Cyclic threshold shear strains for pore water pressure generation and stiffness degradation in marine clay

The cyclic threshold shear strain is a fundamental property of saturated soils under cyclic loading. To investigate the cyclic threshold shear strains for pore water pressure generation (γ_tp) and stiffness degradation (γ_td), a series of strain-controlled multistage undrained cyclic triaxial tests are carried out on the in-situ saturated marine clay in the Yangtze River estuary with different values of plasticity index I_p. The test results show that both γ_tp and γ_td increase with the increasing I_p of the marine clay, and γ_tp is greater than γ_td for the same marine clay tested under the same conditions, with γ_tp = 0.018% ~ 0.019%, γ_td = 0.011% ~ 0.012% for I_p of 17, and γ_tp = 0.037% ~ 0.041%, γ_td = 0.022% ~ 0.027% for I_p of 30. Moreover, the development of stiffness degradation may not necessarily require the pore water pressure generation, but can be aggravated by it. The γ_tp and γ_td of the marine clay are compared with those of the terrestrial soils cited from the published literatures, indicating that the special marine sedimentary environment causes the γ_tp and γ_td of the marine clay in the Yangtze estuary to be smaller than those of the undisturbed terrestrial clay.