The annual International Conference on Offshore Wind Substructures was held in Edinburgh last week, where the latest trends in foundation design and innovations in substructure technology were discussed.
This two day event facilitated lively discussion among attendees from across the supply chain regarding how foundation engineering may contribute to reducing the levellised cost of energy. All stakeholder classes were represented including developers, contractors and designers.
Engineering Consultant, Gavin and Doherty Geosolutions Ltd. (GDG), delivered a presentation on Novel Pile Installation Methods, which covered vibratory piling techniques and the impact of pre-drilling on pile capacity.
Vibratory pile installation is gaining traction across the offshore sector due to advantages that can include lower noise levels, reduced installation time, and less discrete marine operations. The benefits of vibratory pile installation are often not fully realised due to uncertainties about their load carrying behaviour.
At the Edinburgh conference, Paul Doherty, Managing Director at GDG, presented the preliminary findings from a research study funded by the Deep Foundations Institute in the USA which investigated this topical issue. The outcome of this work suggests that vibratory piles have axial resistances which are typically 25% lower than impact driven piles with the same geometry.
This study also highlighted that in the offshore environment, where installation time can be critical and lateral loads often dominate (rather than axial capacity), vibratory installed pile foundations may still offer significant cost savings. It should always be noted that the value of a specific pile installation technique is very site specific and needs to take into account the local ground conditions and geohazards.
In ground where shallow bedrock or very hard soils may pose a challenge to traditional pile driving or vibratory pile installation, pre-drilling may prove a cost effective means of optimising the foundation design. By using a pilot drill ahead of the pilot toe to reduce stresses, open ended steel piles may be driven to high tensile resistances despite hard layers that may otherwise cause refusal.
The term “Drive-Drill-Drive” has been used to describe this installation technique, which is currently being considered for a wind farm project in the Firth of Forth in Scotland. GDG are currently working with developers Mainstream Renewable Power to develop empirical correlations form a suite of onshore pile tests in representative geological conditions, which will allow further optimisation using the Drive-Drill-Drive installation technique.