Shale geochemistry as a proxy for shear strength

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. Mine wall failures most commonly occur in weaker shale units with low shear strengths. However, the principal controls on shale shear strength are poorly constrained. This study shows that shale geochemistry, particularly alumina content, is a good proxy for shear strength. The metasomatism that enriched iron in the BIFs has variably strengthened or weakened the interbedded shales. This study finds alumina, silica, and iron oxide (III) to be the dominant element oxides in 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 than with defect surface conditions, previously thought to be the controlling factor on strength. This research permits shale shear strength to be estimated in a field environment by using a portable Xray fluorescence analyser to determine the alumina content, helping to narrow the selection of shale samples in need of direct shear testing.