Bending moment capacity and failure mechanism of caisson foundations under monotonic and cyclic loading in clay

During service period, offshore wind turbines are subjected to both monotonic and cyclic loads, causing the rotation or translation of caisson foundations in the seabed. However, most of the existing studies focused on the performance of caisson foundations under monotonic static loading, and there are few studies about the effects of caisson-soil contact mode and soil strength reduction during installation in numerical simulations. This paper therefore systematically investigates the bending moment capacity and failure mechanism of caisson foundations under monotonic and cyclic loading in clay using finite element analyses. Three typical caisson-soil contact modes in different loading scenarios are considered, and the influence of soil strength condition, caisson aspect ratio on the bending moment capacity and failure mechanism of caisson foundations is explored. It is found that under monotonic loading, the bending moment capacity in the tensionless mode and the fully-bonded mode could be used as the lower and upper limit, respectively. Under cyclic loading, the fully-bonded mode always yields the highest moment capacity, while the frictionless mode and the tensionless mode produce the lowest in the case with small loading amplitude and the case with large loading amplitude, respectively. In addition, the behavior of cumulative angular displacement under combined load of wind and wave is also studied to provide insight for caisson foundation design.