The p-y curve method provides a relatively simple and efficient means for analyzing the cyclic response of horizontally loaded piles. This study proposes a p-y spring element based on a bounding surface p-y model, which can be readily implemented in Abaqus software using the user-defined element (UEL) interface. The performance of these p-y spring elements is validated by simulating field tests of laterally loaded piles documented in the literature. The developed spring element effectively replicates the nonlinear hysteresis, displacement accumulation, and stiffness degradation observed in soft clay. Subsequently, a finite element model of a large-diameter monopile is established using the proposed spring element. A comprehensive numerical investigation is conducted to explore both the monotonic and cyclic responses of large-diameter monopiles in soft clays. The results are presented and discussed in terms of pile head load-displacement curves, the evolution of rotation angles at the mud surface, and cyclic p-y curves. Additionally, empirical formulas are proposed to predict the evolution of cumulative rotation angles and peak bending moments under both one-way and two-way cyclic loading conditions. The results provide valuable insights into the mechanism of pile-soil interaction under lateral cyclic loading.

Resonance occurs when the natural frequency of an offshore wind turbine matches its rotational or blade passing frequency, potentially causing severe structural damage. Existing research on the resonance frequency characteristics of offshore wind turbines has mainly focused on elastic analysis, neglecting the nonlinear dynamic interaction between the foundation and soil. Based on the dynamic Winkler foundation model, the hyperbolic soil resistance around the pile-lateral displacement (p-y) backbone curve was used to consider the stiffness nonlinear of the pile-soil system. The shear strain-dependence of hysteretic damping was considered for soil energy dissipation. A simplified nonlinear frequency domain analysis method for calculating the resonance frequency of monopile-supported offshore wind turbines was proposed. The validity of the method was confirmed through comparisons with model test results and field measurements from the Lely (A2) offshore wind turbine. A parametric study was conducted to investigate the influence of sand density, pile length and pile diameter on the nonlinear resonance frequency of offshore wind turbines. The results show that the resonance frequency of the offshore wind turbine system decreases with increasing loading amplitude. Additionally, the influence of soil nonlinearity on the resonance frequency for systems is more obvious when the sand is looser, the pile length is shorter, or the pile diameter is smaller.

Accurate prediction of excavation deformation and stress affects the safety of excavation engineering and the surrounding environment. However, the traditional calculation method ignores the influence of soil shear action and its nonlinear deformation characteristics. Therefore, this paper proposed a coupled analytical method for braced excavation considering the continuity of soil deformation and nonlinear pile-soil interaction. A nonlinear Pasternak two-parameter foundation model was developed based on the Pasternak foundation model and nonlinear p-y curves. The control differential equations for the excavation in the critical and embedded sections were derived. Also, the numerical solutions of excavation deformation and force under different boundary conditions were obtained by the finite difference method and Newton's iteration method. Further, the excavation calculation procedure considering the construction process and nonhomogeneity of soil was suggested. Through finite-element (FE) and engineering case analyses, the traditional calculation method overestimated the excavation deformation and internal force, while the proposed methods were consistent with the measured results. Finally, the effects of soil shear stiffness and initial foundation reaction modulus on the excavation were discussed, and we found that the two parameters had more significant impact on the wall bending moment than displacement. The results provide some reference for the design calculation of braced excavation.

Piles supporting large structures are often subjected to cyclic lateral loads due to natural phenomena, including earthquakes, winds, and waves. Such loads are main causes of progressive deterioration in the stiffness and reduce the lateral capacity of piles. However, the effects of unsaturated soil conditions on the lateral cyclic response of piles are not yet fully understood, and the p-y curves used in engineering practice are merely based on the assumption of full saturation or complete dry conditions. This study is aimed to investigate the pile performance under unsaturated soil conditions by performing monotonic, cyclic, and post-cyclic loading tests on piles installed in sand with a varying water table. A loading system was designed and constructed to carry out different types of cyclic loadings. It was observed that the lateral capacity of the pile is influenced by the average suction stress along the pile which increases with the depth of the water table. During the cyclic loading, gap formation is noticed around the pile head for tests conducted in unsaturated conditions, which results in significant stiffness degradation compared to the saturated state. However, post-cyclic loading tests showed that the ultimate lateral capacity of the pile is not affected by the cyclic loading history. Finally, a modified p-y curve is proposed for the piles embedded in unsaturated sandy soils, and a comparison of its performance with the observed results is promising.

Offshore wind farms are located in marine environments with complex hydrological, meteorological and submarine geological conditions, which pose difficulties for wind turbine foundation design and construction. Therefore, the study of the key technologies of offshore wind turbine foundation design has important theoretical value and practical significance for the assurance of structural safety, the optimization of structural design and the extension of structural service life. In this paper, a numerical simulation model of three pile foundation is established, and a detailed FEA model of grouted area is calculated and analyzed, and influence of grout on performance under different loading conditions is calculated and analyzed. The results show that it is feasible to use the p-y curve method to describe the pile-soil interaction of the three-pile foundation of the offshore wind turbine, the stress check of the whole foundation structure under ultimate load conditions and normal load conditions meets the requirements of the DNV specification, and the result of the fatigue damage check is that the fatigue strength requirement is met in 26.7 years, which indicates that the three-pile foundation structure of the offshore wind turbine is safe and reliable and can be operated safely.

Field measurements in offshore areas have revealed that wind turbines are mainly exposed to large-amplitude and long-duration horizontal environmental loads, and those located in high-intensity areas are particularly susceptible to seismic activity during service life. However, it remains unclear how the initial horizontal loads affect the seismic behaviour of offshore wind turbines, particularly with respect to pile-soil interaction. To address this issue, this study employed the ZJU-400 centrifuge shaking table to conduct a series of centrifuge tests on a single rock-socketed pile in liquefiable saturated sands, with its prototype located in the offshore wind farm in Fujian waters. The experimental results demonstrate the bending moment and displacement responses of the pile subjected to the seismic loads in combination with initial horizontal loads. The proposed p-y model introduces a parameter related to excess pore water pressure to quantify its weakening effects on pile-soil interaction as observed in the centrifuge tests. When combined with the nonlinear Winkler foundation beam model, the proposed post-earthquake prediction model can effectively estimate the bending moment and displacement distribution of monopiles under initial lateral loading.