This paper presents a synthesis of recent and new research conducted by the authors on laterally loaded monopiles in drained sand. The research involved reduced-scale field tests, centrifuge model tests, finite element (FE) simulations and comparisons of design approaches with published experimental data. The influence of the monopile base on lateral response is first discussed by drawing on field tests and numerical simulations and it is shown that the base generally provides a negligible contribution. The applicability of the API p-y formulation is then investigated through systematic FE analyses. The results show that this formulation leads to inaccurate predictions largely due to the assumption of a high initial stiffness varying linearly with depth and an unrealistic hyperbolic tangent back-bone function. Based on new insights into pile-soil interaction together with elastic simulations of laterally loaded rigid piles and new observations based on 26 pile tests, a simple rotational spring model is proposed to allow rapid quantification of the non-linear response of rigid monopiles in uniform sand. The effect of monopile flexibility is then added through a new straightforward correction factor based on 80 extra FE simulations. Finally, an example application of the proposed approach for a typical monopile design is presented.

Piles have been widely used as foundations to resist lateral loads. For the design of a laterally loaded pile, one of the most important inputs is the ultimate soil resistance (p_ult = K_ultDσv^′,whereKult is the ultimate lateral soil resistance coefficient, D is the pile diameter, and σv^′ is the vertical effective stress). However, great discrepancy can be found in the existing design equations for piles in sand. To provide new insights and clarify the discrepancy in previous studies, in this study, a series of numerical simulations were performed on piles of different configurations using the finite element model validated by centrifuge pile tests. The computed results suggest that K_ult is a function of depth ratios z/D and z/L for the flexible and rigid piles, respectively (where z is the absolute depth and L is the embedded pile length), and all existing design equations failed to reproduce the magnitude and distribution of K_ult . Additionally, the K_ult of horizontally translated fixed-head rigid piles exhibits the same pattern as that of free-head flexible piles, suggesting that the difference between free-head flexible piles and rigid piles is caused by the change of failure modes.

Monopiles are the most popular foundation for offshore wind turbines. These foundations typically have a low length to diameter ratio and undergo a rigid body rotation when subjected to lateral load. This paper presents results from an extensive numerical investigation involving 3D finite element analyses to demonstrate that the lateral moment-rotation response of a monopile in sand can be represented using a single non-linear rotational spring located at a depth of about 0.75 times the pile embedment. Expressions for the elastic rotational stiffness of a monopile under very low rotations are developed and these combined with observations from measured non-linear variations of rotational stiffness, that are supported by the numerical analyses, are used to develop a simple approximate expression that can be used to determine the response of a monopile to a monotonic lateral load in sand.