A composite bucket shallow foundation (CBSF) has been proposed by Tianjin University to adapt the offshore soft geological conditions of China for wind turbines. Vertical bearing capacity modelling and observation tests regarding the CBSF are performed. The test results are accompanied by numerical simulations to provide a better understanding of the failure mechanism of the CBSF. The upper limit of the vertical bearing capacity of the bucket foundation is derived through the upper bound theorem of classical plasticity theory according to the failure mechanism. The soil damage rate is specified as a new empirical parameter in the formula and is defined as the rate between the thickness of the soil that is broken inside the foundation and the radius of the foundation, which indicates the range of soil failure. Furthermore, the range of the soil damage rate is obtained through vertical bearing capacity model tests. The relationship of the bearing capacity factor Nq and Nγ with the friction angle is also discussed under the specific soil damage rates.
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