3D finite element analysis on large-diameter monopiles in dense sand and evaluation of soil degradation under cyclic loading in short-term

This study investigates the applicability of three-dimensional finite element analysis (3D FEA) for large-diameter monopiles under short-term loading conditions. Centrifuge model tests conducted on saturated, dense Toyoura sand were used to validate the 3D FEA, with a focus on load–displacement response, bending moment distribution, and excess pore water pressure. The results demonstrate that 3D FEA accurately captures the initial loading behavior and key resistance mechanisms, particularly for large-diameter monopiles. Furthermore, the Cyclic Contour Diagram (CCD) concept, a practical approach for predicting degradation due to cyclic loading, was examined. CCD-based degradation factors were compared with stiffness reductions observed in centrifuge model tests, showing reasonable agreement under nearly undrained conditions. Additional FEA incorporating degraded parameters derived from the CCD confirmed the potential of this approach, although limitations remain in accurately reproducing detailed monopile responses. This study highlights the effectiveness of the CCD concept, while emphasizing that its application in practical design requires careful consideration of factors such as load levels, drainage conditions, and the degree of soil mobilization along the monopile. The results provide additional insight into the applicability of CCD-derived degradation characteristics to large-diameter monopiles, highlighting the importance of drainage conditions and pile geometry when interpreting cyclic degradation at the pile level.