Installation Effects on Axial Performance of Monopiles in Chalk for Offshore Renewables

Chalk is present in a large area of the East and South coast of the UK where renewables are now being installed. In most cases they are founded on displacement piles, whose design uses empirical methods or partial (wished-in-place) simulations. Installation effects such as grain crushing and pore pressure generation are conservatively estimated (if considered at all), as the change of chalk properties around the pile due to pile installation cannot be easily quantified. In this paper it is shown how advanced numerical modelling can be used to guide the design of the foundations in these complex soils. The coupled hydromechanical effects developing during pile installation in chalk are investigated numerically using a robust and mesh-independent implementation of an elastoplastic constitutive model at large strains. The model, implemented into an open source Particle Finite Element (PFEM) code, is able to capture the damage of the rock until the formation of a chalk putty layer around the shaft of a small diameter pile. Installation effects are highlighted by comparing the axial performance between wished in place piles and piles which considered the full installation process. The numerical results are then used to critically address the limitation of current design specifications of displacement piles in chalk.