Coupled DEM-FDM analysis of open-ended pile installation and lateral loading in chalk

The in-service capacity of monopiles for offshore wind turbines founded in chalk is a function of the state of the rock around the pile. The installation of a displacement pile affects the fabric of the surrounding rock (‘installation effects’). In this paper, a coupled DEM (discrete element method)-FDM (finite differential method) 3D model is used to investigate the installation effects of open-ended displacement piles and their influence on subsequent lateral behaviour in soft rocks. Results indicate that the axial resistance of installed piles in chalk is primarily provided by external shaft and base resistances. In addition, rock material inside the pile cavity experiences a significant dilation behaviour, suggesting traditional indices (incremental filling ratio (IFR) and plug length ratio (PLR)) are unsuitable for evaluating pile installation mode. Considering the distribution of the principal stress, the axial load transfer mechanism for the pile is proposed from a microscale perspective. Subsequent lateral loading simulation reveals that changes in the rock state around the pile due to installation strongly influence lateral stiffness. Finally, macro- and micro-scale analyses confirm that the dominant contributors to lateral resistance are the pile tip region on the passive side and the near-surface region on the active side of the pile.