Numerical study of borehole instability of large diameter monopile during the window period between drilling and pile driving

Abstract

Large-diameter monopiles are among the most widely used offshore wind turbine foundations. Implementing drive-drill-drive construction techniques in a decomposed granite seabed may lead to borehole instability during the window period between drilling and pile driving. In this study, the borehole stability of a large-diameter monopile in a weathered granite seabed was investigated using the discrete element method. A linear parallel bond model was employed to simulate the bonding effect among weathered granite particles, and the influence of pore water was considered. The results indicated that soil shear failure caused by excavation unloading was the primary cause of borehole instability. The exposed height of the unsupported borehole significantly influenced the occurrence of borehole instability. The greater the exposed height of the unsupported borehole, the larger the soil arch required to maintain the stability of the borehole, and the less stable the soil arch becomes. Moreover, the susceptibility to borehole instability and collapse also increased.