The bearing capacity of offshore single pile composite foundations can be significantly affected by the spatially variable soil properties and the different soil layers installing the pile. The previous research mainly focuses on effects of isotropy or transverse anisotropy spatial variable soil on the bearing capacity and failure mechanism of piles embedded in a single soil layer. The practical sites generally contain multiple soil layers and the soil properties may exhibit strong rotated anisotropy characteristics due to the complex geological movements. However, how the rotated anisotropy spatial variability of soil property affects the bearing capacity of the offshore single pile composite foundation embedded into multiple soil layers remains unclear. This study aims to systematically investigate the effects of rotated anisotropy three-dimensional spatial variability of soil properties on the vertical bearing capacity of the offshore single pile composite foundation embedded into two soil layers. The three-dimensional random finite element is used to simulate the pile-soil response of the offshore single pile composite foundations under vertical static loads. The influence of the scale of fluctuation delta, rotated angle of anisotropy, and coefficient of variation of different soil parameters including elastic modulus E, cohesion c, and internal friction angle phi are investigated. The results show that the COV of E and c have a larger influence than that of phi. The rotated anisotropy of the upper-layer soil generally has a prominent effect on the bearing capacity of the pile compared with the lower-layer soil especially when the horizontal scale of fluctuation is large. These findings underscore the importance of accounting for rotated anisotropy spatial variability in the design of offshore single pile composite foundations.

The overconsolidation ratio considerably affects the physical and mechanical properties of soil as well as the interaction between structures and soil. Scale and consolidation time limitations render the preparation of overconsolidated soil for small-scale model tests difficult. Therefore, studying structure-soil interactions, especially the vertical bearing capacity of pile foundations in overconsolidated soil becomes challenging. Given the importance of reliable overconsolidated soil in physical model tests for studying soil-structure interactions, this study, based on the fundamental of the overconsolidation ratio, established a reliable method for preparing overconsolidated soil by altering centrifuge acceleration. Piezocone penetration tests were conducted to validate the accuracy of this method. Furthermore, vertical bearing capacity of pile foundations was evaluated in various overconsolidated soils. The vertical ultimate bearing capacity of pile foundations, cone penetration resistance, pore water pressure, and sleeve friction resistance were obtained in soils with various overconsolidation ratios. Based on the results of both tests, a formula was developed to calculate the vertical ultimate bearing capacity of pile foundations, taking into account the overconsolidation ratio of soil. This proposed method for evaluating vertical bearing capacity of pile foundations in overconsolidated soil can also be applied to study interactions between other marine structures and soil. The results of the study can provide technical support for designing the foundations of offshore oil and gas facilities, wind power, and other structures.

With the rapid development of infrastructure construction on oceanic reefs, calcareous sand, as the primary medium of these reefs, exhibits unique physical and mechanical properties such as high void ratio, low strength, and susceptibility to particle breakage. These characteristics reduce the bearing capacity and stability of pile foundations in calcareous sand foundations. This study investigates the bearing characteristics of high-strength preloaded expansion piles in calcareous sand foundations, taking into account the influence of HSCA high-performance expansion agent dosage through a series of indoor model tests and in-situ tests. The research delves into the load-settlement curves of expansion piles, the distribution of axial force and side resistance of piles, and the effects of Calcareous sand compaction and reinforcement around the piles. The results indicate that adding the HSCA high-performance expansion agent results in compaction preloading of the Calcareous sand around the pile, significantly increasing the expansion stress on the pile side, thereby enhancing the resistance on both the pile side and pile tip. When the expansion agent dosage is 20%, the ultimate bearing capacity can be increased by 56%, and the ultimate side resistance by 63%. The Coulomb strength theory of non-cohesive soil is employed to accurately calibrate the incremental side resistance of the expansion section. A prediction model for the bearing capacity of the expansion pile is established by combining the side resistance prediction model with the ultimate side resistance load-sharing ratio. The research outcomes provide important guidance for the optimization, design, and construction of high-strength preloaded expansion piles in calcareous sand foundations.

Commonly encountered problems, such as insufficient bearing capacity of the foundation and significant soil deformation, typically necessitate improvements to sandy soil. The excessive use of traditional soil improvement materials, such as cement and lime, causes irreversible damage to the ecological environment. As a sustainable soil reinforcement material, xanthan gum has broad application prospects with respect to its effects on the bearing capacity and deformation of sandy soil foundations. In this study, scanning electron microscope tests and cone penetration model tests based on particle image velocimetry technology were conducted to investigate the microstructure, mechanical behavior, and deformation characteristics around cones in sand treated with different xanthan gum rates. The test results show that the xanthan gum exerts cementation and filling effects between sand particles, enhanced the bearing capacity of sand. The displacement field around the cones in xanthan gum-treated sand during the penetration exhibits good symmetry. With increasing xanthan gum rate, the maximum displacement value and vertical influence range around the cone of xanthan gum-treated sand decrease, while the horizontal influence range increases. On the basis of the cone penetration test result, a predictive model for the vertical bearing capacity incorporating the xanthan gum rate is proposed using the Laboratoire Central des Ponts et Chauss & eacute;es (LCPC) model. The research results can provide a scientific basis for using xanthan gum when designing and constructing sandy soil foundations.

Current calculation methods for the vertical bearing capacity of steel pipe piles are predominantly designed for smaller diameters and do not account for the soil inside the pile. This necessitates an evaluation of their applicability to piles with diameters exceeding 2.0 m. This study aims to refine the existing formula for calculating vertical bearing capacity, as outlined in the Port Engineering Foundation Code of China, by investigating the vertical bearing capacity of large-diameter steel pipe piles through numerical simulations. By analyzing the relationship between the internal friction resistance of the soil core within the pipe and the bearing capacity for diameters ranging from 2 m to 10 m, this paper proposes a revised formula specifically tailored for steel pipe piles with diameters greater than 2 m, incorporating the effect of the soil core. The validity of the proposed formula is then confirmed through comparison with field data from four large-diameter steel pipe piles. The results demonstrate that the modified method proposed in this study performs better than the original formula when compared with the measured data.

Offshore wind turbines are often constructed on the foundation of soft clay, which often experience strength and stiffness weakening and internal plastic strain under cyclic loading. These changes in the mechanical properties of soft clay foundation can negatively impact the normal operation of the upper turbine. Due to the complexity of the mechanical properties of soft clay, most of the existing cyclic pile-soil load transfer models for soft clay foundations are based on hyperbolic models, which are more complicated and less efficient. To address this problem, this paper introduces a mechanical model that can describe the cyclic weakening characteristics of soft clay in pile-soil interaction, and realizes the weakening characteristics of the soil with the cyclic loading by using ABAQUS subroutine secondary development. By introducing the cyclic weakening model of soft clay into the numerical simulation of pile-soil model, the weakening law of side resistance of pile with the number of cycles is obtained. The pile side load transfer model under the traditional static condition is improved, and a trilinear model of pile side load transfer that can consider the cyclic weakening effect of soft clay is proposed; in addition, the calculation method of pile foundation vertical bearing capacity is established according to the load transfer method which can consider the cyclic weakening effect of soft clay, and compared with the model test results in the existing literature, the correctness and effectiveness of the calculation method of pile foundation vertical bearing capacity considering the cyclic weakening effect of soft clay proposed in this paper are verified. Finally, the method is applied to the actual project of an offshore wind power pile foundation, and the vertical bearing capacity of offshore wind power pile foundation is calculated.