Offshore wind turbine is a new solution of the energy crisis, and stable foundation is an important guarantee for its safety. An important characteristic of wind turbine foundation is to bear the vertical load, horizontal load and moment at runtime. A large diameter wide-shallow bucket foundation, as a new foundation type, is proposed by Tianjin University for applying to the specific load conditions. The diameter of the foundation is generally more than 30 m, and its embedment ratio is between 0.2 and 0.5. Compared with other traditional forms of ocean engineering foundation, there have been no standards and theories for bucket foundation designing currently. The vertical bearing capacity is an important basic indicator in the process of offshore wind turbine foundation designing, which determines the size of wind turbines and the cost of engineering. This paper proposes two failure mechanisms about vertical bearing capacity of bucket foundation through the experiment. The upper value of vertical bearing capacity of bucket foundation has been derived through the upper bound theorem of classical plasticity theory. Soil damaging rate is specified as a new empirical parameter in the formula. It is defined as the rate between the thickness of the soil which is broken inside the foundation and the radius of foundation, which indicates the range of soil failure. And the number range of soil damaging rate is obtained through the vertical bearing capacity experiments. Relationship between bearing capacity factor Nγ and friction angle also discussed under the specific soil damaging rates. These results are helpful in reducing the uncertainties related to the method of analysis in bearing capacity calculations, paving the way for more cost-effective foundation design.

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