Increasing demand on clean energy leads to the expanded construction of offshore wind turbines (OWT) worldwide. Different types of foundations of OWTs includes gravity, jackets, monopiles etc. When functioning, OWTs face severe conditions with complex loadings (e.g. varying loading amplitudes and loading frequencies). In this study, the influence of the loading amplitude and loading frequency on the lateral displacement of monopiles in marine clay was investigated by conducting 1-g physical model tests at a scale of 1:30. The p-y curves at different depths were derived as well from the recorded moment distribution along the monopile. According to the results, the lateral displacement increases with the loading amplitude and frequency and the monopiles experience response of shakedown under cyclic loading. The lateral displacement after N cycles is related to the initial displacement via an extended logarithmic function. Besides, the p-y curves available in literature underestimate the soil resistance but hyperbolic functions provide comparatively closer predictions.

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.

The paper introduces a semi-analytical approach for predicting the pile-soil response under cyclic lateral loads in sands, incorporating the cavity expansion/contraction theory with an anisotropy and non-associated constitutive model, Simple ANIsotropic SAND (SANISAND). The pile hole is regarded as a cylindrical cavity, and the cyclic loading process is reasonably treated as a cavity expansion/contraction problem. A superposition principle is introduced to determine the superimposed stress states around the cavity. The geometric relationship, quasistatic equation, and boundary conditions are integrated into a standardized solving procedure to obtain the stress-strain distribution surrounding the pile. Subsequently, the derived cyclic p-y curve is used in conjunction with the deflection equilibrium differential equation and finite-difference method to determine the pile-soil response under lateral cyclic load. The method's validity and capacity are further demonstrated through two well-examined centrifuge tests, which shows a good agreement with the experimental data. The cumulative deformation, hardening and ratcheting behaviors of pile-soil system can be captured in this study, which provides a novel approach to figure out the pile-soil response in sands under cyclic lateral loads.

Long-span river or sea crossing bridge projects are commonly subjected to significant and complex long-term cyclic loads driven by factors such as wind and waves. The lattice-shaped diaphragm wall (LSDW) foundation, a novel and promising solution for long-span bridges, offers advantages in construction safety, adaptability, costeffectiveness, and stiffness. However, research in this area remains limited, impeding the further application of LSDWs. This study comprehensively investigates the bearing behavior of LSDWs under horizontal cyclic loads in soft soils. It introduces a detailed methodology utilizing a dual-layer wall setup and mathematical calculations to measure key parameters such as wall bending moments, displacements, and soil pressures. The research explores LSDW behavior throughout cyclic loading cycles, revealing trends in soil compression, densification, and displacement growth rates. Analysis of cumulative displacement and rotation patterns underscores the influence of load amplitudes and wall stiffness. The findings highlight the significant impact of cyclic loading cycles on cumulative displacement, with curve fitting revealing a logarithmic function with a strong correlation. Additionally, the study delves into the reduction in lateral soil resistance with increasing cyclic loads, proposing cyclic weakening factors for predicting soil pressure distribution and cyclic p-y curves. This study offers valuable insights into the comprehensive analysis and prediction of the performance of LSDW subjected to horizontal cyclic loading, with potential implications for long-span bridge construction projects.