Precast concrete elements are increasingly used in the construction of Maritime Structures. They offer the prospect of efficient unit production and rapid construction, but that requires
the efficient construction of adequate foundation restraint. Foundation design and constructability for these elements is therefore a critical area, but little guidance is available on the
different forms of foundation available. Typical underwater foundation systems have been:-

Pre-levelled Bed: Stone Layers

Base Infill: Tremie Concrete, Open Grouting, Grouted Fabric Formwork (Grout Bags), Pumped sand

Weak / Inadequate Strata: Piled Foundations, Ground Improvement

This paper will describe these different foundation systems used underwater, and will describe particular experiences of combinations of systems used on completed and current
projects by way of example: Cardiff Barrage, Wales; Second Severn Crossing, UK; Confederation Bridge, PEI, Canada; Central Artery, Boston, USA; Greystones Harbour, Ireland.

Maritime construction is usually a high risk operation that needs efficient and suitably robust design and construction methods to be developed. The relative merits of different foundation systems are also outlined in terms of design and constructability, thus seeking to inform Designers, Contractors and Owners in considering alternative and combinations of
foundation systems.