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Joining the renaissance

MILLENNIUM PROJECTS

Protecting wildlife was an important factor in Portsmouth Harbour Renaissance Project.

Gosport's £1.8M Forton Lake bridge scheme is part of the Portsmouth Harbour Renaissance Project, which aims to revitalise this area of the important naval port. The overall project is part funded by a £40M Millennium Commission grant.

At 180m long and 5.5m wide, the concrete bridge will replace a now demolished structure used by the Royal Navy to link two military dockyards. It will form part of a 3km long pedestrian route and cycle path and is also capable of carrying a road train - a miniature rubber-tyred people mover system. The bridge has a central 20m section that can be opened to allow sailing boats to pass.

The project is being managed by a new form of partnership arrangement between client Gosport Borough Council, main contractor May Gurney Construction and bridge designer Maunsell. May Gurney says the shared method of working has already been recognised as one of national significance and has been accepted as a demonstration project by the Movement for Innovation (M4i).

Southampton University is monitoring the project's process and will produce a report when it is finished. It is hoped that the project's management structure will bring significant benefits to the construction industry, by increasing productivity and profitability of contracts through team work.

Forton Lake is an internationally significant wildlife area so environmental and planning issues were an important consideration in both the design of the bridge and the construction techniques.

May Gurney's first task was to design and build a causeway over the underlying soft alluvial silt to allow machinery to reach pile positions in the tidal areas of the lake. This was built using wraparound geotextile and selected granular fill and forms a base for the 24m high, 65t continuous flight auger rig, a 60t crane and other equipment.

With the causeway in place, May Gurney (Technical Services) could start piling the foundations for nine of the 10 spans. Each span is supported on a pair of 600mm diameter reinforced concrete columns cast directly on to 900mm diameter CFA piles between 12m and 25m deep through the estuarine alluvium and into the London Clay beneath.

May Gurney Construction then installed the two pairs of piles that will support the bridge's centre span. To ensure these piles are flexible enough to withstand thermal variations, a 821mm diameter steel tube was driven 25m into the lake bed with the top 15m filled with reinforced concrete.

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