Strength, Stiffness, and Hysteretic Damping of Steel Pile–Sand Interfaces under Cyclic Axial Loading

The most critical parameters of the soil–foundation interface in any dynamic analysis involving offshore wind foundations are strength, stiffness and hysteretic damping. This paper aims to investigate the behavior of the sand–pile interface under cyclic axial loading. Based on laboratory experiments of sand–pile interfaces involving 35 cyclic shear displacement-control tests, the effects of normal stress, sand relative density, pile surface roughness, and shear displacement amplitude on strength, stiffness, and hysteretic damping are determined. The shear test results indicate that the normalized interface strength is higher for larger values of the sand relative density and pile surface roughness, and that it is independent of the shear displacement amplitude and normal stress. The shear stiffness and damping ratio also show nonlinear dependency on the shear displacement amplitude. Subsequently, a practical model is established for evaluating hysteretic damping and the stiffness versus shear displacement amplitude curve. This analytical model displays satisfactory performance as compared to experimental results from three comprehensive laboratory shear tests. In addition, at a given shear displacement amplitude, the shear stiffness increases and the damping ratio decreases as the pile surface roughness increases. Hence, the base model is extended to describe sand–pile interfaces, incorporating the effects of shear displacement amplitude and pile surface roughness.