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Making an entrance: The Rosyth Shipyard refurbishment

The Rosyth Dockyard on the Firth of Forth is more than halfway through a major refurbishment that will allow it to house two aircraft carriers. Paul Thompson reports.

Temporary works always seems such an arbitrary title to bestow on feats of engineering that can make or break a project. At the Babcock-owned Rosyth site on the north coast of the Firth of Forth near Edinburgh, contractor Bam Nuttall is part way through a project that could easily fit into that category.

In a bid to widen the direct entrance from the Firth of Forth into the non-tidal basin, so that large prefabricated sections of the Royal Navy’s next generation aircraft carrier can be floated through, the contractor is installing an outer cofferdam to hold back the tide while another inner cofferdam retains water within the basin.

A huge gantry crane nicknamed Goliath will straddle the dock and is due to be installed in October. Its delivery has set a deadline for the widening work on the direct entrance.

“It’s going to have to come through the widened entrance,” says Bam Nuttall project manager Dougie Grant. “We started the work in July 2009, so we have a little over a year to deliver.”

The gated direct entrance to the inner non-tidal basin - there is another entrance through lock gates which is being used during the work - has to be widened by 4m from 38.1m to 42.1m to accommodate the new carriers.

A giant cofferdam was built to hold the tide out during widening work.

A giant cofferdam was built to hold the tide out during widening work.

This new 42.1m long sliding dock gate is being built by Babcock at Rosyth. It must be able to draw back fully into its own chamber or “camber” located on the southern side of the entrance.

To accommodate this extra 4m length, an 8m section is being trimmed from the entrance on the northern side while a 4m extension is added to the southern side.

When the dock was first built in the early 1900s the construction team used the Isle of Dhu Crag in the Forth estuary as a launchpad and foundation for the lock and direct entrance.

Proving that Victorian and Edwardian engineers were no slouches, this domed outcrop of dolerite - a medium grained basalt igneous rock - is being used as the foundation for the work.

The Bam Nuttall team is breaking out a large part of the existing north quay down to the dolerite outcrop 25m below the top of the harbour side and 20m below the surface of the Firth of Forth, temporarily leaving a 4m wide section of the existing quayside, which will be demolished once the new section has been rebuilt.

Reinforced concrete propping which will incorporate a sliding lock gate

Reinforced concrete propping which will incorporate a sliding lock gate

The final rebuilt and reprofiled section will tie back onto the dolerite through a huge 2,400m3 mass concrete foundation.

But before it is placed the project team will install a series of 2m square-section reinforced concrete props stretching between the inner face of the temporarily retained section and the outer face of the secant piled wall socketed into the dolerite, which butts against the trimmed original quay.

This is to prevent the combined pressure of millions of gallons of the Firth of Forth overturning the temporarily retained section and pouring through the excavation.

Other features of the temporary work, designed by Bam Nuttall’s temporary works department, include an inner cofferdam that retains water from the basin.

“The client’s driver is the programme and the temporary works are the key to everything”

Dougie Grant, Bam Nuttall

Constructed using four 18.5m diameter cells of straight-web steel sheet piles, which sit on the bed of the basin and three interconnecting arcs between each of them, the cofferdam is fixed to the existing dock through a hinged cell.

These are dowelled into the existing walls through a series of 75mm diameter high tensile steel bars and the cells each filled with 8,000t of free draining granular material.

On the seaward side of the direct entrance, a steel pile arched cofferdam is being installed using the new wing walls as jump off points and a series of locating piles driven into the bed to help anchor the upper and lower walings of steel framework being used to help install the sheet piles.

The temporary works design department has certainly done its homework, with the unmissable deadline of this September to focus the mind.

“The client’s driver is the programme and the temporary works are the key to everything,” says Grant. “Work to the direct entrance is critical to the delivery of the whole project so they had to be as robust as they are.”

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