The project was won as an ICE design and construct contract with caissons proposed as an alternative to traditional piled solutions. This offered environmental and construction/whole life cost benefits.

Regional civil engineering manager for Dean and Dyball, Steve Wilson, said, ‘This project had many challenging features and it is a great credit to our site team that it was delivered to programme and on budget. The whole project team worked very closely together to achieve this result and it is pleasing that their efforts have been recognised with this award.’

Mike Stevenson of Walcon Marine added, ‘This award has very much been the result of skilful design and research done by ABP Marine Environmental Research for the environmental aspects, Rainey Petrie Johns for the architectural planning authority aspects and Evans Grant Opus for the civil engineering design for the contractor.’

The caisson solution was developed because:

The seabed comprised a thin layer of silt overlying stiff to very stiff clays. A traditional piling approach would require large pile sections and heavy driving equipment.

The main risks associated with piling were considered to be potential for pile refusal, nuisance to the public through noise and vibration, and potential detrimental effects to Cowes Castle, a nearby grade II listed structure.

All materials were going to be delivered by sea. This solution minimized barge movements as the majority of work could be completed off site.

The caissons, capping beams, wave wall and feature fort could all be pre-cast off site. Casting components in a controlled environment would ensure quality was not impaired by the marine environment.

The site was in an exposed area of sea, particularly susceptible to north-easterly winds with a 4m tidal range. Utilising pre-cast elements allowed best use to be made of the tidal window.

Corrosion associated with steel piling would not occur. Whole life costs would be considerably reduced with little or no maintenance requirement.

Savings could be made in the dredged arisings dumped at sea. A significant proportion were re-used in the design, filling the caissons and ballasting them down.

The design identified one to three ton rock armour that was sourced locally in Swanage. Generally this over-burden stone is considered a waste product so using it enabled cost savings.

To harmonize with existing stonework, care was taken during pre-casting to cast mock masonry joints into the face of the capping beams, wave wall and feature castle.
By the end of the project 6,000 tonnes of rock armour had been placed and 7,000 tonnes of dredging completed.

The objective of the annual BCI Awards is to recognise excellence in the overall design, construction and delivery of buildings and civil engineering works. The Small Civil Engineering Project Award is for any civil engineering scheme valued at under £3 million. A rigorous judging process assembles a panel of leading clients, designers, contractors and construction industry commentators to choose the winning projects.

MJ Information No: 22707