Pore fluid chemistry, stress–strain behaviour, and yielding in reconstituted highly plastic clay

Extensive deposits of proglacial Lake Agassiz clay in the Red River valley in southern Manitoba, Canada are smectite-rich, expansive, strain-softening, and cemented by gypsum (CaSO4.2H2O). Mechanical properties of the clay are affected by pore fluid chemistry. They control the engineering behaviour of projects such as shallow foundations, riverbank instabilities, landfill liners and other engineered barriers. Reconstituted Lake Agassiz clay is being considered as a component of the sealing system for a Canadian repository for used nuclear fuel.The project that stimulated this study involved intermittent instabilities of retention dykes for the reservoir (forebay) of a hydroelectric generating station. Testing for cation concentrations suggested that natural seepage beneath the dykes removed gypsum cementation and reduced the strength of the foundation soils. To examine this, blocks of reconstituted clay were prepared using different pore fluids. Specimens that were saturated with gypsum displayed similar yield stresses and peak strengths, even though there were other differences in their pore fluid chemistry. Specimens that were under-saturated with gypsum yielded at lower stresses. The results suggest that early straining up to 1 to 2% axial strain was controlled by the presence of gypsum at near-saturation concentrations.

Research Highlights
► Dissolution of gypsum cementation in plastic post-glacial clay.
► Intermittent failure of water retention dykes over a period of sixty years.
► Triaxial tests on reconstituted clays with various pore water chemistries.
► Different chemistries change strengths, stiffness, strain-softening and yielding.