Large-diameter steel pipe pile foundations, typically known as monopiles, are currently the dominant foundation solution for supporting offshore wind turbines. The design of monopiles in sandy seabed is typically based on p-y curves derived for fully drained conditions. However, in reality, the drainage condition around a monopile under cyclic loading, at least during each single loading cycle, is generally undrained. To verify the applicability of the design methods based on fully drained condition, this study conducted a series of finite element analyses examining the effect of drainage condition on the monopile soil-pile interaction in sandy seabed. Based on the analyses in four sands which are of different relative densities and particle size distributions, it is found that, for medium dense to very dense sands that exhibit dilative response upon shearing, the effect of drainage conditions can be practically ignored within the range of load relevant for practical engineering. For loose sands or sands with considerable fines that exhibit contractive response upon shearing, the drainage conditions have negligible effect on the soil-pile interaction stiffness at low to modest load levels; however, the undrained conditions can lead to lower capacities. This implies that the current design approach which assumes fully drained soil response is still acceptable for the FLS design in such soil conditions. However, for the ULS design, assumption of drained soil response may lead to overestimation of the lateral bearing capacity and assessment of the actual drainage condition and its influence on soil-pile interaction on a project-specific basis is warranted for such cases.

Understanding accurately the influence of non-plastic fines on stress-dilatancy of coral sand mixture-packing is crucial for marine engineering in various geotechnical applications. This work experimentally examined the effects of non-plastic fines and initial test conditions on stress-dilatancy behavior of mixture. Based on test results, equivalent void ratio (e*\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${e}{*}$$\end{document}) was determined to quantify the global effect of fines on shear behavior across different shear stages. Test results show that e*\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${e}{*}$$\end{document} exhibits a reduction as the mean effective stress (p '\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${p}{\prime}$$\end{document}) increases, following a power function relationship. Besides, e*\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${e}{*}$$\end{document} variation under phase transformation, peak state, and critical state can be described by a normalized curve. Reduced fines content and increased relative density can contribute to the enhancement of both peak strength and internal friction angle within the mixture. However, the smooth shape and lubrication function facilitated by fines actively contribute to initiation of shear contraction. Furthermore, the stress paths observed in the CD shear tests manifest as a sequence of parallel straight lines within the q\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$q$$\end{document}-p '\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${p}{\prime}$$\end{document} plane. The length of these lines progressively extends as the stress level escalates. Moreover, deviator stress in q\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$q$$\end{document}-p '\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${p}{\prime}$$\end{document} curves under character state presents lower and upper limits which are 0.334 and 0.639 corresponding to tested samples determined by fines content and relative density. Elevated fines content combined with reduced relative density can lead to a reduction in both peak-state friction angle and maximum angle of dilation.

This paper is dedicated to examining the impact of fine particles, specifically stone dust (passing 600 microns), on the shear strength, friction angle, and dilation angle of a subbase mix. To assess these properties, a large-scale direct shear test employing a 300 mm x 300 mm x 230 mm box was conducted. The subbase mix consisted of well-graded aggregate with varying proportions of fines, ranging from 1 to 15% by mass of the mix. The direct shear test was performed at 49.03 kPa, 98.06 kPa, 147.10 kPa and 196.13 kPa of normal stress across different densities. The findings revealed that the inclusion of 15% fine particles in the mix led to an 18% reduction in the friction angle for the loose mix and a 10% reduction for the compacted mix. Notably, the friction angle of the subbase mix proved to be influenced by factors such as normal stress, density, void ratio, and stone dust content. In compacted subbase mixes, the friction angle was predominantly influenced by variations in the mix's void ratio. The average dilation angle was determined to be 7.73 degrees for the loose mix and 16.36 degrees for the compacted mix. The analysis indicated that alterations in the dilation angle were impacted by normal stress, density, and the mean grain size of the mix. Furthermore, statistical analysis underscored the significant influence of the proportion of stone dust particles on the peak shear stress of the subbase mix. These findings collectively contribute to a comprehensive understanding of how fine particles, specifically stone dust, affect crucial mechanical properties in subbase mixes.

Understanding direction-dependent friction anisotropy is necessary to optimize interface shear resistance across soil-structure. Previous studies estimated interface frictional anisotropy quantitatively using contractive sands. However, no studies have explored how sand with a high dilative tendency around the structural surface affects the interface shear response. In this study, a series of interface direct shear tests are conducted with selected French standard sand and snakeskin-inspired surfaces under three vertical stresses (50, 100, and 200 kPa) and two shearing directions (cranial -> caudal or caudal -> cranial). First, the sand-sand test observes a higher dilative response, and a significant difference between the peak and residual friction angles (phi peak - phi res = 8 degrees) is obtained at even a lower initial relative density Dr = 40%. In addition, the interface test results show that (1) shearing against the scales (cranial shearing) mobilizes a larger shear resistance and produces a dilative response than shearing along the scales (caudal shearing), (2) a higher scale height or shorter scale length exhibits a higher dilative tendency and produces a higher interface friction angle, and (3) the interface anisotropy response is more pronounced during cranial shearing in all cases. Further analysis reveals that the interface friction angle and dilation angle are decreased with the scale geometry ratio (L/H). For L/H values between 16.67 and 60, the interface dilation angle varies between 9 degrees and 4 degrees for cranial first shearing and 3.9 degrees-2.6 degrees for caudal first shearing. However, the difference in dilation angle within the same shearing direction is less than 1 degrees.

The sustained growth of the offshore wind sector is leading to the construction of offshore wind farms in highly seismic regions of the world. Hence, a large proportion of the potential sites may be exposed to liquefaction risk. Monopiles are directly affected by this phenomenon given their preference as a support system for offshore wind turbines over other foundation types. This paper includes a comprehensive study of the lateral response of monopiles against the combined action of earthquake induced liquefaction and environmental loading using centrifuge modelling. The experimental setup was designed to compare the amount of excess pore pressure generated within the soil adjacent to the monopile with and without operational wind/wave loading. The results revealed higher accumulation of excess pore pressure in the non-laterally loaded case, and significant differences in the amount of excess pore pressure recorded in the windward and leeward sides of the monopile in the laterally loaded scenario. In addition, the study provides data on the rotation experienced by monopile supported offshore wind turbines in liquefiable soils.