Characterization of the Reduction in Undrained Shear Strength in Fine-Grained Soils due to Cyclic Loading

To evaluate the effect of plasticity characteristics and mineralogical composition on postcyclic shear strength degradation, 18 soils, prepared as different proportions of kaolinite, montmorillonite, and ground quartz, were characterized using static simple shear and cyclic simple shear tests with postcyclic monotonic loading. The postcyclic undrained strength ratio (s(u,pc)/sigma'(c)) was found to depend on the clay mineral, plasticity index (PI), and cyclic stress ratio. In soils with kaolinite as the clay mineral, a greater reduction in shear strength due to cyclic loading was noted at lower PIs, whereas soils with montmorillonite as the clay mineral typically experienced reductions between 0% and 30% of the static undrained shear strength. A relationship was established between degradation in undrained shear strength and postcyclic effective stress ratio (PC-ESR), which is the ratio of the consolidation stress to the effective vertical stress after cyclic loading and is equivalent to the reciprocal of the difference between the pore pressure ratio and one. A linear relationship between the ratio of s(u,pc)/sigma'(pc) (where sigma'(pc) is the effective vertical stress immediately after cyclic loading) and the undrained strength ratio (s(u)/sigma'(c)) of a normally consolidated soil and PC-ESR was also developed. Results from nine natural soils showed good agreement with the relationships developed for mineral mixtures. (C) 2019 American Society of Civil Engineers.