Effects of oxidation on residual shear strength of shales within Lea Park Formation

Abstract

The residual shear strength of Cretaceous Lea Park Formation shale near Lloydminster, Saskatchewan, was evaluated to determine its suitability for road construction. Exposure of these shales results in oxidation, leaching, and other geochemical changes, which may affect the physicochemical forces that control the shear strength behaviour. A laboratory geotechnical testing program was complemented with geochemical testing and modelling. In situ pore-water chemistry and cation-exchange site composition were determined from soil-water extracts using an equilibrium geochemical model. It was found that leaching by low ionic strength recharge removes dissolved and adsorbed sodium from surficial deposits. Sulfur oxidation overprinted the effects of leaching by releasing acidity and sulfate, increasing stored gypsum, and increasing dissolved and adsorbed magnesium. Shear testing indicated higher apparent residual friction angles for surficial samples with lower exchangeable sodium; however, this relationship is obfuscated by the impact of increased sand contents on increased strength. Samples at depth exhibited lower apparent residual friction angles with higher clay fractions and greater exchangeable sodium. Fully assessing the long-term stability of the material would require further laboratory testing, and a theoretical basis to link the geochemical model results to the calculation of physicochemical forces in multi-ion, multi-mineral systems. Qualitatively, it can be concluded that near-surface oxidized and leached shales have greater shear strength, with less risk of this strength changing as a result of post-construction weathering.