The structural design of offshore wind turbines must account for numerous design load cases to capture various scenarios, including power production, parked conditions, and emergency or fault conditions under different environmental conditions. Given the stochastic nature of these external actions, deterministic analyses using characteristic values and safety factors, or Monte Carlo Simulations, are necessary. This process involves a large number of simulations, ranging from ten to a hundred thousand, to achieve a reliable and optimal structural design. To reduce computational complexity, practitioners can employ low-fidelity models where the soil-foundation system is either neglected or simplified using linear elastic models. However, medium to large cyclic soil-pile lateral displacements can induce soil hysteretic behaviour, potentially mitigating structural and foundation vibrations. A practical solution at the preliminary design stage entails using stiffness-proportional viscous damping to capture the damping generated by the soil-pile hysteresis. This paper investigates the efficacy of this simplified approach for the IEA 15 MW reference wind turbine on a large-diameter monopile foundation subjected to several operational and extreme wind speeds. The soil-pile interaction system is modelled through lateral and rotational springs in which a constant stiffness-proportional damping model is applied. The results indicate that the foundation damping generated by the nonlinear soil-pile interaction is significant and cannot be neglected. When fast analyses are required, the stiffness-proportional viscous damping model can be reasonably used to approximate the structural response of the wind turbine. This approach enhanced the accuracy of the computed responses, including the maximum bending moment at the mudline for ultimate limit design and damage equivalent loads for fatigue analysis, in comparison to methods that disregard foundation damping.

Offshore structures typically experience multiple storms during their service life. The soil around the foundations of offshore structures is subjected to cyclic loading during storm and reconsolidates between storms. Therefore, it is essential to understand the fundamental soil behaviour under episodic cyclic loading and reconsolidation to evaluate the long-term serviceability of offshore foundations. This paper presents experimental results of a comprehensive suite of cyclic DSS tests on a normally consolidated silty clay. The tests explore the soil response under different cyclic loading patterns (e.g., one-way or two-way), different cyclic amplitudes and number of cycles. A theoretical model, which combines the conventional cyclic contour diagram approach and principles of the critical state soil mechanics, is proposed and validated for predicting the cyclic soil response during undrained cyclic loading and hardening after reconsolidation. The model proposed in this paper paves a critical step for developing long-term soil-structure interaction models that are fundamentally linked to soil element level responses.

Using steel slag concrete (SSC) as a pile material not only promotes industrial waste recycling but also improves ground conditions through its distinct hydrological and chemical properties. This study investigated the hydrological processes of SSC piles under no-load conditions, offering new insights into pile-soil interactions. A novel visualization test device was developed to continuously monitor water migration, pore water pressure fluctuations, and soil disturbance over six months. Macro-scale observations and micro-scale analyses were conducted to elucidate physical and chemical reactions at the pile-soil interface. Compared to ordinary concrete piles, SSC piles demonstrated superior expansion and drainage capabilities, characterized by enhanced radial and vertical water flow, increased surface porosity, and the formation of a distinct interface layer enriched with calcium carbonate and cementitious hydration products. These improvements facilitate effective water distribution and drainage while reinforcing the pile-soil bond, thereby contributing to a more robust composite system for ground improvement. This integrated approach and its findings offer valuable contributions to the broader field of soil-pile interactions by detailing the multi-scale mechanisms governing the hydrological behavior and interface evolution of composite foundation systems.

Composite reinforcement concrete square piles exhibit excellent bending resistance and deformation capacity, along with construction advantages such as ease of transportation. In recent years, they have been widely adopted in building pile foundation applications. However, their seismic behavior, particularly under multi-directional excitation, remains inadequately explored. This study employs large-scale shaking table tests to evaluate the seismic response of a single composite reinforcement square pile embedded in a soft clay foundation under different horizontal excitations (0 degrees and 45 degrees) and two distinct ground motions (Wenchuan Songpan and Chi-Chi) to assess directional anisotropy and resonance effects, with explicit consideration of soil-structure interaction (SSI). The key findings include the following: the dynamic earth pressure along the pile exhibits a distribution pattern of large at the top, small at the middle and bottom. And SSI reduced pile-soil compression by 20-30% under 45 degrees excitation compared to 0 degrees. The dynamic strain in outer longitudinal reinforcement in pile corners increased by 30-60% under 45 degrees excitation compared to 0 degrees. Under seismic excitation considering SSI, the bending moment along the pile exhibited an upper-middle maximum pattern, peaking at depths of 3-5 times the pile diameter. Axial forces peaked at the pile head and decreased with depth. While bending moment responses were consistent between 0 degrees and 45 degrees excitations, axial forces under 45 degrees loading were marginally greater than those under 0 degrees. The Chi-Chi motion induced a bending moment about four times greater than the Songpan motion, highlighting the resonance risks when the ground motion frequencies align with the pile-soil system's fundamental frequency.

The use of permeable piles as an effective drainage method in liquefiable sites has become widely accepted. In this study, the seismic response of both the liquefiable soil and the pile was simulated using FLAC3D software to validate the anti-liquefaction performance of the permeable pile. A group of permeable piles designed according to the China foundation code were numerically modeled with various opening ratios (i.e. area of openings/total surficial area). The numerical results showed that the permeable pile is able to enhance liquefaction resistance by dissipating excess pore water through the drainage holes. The bending moments and axial force of the permeable pile decrease but the ultimate bearing capacity increases in the process of drainage. It is found that the excess pore water pressure ratio (EPWPR) of soil around permeable pile under seismic loading reduces rapidly with increasing opening ratio, but the excess pore water pressure tends to keep nearly a stable level once the opening ratio is beyond a critical value of 0.5%. As a result, the critical value of the opening ratio may be considered as the optimum parameter to design the permeable pile against liquefaction.

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.

The kinematic interaction between piles under seismic loading has been extensively studied from analytical, experimental, and numerical perspectives. Of note, within numerical modeling, the majority of the existing literature relies on simplified approaches for characterizing the soil-pile interaction, which leads to the requirement for more reliable and comprehensive research. In this paper, using FLAC3D, the seismic response of the soil-pile system was investigated with a set of fully nonlinear three-dimensional (3D) numerical analyses in the time domain. This model simulated the soil strength and stiffness dependency on the stress level and soil nonlinear behavior under cyclic loading. The Mohr-Coulomb (M-C) constitutive model described the soil's mechanical behavior, which was used with additional hysteretic damping to suit the dynamic behavior. In the framework of a parametric study, the effects of loading frequency on the response of a soil-pile system that was subjected to seismic loading were studied. The results showed that the pile response and soil characteristics, as well as the natural frequency mode of the system's dynamic behavior, are strongly affected by the frequency of the seismic loading. Therefore, the bending moment and lateral displacement along the length of a pile increase as the loading frequency approaches the natural frequency of the system. In addition, when the loading frequency reaches a threshold value far from the fundamental frequency of the system, the effect of loading frequency on the soil-pile system response becomes negligible. In addition, the relationship between the pile diameter and maximum pile bending moment at different loading frequencies is affected by the soil properties.

The prediction of time-dependent behavior of axial capacity for jacked piles are essential for coastal pile engineering. This study develops a numerical model to simulate the entire process of pile installation, soil consolidation, and loading, incorporating soil-pile interaction effects on excess pore pressure and effective stress distribution in the surrounding soil, which influence the bearing performance of jacked piles in saturated clay. The well consistency between the predictions from the presented approach and the experimental measurement data validate the applicability of the proposed model. The mechanism of set-up effects on the pile axial capacity is elucidated through the evolution of excess pore pressure. A parametric study is performed to assess the influence of the permeability coefficient (k) and length-to-diameter (L/De) ratio on the axial capacity of jacked piles. The findings demonstrate that the proposed model accurately predicts the set-up effects of jacked piles. Specifically, the permeability coefficient primarily impacts the rate of capacity increase, while the axial capacity exhibits a significant rise with an increase in L/De. The derived empirical formula can reasonably guide the design of the axial bearing capacity of piles in saturated clay.

Shield tunneling adjacent to existing piles is common occurrence in subway construction. This study proposes a novel tunnel model capable of simultaneously simulating ground loss, unloading effects, and void grouting under in-flight conditions. Several three-dimensional (3D) centrifugal scale model tests are implemented in a silty-silty clay composite to investigate the response of a single pile (Test SP) and pile group (Test GP) with a sinking low cap subject to adjacent tunneling. The results indicate a critical influence area, i.e., 0.75D in front and 0.25D behind the centerline of the existing piles, is observed for the pile head settlement, in each test, and the induced bending moment in the piles above the tunnel spring line is more sensitive to tunneling than that below it. A decreasing trend in axial force along the pile shaft is observed in Test SP, whereas Test GP shows the opposite behavior. The maximum variations in axial force and bending moment occur near the tunnel invert and crown in Test SP, respectively, however, they all appear near the tunnel spring line in Test GP. There is a law of load transfer for downward migration in Test SP. In Test GP, however, the load on the upper part of pile P1 decreases and shifts to the lower of pile P1 and the whole pile P2. Subsequently, the load on the upper part of pile P2 reduces and transfers to the lower part of pile P2 and the whole pile P1. A significant increment in the earth pressure near the pile toe is observed. The pore water pressure increases slightly at first and then dissipates. Digital image correlation (DIC) has been preliminarily demonstrated as a valuable tool for visually capturing the progressive behavior of pile-soil interactions during in-flight tunneling, proving advantageous for analyzing tunnel-soil-pile interaction issues under centrifugation conditions.

Local scour has been reported as a common phenomenon for pile-group foundations in marine, coastal, and riverine sites, while its effects on the seismic behavior of pile-group foundations are yet to be well documented. This paper reported a pair of quasi-static cyclic loading tests on 2 x 3 scoured pile-group foundation specimens, one in global scour and the other in local scour, to understand the influence of local scour on the seismic behavior, particularly in terms of soil-pile interaction features and failure mechanisms. Test results indicate a flexural failure mode for both specimens. Local scour does not change the order of limit states but postpones the occurrence of concrete cover cracking and rebar yielding due to the reduced lateral stiffness of the locally scoured piles. Moreover, local scour rarely changes the aboveground damage regions at the top of outer piles (one time the side length of squared pile section, D ), but significantly aggravates and deepens the underground damage regions at outer piles (from 4 5D D for global scour to 3.7 6D D for local scour). Besides, local scour reduces the displacement ductility factor from 2.50 to 1.25 for the Easy-to-Repair limit state, resulting in adverse impacts on pile-group foundations in general.