As a newly emerged solution for supporting the new generation of offshore wind turbines (OWTs), the pile-bucket foundation has received wide attention. However, little attention has been paid to the grouted connection that connects the monopile and bucket foundation. As the loadtransferring, yet vulnerable component, the fatigue mechanism of the grouted connection and its influence on the cyclic laterally-loaded response of OWT foundation are still not clear. In this study, a sophisticated three-dimensional (3D) finite element (FE) model of the pile-bucket foundation with grouted connection is constructed, which incorporates a hypoplastic clay model and the concrete damage plasticity (CDP) to consider the cyclic load effect on both soil and grout material. A modal analysis is first performed to verify the rationality of the proposed model. Then the influence of cyclic load frequency, load amplitude and stiffener arrangement on the accumulation of pile head displacement, stress distribution and crack development of the grouted connection is systematically analyzed. Results indicate that as load frequency approaches the eigen-frequency, the OWT structure tends to vibrate more intensively, leading to stress concentration and fatigue damage of the grouted material and rapid accumulation of the pile-head displacement. The influence of load amplitude on grout damage seems to be limited in the contact area in the simulated cases. Meanwhile, the installation of stiffeners slightly mitigates the pile head displacement accumulation, but also raises the risk of stress concentration and fatigue damage of the grouted connection. The numerical results reveal the load-transferring function and fatigue damage of the grouted connection, which could provide some reference for an optimized structure and dynamic design for the pile-bucket foundation under cyclic load.

Piled raft foundations are increasingly used in construction due to their cost efficiency, requiring fewer piles than traditional pile foundations. Their ability to withstand cyclic lateral loads, such as those from earthquakes and wind forces, is crucial for structural stability. Understanding their response under cyclic loading conditions is essential, and finite element modeling (FEM) is a valuable tool for analyzing these behaviors. A recent 3D FEM study examined the performance of piled raft foundations in clay soils, focusing on loading pattern, frequency, and number of cycles. Results showed that lateral load capacity decreased as cycle count and frequency increased, with full cyclic loading (FCL) having a more pronounced effect than half cyclic loading (HCL). The raft shared 20.57-39.07 % (HCL) and 27.68-55.13 % (FCL) of lateral loads at frequencies of 0.1-10 Hz over 20 cycles. Additionally, locked-in moments increased by 21 %, and the degradation factor ranged from 65 to 80 % for HCL and 70-90 % for FCL. These findings provide valuable insights into pile-soil interaction and foundation stability under cyclic lateral loading, ensuring more effective design strategies for structures exposed to dynamic forces. Future research should explore long-term cyclic effects to further optimize foundation performance.

It is well known that piles embedded in sand accumulate lateral deformation (displacement and rotation) when subjected to horizontal cyclic loading. The rate of accumulation depends on various parameters, such as loading conditions and properties of the pile-soil system. For nearly rigid piles, such as monopile foundations for offshore wind turbines, an essential aspect is the type of loading, which is determined by the ratio of the cyclic minimum load to cyclic maximum load. Several studies have shown that asymmetric two-way loading generally results in larger accumulated pile deformation compared with other types of loading, especially oneway loading with complete unloading in each cycle. This article presents the planning, execution, and evaluation of physical 1g small-scale model tests on the deformation accumulation of laterally loaded rigid piles due to cyclic loading focusing on soil deformations resulting from various cyclic load ratios. To visualize soil deformation fields and rearrangement processes within the soil profiles, particle image velocimetry (PIV) was applied in the tests. The evaluation of the model test results provides insights into varying accumulation rates and highlights the capabilities as well as limitations of PIV. The observations are summarized under the of findings, which may assist in planning future PIV experiments.

The monopile response to lateral load under different liquefaction phases via centrifuge modeling tests is reported in this paper. The test was performed in viscous scaling mode to match the loading time and flight speed at a centrifuge acceleration of 80 g. A mixture of methylcellulose powder in water increased the viscosity by 80 times compared with water at a concentration of 0.47%. The seismic input is applied with a frequency of 1 Hz to achieve soil liquefaction. During the test, the monopile received the lateral load. The liquefaction regime was divided into four distinct phases based on the excess pore water pressure ratio during loading: total liquefaction, two tests during the period of degradation of pore pressure when the excess pore water pressure ratio was 0.67 and 0.32, and the last test at the end of liquefaction. The results reveal that pore water pressure distribution slightly differs in the free field and surrounding pile. The shallow layer started to liquefy early in response to the ground acceleration. The entire monopile body rotated owing to 0.48 m of pile head displacement during full liquefaction when the lateral load applied reached 290 kN. During the liquefaction regime, the profile of the pile behaviors migrating from a slender pile to a rigid pile is a significant discovery in this study. A large shear force appeared one-third of the pressure to the bottom of the monopile, and the maximum bending moment location became deeper, with a value of 65% greater than at the end of liquefaction.

Most previous experimental studies on the behavior of piles subjected to lateral loading have focused on testing model piles embedded in dry or fully saturated soils, and little attention has been paid to the impact of the soil partial saturation on the results. This paper presents results of 1g model tests on a single pile embedded in dry and unsaturated sand subjected to two-way constant displacement amplitude loading. The tests were aimed at examination of the effects of degree of soil saturation and density on the pile internal forces and lateral capacity and the deformation patterns of the adjacent soils. Five degrees of saturation (Sr = 0, 10, 20, 35 and 50%) for loose and medium-dense sand (Dr = 20% and 50%) were chosen and a 65-mm-diameter and 900-mm-long polyethylene model pile was used. Test results indicated that at each soil relative density, the pile head horizontal load, and the maximum bending moment, shear force, and soil reaction in the pile increase with increase in the degree of saturation up to about Sr = 35%. However, further increase in Sr led to decrease in these values. Moreover, the cyclic loading led to depressions in the surface of the dry sand and bulging associated with soil-pile separation in the unsaturated sand. For the model testing conditions used, results indicated that the sand degree of saturation can have a greater impact on the pile behavior than its density.

Concrete-filled FRP (Fiber Reinforced Polymer) tube composite piles offer superior corrosion resistance, making them a promising alternative to traditional piles in marine environments. However, their performance under cyclic lateral loads, such as those induced by waves and currents, requires further investigation. This study conducted model tests on 11 FRP composite piles embedded in sand to evaluate their behavior under cyclic lateral loading. Key parameters, including loading frequency, cycle count, loading mode, and embedment depth, were systematically analyzed. The results revealed that cyclic loading induces cumulative plastic deformation in the surrounding soil, leading to a progressive reduction in the lateral stiffness of the pile-soil system and redistribution of lateral loads among piles. Higher loading frequencies enhanced soil densification and temporarily improved bearing capacity, while increased cycle counts caused soil degradation and reduced ultimate capacity-evidenced by an 8.4% decrease (from 1.19 kN to 1.09 kN) after 700 cycles under a 13 s period, with degradation rates spanning 8.4-11.2% across frequencies. Deeper embedment depths significantly decreased the maximum bending moment (by similar to 50%) and lateral displacement, highlighting their critical role in optimizing performance. These findings directly inform the design of marine structures by optimizing embedment depth and load frequency to mitigate cyclic degradation, ensuring the long-term serviceability of FRP composite piles in corrosive, high-cycle marine environments.

Prolonged lateral cyclic loading leads to soil stiffness degradation around offshore wind turbine (OWT) foundations, which reduces the system's natural frequency and increases the accumulation of foundation rotation angle. Proper evaluation of natural frequency and rotation angle is crucial for the design of OWT foundations. This study develops a two-stages numerical approach to calculate the fundamental frequency of OWT systems considering the foundation stiffness degradation by integrating a stiffness degradation model of soft clays with a simplified three-spring model. Subsequently, it investigates the evolution of accumulated rotation and natural frequency for three foundation types-monopile, monopod bucket, and hybrid monopile-bucket-throughout their service life. It is observed that the hybrid foundation shows the smallest rotation in the first cycle, attributable to its relatively high initial stiffness compared to the other two foundations with the same steel consumption. However, it also exhibits the highest rate of cumulative rotation growth. At the same load level, the monopod bucket foundation exhibits the largest cumulative rotation angle due to its lower bearing capacity, and the degradation of natural frequency is most pronounced for monopod bucket OWT. For all three foundation configurations, increasing either the pile diameter or the bucket diameter is the most effective approach to reduce the cumulative rotation angle and improve natural frequency degradation, while maintaining the same steel consumption. These findings should be considered in the design of OWT foundations.

Large-diameter steel pipe pile foundations, typically known as monopiles, are currently the dominant foundation solution for supporting offshore wind turbines. The design of monopiles in sandy seabed is typically based on p-y curves derived for fully drained conditions. However, in reality, the drainage condition around a monopile under cyclic loading, at least during each single loading cycle, is generally undrained. To verify the applicability of the design methods based on fully drained condition, this study conducted a series of finite element analyses examining the effect of drainage condition on the monopile soil-pile interaction in sandy seabed. Based on the analyses in four sands which are of different relative densities and particle size distributions, it is found that, for medium dense to very dense sands that exhibit dilative response upon shearing, the effect of drainage conditions can be practically ignored within the range of load relevant for practical engineering. For loose sands or sands with considerable fines that exhibit contractive response upon shearing, the drainage conditions have negligible effect on the soil-pile interaction stiffness at low to modest load levels; however, the undrained conditions can lead to lower capacities. This implies that the current design approach which assumes fully drained soil response is still acceptable for the FLS design in such soil conditions. However, for the ULS design, assumption of drained soil response may lead to overestimation of the lateral bearing capacity and assessment of the actual drainage condition and its influence on soil-pile interaction on a project-specific basis is warranted for such cases.

This study employs variance-based and parametric analyses to quantify the impact of geometric and mechanical properties on the performance of pile foundations under axial tensile (Pa) and lateral (PL) loading. Utilizing 3D finite element analysis with the Drucker-Prager model, the research investigates pile-pile cap interaction across varying soil moduli (Es = 5, 20, & 50 MPa) and length-to-diameter (L/D) ratios (10, 20, & 33). The sensitivity analysis identifies the friction coefficient between sand and pile, as well as pile diameter, as the most influential factors, followed by pile length and the Young's modulus of both the pile and the sand. Parametric analysis reveals that pile deformation, contact pressure (Pc), and shear stresses (fs) are strongly affected by Es and the L/D ratio. Under Pa loading, as Es and L/D decrease, fs increases up to a certain depth before decreasing. Additionally, the normalized Pa to axial deformation ratio (Pa/delta a) decreases with increasing relative stiffness of the pile to soil (Ep/Es), with L/D becoming increasingly influential as Ep/Es decreases. Under PL loading, increased L/D and Es result in greater pile flexibility and a concentration of Pc at the top. The pile's lateral deformation behavior with depth mirrors the Pc distribution.

The piles are structural elements in a foundation that transfer weight from the superstructure to the soil. The behaviour of pile foundations under lateral loading is critical. The pile needs to have enhanced tensile strength and ductility by adding supplementary material to withstand the lateral loads. There were many research studies done to improve these properties in concrete, and the addition of fibre to the concrete is one among them. Fibre-reinforced concrete is classified into numerous categories depending on the type of fibre used. This study is to use the combination of Basalt and E-glass fibre i.e., hybrid in the full-scale pile foundation under combined axial and lateral cyclic loadings. The experimental investigation was conducted on the full-scaled Conventional Concrete (CC) and Hybrid Fibre Reinforced Concrete (HFRC) piles to understand the behaviour under static and cyclic lateral loads. The lateral displacement on the piles was measured at each level of loadings using a Linear Variable Differential Transformer (LVDT). The load-displacement behaviour of CC and HFRC piles was compared under different loading conditions. The HFRC pile exhibits a 40% reduction in displacement and a 10% increase in ultimate carrying capacity compared to the CC pile. HFRC piles tend to have more load-carrying capacity than the CC piles under all types of loadings.