Authors: Kavanagh, C; Daczko, N; Eggers, MJ

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Kavanagh, C, Daczko, N & Eggers, MJ 2023, 'Shale geochemistry: a proxy for shear strength in the Pilbara?', in PM Dight (ed.), SSIM 2023: Third International Slope Stability in Mining Conference, Australian Centre for Geomechanics, Perth, pp. 435-448,

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Australian iron ore is mined across the Hamersley Province, Western Australia, from open pit mines composed of mineralised interlayered strong banded iron formations (BIFs) and weaker shales. Slope design and failures are commonly controlled by shale units with low shear strengths. However, the principal controls on shale shear strength are poorly constrained. The purpose of this study is to investigate the potential relationship between shale geochemistry and shear strength. The metasomatism that enriched iron in the BIFs has variably strengthened or weakened the interbedded shales. This study finds alumina (Al2O3), silica (SiO2) and iron oxide (III) (Fe2O3)to be the dominant element oxides in the shales. Shales with an alumina content <10 wt% are likely to have high strength and those >16 wt% are likely to be classified as weak. The alumina content of the shales was found to better correlate with shear strength than the defect surface condition; the latter previously having been considered the controlling factor on strength. This research permits shale shear strength to be estimated in a field environment using a portable X-ray fluorescence analyser to determine the alumina content, assisting selection of shale samples for direct shear testing.

Keywords: shale, shear strength, geochemistry, Hamersley Province

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