A unified finite element upper-bound framework for the lateral bearing capacity of monopiles in undrained clays considering different failure mechanisms

Ocean Engineering, 367:128120, 2026.

• Manuscript

Large-diameter monopile foundations are generally subjected to severe lateral loading during the construction of offshore wind farms. The failure of rigid monopiles can typically be divided into two mechanisms: one involves the formation of a passive wedge in front of the pile accompanied by soil-pile separation near the mudline behind it (one-sided wedge failure mechanism), while the other involves the development of an active wedge behind the pile without soil-pile separation (two-sided wedge failure mechanism). However, these two mechanisms are typically considered in isolation in existing studies, thereby introducing additional uncertainty and conservatism into practical engineering design. In this study, a unified finite element upper bound framework for evaluating the lateral bearing capacity of rigid monopiles in undrained clays considering both mechanisms is developed. First, the Total-displacement-loading Extended Mobilized Strength Design (T-EMSD) method is used to evaluate the ultimate lateral bearing capacity of rigid monopiles. The results indicate that once an active wedge has formed, the ultimate bearing capacity becomes insensitive to the soil unit weight. In addition, a transition criterion between the two failure mechanisms is identified, which is related to the slenderness ratio ($L/D$), adhesion coefficient ($\alpha$), and dimensionless strength coefficient ($\eta$). Based on these findings, a unified upper-bound empirical equation is proposed. Using this equation, the lateral bearing capacity can be determined directly without the need for any priori assumption regarding the failure mechanism. Finally, the reliability of the proposed method is validated by comparing its predictions with numerical simulation results, laboratory test data, and field test data reported in the literature.