Ipswich barrier: design of piled foundations for the main barrier structure

The site of the Ipswich Barrier project is underlain by River gravel overlying chalk that was proved to 60 metres depth. The contract was awarded in 2015 to VBA (Volker, Boskalis Westminster and Atkins) as a Design & Build Project for the UK Environment Agency. The reference design indicated the Main Barrier would be founded on 49no driven tubular steel piles 55 metres long. CIRIA C574 (2002) determines the design unit skin friction for driven tubular steel piles in chalk as 20kN/m² but suggests that higher unit skin friction is likely to develop in time after installation due to set-up effects. To give confidence to the design of the foundation piles a trial pile was installed early in the construction programme and dynamically tested over time to get insight in the mobilised shaft friction in chalk. Additionally, the same pile testing was carried out on the main work piles for a period of up to 103 days after installation. This paper presents an overview of the design method and the results of the dynamic pile testing in order to provide a case history example that can be used in the future to optimise the length of tubular steel piles for foundations in chalk. The final design of the Main Barrier piling resulted in a saving of some 30% on steel from the reference design and also saved on the installation programme in reducing the amount of on-site welding required.