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Material savings: Mersey Gateway

It has been revealed that the Spanish-led Merseylink consortium is to build the 2.13km-long Mersey Gateway bridge for £250M less than budgeted. Mark Hansford talks to the design team behind the saving.

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The four-span cable-stayed bridge will cross the River Mersey

Late last month, client Halton Borough Council (BC) and its design and build consortium Merseylink announced that a whopping £250M had been knocked off the cost of the new Mersey Gateway bridge, a six-lane toll bridge linking Runcorn and Widnes.

It was common knowledge that the saving had been made by switching from steel to concrete box girder for the bridge’s deck; that much had been revealed when the Merseylink consortium was named as preferred bidder last summer (NCE 25 June 2013). But it was made clear that the detail - and extent - of the savings could only be revealed once Halton BC and Merseylink reached financial close on the privately financed project.

The consortium comprises Australia’s Macquarie Capital, Germany’s Bilfinger Project Investments Europe and Spain’s FCC.

A driving factor for the consortium was to minimise long term maintenance costs and to enable rapid construction

Late last month, that hurdle was overcome, clearing the way for the Merseylink construction joint venture - made up of FCC, Samsung and Kier, and its design team Flint & Neill/URS joint venture (JV) - to reveal all. And there is more to it than meets the eye.

Put simply, the cost of the construction phase of the project, including land, is now estimated at £600M, with Halton BC stating this is an impressive £250M below the budget set by the government in 2011.

With that £600M, Merseylink will build a 1km-long, four-span cable-stayed bridge. Elegant mono-towers form the iconic features of the crossing, with a shorter central tower at 80m being dictated by the restrictions on span arrangements across the environmentally sensitive estuary. The two outer towers will be 110m and 125m tall. The bridge, along with just over 1km of approach viaducts, amounts to a 2.13km-long crossing.

The saving has come largely from three design changes, the most obvious of which is the change from steel to concrete for the main bridge deck.

Flint & Neill led on the bridge design, and developed a competitive design that features a post-tensioned concrete box supported on a single line of central stays.

Its project manager Paul Sanders explains the reasons behind the switch: “It terms of form, the bridge is now concrete construction. A driving factor for the consortium was to minimise long term maintenance costs and to enable rapid construction.”

Need for speed

Speed is truly of the essence, with the 30-year contract now running and Merseylink unable to claim payment from Halton BC until the bridge is operational. “The concession company is absolutely focused on time,” says Sanders.

The switch plays to the strengths of consortium leader FCC, and to a lesser extent Samsung, both of which have a strong track record of rapidly building reinforced concrete cable-stayed bridges.

“A key factor throughout was developing the design with the contractors and making sure the design team understood what was required from the permanent works, and also what was required from the construction side,” explains Sanders.

“The main driver in terms of choice of material was the international experience in the construction joint venture,” Sanders continues. “The JV has international contractors, and they are bringing in expertise gained on other projects.

“The UK is much more focused on steel or composite bridges. But you have only got to cross the channel to see the difference in approach in continental Europe,” he adds.

FCC’s recently completed Corgo viaduct in northern Portugal is a classic example: at 2.78km long and a maximum height of 230m, it is now the second highest viaduct in Europe.

It’s an evolution of techniques that I have seen used before, but never on this scale

“The contracting JV has got a lot of big bridge construction experience, and it had the confidence in using concrete,” says Sanders. “We married that up with our design.”

He adds: “They are used to these big construction projects and are comfortable with innovative construction techniques.”

This leads on to the second design change, and this too is focused around speed of construction.

“Working with the contracting JV, we have got the construction process down to a fast, repetitive operation,” Sanders explains. This means the main bridge spans are to be erected as a series of balanced cantilevers off of the three towers.

Nothing unusual there. But on the approach viaducts, Sanders thinks the project team has come up with something unique. The viaducts are all now 70m spans and each span will be cast in one go, using a giant 1,500t piece of moving formwork.

“It’s an evolution of techniques that I have seen used before, but never on this scale,” says Sanders. “And because this is a bigger structure, we are getting bigger benefits.”

Significant change

The third design change - to the foundations - is also significant.

“A key thing was to find an efficient foundation solution,” says Sanders. “We came up with having large diameter spread footings. It is a very efficient design as it allowed us to keep foundation depths down to 20m, with some just 8m deep.”

These footings will be cast inside sheet piled cofferdams - and again, this will speed up construction.

URS, meanwhile, has led on all landside works, and it has contributed to the savings pot by tweaking the road alignment and improving the earthworks schedule.

The scheme includes 7km of new roads, two major new junctions on both sides of the bridge, and extensive improvements to the existing road network.

Explains URS technical director and project manager, Darren Kimberley: “We did a lot of work through scheme development with the contractor on how to best integrate the bridge alignment with the existing road network.”

The client takes credit for setting itself up in such a way that allowed maximum flexibility to value engineer

In addition, URS is introducing innovative technology to the scheme through its experience on a number of smart motorway projects for the Highways Agency.

Both Kimberley and Sanders have high praise for the decision of the contracting JV to go for co-located integrated working from day one, and also for Halton BC for being open to such extensive value engineering.

“Halton have been very accommodating and encouraging to ensure we put aesthetics high on the agenda,” notes Sanders, referencing the client’s insistence throughout the tender process that whatever design changes are made, the bridge must remain “iconic” (NCE 25 April 2012).

“But the client takes credit for setting itself up in such a way that allowed maximum flexibility to value engineer,” he says, adding that workshops during the competitive dialogue phase allowed the design team to present its ideas and “understand what they really wanted”. “It took a great degree of courage,” he says.

The savings, meanwhile, are being shared between central government and Halton BC. The council has already committed to reinvesting its 30% share of the savings - which amounts to around £4M a year over the lifetime of the project - to maximise a discount scheme for eligible Halton residents, which will give them up to 300 free trips a year across the river.

Financial close triggers start of work on the Mersey Gateway

Under budget

Costs have been slashed by switching from steel to concrete for the main bridge deck

Financial close means all the 30-year contracts between Merseylink and Halton Borough Council have been agreed, and funding committed to allow construction to begin.

The next few months will see work starting on:

  • setting up the accommodation compounds on both sides of the river
  • final demolition work around Ditton, Astmoor, Catalyst Trade Park, Victoria Road and Hutchinson Street
  • access roads across the saltmarsh to the river in both Widnes and Runcorn
  • construction of two pylon cofferdams in the River Mersey from floating barges
  • two floating trestles - one from each side of the river - to take vehicles to the two pylons
  • upgrading the road network in Runcorn, starting at the Bridgewater Interchange and M56 roundabout.

Work will continue through to an anticipated bridge opening date of autumn 2017.

Once it is open, there will be an opportunity to close the Silver Jubilee Bridge 1.5km away, while it undergoes essential maintenance and is remodelled as a local bridge, with improved facilities for pedestrians and cyclists, as well as better public transport links.

The project is being funded over the 30-year period by a mix of tolls paid by users and grants from the UK government.

The council will not make any payments to Merseylink until the road is open and toll revenues and government grants are available to fund these payments.

To finance the delay in receiving revenue, the Merseylink consortium has put in place finance arrangements that include making use of the new UK Government Guarantee Scheme. This will be used to guarantee £260M of the debt required, with the balance of the finance provided by four banks and the Merseylink sponsors.

Halton Borough Council has set up a new company - the Mersey Gateway Crossings Board - which has the delegated authority to deliver the Mersey Gateway Bridge project, and to administer and oversee the construction and maintenance of the new tolled crossings, including the tolling of the existing Silver Jubilee Bridge.

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