Influence of hook load on vibro-driven pile installation in sand

Vibro-driving is increasingly being considered as an installation method for monopiles of offshore wind turbines. It is a dynamic installation method with lower acoustic emissions than the traditional impact driving, and vibrodriving has been reported to result in rapid yet controlled pile installation in sand. The risk of pile run is mitigated as the pile is clamped to the vibro-driver, which is connected to the crane. Besides cyclic degradation (friction fatigue) and tip reversal, dynamic effects and, in saturated conditions, excess pore pressures are important. Pile installation is performed with the applied dynamic force resulting from the product of the vibration frequency, the eccentric counter-rotating masses and their eccentricity. Pile mass and (parts of) the vibro-driver contribute to the static force. The interplay between these inputs and vibro-driven pile response is complex and non-unique. This contribution focuses on the influence of the crane tension or hook load on the vibro-driving outcome. The hook load reduces the static force but not the dynamic mass and becomes particularly important near refusal. Available physical data from vibro-driven pile installations in sand at different scales (small scale tests performed in the centrifuge and medium scale tests performed in a test pit) are drawn on. results from postdiction modelling are presented before the effect of hook load is explored.