In this study, centrifuge model test, numerical simulation and hydraulic gradient test were employed to investigate the cyclic behavior of tripodpile foundation for offshore wind turbines. The short-term cyclic response of soil-pile system were investigated by centrifuge model test and finite element simulation. The "load conversion" by the superstructure was observed. The compaction densification of sand during cyclic loading was analyzed. Besides, the long-term cyclic responses were observed by hydraulic gradient test. The accumulative deflection of piles with three stages were observed. The change of natural frequency of the soil-pile system was calculated and analyzed during long-term cyclic loading.
Nowadays, tripod-pile foundations are widely used in the offshore wind industry in China because of the advantage of better stability, which experience millions of laterally cyclic loads from wind and wave in the service life. Because of the strict requirement on the pile deformation for the normal operation of wind turbines, the cyclic behavior of tripod-pile foundation in its entire service life should be thoroughly studied.
Regarding the long-term behavior, the deflection accumulation and the natural frequency offset are two vital issues (Cuellar, 2011). The deflection accumulation of piles under laterally cyclic loading has been studied by many researchers (Swane and Poulos, 1982; Long and Vanneste, 1994; Nicolai et al., 2017). However, most of the existed studies are restricted to a short term with the loading cycles from hundreds to thousands, which are not able to describe the soil-pile response under millions of loading cycles during the service life. A series of 1g model tests on the laterally loaded pile with up to six million load cycles was conducted by Cuellar (2011) to investigate the long-term behavior of soil-pile system. The displacement accumulation of the pile and the densification effect of sand were observed in the tests. But these results remain to be verified due to the limitation of 1g model test.