The new boathouse, which will replace the existing boathouse built in 1905, is required to accommodate the new Tamar class slipway lifeboat due in service from 2005 onwards. The existing boathouse has been adapted on several occasions to accommodate several classes of lifeboat and the decision to rebuild and relocate the boathouse was taken following lengthy studies.

The existing slipway has been suffering from increasing levels of siltation, which restricts the launch and recovery of the lifeboat during low spring tides.

The new boathouse, located just to the west of the existing one and on the site of the old Royal Victoria Pier, will give much better access to deep water as well as providing increased accommodation for both the new lifeboat and modern crew facilities. The long term future of the existing boathouse is undecided, but in the short term it will continue to house the current lifeboat. The planning authority will wait until the new boathouse is completed before considering any possible application for demolition or change of use.

The initial works for the new boathouse were focused on the piling, and Seacore's drive, drill, drive installation technique was varied and adapted to accommodate the depth of overburden and quality of rock. Seacore worked closely with Dean & Dyball and the design consultants Posford Haskoning to overcome the difficulties encountered with the variation in rock quality.

'The aim was to establish a rock socket of sufficient depth to obtain the required level of sleeve friction and end bearing to cater for the expected forces transmitted by the structure and environmental loadings, said Seacore divisional manager Rob Maynard. 'The first piles were intertidal and largely straight onto bedrock. It was a great benefit to work from the jack-up, which provided a stable platform at all states of tide.'

The highly irregular surface of the bedrock on the foreshore needed some initial preparatory work prior to pitching the 813 mm diameter monopiles and enable accurate drilling of the rock socket. After pitching the Seacore T3 drill rig was fixed to the pile top and the drill bit lowered down inside the tube.

Using reverse circulation, the socket was drilled out ahead of the pile toe through the limestone bedrock, which varied in compressive strength from 40MPa to 50MPa but localised cemented layers were closer to 95MPa.

On reaching the required depth, ranging from 2m to 7.5m, the drill was replaced with a hydraulic hammer to drive the pile down into the open socket to its final penetration. As the work progressed to a fully marine environment, the pitch, drill, drive method was replaced by a pitch, drive, drill, drive method.

Seacore had to install 41 piles up to 18m long and the company steadily improved its installation technique and was able to complete the project's last ten piles at a rate of one every 24 hours.

MJ Information No: 19517