Authors: Maldonado, A; Dight, PM

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Maldonado, A & Dight, PM 2020, 'The shear strength of bedding partings in shales of the Pilbara: the similarity of non-dilatational angles, mineralogy relationships, and nominal roughness', in PM Dight (ed.), Slope Stability 2020: Proceedings of the 2020 International Symposium on Slope Stability in Open Pit Mining and Civil Engineering, Australian Centre for Geomechanics, Perth, pp. 551-564,

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Laboratory direct shear test results for natural bedding and sawcut defects have been statistically analysed in shales and banded iron formation materials across BHP Western Australian Iron Ore deposits (BHP WAIO) to prove the hypothesis of similarity of shear strength of defects. The following paper provides typical Hencher corrected friction angles based on statistical methods to group data sharing similar characteristics. This paper justifies the existence of similar populations of shear strength of defects based on rigorous statistical analysis of valid direct shear test samples collected from multiple projects of the Pilbara. Spatial distribution bias effects and normality tests have been checked. Surface ‘geotechnical’ weathering grades do not explain the reduction of the shear strength of defects in shales. The use of base of completed oxidation and spectral signatures have provided a better explanation for the variability observed on the shear strength of defects. Where weathered shale samples from the Brockman column have greater frictional strength than fresh samples, a paradoxical phenomenon that can be attributed to chemical alteration generating additional textural roughness.

Keywords: shear strength, shale, bedding partings

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