Sand bed deformation around monopile foundations subjected to cyclic lateral vibration

Cyclic lateral vibration on monopile foundations often causes deformation and convective motion of the surrounding sand, affecting the bearing capacity and structural safety of the foundation. Previous research relied on simplified two-dimensional tests to explore sand deformation around vibrating monopile foundations, leaving the characteristics of sand motion in three-dimensional (3D) conditions insufficiently understood. To address this gap, a 3D experimental system featuring a semi-cylindrical monopile model was developed to investigate the effects of cyclic lateral vibration and sand relative density on sand deformation around vibrating monopile foundations. Sand deformation processes were recorded using multiple cameras, and the 3D terrain around the foundation was scanned. The results reveal that 3D plastic deformation of the sand starts with shear shrinkage, followed by shear dilation. The displacement amplitude at the monopile top was identified as the primary factor influencing sand deformation, while vibration frequency accelerates this process. In addition, an empirical equation with an angular conversion coefficient is proposed to describe the temporal evolution of 3D sand deformation around vibrating monopiles.