As the monopile supported offshore wind turbine (OWT) is a dynamic sensitive structure, one of the major challenges in its design is the assessment of the natural frequency to avoid resonance during the lifetime. Since the characteristics of OWTs under dynamic loading and their long-term behavior are not fully understood, to study their natural frequency considering soil-monopile interaction, a series of scaled model tests in sand were performed. The first part was about the initial resonant frequency subjected to different forcing amplitudes and the second part was about the change of the natural frequency under long-term horizontal cyclic loadings. Based on the test results, the effects of pile-soil interaction, related to the loading amplitude, embedment depth, soil density, and cyclic numbers, on the natural frequency of OWTs are presented by a non-dimensional group based on the explanation of the governing mechanism. As the soil nonlinearity leads to a degradation in the natural frequency of monopile supported OWTs in the sand and the cyclic loading results in an increase, the choice of the natural frequency closer to the upper limit of the 1P band is suggested in practice based on the tradeoff of the two above effects.

Limited laboratory studies have investigated the cyclic behavior of sands under plane strain state, despite the current extensive applications of the plane strain hypothesis in modeling the behavior of subgrade soils beneath long road embankments. This study aims to explore the traffic-induced deformation behavior of sand under plane strain state and compare it to the conventional triaxial stress state. A series of one-way high-cyclic tests were performed on Fujian sand under both states using a true triaxial apparatus, considering different cyclic stress levels, consolidation stresses, consolidation anisotropies, and relative densities. In the plane strain scenario, the deformation of the specimen in the direction of intermediate principal stress was restricted when the cyclic major principal stress was applied. The test results indicate that during long-term cyclic loading, the sand exhibits substantially lower accumulated axial and volumetric strains when subjected to plane strain state as opposed to the conventional triaxial state. The reduction effect of plane strain state on the accumulated axial strain was found to be distinctively correlated with the strain levels, regardless of the cyclic stress amplitude and relative density. A practical formula was developed to estimate the difference in accumulated axial strain between the plane strain and triaxial states. Additionally, the intermediate principal stress of specimens under plane strain state was observed to oscillate cyclically in accordance with the one-way vertical cyclic stress. The intermediate principal stress coefficient, triggered by vertical cyclic loading, is more pronounced under high deformation, with its magnitude dependent on the specific loading conditions.

Geosynthetic-reinforced pile-supported (GRPS) embankments are a primary method for mitigating subgrade settlement. However, the load transfer mechanism between piles and soil remains incompletely understood, with the load sharing ratio (LSR) between piles and soil serving as a critical indicator for this mechanism. This study conducted a model test at a similarity ratio of 1:10 to investigate the effects of load amplitude, load frequency, number of geogrid layers, and pile types on the LSRs of piles and soil in GRPS embankments. The test results show that the pile's LSR increases with rising values of these parameters, while the corresponding LSR of the soil decreases. Among these parameters, the number of geogrid layers has the least effect on the LSRs of both piles and soil. Furthermore, the rigid long pile demonstrates a higher LSR than the flexible short pile, attributed to its greater stiffness. The influence of load frequency on the LSRs of the rigid long pile is also less significant compared to the flexible short pile. Variations of LSR increment can be predicted using a formula that incorporates the number of loading cycles. These findings provide deeper insights into the load transfer mechanism in the pile-soil system, contribute to the optimization of GRPS embankments design practice, and ultimately enhance performance and reliability of the GRPS embankments in geotechnical engineering applications.

Natural soft soils beneath transportation infrastructures sustain typical structured properties and are characterized by high sensitivity and poor engineering performance, which pose great challenges for the efficient operation of high-speed trains. Under traffic loading, soft subsoils present shakedown response and accumulate no negligible deformation. While few constitutive models are available both for the long-term behavior description of soft soils under cyclic loading and structure degradation of soil. Herein, a conceptual constitutive model within bounding surface theory framework was established to depict the dynamic behavior of soft clay under high-cycle, low-amplitude loading. The bounding surface could expand due to the hardening effect caused by contractive plastic deformation, and it could simultaneously shrink due to the weakening effect caused by both soil destructuration and excess pore water pressure. To further validate the proposed model, relevant triaxial tests were referenced. The consistent plastic deviatoric strain and excess pore water pressure from tests and the prediction confirmed the effectiveness of the proposed model. Following a comprehensive analysis of the varying internal variables during cyclic loading and a thorough investigation into the damage effects related to plastic strains, the model was considered capable of reasonably describing the structure destruction of soft soil to long-term cyclic load.

Long-term cyclic loading applied to clay at stress levels lower than the critical cyclic stress leads to soil deformation without inducing damage. The Monismith model is well-known for its simplicity and ability to describe the trend of cumulative plastic strain under cyclic loading. However, the simulated cumulative plastic strain increases indefinitely with the number of cycles until damage occurs. At lower cyclic stress levels, the cumulative plastic strain tends to be stabilized with an increasing number of cycles, ultimately limits the applicability of the model. To address this issue, a series of axial-torsional bi-directional cyclic loading tests are conducted on saturated clay using a hollow cylinder torsional shear apparatus. An empirical three-parameter mathematical prediction model is proposed by analyzing the development of cumulative generalized shear strain based on test results. The relationships of model parameters a with plasticity index, frequency, generalized shear stress, and mean effective stress; b with plasticity index, and c with frequency and plasticity index are presented as functional expressions. Finally, the predicted results of the empirical model are compared with test results to verify its effectiveness, providing a basis for calculating cumulative deformation in clay under long-term low cyclic stress levels.

Skirt-pile foundations have gained widespread attention in the field of offshore engineering due to their ease of installation and high bearing capacity. In this study, the ultimate bearing capacity, pile bending moment distribution and development, cumulative deformation characteristics, and cyclic stiffness development of skirt-pile foundations were investigated using physical model tests. The experimental results indicate that the ultimate bearing capacity and deformation resistance of the foundation can effectively be improved by increasing the skirt diameter. The cumulative deformation of the skirt-piles exhibited rapid development during the initial stages of cyclic loading, eventually stabilizing. Under long-term cyclic loading, the existence of the skirt can share the bending moment, which then affects the internal force distribution of the pile foundation along the axis. The pile foundation's cyclic stiffness reduces as the loading cycles increase and increases as the skirt diameter and length grow. Meanwhile, the horizontal cyclic stiffness decreases as the number of cycles increases, stabilizing after 3000 cycles. This study can not only deepen the understanding of the deformation laws of skirt-pile foundations in clay soil but also offers some references for the design of offshore pile foundations.

Offshore wind turbines are often supported on monopiles and are always subjected to long-term cyclic loading during their service life. This cyclic loading induces changes in the damping, stiffness and permanent accumulated rotation of the monopile foundation. The main purpose of this paper is to investigate the effect of these three changes occurring simultaneously on the dynamic response and fatigue life of offshore wind turbines in sand. To this end, an integrated methodology is presented based on time-domain finite element model and small-scale model tests of rigid piles. Three states of the monopile foundation are selected and defined based on the operational time of the turbine. The results show that these three changes have a slight effect on the dynamic response of offshore wind turbines, but have a significant effect on the fatigue life. The fatigue life decreased from 23.3 years for the initial state to 20.99 years for the medium state and 19.45 years for the ultimate state, a decrease of 10% and 16.5%, respectively, indicating that these changes should be addressed in the design of the fatigue life calculation of offshore wind turbine structures. The systematic parametric analysis shows that soil damping has the greatest effect on the dynamic response and fatigue life, followed by soil stiffness, which is less affected by permanent accumulated rotation.