Vertical Bearing Capacity for Pile-Ring Composite Foundations in Clay

Evaluating the vertical bearing capacity of offshore wind turbine pile-ring composite foundations under complex marine environmental loads is critical for ensuring engineering safety. This study employs the rigorously validated T-EMSD upper-bound method to conduct a three-dimensional numerical analysis of the vertical bearing capacity of pile-ring composite foundations in saturated clay. It systematically investigates the influence of soil homogeneity (η, diameter ratio (D/B), embedment ratio (L/B), and external shaft friction coefficient (α) on the bearing capacity factor Nc, and reveals the associated failure mechanism through velocity field analysis. The results indicate that the bearing capacity factor Nc increases significantly with the diameter ratio D/B. The system exhibits optimal bearing performance when the pile shaft friction is fully mobilized (α = 1) in homogeneous soil (η=1). Moreover, as the embedment ratio L/B increases, the plastic zone extends downward along the pile shaft, enhancing the deep foundation effect. Based on parametric analysis, a predictive formula for the net bearing capacity factor of the pile-ring composite foundation under homogeneous conditions is established. Verified against existing numerical methods and experimental data, the formula demonstrates an error margin within ±5%, indicating its good suitability for engineering applications. Furthermore, by establishing a ratio relationship, the net bearing capacity factor under heterogeneous conditions is correlated with that under homogeneous conditions. This enables a more in-depth analysis of the influences of soil strength heterogeneity and external shaft friction coefficient on the vertical bearing capacity of the pile-ring composite foundation. The work presented in this paper provides a theoretical basis for the design and bearing capacity assessment of this type of composite foundation.