Kier was commissioned by the Cornwall Council to design and build an extension and widening of the quay at St Mary’s in the Isles of Scilly. This was to allow larger passenger vessels to visit the islands and increase freight capacity. Work needed to be done with minimal disruption to the seabed because of environmental factors.
Kier designed and built the 40m long quay, wave wall, and harbour extension, using off-site manufacturing methods.
Modular reinforced precast concrete units were used for the quay and wall and a pre-cast caisson gravity structure for the harbour extension. We were after a solution which minimised construction on site, required less marine plant, was better for the environment and also safer for the workforce.
An important aspect of the project was delivering a solution which required minimal disruption to the seabed, particularly when the extension was being secured into position. This was because any underwater drilling would result in the release of high levels of contamination found within the marine sediments. Traditional encapsulation using close-centred piles, topped with precast concrete slabs, was not an option because of the hazardous waste and the time and the cost associated with managing this. So instead we pioneered a solidification methodology within the tidal zone –the first time this was achieved in the UK.
By opting for a solution which eliminated the need for piling and the subsequent reduction in the volume of contaminated waste, this reduced carbon emissions by 42%, saved £1.8m on the cost and reduced the length of the programme by about six months.