Pore pressure generation in sand with bentonite: from small strains to liquefaction

The paper examines the effect of small percentages of bentonite on pore pressure generation in loose sands, from small strains all the way to liquefaction. It relies on resonant column, static triaxial and cyclic triaxial tests conducted on specimens prepared by dry-mixing Ottawa sand and bentonite, at a skeleton relative density of 35% +/- 5%. Two main variables are investigated: the percentage of bentonite (3% and 5% by dry mass of the sand), and the duration of the ageing period preceding shear (1 to 10 days). The presence of bentonite increases the shear strain needed to initiate the generation of excess pore pressures in resonant column tests; in cyclic tests it reduces the mean pore pressure generated per loading cycle and allows the specimen to sustain an increased loss of effective stress before liquefaction initiates, both effects contributing to an increased resistance to liquefaction. These effects are further enhanced with prolonged pre-shear ageing. Additionally, under static conditions, the behaviour of the sand is found to become increasingly stable as the ageing duration is extended. Given the role played by the ageing period, the effects observed cannot be simply ascribed to the increased bulk density of the specimens with bentonite. Instead, they are attributed to the pore fluid formed in presence of bentonite: a concentrated clay gel with solid-like properties. This pore fluid increases the sand's threshold shear strain, which is shown to have a strong correlation with the resistance to liquefaction.