Undrained Behavior and Strength Degradation of Undisturbed Marine Clay under Various Stress Levels

This study investigates the undrained behavior and strength degradation of marine clay under cyclic loading, which is critical for assessing the performance of coastal and offshore structures. A series of dynamic triaxial tests were conducted on undisturbed marine clay to analyze the evolution of double amplitude strain, pore water pressure, and the degradation index under varying stress levels. Post-cyclic undrained shear tests were also performed to evaluate strength degradation. The results show that a higher cyclic stress ratio (CSR) accelerates the development of double amplitude strain and pore water pressure, with an inflection point in the double amplitude strain evolution curve at approximately 3 %. The critical CSR ranges from 0.15 to 0.20 for clay at depths of 50-60 m, whereas the clay at depths of 70-80 m exhibits a higher critical CSR ranging from 0.20 to 0.25. The cyclic loading effect leads to a reduction in the undrained shear strength of the soil samples, which shows a strong correlation with post-cyclic pore water pressure and double amplitude strain. These findings provide valuable insights for marine engineering design in clay-rich regions.