Axial load-transfer behavior of offshore monopiles in soft clay: A modified Q-z and t-z curve approach

Current design practice for axially loaded piles commonly adopts the load-transfer method, in which shaft friction and base resistance are represented by t-z and Q-z springs. However, existing code-based curves remain insufficiently validated for saturated soft clays and often neglect the effects of pile configuration, pile-soil stiffness ratio, and soil parameter dependency. This study investigates the vertical static response of large-diameter monopiles in saturated soft clay through 1:50 model tests and finite element simulations. The model tests measured pile-head load-displacement response, axial force distribution, shaft resistance mobilization, and base resistance development under different consolidation pressures. The results reveal a shaft-resistance-dominated load-transfer mechanism, in which shaft friction is mobilized rapidly while base resistance develops with an obvious delay and contributes less than 10% of the total capacity. Numerical analyses further clarify the effects of soil stiffness and embedment ratio on load-transfer behavior. Based on the experimental results and numerical inversion, a refined Q-z model incorporating soil stiffness and an enhanced t-z model accounting for soil stiffness and embedment effects are proposed. Compared with the API model and an independent centrifuge test case, the proposed curves better reproduce the nonlinear axial response of monopiles under the investigated soft clay conditions.