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.

Stiffness degradation of soft clay around offshore monopile is caused by the long-term effect of lateral complex cyclic loading such as wave and wind. Offshore wind turbine structure is a dynamic sensitive structure. It is urgent that the effect of complex cyclic loading on stiffness degradation of soft clay around pile and natural frequency of offshore wind turbine. A series of variable cyclic dynamic shear tests were conducted. The effect of initial shear stress and cyclic shear stress on soften characteristics of soft clay was investigated. It was found that as the initial shear stress is less than the cyclic shear stress, softening index decreases with the increase of cyclic stress ratio. Based on the test results, a soften model of soften clay with considering the effect of initial stress and cyclic shear stress was then built. By combining dynamic motion equation and this soften model of soft clay, a calculation method of natural frequency for offshore wind turbine structure was established to consider the effect of initial shear stress and cyclic shear stress. This method is verified by combining with the results of practical engineering and numerical data. Some parameters influence analysis were performed to explore the effect of amplitude and number of shear stress on the natural frequency of offshore wind turbine structure. The results showed that natural frequency of offshore wind turbine structure decreases with the increase of initial shear stress. As the amplitude and number is increased, the natural frequency decreases.

Vibration-based damage detection techniques have gained popularity in structural health monitoring due to their non-destructive nature. Most of such damage detection techniques on buildings have developed considering fixed base foundation, that is without considering effect of soil underneath. The objective of the present study is to develop a closed-form expression for determining damage severity in a shear building considering flexible boundary condition that is soil-structure interaction (SSI) using frequency response function (FRF)-based approach. The main concern is to understand the influence of SSI on structural damage quantification during post-seismic mitigation through numerical as well as experimental study. A numerical simulation has been performed on a 14-storey shear building with various soil conditions, namely fixed, dense, medium and soft soil. In the experimental investigation, the dimensions of the soil mass have been considered in such a way that free-field response can be replicated. By similitude laws, a geometric scale factor has been applied to develop a small-scale model and an equivalent shear beam (ESB) container. Damage severity has been determined for both numerical and experimental studies. The effectiveness of the proposed approach has been further studied for a real structure. The novelty of the study lies in the mathematical development involving minimum number of sensors as well as in modelling the effect of semi-infinite soil layer under a scaled-down model. The proposed approach is effective in identifying intermediate and ground storey damage. However, further investigation is required for quantifying complex damage patterns.

Reservoir water fluctuation is the key factor affecting the stability of reservoir landslides. Existing research on the evolution of landslides under cyclic reservoir water fluctuations is limited. However, further research is needed focusing on the evolution of the first-order natural frequency of reservoir landslides. In this study, model tests were conducted to investigate the evolution of the stress, displacement, inclination angle and first-order natural frequency of reservoir landslides under different rates of water level fluctuations during cyclic reservoir water fluctuations. The tests demonstrated that cyclic fluctuations in the reservoir water level resulted in oscillatory increases in the pore water pressure and soil pressure; while, the effective stress exhibited an oscillatory decrease, leading to a reduction in the landslide stability. The landslide displacement and inclination angle exhibited periodic increases, without distinct stages of initial deformation, uniform deformation, or accelerated deformation. Regarding landslide failure below the water surface, the inclination angle was more sensitive than the displacement. Changes in the inclination angle preceded changes in the displacement, making this approach highly suitable for early warning of reservoir landslide instability. Before the occurrence of landslide failure, the development and connection of cracks led to fragmentation of the sliding mass into multiple smaller blocks with reduced masses, resulting in a drastic increase in the first-order natural frequency of the landslide. Changes in the first-order natural frequency preceded changes in the inclination angle and displacement, rendering this approach very suitable for early warning of reservoir landslides.