The cyclic behavior of clay significantly influences the dynamic response of offshore wind turbines (OWTs). This study presents a practical bounding surface model capable of describing both cyclic shakedown and cyclic degradation. The model is characterized by a simple theoretical framework and a limited number of parameters, and it has been numerically implemented in ABAQUS through a user-defined material (UMAT) subroutine. The yield surface remains fixed at the origin with isotropic hardening, while a movable projection center is introduced to capture cyclic hysteresis behavior. Cumulative plastic deviatoric strain is integrated into the plastic modulus to represent cyclic accumulation. Validation against undrained cyclic tests on three types of clay demonstrates its capability in reproducing stress-strain hysteresis, cyclic shakedown, and cyclic degradation. Additionally, its effectiveness in solving finite element boundary value problems is verified through centrifuge tests on large-diameter monopiles. Furthermore, the model is adopted to analyze the dynamic response of monopile OWTs under seismic loading. The results indicate that, compared to cyclic shakedown, cyclic degradation leads to a progressive reduction in soil stiffness, which diminishes acceleration amplification, increases settlement accumulation, and results in higher residual excess pore pressure with greater fluctuation. Despite its advantages, this model requires a priori specification of the sign of the plastic modulus parameter cd to capture either cyclic degradation or shakedown behavior. Furthermore, under undrained conditions, the model leads pstabilization of the effective stress path, which subsequently results in underestimation of the excess pore pressure.

The tetrapod jacket-supported offshore wind turbine is subjected to marine environmental loads, resulting in monotonic and cyclic lateral-compression-tension interaction behavior of the pile-soil system. Although the excellent applicability that has been demonstrated by three-dimensional numerical simulation for aiding the revelation of the mechanism of jacket foundation-soil interaction, a significant challenge remains in accurately reflecting the nonlinear stress-strain relationship and cyclic behavior of the soil, and others. Finite element numerical models are therefore established for laterally loaded tetrapod jacket pile foundations in this study, and a bounding surface model is adopted to simulate the elastoplastic characteristics and cyclic ratchet effect of the soil. Subsequently, a parametric analysis is conducted on different net spacings and aspect ratios of the jacket base-piles to investigate the pile deformation characteristics, bearing mechanisms, evolution of pile-soil interaction, and the internal force development under monotonic and cyclic conditions, respectively. The results indicate that under monotonic loading, the pile deformation pattern transitions from a flexible pile mode to a rigid rotational deformation mode as the aspect ratio decreases. Under cyclic loading, attention should be paid to the asynchronous accumulation of axial forces within the base-piles and its impact on overall bearing performance.

The p-y curve method provides a relatively simple and efficient means for analyzing the cyclic response of horizontally loaded piles. This study proposes a p-y spring element based on a bounding surface p-y model, which can be readily implemented in Abaqus software using the user-defined element (UEL) interface. The performance of these p-y spring elements is validated by simulating field tests of laterally loaded piles documented in the literature. The developed spring element effectively replicates the nonlinear hysteresis, displacement accumulation, and stiffness degradation observed in soft clay. Subsequently, a finite element model of a large-diameter monopile is established using the proposed spring element. A comprehensive numerical investigation is conducted to explore both the monotonic and cyclic responses of large-diameter monopiles in soft clays. The results are presented and discussed in terms of pile head load-displacement curves, the evolution of rotation angles at the mud surface, and cyclic p-y curves. Additionally, empirical formulas are proposed to predict the evolution of cumulative rotation angles and peak bending moments under both one-way and two-way cyclic loading conditions. The results provide valuable insights into the mechanism of pile-soil interaction under lateral cyclic loading.

To accurately predict soil thermal effects is of great importance for simulations of complex boundary value problems such as the energy foundations and nuclear waste disposal. Existing thermo-mechanical constitutive models only account for clay, and cannot simulate the sand's behaviour under thermo-mechanical conditions. In this study, a unified thermo-mechanical bounding surface (UTMBS) model is proposed for saturated clay and sand. Based on the thermal effects on the isotropic compression line, the model proposes a new unified thermal softening relationship and a plastic modulus for clay and sand, with the thermal cyclic behaviour replicated by a memory surface. The unified model considers the thermal effects on the critical state line and the shape of the bounding surface, accounting for both drained and undrained shearing of clay and sand at different temperatures. In addition, the non-linear elasticity relationship represents the hysteresis loops of the stress-strain relationship in the mechanical cycles. The performance of the proposed model is evaluated against existing experimental results for clay and sand in terms of their thermal cyclic behaviour, drained/undrained triaxial compression, and mechanical cyclic behaviour at different temperatures. It is evident that the UTMBS model is able to simulate various thermo-mechanical behaviours of clay and sand.

Vibratory compaction significantly affects the construction quality of the subgrade in the road construction. Establishing a numerical analysis model for the subgrade compaction process helps visualize the compaction process and enhances the quality of compaction for the subgrade. Nowadays, such nonlinear characteristics between the vibratory roller and subgrade could be captured by establishing numerical simulation methods via finite element analysis, which effectively reduces the difficulty of the solution. In these methods, however, the elastoplastic model for the subgrade material tends to ignore the plastic accumulation characteristics of the soil under cyclic loading. Aiming at this problem, a finite element simulation method is proposed for the compaction process of subgrade. In the method, a bounding surface model considering plastic accumulation effect under cyclic loading is used for modelling the compacted soil material. Consequently, a three-dimensional finite element simulation model of drum-soil was established by using UMAT in the secondary development of ABAQUS. Compared with experimental data and popular models like the modified Cambridge model, the DruckerPrager criterion and the Mohr-Coulomb model, the finite element simulation method in this study demonstrates higher accuracy in terms of soil stress, settlement and drum acceleration, confirming its effectiveness. Finally, the dynamic changes in stress and strain during the compaction and the effects of the excitation forces on compaction were analyzed by the finite element simulation method.