Authors: Condon, K; Martin, J; Valerio, M

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Condon, K, Martin, J & Valerio, M 2023, 'Considerations for developing intact rock strength parameters for open pit applications', in PM Dight (ed.), SSIM 2023: Third International Slope Stability in Mining Conference, Australian Centre for Geomechanics, Perth, pp. 373-386,

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Intact rock strength is a fundamental parameter used in rock mass characterisation for open pit slope design. Obtaining reliable intact strength parameters is dependent on the quality of data collected, characterisation of the failure behaviour considered and implementation of appropriate strength criteria. The strength results from laboratory testing can vary significantly due to natural sources of variability (e.g. lithology, defects, alteration), variability introduced through specimen preparation and testing techniques, and variability resulting from the failure behaviour exhibited by tested specimens. Laboratory strength testing must be performed to an industry standard to mitigate laboratory-induced sources of variability. Natural variability is captured through development of geotechnical domains based on rock mass characteristics. Laboratory specimen failure behaviour can be captured through the characterisation of failure modes (e.g. specimen rupture through shearing, axial splitting, spalling etc.) and failure types (e.g. failure though homogeneous rock matrix, along defects or through a combination of both). Classification of these failure behaviours aids in filtering out invalid test results and non-applicable failure behaviours that can introduce additional strength variability and impact design parameters. In heterogeneous rocks comprising healed discontinuities such as veins, the impact of these features on strength can be investigated through failure type classification. Consideration of the failure behaviour exhibited by laboratory specimens can aid in explaining strength variability and yield higher confidence in the parameters selected for design. The Hoek–Brown failure criterion is typically used to estimate the intact strength of hard rock by fitting a strength envelope to laboratory testing data. Variability among laboratory testing results can make it difficult to obtain reliable strength parameters even when failure behaviour, induced variability and natural variability are accounted for. Additionally, the approach used to fit the Hoek–Brown envelope to a dataset can result in different intact strength parameters depending on the selected optimisation algorithm and treatment of residuals. Thus it is often desirable to estimate a range of likely parameters and incorporate these ranges as sensitivities in modelling. Discussed in this paper are considerations for estimating intact strength parameters using the Hoek–Brown criterion for open pit applications.

Keywords: intact rock strength, pit slope design, Hoek–Brown criterion, failure behaviour, failure type, core damage

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