Large-diameter monopiles of offshore wind turbines are subjected to continuous multistage cyclic loads of different types (one-way or two-way) and loading amplitudes over time. The loading history is likely to affect the lateral response during the subsequent loading stage. This paper conducts a systematic study on the lateral response of monopiles with and without reinforcement in multilayer soil. Two groups of monotonic centrifuge tests of monopiles with and without reinforcement are carried out to compare and study the influence of reinforcement on the displacement, bending moment and earth pressure of monopile foundations. Local reinforcement in the shallow layer effectively improved the bearing capacity of the monopile foundation. The ultimate bearing capacity of monopile foundations in monotonic tests provides a load basis for cyclic tests. Four groups of continuous multistage cyclic centrifuge tests of monopiles with and without reinforcement with different cyclic modes and loading amplitudesare carried out to investigate the influence of loading history on the lateral cumulative displacement, unloading secant stiffness and bending moment. Empirical design recommendations for monopiles under continuous multistage cyclic loads with different cyclic modes and loading amplitudes are provided based on the results of the tests.

This article introduces a novel system identification technique for determining the bulk modulus of cohesionless soils in the post-liquefaction dissipation stage following seismic excitation. The proposed method employs a discretization of Biot's theory for porous media using the finite difference method. The technique was validated using synthetic data from finite elements simulations of an excited soil deposit. These numerical simulations were performed using an advanced multi-yield surface elastoplastic model. Additionally, the technique was used to analyze a series of high-quality dynamic centrifuge tests performed on Ottawa F-65 sand as part of the LEAP- 2020 project. A comparative analysis between recorded and identified bulk modulus values highlights the effectiveness of the proposed technique across a wide range of conditions.

The dynamic response of piles is a fundamental issue that significantly affects the performance of pile foundations under vertical cyclic loading, yet it has been insufficiently explored due to the limitations of experimental methods. To address this gap, a hydraulic loading device was developed for centrifuge tests, capable of applying loads up to 2.5 kN and 360 Hz. This device could simulate the primary loading conditions encountered in engineering applications, such as those in transportation and power machinery, even after the amplification of the dynamic frequency for centrifuge tests. Furthermore, a design approach for model piles that considers stress wave propagation in pile body and pile-soil dynamic interaction was proposed. Based on the device and approach, centrifuge comparison tests were conducted at 20 g and 30 g, which correspond to the same prototype. The preliminary results confirmed static similarity with only a 1.25% deviation in ultimate bearing capacities at the prototype scale. Cyclic loading tests, conducted under various loading conditions that were identical at the prototype scale, indicated that dynamic displacement, cumulative settlement, and axial forces at different burial depths adhered the dynamic similarity of centrifuge tests. These visible phenomena effectively indicate the rationality of centrifuge tests in studying pile-soil interaction and provide a benchmark for using centrifuge tests to investigate soil-structure dynamic interactions.

The significant reduction in the stiffness of liquefied soil is accompanied by a decrease in the shear wave velocity, which ultimately results in the softening of the liquefied site. Time-frequency response analysis can identify the sudden drop in the frequency of the liquefied site, which has been widely employed to determine the onset of liquefaction. However, using the modal frequency (corresponding to the maximum power at each time step) to identify the timing of liquefaction (tL) captures the reduction in frequency during earthquakes, but it does not encompass the entire range of frequencies that have changed. Furthermore, previous literature defines tL as the boundary separating the modal frequency into pre- and postliquefaction time segments, but this estimate does not consider the generation of pore water pressure. Two representative case histories are presented to highlight the limitations of identifying tL by solely relying on the modal frequency approach that uses a two-step function. As a result, this study introduces an innovative method to identify tL utilizing the spectral energy ratio (SER), which captures the entire frequency shift. A step-by-step procedure using SER is detailed, and the new estimates of tL are compared with those derived from previous literature using 30 case histories. To validate the approach, a sensitivity analysis was performed using centrifuge test data from the Liquefaction Experiment and Analysis Projects. Results indicated that incorporating a ramp that accounts for pore water pressure buildup in the trilinear function improved tL estimation. An optimized SER value of 0.92 was determined for the proposed method. The notable contribution of this study is an enhanced approach of identifying the timing of liquefaction triggering by only utilizing acceleration records without requiring pore water pressure responses.

This study presents the numerical results of a series of laboratory and dynamic centrifuge tests conducted by the team at Universidad del Norte, as part of the LEAP -2022 project. The soil ' s mechanical behavior was simulated using a pressure -dependent multi -surface plasticity constitutive model, which was carefully calibrated based on cyclic soil tests performed on Ottawa F-65 sand. These tests covered a wide range of initial densities, initial effective stresses, and cyclic stress ratios. The comparison between laboratory and numerical element tests revealed that the adopted constitutive model reasonably replicated most features of the material ' s undrained cyclic response, including liquefaction occurrence and the progressive development of double -amplitude permanent shear strains. The calibrated constitutive model was then used to blindly predict the dynamic behavior of centrifuge experiments composed of a sheet pile -soil system using the OpenSees finite elements software framework; these simulations are referred to as Type -B predictions. The numerical simulation showed that the model provided reasonable representation of soil responses in terms of accelerations and pore water pressure build-up; however, the simulations consistently overpredicted the displacements of the sheet piles. Therefore, based on the centrifuge experimental results, minor adjustments of the material parameters were performed, and the centrifuge tests were re -simulated; these simulations are referred to as Type -C predictions. The comprehensive evaluation exposed both the strengths and weaknesses of the modeling approach for the simulation of liquefiable deposits. Despite the discrepancies in sheet pile displacement, the study instills confidence in the model ' s applicability to liquefaction -related projects with similar conditions.

Retaining walls and other waterfront structures were seen to suffer severe damage due to soil liquefaction in previous earthquakes. As part of the LEAP project, cantilever retaining walls with loose, saturated backfill were tested at various centrifuge centres participating in this endeavour. The toe of the retaining wall penetrated about 0.5 m into the dense sand layer underlying the loose sand layer. Retaining walls with different ratios of the retained height h over the penetration depth d were tested. As part of the LEAP project, additional testing was carried out at Cambridge to consider the effect of the wall size on its deformation following liquefaction. It will be shown that a larger wall will suffer more rotation and wall top displacement than a smaller wall with the same h/d ratio. This can have implications for numerical modelling in terms of how well the constitutive models capture the suppressed soil dilatancy at higher confining pressures.