UCLA

A series of cyclic and monotonic direct simple shear experiments was conducted on mixtures of non-plastic silt and bentonite, prepared with two different pore fluids: fresh deionized water, and saline-water with a 35 g/L concentration of NaCl (consistent with sea water). The clay fractions were adjusted to achieve a plasticity index of PI = 9, with the fresh-water blend requiring 5% bentonite and the saline water blend requiring 10% bentonite. Though both blends have the same plasticity index, differences in cyclic behavior were observed. The relationship of cyclic stress ratio versus number of cycles and the ratio of cyclic stress to monotonic undrained strength was higher for the saline water blend than for the fresh water blend. Results indicate that plasticity index alone is an insufficient indicator of cyclic strength. Corrections for pore fluid chemistry (and by extension, depositional environment) may be necessary, in addition to possibly other factors, for procedures to assess the potential for liquefaction susceptibility and cyclic softening.