Deflection-Based Bearing Capacity of Suction Caisson Foundations of Offshore Wind Turbines

The monocaisson foundation has become an important choice with the increasing demand of offshore wind turbines constructed in silt and sand seabeds in China. To guarantee the normal operation of the wind turbine, the angular rotation of the foundation should be strictly controlled. It is necessary to predict the bearing capacity of caisson foundations at an allowable angular rotation. Several series of model tests were carried out in sandy silt under 1g gravity on suction caissons with different sizes subjected to overturning loading. The test results revealed the effect of the diameter, aspect ratio, and loading eccentricity on normalized moment capacity of caissons. The instantaneous rotation centers of model caissons were simplified to be at the depth of about four-fifths of the skirt length, almost directly below the lid center. Apart from the available ultimate moment capacity prediction approaches, a new calculation approach of deflection-based bearing capacity was proposed based on the mechanisms of soil-structure interaction and deflection of the caisson. Furthermore, determination of the model parameters is discussed. The present analytical approach can predict the moment-rotation curves of the model tests in silt and of the field test in a sand well. This deflection-based approach makes it possible to calculate bearing capacity at any allowable angular rotation. Further studies show that soil pressures and friction on the outer skirt at the passive zone contribute most to the moment capacity of the caisson, specifically the aspect ratio and the horizontal subgrade reaction constant. Finally, design charts of monocaisson foundations in silt and sand for offshore wind turbines are given.