Effect of Initial Static Shear on Cyclic Softening Behavior of Very Soft Clay

Abstract This study presents a comprehensive experimental investigation into the cyclic softening behavior of very soft clay under varying initial static shear conditions by conducting two extensive series of strain-controlled undrained cyclic simple shear tests. Cyclic softening, interpreted through effective stress–based or pore pressure–based criteria, revealed that initial static shear amplifies the softening response of very soft clay under continued cyclic loading. A strain-based empirical model was proposed to estimate the cyclic pore water pressure generation in soft and very soft clays under varying static–cyclic loading conditions. Its predictive capability was further validated against different soft clays from the literature. The application of initial static shear prior to cyclic straining was observed to reduce soil stiffness at each cyclic strain amplitude and overburden pressure. However, its absence resulted in the greatest stiffness degradation and the lowest total shear stress mobilization across all confinement levels. The combined effect of initial static shear and overburden pressure on stiffness behavior was characterized using a power-law relationship. Strain-dependent design charts were further generated to assess cyclic softening potential under combined static–cyclic loading, offering design guidance for coastal and offshore foundations on soft and very soft clay deposits.