Dynamic behaviour of very soft clay under different earthquake loading conditions

Very soft clays present critical challenges in geotechnical engineering, particularly under earthquake loading. This study explores the rarely investigated dynamic response of very soft clay under high-strain, long-duration far-field earthquake loading conditions through undrained cyclic triaxial tests over a wide range of cyclic strain amplitudes (εa = 0.01%−2%). The results showed that increasing strain amplitude led to greater hysteresis loop asymmetry. Assuming symmetrical loops while evaluating dynamic properties underestimated the damping ratio by 6–24%. Prolonged cyclic loading caused significant cyclic strength degradation of 79% and stiffness loss of 88% at the highest εa of 2%, with most strength loss occurring within the first 100 loading cycles. The clay lost almost 78% of its stiffness within the initial 20 sec for εa ≥ 1%. Pore pressure ratios also exceeded 0.95 at strain amplitudes above 0.2%. A notable 19-fold increase in the cumulative energy dissipation was further observed during the first cycle as εa increased from 0.01% to 0.05%.