Undrained shear and stiffness degradation of intact marine clay under monotonic and cyclic loading.

Optimal whole-life design for marine structures requires an accurate assessment of how their underlying geo-materials respond to various static and cyclic loads. This study experimentally investigates the undrained shear behavior and stiffness characteristics of intact, soft marine clay that outcrops an offshore wind farm in southeast China. A series of monotonic and cyclic triaxial tests were conducted on the highly compressible marine clay, and the test results were interpreted within the critical state framework. The elastic shear modulus measured using bender elements in triaxial tests was compared with the secant modulus that gradually degraded with the strain during monotonic and cyclic loading. An interesting finding was that the elastic stiffness measured at various loading stages increased exponentially with the mean effective stress; a power-law increasing trend was also observed in the secant modulus normalized by the void ratio at various strain levels. It was also demonstrated that the intact sample under cyclic loading is stiffer than that under monotonic loading at the same strain level. Furthermore, a close correlation between the nonlinear undrained shear stiffness and the elastic stiffness measured during monotonic or cyclic loading was observed, which is of vital importance when evaluating the stiffness degradation behavior of natural soils over strain ranges relevant to major engineering works. • Undrained monotonic and cyclic behavior of intact soft marine clay is experimentally studied. • The elastic modulus measured using bender elements is compared with the secant modulus. • The shear moduli at different strain levels correlate well with the mean effective stress. • The nonlinear undrained shear stiffness has a close correlation with the elastic stiffness. [ABSTRACT FROM AUTHOR]