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Basin Cut

The Rosyth Dockyard on the Firth of Forth is more than halfway through a major refurbishment project that will allow it to accommodate two of the latest class of 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.

And that is aside from the fact that on many schemes these temporary works would be better labelled as semi-permanent.

Take the Camp Hill fly-over in Birmingham.

This was a steel bridge that was knocked up over a weekend to help ease the morning commute into the city centre; it stayed insitu for almost 30 years as town planners prevaricated about how best to link in the suburb of Sparkbrook.

There are also examples where the actual engineering complexity of the temporary works dwarfs that of the final structure.

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 (see box overleaf), the contractor is installing an outer cofferdam to hold back the tide while another inner cofferdam retains water within the basin.

The contractor is also installing reinforced concrete channel struts, manufactured using steel shipping containers welded end-to-end and acting as sacrificial formwork, across the bed of the entrance.

This will help prevent the wing walls sliding inwards when dewatered.

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Bam Nuttall has already carried out refit and refurbishment work on Number 1 Dock at the yard in readiness for the aircraft carriers.

A huge gantry crane nicknamed Goliath, which will straddle the dock, is due to be installed in October and 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 widening work in July 2009, so we have just a little over a year to deliver. It’s quite a tight programme, but I am extremely confident that we will complete the whole job on time.”

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 aircraft carriers.

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.

“We started the widening work in July 2009, so we have just a little over a year to deliver. It’s quite a tight programme,”

Proving that Victorian and Edwardian engineers were no slouches, this domed outcrop of dolerite - a medium grained basalt igneous rock - is again 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.

The final rebuilt and reprofiled section will tie back onto the dolerite through a huge 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.

The four levels of reinforced concrete props give the excavation the feel of a city centre car park, but these robust temporary works will soon be engulfed by the final structure.

An alternative plan to pile foundations beneath the quay wall was rejected as the team decided that it would be unnecessarily time consuming and costly.

“Basically we needed to ensure that we had a contingency plan that would be just as successful. This wall is very important as it maintains the basin at a constant level during all conditions of the tide and success is the only option.

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“We also thought piles might save some money but when we did the sums they just didn’t offer us enough. What it showed was that it had been engineered as tightly as possible during the detailed design process,” says Grant.

Poured directly onto the dolerite the only preparation the surface of the rock has is a hose down from high pressure water jets to clean it up before 2,400m3 of C30 mass concrete, supplied by local supplier Skene Group with 35% PFA replacement to help keep the heat of hydration under control, is poured on top, bringing the new quay core up to level.

A final concrete capping section will be placed above.

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

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 at each end.

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

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“In total there is around 40,000 tonnes of material in the inner cofferdam, even then though it can move as much as 225mm when dewatered,” says Grant. “If those tie-ins were not hinged then it would be unable to accommodate that movement.”

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.

These piles are driven through the overlying boulder clay and socketed around 500mm into the rockhead or until refusal.

When the new gate is installed, this cofferdam will be working on a reverse head and the upper waling will be working in tension, which is why it is fixed to the wing walls.

The temporary works design department has certainly done its homework but with the unmissable deadline of this September to focus the mind it is hardly surprising.

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

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