Cyclic Response of a Sand with Thixotropic Pore Fluid

Saturated specimens of Ottawa sand prepared with 0%, 3% and 5% bentonite by dry mass of sand are tested under cyclic loading to investigate the effects of bentonite on the cyclic response. For the same skeleton relative density and cyclic stress ratio (CSR), the cyclic tests on the sand-bentonite mixtures show a significant increase of the number of cycles required for liquefaction compared to the clean sand. This is caused, as observed in resonant column tests, by an increase of the elastic threshold due to the presence of bentonite, which delays the generation of excess pore pressure. Such behavior can be explained by the rheological properties of the pore fluid. Oscillatory tests conducted with a rheometer on bentonite slurries show that for shear strains as large as 1% these materials exhibit elastic behavior with a constant shear modulus. Moreover, due to the thixotropic nature of the bentonite slurries, their storage modulus shows a marked increase with time. This observation is consistent with the increase in the liquefaction resistance of the sand-bentonite mixtures with time also observed in cyclic triaxial experiments.