Insights into pile vibro-driving in sand through numerical modelling

Abstract

Monopiles installed by impact-hammering into the seabed have been the most commonly used foundation type for offshore wind turbines to date. However, acoustic emissions due to impact-hammering have recently motivated the uptake of alternative, quieter installation methods. Vibro-installation is one such approach that has been increasingly used in recent years, offering potentially quicker installation as well as reducing the risk of pile run. However, the soil mechanisms governing vibratory installation and its effects on the post-installation response are not yet fully understood. Improving this understanding could guide industry recommendations on vibro-driven pile installation. Therefore, the present study focuses on evaluating soil mechanisms underpinning resistance against vibro-driving in sand by performing large deformation numerical analyses of displacement-controlled vibro-driving of large-diameter cylindrical open-ended piles (monopiles). The results indicate that dynamic components of total soil resistance acting out of phase with static ones together with reductions in arching stresses inside the pile, can significantly decrease the penetration resistance compared to the monotonic jacking resistance. Furthermore, the total soil resistance against vibro-driving reduces if the upward displacement is large enough for full reversal of the shaft resistance but its downward movement is not sufficiently large for the full remobilization of tip resistance. In addition, shear-induced positive excess pore pressures around the pile tip during vibro-driving can facilitate easier pile penetration through reduction in effective stresses around the pile tip and more significant phase shifts.