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.

The Cyclic Interface Shear Test (CIST) device was recently developed to evaluate the response of soil-structure interfaces subjected to monotonic or cyclic loading. Numerical models of the CIST have not been documented. Such simulations may be beneficial to help guide the design of experiments, interpret results, and inform the development of further experimental device modifications. In the present paper, a series of interface shear tests utilizing the CIST system on a cohesive soil under monotonic loadings were simulated using a proposed three-dimensional model in the commercial finite element analysis software ABAQUS/Standard. Comparisons of simulations with experimental results are presented for the Mohr-Coulomb and hypoplasticity models for cohesive soils. It is found that (i) the clay-based hypoplasticity model outperformed the simpler Mohr-Coulomb model in terms of predicting the interface shear stress evolution and the soil volume change and (ii) the clay-based hypoplasticity model allows for identification of trends in shear response as a function of normal confining pressures at the soil-structure interface (e.g. soil-structure interface shear zone thickness). Neither of these capabilities have previously been documented or experimentally validated for cohesive soil-structure interface simulations using clay-based hypoplasticity models.