Analytical method for synergistic cyclic degradation of base and side resistances on large diameter monopiles

Large-diameter monopiles are the favored foundation type for offshore wind turbines. However, their long-term stability may be degraded due to the cyclic nature of environmental loads. To evaluate the potential degradation of monopile stiffness, this study develops a cyclic analytical method by extending the Double Conical Strain Wedge (DCSW) framework to account for the cyclic degradation of shaft and base stiffness. The developed model explicitly considers the synergistic degradation of shaft and base stiffness, which is defined in this study as the coupled stiffness reduction of the shaft soil and pile-tip soil induced by cyclic pile displacement and rotation. The proposed method is validated against low-cycle and high-cycle model tests reported in the literature. The analysis demonstrates that: (1) the synergistic degradation of shaft and base resistances is critical, as neglecting base weakening significantly underestimates long-term deformation; (2) cyclic load amplitude has a more significant influence on the degradation rate than the number of cycles; (3) internal forces redistribute toward deeper soil zones due to the progressive weakening of shallow layers; and (4) the initial restraining effect of a high soil friction angle is gradually diminished by long-term cumulative cyclic effects.