Horizontal cyclic response of bucket caisson for offshore wind turbines in over-consolidated clay

Abstract

Compared with pile foundations, bucket caisson has the advantages of convenient installation and recyclability, and is more suitable as foundation for offshore wind turbines. Horizontal cyclic response is very important for the stability analysis of bucket caisson as it suffers long-term wave loadings in service period. This study presents the results of a series of 1 g model tests under horizontal cyclic loadings in over-consolidated clay. The cumulative deformation, stiffness changes and rotation centre variation of caisson model were discussed in detail. Test results indicated that the horizontal displacement of caisson model in over-consolidated clay accumulated with the increase of cyclic number and horizontal loadings. However, the increase of horizontal displacement gradually decreases for each cycle. Based on this, an empirical equation between Tb and ζb was proposed to predict the accumulated rotation of bucket caisson in over-consolidated clay. The position of rotation centre for caisson model in this study had the tendency to move up gradually. When the caisson model was under fatigue limit state (ζb=0.4), the type of rotation changes from deep to shallow certre. Under the state of ULS/1.35 (ζb=0.6), the type of rotation is always shallow centre during the whole cyclic loadings. The unloading stiffness of caisson model decreases logarithmically and tends to be stable with the increase of loading cycles. Due to the change of unloading stiffness, the system natural frequency of bucket caisson is reduced by about 37%. This study can provide theoretical basis for the design of bucket caisson as offshore wind power foundation in over-consolidated clay.