Shale geochemistry as a proxy for shear strength

Western Australian iron ore is mined from open pit mines composed of mineralised interlayered strong Banded Iron Formations (BIFs) and weaker shales. Mine wall failures most commonly occur in weaker shale units with low shear strengths. However, the principal controls on shale shear strength are poorly understood. This study will build on a pilot study that showed shale geochemistry, particularly alumina content, is a good proxy for shear strength. The metasomatism that enriched iron in the BIF’s has variably strengthened or weakened the interbedded shales. The pilot study found that alumina, silica, and iron oxide (III) to be the dominant element oxides comprising the shales, where shales with an alumina content <10 wt% Al2O3 N are likely to be strong and those >16% wt% Al2O3 N are likely to be weak. The alumina content of the shales better correlates with shear strength compared with defect surface conditions, previously thought to be the controlling factor on strength. This proposed PhD project aims to build on the pilot study by addressing:
(1) Laboratory test and characterise geochemistry, mineral assemblage, microstructure & shear strength of representative samples of unmineralised and mineralised shales.
(2) Assess any relationship between shale geochemistry and shear strength.
(3) Evaluate key mineral assemblages that control geochemical patterns among shales.
(4) Explore the usefulness of field-acquired geochemical data as a proxy for inferring shear strength.
(5) Analyse the applicability of the established data, knowledge, and methods in real-world geotechnical issues, such as slope stability, geo-model refinement, and waste mitigation.