determine the rock conditions and the effectiveness of various support designs in two and three dimensions where this type of analysis is essential to safely work in underground conditions that are adequately supported, which can be determined quickly and effectively using the mixed-method approach for evaluating ground support and rock mass conditions. Further steps are suggested to improve the approach’s usefulness subject to more data and analytic capabilities, including downhole geophysics, numerical modelling with a discrete fracture network, and synthetic rock mass modelling.

Keywords: ground support, numerical modelling, sublevel cave

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