Offshore wind turbines, crucial for global electricity generation, face significant challenges from harsh marine conditions, including strong wind, waves, and uneven seabeds. To optimize the foundation solution, this study investigates the lateral performance of helical monopiles, comparing conventional monopiles under cyclic loading, with a focus on variations in pile configuration and soil conditions. Model-scale experiments were conducted with helical piles subjected to both monotonic and one-way cyclic loading conditions. Key variations in the study include three soil densities (Dr = 35 %, 55 %, and 75 %), along with different slope conditions (Flat, 1V:5H, 1V:3H, 1V:2H) and pile positions (c = 0Dp, 2.5Dp, 5Dp, 7.5Dp). Additionally, the effect of load amplitudes (xi b = 50 %, 40 %, and 30 %) applied at a frequency of 0.25Hz for over 1000 cycles was examined. Results showed that helical piles outperformed conventional monopiles, exhibiting up to 25 % higher lateral load capacity, 30 % less accumulated rotation, and 20 % greater cyclic stiffness, especially in dense soils. Furthermore, the analysis revealed that the performance of helical piles significantly improved when placed nearer to the slope crest and in denser soils. Numerical simulations using PLAXIS 3D confirmed these experimental findings, demonstrating that helical piles consistently maintain superior lateral resistance and cyclic performance under varying loading conditions and slope configurations. This study underscores the potential of helical piles to enhance the stability ad performance of offshore wind turbine foundations, offering a more robust and efficient alternative to monopile systems.

This study explores the performance of finned monopiles as an innovative foundation solution for Offshore Wind Turbines subjected to cyclic loading under varying seabed conditions. Traditional monopiles face challenges related to stability when installed on sloped terrains, which are common in offshore environments. To address this, the research investigates the effectiveness of rectangular fins attached along the monopile's length to improve lateral resistance and reduce accumulated rotation. Experimental and numerical analyses were conducted across different slope gradients (flat, 1V:5H, 1V:3H, 1V:2H), pile positions (0Dp, 2.5Dp, 5Dp, 7.5Dp), and soil densities (35%, 55%, 75%), applying cyclic loading at 0.25 Hz over 1000 cycles with lateral load amplitudes (xi b) of 30%, 40%, and 50%. This study is the first to investigate finned monopiles under cyclic loading on sloping seabed conditions, demonstrating a 30-60% improvement in lateral resistance by increasing the passive soil resistance by reducing the rotation compared to monopiles. This work addresses the challenges of Offshore Wind Turbine foundations in complex topographies. Numerical modeling using PLAXIS 3D closely aligned with experimental findings, confirming the effectiveness of finned monopiles in enhancing stability on sloped seabeds. These findings suggest that finned piles offer a robust foundation alternative for Offshore Wind Turbines, particularly in challenging environments with variable seabed topography.

The present study aims to examine the behavior of sand reliquefaction phenomenon on gently inclined sloping ground subjected to repeated seismic events. The seismic sequence represents the combinations of foreshocks and aftershocks associated with mainshock events. Free field and pile group models were experimented with in a sloping ground with 5 degrees inclination using centrifuge modelling. A 2 x 2 pile group model was inserted in Toyoura sand, and results were compared with that with free field model ground. Tapered sinusoidal waveform was inputted at a constant 1 Hz shaking frequency, whereas the acceleration amplitude and shaking duration for the mainshocks were considered twice that of foreshocks and aftershocks. Two different seismic sequences with six shaking events were imparted to the model grounds to investigate the influence of slope and presence of pile group on sand reliquefaction behavior. The time history responses were recorded in the form of acceleration, excess pore pressure (EPP), bending moment, and lateral displacement and presented. The response indicates that sloping ground was stable under the action of foreshocks, whereas it collapsed during mainshocks, primarily due to liquefaction. The mainshock has transformed the gently inclined sloping ground to levelled ground model. This transformation has resulted in increased bending moment values in the pile group. Resistance to reliquefaction was smaller compared to first liquefaction, primarily due to change in soil state and increase in magnitude of anisotropy. Presence of slope has resulted in higher EPP response and bending moment values compared to levelled ground. GeoPIV analysis and visualization show the flow of sand particles from upside to downside due to lateral spreading at shallow depths that initiated the slope failure. The foreshocks and aftershocks are not very significant in increasing the resistance to reliquefaction. Meanwhile the presence of the pile group has reduced the EPP generation during repeated shaking events.

This paper presents a computational sand model based on the well-known pressure dependent multi-surface constitutive model to solve the necessity of employing a separate set of model parameters for each soil relative density change. The proposed model correlates the original model parameters with the soil relative density through critical-state-based soil mechanics formulations to provide a single set of model constants that adapt to different soil states. Model formulation updates are performed for the flow rules, material moduli calculations, and the computation of stress ratios at the phase transformation and failure stages. The model parameters are calibrated for Ottawa F-65 sand against cyclic soil element tests with different stress levels and various soil densities. Thereafter, numerical simulations are conducted for centrifuge experiments of gently sloped grounds to validate the proposed model. Throughout numerical simulations, the proposed model accurately replicates the sand cyclic undrained behavior as similar to laboratory-measured responses for different soil relative densities with a single calibrated set of model parameters and provides reliable numerical predictions in finite element simulations of the engaged centrifuge experiments. Overall, the proposed model robustly simulates the saturated sand seismic response, which can improve the numerical prediction accuracy of liquefaction-induced damage in different engineering applications.

Liquefaction-induced large deformations in sloping ground caused heavy damage to buildings and infrastructures during earthquakes, and its evaluation and mitigation challenge. In this study, a series of soil element tests using hollow cylinder apparatus (HCA) were conducted to investigate the relationship between residual volumetric strain and residual shear strain of medium dense to dense saturated sand with moderate initial static shear stress. The soil element tests indicate that the developments of residual volumetric strain and residual shear strain are dominated by the Post-liquefaction Deformation Potential (PLDP) of soil, which is well correlated to the maximum cyclic shear strain developed during cyclic loading. Then, the applicability of PLDP to characterize the post-liquefaction deformation response in gently sloping ground was investigated by centrifuge model tests without and with stone column improvement. The model tests of medium dense and dense sand slopes proved the applicability of PLDP preliminarily. The mitigation mechanisms against settlement and lateral spreading in gentle slopes by densification and drainage effects induced by stone columns were also observed and discussed. The present study provides the conceptual term of PLDP for evaluating post-liquefaction deformations of natural and stone column-improved gently sloping grounds, which helps to develop mitigation techniques for liquefiable sloping ground subjected to earthquake loadings.