With the advantages of low construction costs and rapid installation, suction caissons are widely used as foundations in offshore engineering. This paper investigates the behavior of suction caisson foundations located in sandy soil under horizontal cyclic loads. The upgraded simple anisotropic sand constitutive model with memory surface (SANISAND-MS model) is employed to accurately capture the sand's cyclic behavior. To calibrate the parameters of the upgraded SANISAND-MS model, a series of triaxial drained monotonic and cyclic tests was performed. The effects of load idealization and loading sequence on the cyclic behavior of sand are studied based on the element test results, and the effects of load idealization on the cyclic response of suction caissons are studied from a finite-element simulation perspective. The triaxial test results indicate that load idealization slightly affects strain accumulation in both loose and dense sand. Based on simulation results, it is found that the loading sequence of load packages with varying amplitudes has a minor effect on the rotation accumulation of the suction caisson. The current load idealization method used in the engineering design practice of suction caissons is acceptable under drained conditions.

Suction caissons have been used in floating offshore wind farms worldwide with new types of finned suction caissons emerging to resist torque-loading. The additional fins attached to the caisson are expected to improve the torque-bearing performance, but the mechanism is yet to be clarified. Therefore, a novel torsional centrifugal modelling test system is developed to investigate the interaction between finned caisson and soil. The system is composed of the interacting chamber, the loading module, the transmitting module, and the measuring module. It allows precise control of the suction caisson penetration and pure torsional loading, which is validated by two torsion tests on a traditional caisson and a finned caisson. The results show that the torque-bearing capacity of the finned caisson is about 9.7 times that of the traditional caisson. The existence of the fins changes the failure mode from the interfacial friction failure between the caisson and the soil to the global soil-soil shear failure. The development of pore water pressure in soil was significantly changed by fins during torsional loading. The sudden change in the pore water pressure and soil pressure on the rear side of the fins indicates that tension gaps can be produced. The test results indicate that the developed test system is capable of evaluating the torsional performance considering foundation-soil interaction effectively.

As an innovative foundation used in the offshore floating platform, the scaled suction caisson (SSC) has a greater advantage in the installation and service than the traditional suction caisson (TSC). To investigate the pull-out capacity of the SSC, the numerical simulations are carried out under static and cyclic loads. The study shows the inclined pull-out capacity of the SSC increases with increasing the loading angle., but firstly increases, then decreases with increasing the padeye depth. Compared with the TSC, the inclined pull-out capacity of the SSC increases by 20 similar to 40% when the loading angles. are in the range of 0 similar to 30 degrees, but its values increase by 100% when the loading angles exceed 60 degrees. Under the combined static and cyclic loads, the vertical cumulative displacement of the SSC decreases, but horizontal cumulative displacement increases. The total cumulative displacement of the SSC decreases by 13% compared with the TSC. It can be concluded that more soils can mobilized by the bio-scale surface to resist the pull-out loads. As a result, the SSC significantly improves inclined pull-out capacity and decreases the cumulative displacement.

Suction caisson is a new offshore wind power foundation structure developed in recent years. Understanding its penetration characteristics is crucial to the successful application. A field test was conducted in the eastern waters of Rudong, Jiangsu Province, China, to investigate the penetration process of suction caisson. The test results demonstrate that suction caisson can penetrate smoothly to a predetermined elevation of seabed soil under the complex environmental loads, such as natural wind and wave currents. The inclination of the caisson is only 0.018 degrees degrees once the penetration process is complete. The pore water pressure at the outer skirt is primarily influenced by the tide level and drainage conditions of the contact soil layer during penetration, and the peak effective interface pressure at the skirt-soil interface decreases by about 46.7% due to negative pressure inside the skirt. Frictional fatigue effects are found at the skirt-soil interfaces. Meanwhile, the seepage reduction and squeezing induced resistance effects at the skirt-soil interface are the leading causes for the different distributions of effective interface pressure between the inner and outer skirts of the caisson. The findings of this study can guide the future penetration of suction caisson under challenging geological circumstances.

Inspired by the anisotropic shear behavior of the snakeskin, an innovative suction caisson with a bio-scale sidewall of the snakeskin is proposed, which is called the scale suction caisson (SSC). Compared with traditional suction caisson (TSC), the interface friction decreases when the SSC penetrates to the seabed, but increases as the SSC is pulled out. Therefore, the SSC has better installation and service performance than the TSC. The model tests are carried out to investigate the penetration behaviors of the SSC. The study shows the penetration resistance depends on the aspect ratio of the bio-scale surface, and the corresponding value increases with increasing the diameter of bio-snakeskin surface D1, but firstly decreases and then increases with increasing the height of bio-scale surface H1. More sand bypasses the end of the sidewall into the SSC and the sand inside the SSC is loosened under the seepage, causing the soil plug inside the caisson. In the half-caisson model tests, the soil heave inside the caisson under the different penetration depths is captured with an HD camera, and the permeability coefficient k can be further calculated. Considering the variation of the permeability coefficient k, a method of calculating the critical suction is provided.

The interface resistance during installation is crucial for the stability and safety of suction caisson in offshore geotechnical engineering, which is strongly affected by the penetration rate and soil-structure interface mechanical properties. This research conducts a series of clay-structure interface shear tests using modified direct simple shear device to fully study the mechanical behavior of clay-suction caisson interface. The effect of shear rate, over consolidation ratios (OCRs), interface boundary conditions, stress levels, and interface roughness were considered. Results show that as the OCR increases, the strength of both the clay and interface increase but show distinct patterns under constant volume (CV) and constant normal load (CNL) boundary condition. It was found that the interface strength is positively related to interface roughness and shear rate impact both the clay and corresponding interface strength. Under CNL conditions, the strength of normally consolidated (NC) clay decreases with rising shear rate, while the over consolidated (OC) clay demonstrate a opposite trend. In contrast, the effect of shear rate on interface behavior gets complicated owing to the combination of roughness, stress levels, and OCRs. Under CV conditions, the shear strength of clay and interface exhibits a logarithmic growth relationship with shear rates. The result of this work can provide a basis for interface resistance evaluation for suction caisson installation in clay.

Cyclic loading features in many applications. Questions important for design include: Does the monotonic capacity increase or decrease following cyclic loading? How does foundation rotation, stiffness and damping evolve? This is investigated here for suction caissons in sand, looking to applications as foundations for offshore wind turbines where changing stiffness, capacity and accumulated rotation can be critical, and soil damping is being looked at more closely. The problem is investigated experimentally through a series of single gravity monopod caisson tests in saturated sand subjected to unidirectional or multidirectional cyclic loading with between 360,000 and 106 cycles applied in each test. Results from the unidirectional tests are consistent with previous experimental studies, whilst also demonstrating the expected changes in damping ratio during cyclic loading for a monopod caisson in sand. The multidirectional tests reveal more significant and potentially important findings, particularly on the very significant increases in unloading stiffness and damping ratio associated with load direction changes.

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%.