To meet growing energy demands, offshore wind turbines (OWTs) with higher energy outputs are being developed, presenting increased challenges for their foundation design. Over the past decade, extensive research on the design optimization of OWT support structures has significantly reduced the anticipated costs of offshore wind farm development. Various design methods have been developed and applied in practice, each with its own advantages and limitations. In this study, 3D finite element (FE) modeling, validated against the measured response of a large-scale test monopile, is used to investigate the lateral load response of monopiles with different geometries and slenderness ratios in smaall and large displacements. The results are compared to the standard p–y method, and specific behavioral and design aspects of large-diameter monopiles, such as the moment contribution ratio from different resisting components and the minimum embedment length criteria, are evaluated and discussed. The results showed that the maximum and minimum differences between the 3D FE modeling and one-dimensional (1D) DNV p–y method are 41% and 11% for large displacements, and 32.5% and 13.3% for small displacements, respectively. As the slenderness ratio increases, the discrepancy between the finite element (FE) modeling results and the 1D DNV p–y method decreases, with an average difference of about 13% across all monopile diameters at an L/D ratio of 10, in both small and large displacements, indicating the reasonable accuracy of the 1D method for slenderness ratios of 10 and above. Among the three minimum embedment length criteria examined, the DNV recommended and vertical-tangent criteria offered shorter embedment lengths. The primary resisting moment across all slenderness ratios comes from the distributed lateral load along the monopile shaft (${\mathrm{M}\mathrm{C}\mathrm{R}}_{\mathrm{p}-\mathrm{y}}$), which increases as the L/D ratio increases.

Sources such as wind or severe seismic activity often exert extreme lateral loading onto the shallow foundations supporting high-rise structures such as bridge piers, buildings, shear walls, and wind turbine towers. Such loading conditions may cause the foundation to exhibit nonlinear responses such as uplift and bearing capacity mobilization of the supporting soil (i.e., rocking behavior). Previous numerical and experimental studies suggest that while such inelastic behaviors may engender residual deformations in the soil–foundation system, they offer potential benefits to the overall integrity of structures through dissipating energy and reducing inertia forces transmitted to the superstructure, thereby limiting seismic demand on structural elements. This study investigates the effect of footing shape on the rocking performance of shallow foundations in different subgrade densities and initial vertical factor of safety (FSv). To this end, a series of reduced-scale slow cyclic tests under 1 g condition were conducted using a single degree of freedom (SDOF) structure model. The performance of different footing shapes was studied in terms of moment capacity, recentering ratio, rocking stiffness, damping ratio, and settlement. For three foundations with different length-to-width ratios, the results indicate that increasing the safety factor and length-to-width ratio leads to thinner, S-shaped moment–rotation curves, mainly owing to the enhanced recentering capability and the P-δ effect. Moreover, across all foundation types, the repetition of a limited loading cycles with consistent rotation amplitude does not cause stiffness degradation or moment capacity reduction.

Ground hydrocarbon contamination is a grave consequence of fossil fuel consumption, making it increasingly unsustainable. Consequently, researchers worldwide have been compelled to conduct a large number of studies on the geotechnical aspects of hydrocarbon-contaminated soils. Still, the complications arising from the integration of geotechnical complexities with diverse hydrocarbon properties present a substantial research need. The assessment of the foundation bearing capacity in hydrocarbon-contaminated soils is of paramount importance, especially given that numerous contaminated sites either house significant industrial facilities or are earmarked for critical infrastructure projects. This study investigates the shear strength and bearing capacity of gas oil-contaminated coastal sands using laboratory testing and physical modeling, with a special focus on the combined influence of the degree of saturation and relative density. Footing’s shear failure mechanisms were analyzed based on predicted and measured bearing capacity values. Findings show a decrease in the friction angle with a contamination increase, while the apparent cohesion initially rises before declining. Furthermore, the study revealed that the foundation’s bearing capacity exhibited an increase up to a specific contamination level, followed by a subsequent decrease. This increase is circa 90 and 100 percent at a 5 percent contamination content for loose and dense sand, respectively. Comparing the predicted and measured bearing capacity results shows that the general shear failure mechanism has occurred in the dense subgrade, while the loose subgrade’s failure mechanism is more inclined towards local shear failure.

Shallow foundations supporting high-rise structures are often subjected to extreme lateral loading from wind and seismic activities. Nonlinear soil–foundation system behaviors, such as foundation uplift or bearing capacity mobilization (i.e., rocking behavior), can act as energy dissipation mechanisms, potentially reducing structural demands. However, such merits may be achieved at the expense of large residual deformations and settlements, which are influenced by various factors. One key factor which is highly influential on soil deformation mechanisms during rocking is the foundation embedment depth. This aspect of rocking foundations is investigated in this study under varying subgrade densities and initial vertical factors of safety (FSv), using the PIV technique and appropriate instrumentation. A series of reduced-scale slow cyclic tests were performed using a single-degree-of-freedom (SDOF) structure model. This study first examines the deformation mechanisms of strip foundations with depth-to-width (D/B) ratios of 0, 0.25, and 1, and then explores the effects of embedment depth on the performance of square foundations, evaluating moment capacity, settlement, recentering capability, rotational stiffness, and damping characteristics. The results demonstrate that the predominant deformation mechanism of the soil mass transitions from a wedge mechanism in surface foundations to a scoop mechanism in embedded foundations. Increasing the embedment depth enhances recentering capabilities, reduces damping, decreases settlement, increases rotational stiffness, and improves the moment capacity of the foundations. This comprehensive exploration of foundation performance and soil deformation mechanisms, considering varying embedment depths, FSv values, and soil relative densities, offers insights for optimizing the performance of rocking foundations under lateral loading conditions.

The design of offshore monopile foundations typically follows an iterative process aimed at optimizing key geometric parameters—namely, pile diameter, wall thickness, and embedded length. Among these, selecting an appropriate embedded length is a critical step in geotechnical design, as it must satisfy both stability and serviceability requirements. The critical pile length is defined as the embedment depth beyond which additional penetration yields no significant improvement in lateral capacity and at which the pile reaches its critical lateral capacity. From a design standpoint, extending the pile beyond this length offers no further gain in resistance, rendering such an approach both inefficient and uneconomical. To evaluate and characterize the critical length of offshore monopile foundations, three-dimensional finite element (3D FE) analyses were performed on laterally loaded monopiles using the NGI-ADP constitutive model. The analyses considered a wide range of pile geometries, load eccentricities, and soil properties. This study first investigate how geotechnical parameters affect lateral response, then characterizes the critical lateral capacity ($H_{crit}$) and critical pile length ($L_{crit}$) based on the analyzed cases. Finally, an empirical equation was developed to estimate the critical embedment depth of monopiles in clay. Results indicate that higher undrained shear strength ($S_u$) or lower ultimate plastic shear strain (${\gamma }_f$) considerably reduce the critical pile length, whereas it is increased with greater pile head rotation. The normalized critical length is largely independent of pile diameter and load eccentricity. These insights provide practical guidance for geotechnical design by offering an efficient method to estimate critical pile length, supporting informed decisions on the required embedment depth.

Modern offshore wind turbines require increasingly larger monopile foundations, often with low embedded length-to-diameter ratios that fall outside the calibration range of the conventional standard p–y approach. Consequently, existing code-based p–y methods may provide an incomplete representation of the three-dimensional soil–structure interaction mechanisms mobilized under lateral loading. This study presents a comprehensive assessment of current geotechnical design concepts for laterally loaded monopiles embedded in clay and sand. To this end, three-dimensional finite element (FE) analyses were performed and used as the benchmark for comparison. The analyses employed the NGI-ADP constitutive model to simulate clay and the HSsmall constitutive model to represent sand. The FE models were validated against the PISA field test results from the Cowden and Dunkirk sites prior to their use in the subsequent evaluation. The validated FE models were then used to extract PISA-based p–y curves, which were compared with the code-based formulations for sand and clay at both the local p–y level and the global load–displacement level using 1D Winkler analyses. In addition, a moment contribution ratio (MCR) analysis was carried out to quantify the relative contributions of the individual resistance mechanisms governing monopile behaviour in both sand and clay. FE-extracted PISA p–y curves consistently produced the closest predictions of monopile behaviour captured in the FE analysis, while conventional code-based p–y formulations underestimated resistance in clay and overestimated it in sand. MCR analysis confirmed that lateral p–y reaction is the dominant resistance component, but shaft friction and base shear are important for short monopiles, especially at small displacements; incorporating these components into FE-calibrated 1D Winkler models substantially improves agreement with 3D FE predictions while retaining computational efficiency.