Steel piles driven into the seabed for offshore structures regularly experience monotonic and cyclic axial loading. The bearing capacity of these piles under cyclic loading degrades with the number of cycles due to the reduction in skin friction. Limited experimental data has led to the development of interaction diagrams, which predict the number of loading cycles until failure based on the mean load and the amplitude of the cyclic load, both often normalized through the static pile bearing capacity. However, these diagrams do not account for varying soil conditions or pile geometries. In this paper, the authors extend the previously developed Capacity Degradation Method (CDM) by incorporating the hypoplastic material law, which accounts for loading and unloading paths, stress levels, and the change of soil void ratios. New interaction diagrams have been developed for different pile geometries. Additionally, the pullout capacities of piles with varying diameters and embedded lengths under different loading cycles are investigated.

Foundation elements with rough (textured) surfaces mobilize larger interface shear resistance than ones with conventional smooth or random rough surfaces when sheared against soils under monotonic loading. The overall performance of foundation elements such as piles supporting offshore wind turbines, suction caissons supporting tidal energy converters, soil nails, and soil anchors installed in cohesive soils could be enhanced through utilizing rough (textured) surfaces to resist applied static and/or cyclic loading. This paper describes the shear behavior of smooth and rough (textured) surfaces in kaolinite clay and kaolinite clay-sand mixture soils under static and cyclic axial loading. The experimental investigation presented herein consists of a series of interface shear tests performed on 3D printed rough (textured) surfaces and a 3D printed smooth reference surface utilizing the Cyclic Interface Shear Test system. The paper includes a description of the interface testing system components, cohesive soil specimens' preparation procedure, smooth and rough (textured) surfaces details, testing procedure, and results of static and cyclic tests. Test results indicate that kaolinite clay-sand mixture soil mobilized larger static and post-cyclic interface shear resistance and volume contraction relative to kaolinite clay soil when sheared against the smooth reference surface. When tested against rough (textured) surfaces with variable asperity height, larger shear resistance was mobilized and larger soil dilation greater than that mobilized by the reference untextured surface in both soils. The results also indicate rough (textured) surfaces exhibited a prevalent frictional anisotropy increases with asperity angle and height in cohesive soils, the surfaces mobilized larger shear resistance and volume change in one direction (i.e., against the asperity right-angled side) than the other direction (i.e., along the asperity inclined side).