A unified lateral soil reaction model for monopiles in soft clay considering various length-to-diameter (L/D) ratios

Abstract

Large-diameter monopiles are the most commonly used foundation to support offshore wind turbines. Early designs usually adopted pile diameters (D) between 4 and 6 m, which is recently extended to 8 m and will target 10 m in the future. It is increasingly evident that the existing design method (i.e., API's p-y model) can significantly under-predict the lateral stiffness and capacity of large-diameter monopiles in soft clay, due to ignoring the soil resistances from base shear and base moment which become more pronounces as L/D reduces. In this study, a two-spring approach is proposed, aiming to predict the lateral behaviour of monopiles with varied L/D ratios in a unified manner. In light of the soil flow mechanisms around monopiles, the pure lateral soil resistance above the rotation point (RP) is quantified using a p-y model, while the resistances below the RP including the base shear and base moment are integrated into a moment-rotation spring (characterized by a MR-θR model) at the RP. It can naturally recover to a p-y model while analyzing flexible piles, where θR= 0 at RP. Formulations of the ‘p-y + MR-θR’ model (including diameter-related p-y and MR-θR models, and the depth of the RP) are proposed based on the results of a series of well-calibrated 3D numerical models. The proposed model has satisfactorily reproduced a number of field and centrifuge test results on laterally loaded monopiles with a wide range of L/D ratios (including flexible, semi-rigid and rigid piles), using a unified set of parameters. Compared to the standard p-y model, the adoption of the proposed ‘p-y + MR-θR’ model is shown to substantially reduce design conservatism.