Ageing/thixotropy effects on strength and compressibility in Leda clay and polymer-amended clayey tailings

Abstract

Evidence of ageing has been reported in natural deposits of sedimentary clays, dredged sediments, and in clay-rich oil sands tailings, and such time-dependent behaviour has been observed to influence their long-term settlement and strength development characteristics. The ageing phenomenon can, therefore, have important implications for the reclamation of such deposits. This research investigated ageing in polymer-amended tailings and Leda clay monitoring relatively small (10 cm high) laboratory samples for over 100 days. Both tailings and natural clay were found to develop increased strength, sensitivity, and higher pre-consolidation pressure and reduced compressibility. The correlation between fall cone shear strengths and pre-consolidation pressures was strong for both the natural clay and the clayey tailings, though this ratio was different in the tailings and the clay. The Leda clay appeared to reabsorb some of its bleed water, which was found to be due to the transition of some free water to absorbed water after 80 days. Using a concept developed from ageing experiments in artificial clays, it was found that shear strengths measured after an “early equilibrium state” correlated strongly with shear strengths measured near the end of ageing. This is promising for quantifying the potential for ageing on compressibility without having to resort to year-long experiments.