Impact pile driving is widely employed in various environments. The soil surrounding driven piles undergoes large shear displacements and highly cyclic loads, leading to significant strength degradation. This paper introduces a novel soil reaction model with easily calibrated parameters to estimate the pile penetration performance under continuous impact driving, incorporating both cyclic degradation and base gap. Soil cumulative plastic displacement is utilized to quantity the degradation, enabling more accurate simulation of cyclic pile response. The model is integrated into the pile driving system and applied in multiple-blow analysis. Non-linear cumulative displacement-blow count curves are analyzed and the development of residual stress varies between the pile upper and lower sections. It is found that lower blow counts are required when cyclic degradation is considered, although the increased rebound effect may counterbalance this benefit. Comparative analyses for degradation constants further demonstrate that early-stage degradation has a more pronounced impact. Finally, the proposed model is also adopted to predict blow count in field practice, offering valuable insights for driveability analysis.

Prolonged lateral cyclic loading leads to soil stiffness degradation around offshore wind turbine (OWT) foundations, which reduces the system's natural frequency and increases the accumulation of foundation rotation angle. Proper evaluation of natural frequency and rotation angle is crucial for the design of OWT foundations. This study develops a two-stages numerical approach to calculate the fundamental frequency of OWT systems considering the foundation stiffness degradation by integrating a stiffness degradation model of soft clays with a simplified three-spring model. Subsequently, it investigates the evolution of accumulated rotation and natural frequency for three foundation types-monopile, monopod bucket, and hybrid monopile-bucket-throughout their service life. It is observed that the hybrid foundation shows the smallest rotation in the first cycle, attributable to its relatively high initial stiffness compared to the other two foundations with the same steel consumption. However, it also exhibits the highest rate of cumulative rotation growth. At the same load level, the monopod bucket foundation exhibits the largest cumulative rotation angle due to its lower bearing capacity, and the degradation of natural frequency is most pronounced for monopod bucket OWT. For all three foundation configurations, increasing either the pile diameter or the bucket diameter is the most effective approach to reduce the cumulative rotation angle and improve natural frequency degradation, while maintaining the same steel consumption. These findings should be considered in the design of OWT foundations.

Fluidisation in saturated subgrades of transport infrastructure is a huge problem in many countries around the world caused by dynamic cyclic loads due to heavy haul trains on railways and heavy trucks on highways. The mechanism of subgrade fluidisation has been experimentally studied to a significant extent. However, numerical studies that have been carried out for studying fluidisation are limited. The first part of the paper includes a critical review of previous studies on the mechanism and the effect of cyclic loading factors on fluidisation. It is vital to conduct a comprehensive study with numerical modelling to simulate the actual field conditions of transport infrastructure to find reliable and cost-effective solutions to mitigate subgrade fluidisation. This goal can be achieved only by choosing a soil constitutive model that can capture the changes to the soil stiffness and strength due to excess pore pressure generation and dissipation, along with accumulated deformations in clay soil subjected to cyclic loading. Therefore, in this study, the SANICLAY constitutive model is selected as the suitable candidate to fulfil those requirements. It is implemented in the ABAQUS/Standard finite element program using the user-developed material subroutine UMAT. In the second part of the paper, the validation of the SANICLAY model that accounts for the anisotropy and structure of natural clay was presented using triaxial test data found in the literature for undisturbed clay. Application of the model to simulate cyclic loading shows that the version of SANICLAY used in the simulations needs modifications to capture the stiffness and strength degradation during cyclic loading.

The paper studies the effect of soil strength and stiffness degradation on the undrained cyclic performance of offshore foundations in low-plasticity cohesive soil using 3D finite element modelling. Cyclic triaxial tests on reconstituted kaolin are conducted at the ETH Zurich laboratory, providing insights into key parameters affecting the degradation process. A simplified soil constitutive model accounting for cyclic degradation is developed and encoded in Abaqus via a user subroutine. The model is calibrated against experimental results and validated with published centrifuge model tests of monopiles under cyclic lateral loading. It is subsequently used to evaluate the performance of suction caisson foundations with different aspect ratios (L/D = 0.5 and 2) under short-term cyclic and seismic loading. Due to its ductile resistance mechanism, the L/D = 0.5 caisson exhibits superior perfor-mance under vertical cyclic loading in fast-degrading soil. Under inclined cyclic loading, the slower degradation rate of the L/D = 2 caisson governs response, reversing the trend. Under seismic shaking, the degradation-induced resistance imbalance amplifies the irrecoverable settlements produced by kinematic shearing at the caisson sidewalls. For the fast-degrading soil examined, degradation is shown to increase settlements by up to 50%.