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To be competitive in offshore wind energy production, safe and economical foundation design is essential. In recent years, tripod suction bucket foundations have been considered as an alternative to conventional foundations owing to their unique features suitable for offshore construction environments, economic installation, and high overturning resistance. However, it is difficult to accurately predict the behavior of tripod foundation because the load acting on the tripod is complex in HVM (i.e., horizontal, vertical, and moment loads) and the response varies depending on the size and direction of the load. Moreover, it is harder to analyze because the effects of cyclic loads must be considered in an offshore environment. This study, therefore, has investigated the behavior of the tripod suction bucket foundation under cyclic loadings. To analyze the complex responses of the tripod foundation in detail, the overall behavior of the tripod foundation system was observed based on the compression–pullout behavior of a single bucket. Moment–rotation responses, the cyclic stiffness, and permanent displacements of the tripod foundation are evaluated by analyzing the vertical behavior of the single‐bucket foundations as well as the rotational behavior of the tripod foundation. A number of centrifuge model tests were carried out with different loading conditions (i.e., loading amplitudes and directions). It was confirmed that the cyclic behavior of the tripod bucket foundation is significantly affected by loading amplitudes and directions. Furthermore, this study emphasized the importance of considering load characteristics when designing the tripod foundation.

A disconnected piled raft (DPR) foundation has been introduced as an effective pile design to reduce the vertical loading experienced by the pile. The characterization of DPRs has focused on the load transfer mechanism, foundation and soil settlement, bearing capacity, load distribution, and bending moment of the piles. DPR piles can act to increase the bearing capacity of the ground, and DPRs can reduce settlement while securing the bearing capacity. In this study, centrifuge model tests are performed to simulate the static behavior of DPRs under actual stress conditions. The behaviors of the DPR foundation for axial load, axial load distribution among the piles, and bending moment are compared to those of the connected piled raft foundation to understand the complex behaviors of DPRs. The centrifuge test results show that DPRs help reduce the pile axial load and bending moment during vertical loading. In addition, DPRs show smaller vertical settlement than shallow foundations. Therefore, we confirm that DPRs can be applied in foundation design as settlement reducers.

An unconnected pile foundation allows separation between the lower pile and the pile cap, and it has been proposed as an effective foundation type for reducing the seismic load during strong earthquakes. However, previous quantitative evaluations of unconnected piles with various foundation types and earthquake intensities are inadequate. In this study, the influence of base shaking level and the material of the interposed layer between pile and pile cap on the seismic behaviour of unconnected piles were evaluated using a centrifuge model test to reproduce the field stress conditions. A dynamic centrifuge model test was completed on an experimental model consisting of dry sandy soil, a foundation and a single degree-of-freedom structure. The acceleration of the structure and the settlement of the foundation system were measured during base shaking. For the unconnected pile system, the structural seismic load reduction effect due to rocking behaviour was confirmed, and the unconnected pile foundation with the interposed layer with large stiffness had less vertical settlement than the conventional shallow foundation. Finally, the rotational stiffness and damping ratio for the foundation system used in the centrifuge model tests were derived and discussed.

Reconstitution of the soil model is a basic step of physical modeling. Sand pluviation has been introduced as an effective way to prepare the soil specimen. However, it is difficult to create a model within the rigid soil container at a target soil density while ensuring the homogeneity of the entire soil model. To evaluate the spatial variability inside the container when the specimen was created using the pluviation method, a series of performance tests was conducted. In this study, two pluviation methods, sieve and curtain pluviations, were selected and examined in aspects of the target relative density of soil and its spatial variation. A bespoke sieve and commercially available curtain pluviators were employed. Each method was investigated by varying the test parameters, pouring mass rate, drop height, and sweep rate. Moreover, nine samples in a model box were used to evaluate the two-dimensional spatial variation of the relative density. Test results showed that the relative density increased with the drop height and sweep rate, but decreased with the pouring mass rate. Furthermore, the spatial variability reduced as the relative density increased. It was also observed that the curtain pluviator was more effective than the sieve pluviator in reducing spatial variability, with the exception of the high variability of soil density near the boundary wall of the box.

Designs allowing the rocking behavior of the foundation during earthquake have been introduced to reduce the seismic load on the superstructure and the ductility demand on the structural column. In addition, several studies have been conducted on rocking foundation based on the slow cyclic and dynamic tests by assuming the structure as a rigid oscillator. However, when structural bending is included, the rocking behaviors of the foundation for the slow cyclic and dynamic tests are different. Therefore, a clear description of each method and how each behavior is different should be investigated by considering structural bending motion. To fill the gap between cyclic and dynamic rocking behaviors, embedded foundation models with various slenderness ratios of the systems were investigated using horizontal slow cyclic tests and dynamic tests in a centrifuge. Test results show that the rocking foundation was affected by structural bending. The overturning moment in the dynamic test determined by the conventional method was different compared with results obtained from the slow cyclic test due to the structural bending motion. Finally, the overturning moment was re-evaluated by considering structural net displacement, and the re-evaluated dynamic overturning moment matched the results from the slow cyclic tests.

The response of the structure subjected to an earthquake load is greatly affected by the properties of the structure and soil so it is very important to accurately determine the characteristics of the structure and soil for analysis. However, studies on the effective profile depth where soil properties are determined, have been conducted in the presence of restricted conditions (i.e., surface foundation, linear soil properties), and without any considerations on damping. In case of the effective height of structure that affects its rocking behavior, it was only theoretically or empirically determined. In addition, most previously published studies on soil–structure interaction (SSI) focused on limited effects and parameters (e.g., rocking behavior, embedment effect, effective profile depth, spring constant, and damping coefficient) and not on comprehensive SSI parameters. Furthermore, no detailed validation procedure has been set in place which made it difficult to validate the SSI parameters. Since the effective height of structure and effective profile depth are the basis of all the input parameters of SSI analysis, it is important to validate and determine them. Therefore, in this study, the procedure used to optimize the two SSI parameters was established based on an analytical approach that considered all the possible SSI parameters that were investigated from conventional codes and studies and physical model tests. As a result of this study, the optimum values of the effective height of the structure and effective profile depth were respectively determined according to (a) the height from the bottom part of the foundation to the center of the mass of the superstructure, and according to (b) the depth at values equal to four times the radius of the foundation.

Liquefaction-induced settlement of shallow foundations is the result of bearing capacity failure in undrained conditions and sedimentary settlement during the post-liquefaction process. The bearing capacity of a shallow foundation is highly dependent on the size and dimensions of its footprint. In addition, the reduction in shear strength in liquefiable soil, a key parameter for estimating bearing capacity, depends on the excess pore water pressure generated during an earthquake. This study aims to investigate the impact of earthquake motion on the extent of liquefaction-induced settlement in shallow foundations. A parametric study was conducted by varying the input earthquake motions in a three-dimensional response history analysis to directly consider the interaction between the soil and superstructures. The numerical analysis model constructed for the parametric study was rigorously calibrated using a reference dynamic centrifuge test in a prototype scale. The effects of the horizontal boundary and drainage conditions in the numerical model were closely examined during calibration. The parametric study results indicate that the intensity measures of an earthquake, which quantify the energy associated with the number of reversals, exhibit a close correlation with the resulting liquefaction-induced settlement as opposed to other conventional earthquake motion parameters, such as peak acceleration, magnitude, and frequency.

The performance of nonlinear response history analysis was analyzed based on the historic case of the 2017 Pohang earthquake in Korea. The Yeong-il bay container terminal stopped operations for a period after the Pohang earthquake and intensive site and hazard investigations were conducted. Nonlinear response history analyses using various soil constitutive models, including sophisticated liquefaction models, were performed on the caisson quay wall under two-dimensional plane strain condition. The results of analysis were compared to the hazard investigation results. The analysis results exhibit reasonable agreement with the field measurement data for the earthquake-induced permanent displacement of the quay wall and quay crane rails.

Soil liquefaction by earthquake results in significant soil deformation and can cause substantial damage to infrastructure. Accordingly, there is a critical need to analyze earthquake case histories, and such analysis should focus on elucidating ground motion characteristics and field measurement data associated with instances of soil. The most straightforward approach to identifying liquefaction triggering involves examining pore water pressure records, which is challenging in many susceptible areas due to the absence of installed pore water pressure transducers. Therefore, evaluating liquefaction commonly relies on surficial evidence, such as sand boil or lateral spreading. However, this evaluation can miss the liquefaction triggered case without such surficial evidence. This study modeled the 2017 Pohang liquefaction, a first-time occurrence in South Korea, through a centrifuge test replicating the liquefied site based on field investigations, including borehole tests and recorded earthquake motions. We comprehensively assessed liquefaction using the ratio of excess pore water pressure alongside analyses of acceleration time histories, shear stress-strain hysteresis, and time-frequency histories. These results were compared with a conventional method that overlooked pore water pressure, leading to overestimation. Furthermore, using a simplified method, we compared liquefaction triggering evaluation results from the centrifuge cone penetration test and on-site standard penetration test. This, along with the factor of safety, substantiated the validity of the centrifuge results.