Offshore wind turbines are subjected to long-term cyclic loads, and the seabed materials surrounding the foundation are susceptible to failure, which affects the safe construction and normal operation of offshore wind turbines. The existing studies of the cyclic mechanical properties of submarine soils focus on the accumulation strain and liquefaction, and few targeted studies are conducted on the hysteresis loop under cyclic loads. Therefore, 78 representative submarine soil samples from four offshore wind farms are tested in the study, and the cyclic behaviors under different confining pressures and CSR are investigated. The experiments reveal two unique development modes and specify the critical CSR of five submarine soil martials under different testing conductions. Based on the dynamic triaxial test results, the machine learning-based partition models for cyclic development mode were established, and the discrimination accuracy of the hysteresis loop were discussed. This study found that the RF model has a better generalization ability and higher accuracy than the GBDT model in discriminating the hysteresis loop of submarine soil, the RF model has achieved a prediction accuracy of 0.96 and a recall of 0.95 on the test dataset, which provides an important theoretical basis and technical support for the design and construction of offshore wind turbines.

Offshore wind power is a hot spot in the field of new energy, with foundation construction costs representing approximately 30% of the total investment in wind farm construction. Offshore wind turbines are subjected to long-term cyclic loads, and seabed materials are prone to causing stiffness degradation. The accurate disclosure of the mechanical properties of marine soil is critical to the safety and stability of the foundation structure of offshore wind turbines. The stiffness degradation laws of mucky clay and silt clay from offshore wind turbines were firstly investigated in the study. Experiments found that the variations in the elastic modulus presented L-type attenuation under small cyclic loads, and the degradation coefficient fleetingly decayed to the strength progressive line under large cyclic loads. Based on the experimental results, a random forest prediction model for the elastic modulus of the submarine soil was established, which had high prediction accuracy. The influence of testing the loading parameters of the submarine soil on the prediction results was greater than that of the soil's physical property parameters. In criticality, the CSR had the greatest impact on the prediction results. This study provides a more efficient method for the stiffness degradation assessment of submarine soil materials in offshore wind farms.

In recent years, the exploration of seabed has been intensified, but the submarine soils of silt and sand in the Yellow Sea area have not been well investigated so far. In this study, the physical and mechanical properties of silt and sand from the Yellow Sea were measured using a direct shear apparatus and their microstructures were observed using a scanning electron microscope. The test results suggest that the shear strength of silt and sand increases linearly with the increase of normal stress. Based on the direct shear test, the scanning electron microscope was used to observe the surface of sand. It is observed that the surface becomes rough, with many V-shaped cracks. Many particles appear on the surface of the silt structure and tend to be disintegrated. The X-ray diffraction experiment reveals that the sand and silt have different compositions. The shear strength of sand is slightly greater than that of silt under high stress, which is related to the shape of soil particles and the mineral composition. These results can be a reference for further study of other soils in the Yellow Sea; meanwhile, they can serve as soil parameters for the stability and durability analyses of offshore infrastructure construction. In this study, the physical and mechanical properties of silt and sand from the Yellow Sea are measured and microscopically explored. image The physical and mechanical properties of silt and sand from the Yellow Sea are measured and microscopically explored. After the direct shear test, it is found that the surface of sand becomes rough, with many V-shaped cracks. Many particles of the silt structure appear on the surface and tend to disintegrate. The X-ray diffraction experiment reveals that the compositions of sandy and silty soils are different.