Evaluation of the transitional inelastic behaviour of unsaturated clay-sand mixtures

This paper examines and compares the mechanical behaviour of two different unsaturated clay mixtures comprised of bentonite clay (Saskatchewan or Wyoming) and quartz sand. The two mixtures have been proposed as compacted barrier materials for reducing groundwater flow in the vicinity of waste disposal repositories. Triaxial specimens were compacted to consistent properties, and then specified suction conditions were applied to the specimens using the vapour equilibrium technique. Following equilibrium at the specified initial suction, specimens were subjected to isotropic and shear loading in a conventional triaxial cell to measure the mechanical response under selected stress paths. The results are interpreted in terms of the yield, strength, and stiffness behaviour at the various suction levels. Results suggest that the clay component of the mixture dominates the behaviour at suctions less than approximately 30 MPa, and the sand component dominates the behaviour above approximately 30 MPa. The transition from clay- to sand-dominated behaviour is attributed to volume strain during application of the initial suction bringing the sand particles into contact. The discussion highlights how the results can be used to modify constitutive models to incorporate the transitional behaviour in numerical modeling.Key words: inelastic, yielding, unsaturated, stress-strain, triaxial testing.