Thesis: Oliver Cotter

Doctor of Philosophy, Magdalen College, University of Oxford, Michaelmas Term 2009

The Installation of Suction Caisson Foundations for Offshore Renewable Energy Structures

Renewable energy structures, constructed offshore, may use suction caisson foundations. Standards for calculating installation suction do not yet exist, and for the technology to be widely employed, robust installation behaviour must be demonstrated. The soil conditions for these installations will vary significantly. This work describes a programme of physical model tests involving the installation of caissons into soil selected to be representative of conditions present around the coast of the UK. An investigation was undertaken to identify experiments which are non-dimensionally similar to field installations. The soil samples comprised clay over sand, sand over inclined clay and sand with a
hydraulic barrier. The tests investigated the conditions for which installation is possible, and provide guidance for calculating the required suction pressure. The onset of plug lift and caisson refusal was demonstrated in soil profiles comprising clay over sand. A framework quantifying the onset of plug lift is presented and confirmed by the test series. The results of a series of installations are described, during which selective skirt tip injection was undertaken. The objective of these tests was to steer the caisson and investigate the effect of this process on maximum penetration depth. The effect of injection on pore pressure adjacent to the skirt tip is modelled and tested against measured data. A framework for calculating the effect of injection on suction is presented. Experience has shown that suction installation has a detrimental effect on caisson moment resistance. Experiments investigating the effect of pressure grouting caissons after installation are described. Foundation settlement and stiffness during combined V, H, M, loading were measured and compared to jacked footings.