Abstract: As the anchoring foundation of the tension leg platform (TLP), suction caisson foundation is not only subjected to the vertical pullout load but also subjected to the cyclic load. However, there are few studies on the mechanism of the cyclic characteristics of soft clay under unloading condition. In this study, the cyclic triaxial tests under axial unloading were carried out to obtain the cyclic cumulative deformation and strain softening characteristics of soft clay. The results show that the degree of strain accumulation and softening coefficient were low under the condition of low static deviator stress and dynamic deviator stress. With the increase of dynamic deviator stress, the cumulative cyclic deformation gradually increased and rapidly developed in the early stage and tended to be stable in the late stage. Meanwhile, the softening of soil gradually increased. The cyclic cumulative deformation of soil increased with the increase of dynamic deviator stress, and the cyclic cumulative strain curve showed attenuation-stability creeping characteristics. Based on the tests, a formula for softening coefficient related to static deviator stress and dynamic deviator stress was introduced to describe the test results of soft clay under different stress levels. Considering the influence of static deviator stress, dynamic deviator stress and softening coefficient, an equivalent cyclic creep model of soft clay was established. On this basis, a three-dimensional equivalent cyclic creep model was built, and a finite element subroutine was developed. The finite element method for cyclic uplift capacity of caisson was established to analyze the cyclic cumulative deformation of soil. It was verified by comparing with the test results.

Key words: suction caisson foundation, soft clay, strain softening, cumulative deformation, equivalent cyclic creep model, finite element