A cyclic viscoelastic-viscoplastic constitutive model for clay and liquefaction analysis of multi-layered ground

摘要

In order to estimate viscous effect of clay in the wide range of low to high level of strain, a cyclic viscoelastic-viscoplastic constitutive model for clay is proposed. First, we confirm the performance of the proposed model by simulating the cyclic undrained triaxial tests to determine the cyclic strength and deformation characteristics of a natural marine clay. Then, the proposed model is incorporated into an effective stress based liquefaction analysis method to estimate the effect of an intermediate clay layer on the behaviour of liquefiable sand layers. The seismic response against foreshocks, main shock as well as aftershocks of 1995 Hyogoken Nambu Earthquake is analysed in the present study. The difference of shear strength characteristics of the alluvial clay layer is one of the reasons why Port Island has a higher liquefaction potential than that of Rokko Island. The proposed model gives a good description of the damping characteristics of clay layer during large earthquakes. Acceleration responses in both clay layer and liquefiable sand layer just above it are damped due to viscous effect of clay. In the case of main shock and the following aftershocks that occurred within less than 9 days after main event, acceleration responses near ground surface are dc-amplified due to the developed excess pore water pressure, while responses near ground surface are amplified before and long after the main event. Using the viscoelastic viscoplastic model for clay layer, time history of acceleration response in upper liquefiable sand layer can be well calculated, in particular in the range of microtremor process after the main seismic motion.