In reviewing the recent development of the geotechnical engineering aspects of the offshore wind farm industry, it was found that there is an urgent need for more reliable and cost-effective foundation solutions. In this thesis, screw piles (helical piles) have been proposed as a potential innovative alternative foundation for offshore wind turbines in deeper water. This type of pile has been used widely as foundation and anchor for onshore projects due to their ability to provide high compressive and tensile resistance as well as reduced noise/vibration during installation. In order to adopt the screw pile technique as an offshore foundation, the geometry of the piles would need to be scaled up so they can provide the high capacities required for this application. Such change in size and geometry will lead to uncertainties in predicting the required installation torque and the capacity in different soil types and stress histories. For example, without the ability to accurately predict installation torque, it is difficult to design screw piles for offshore use or develop appropriate installation plant with the required torque capabilities in different soils.
In this thesis, non-linear Finite Element Method (2D & 3D) was used to investigate the screw pile behaviour under lateral, vertical and combined loading. The FEM was also used to investigate the optimum spacing of the helical plates and the geometry effects on the screw pile behaviour to meet requirements of the loading conditions experienced for offshore application. In addition, 29 successful centrifuge tests were carried out using a newly developed servo-actuator so that the screw pile models could be installed and tested inflight in one operation at 50g acceleration. The centrifuge tests of screw pile models and CPTs were carried out in sand at three different relative densities (loose, medium and dense). The installation force (Fv) and torque (T) were correlated to the cone resistance (qc) to establish a CPT-based design method to predict the required installation force and torque for the straight-shafted pile and the modified screw pile geometries. A modified theoretical model was used to predict the installation torque of straight-shafted pile and screw pile in sand with different relative densities based on pile geometry characteristics and soil properties. Furthermore, the ultimate compressive capacities (Qc) of straight-shafted piles and screw pile in sand with different relative densities were determined from centrifuge tests and used to develop appropriate design parameters including earth pressure coefficient (k) and bearing capacity factors (Nq) for pushed and rotated piles.