Effect of Material Law on the Behavior of Offshore Steel Piles under Cyclic Axial Loading

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.