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Mersey Gateway nears completion

After a long gestation period, the Mersey Gateway between Widnes and Runcorn is nearing completion.

Readers who remember the 1991 Alan Bleasdale television drama “GBH” will be aware that one of the characters – Jim Nelson, played by Michael Palin – was prone to panic attacks whenever he tried to cross the Silver Jubilee Bridge across the River Mersey between Runcorn and Widnes. Motorists who feel the same way will be delighted to know that relief is in sight, in the form of a new bridge being built 1.5km to the east.

Mss trinity and bridge in winter sun cropped

Mss trinity and bridge in winter sun cropped

The three towers are different heights

The bridge, which is set to open in the Autumn, is part of the Mersey Gateway project, designed to relieve congestion on the old bridge and the surrounding road network. The Silver Jubilee bridge currently carries around 80,000 vehicles a day – 10 times the number it was designed for. All it takes is a very minor incident to cause a traffic jam through the towns on either side.

Plans to build this road crossing of the Mersey go back almost as long as Bleasdale’s political satire. The Mersey Gateway Group was formed to promote the project back in 1994. Spearheading the group was local authority Halton Borough Council, which has to deal with the traffic problems on a daily basis. It was joined by neighbouring councils, agencies, business organisations and developers, who all see the wider benefits of better road access in the region.

It was another 10 years before Halton had a firm project on the table following a design competition. This received approval from the Department of Transport in 2006. There then followed a period of uncertainty after the 2010 General Election, when the project was reconsidered as part of the Comprehensive Spending Review. A year later, however, the government agreed to partially fund the Mersey Gateway, and Halton could look for a consortium to design, build, finance and operate the new bridge for a period of 30 years.

 

Mersey gateway2

Mersey gateway2

The winner, Merseylink, was appointed in June 2013, and a year later the consortium’s construction joint venture of FCC Construcción, Kier Infrastructure & Overseas and Samsung C&T Corporation began work on site. The design joint venture for the project is made up of Cowi (formerly Flint & Neill), which is leading on the bridge design, and Aecom (formerly URS) leading on all the landside works.

The total cost of the Mersey Gateway over the 30-year period from 2014 to 2044 is £1.86bn. Construction and land acquisition accounts for around £600M of this, with the sum split almost equally between the new bridge and all the landside road improvements.

As Merseylink general manager Hugh O’Connor says: “How do you approach a project of this scale?” The answer, he says, is to “split it into chunks you can manage”.

Merseygatewaycentralpylon3to2

Merseygatewaycentralpylon3to2

Main pylon foundation

The largest “chunk” is the main river crossing, a cable stayed, multi-span bridge that will carry six lanes of traffic from north west to south east. It crosses the St Helens Canal, Widnes Warth saltmarsh, Astmoor saltmarsh and Wigg Island, as well as the Mersey Estuary itself, before turning south over the Manchester Ship Canal and across Astmoor Industrial Estate to join the existing Central Expressway in Runcorn. The total length of the bridge, including the approach viaducts on each side, is 2.13km, including a 1km river span.

It was designed to be a landmark structure that is recognisable throughout the North West and beyond, explains Merseylink design manager George Moir: “One of the real drivers for the client was the aesthetic aspect.

“The Silver Jubilee Bridge is iconic, and they wanted the new bridge to lift the profile of the area, so it was specified as cable stayed.”

Moir says the JV did try to “push those boundaries” at the time of tender, and during the competitive dialogue stage it proposed “various radical solutions”.

The contract

The Merseylink consortium was appointed to design, build, finance and operate the Mersey Gateway Project in March 2014, after an 18-month procurement that included a competitive dialogue process.

Merseylink is made up of Macquarie Capital, FCC Construcción and BBGI. The consortium’s contractors are Spanish firm FCC Construcción, Kier Infrastructure & Overseas, and Samsung C&T Corporation from Korea.

When the new Mersey Gateway Bridge opens, it and the Silver Jubilee Bridge will be tolled, with the toll revenue going to Halton Borough Council. The Council will pay Merseylink an annual fee – known as a unitary charge – based on the crossings being available to road users.

Merseylink’s financial arrangement consists of bank loans, a council loan and funding through a bond investment supported by the HM Treasury’s UK Guarantees Scheme, with equity investment.

But they were told it definitely had to be cable stay, in accordance with the Transport & Works Act Order that approved the construction of the new bridge.

However, Halton gave the JV and its designers free rein to come up with the most economical design. The result is a bridge with three pylons of different heights: an 80m high central tower, a 110m high tower on the north side and a 125m high south tower.

“The advantage of having the smallest pylon in the centre is that we don’t have some of the stiffness problems you sometimes get on cable stay bridges,” Moir explains. “If you have a slender deck and no tie down facility, you have to make the deck quite stiff. But the shorter pylon allows us to do that.

Height restriction

“The height was limited by the [nearby Liverpool John Lennon] airport, so we couldn’t have had two 150m high towers,” he adds. “But the main reason for the height differences is a planning restriction on where we could put the pylon foundations in the river.”

At this location, the River Mersey is wide but shallow, so the bridge had to be designed to create as little disturbance as possible to the flow of the river while avoiding scour. The JV was given specific zones where it would be acceptable to build the pylons, and Moir says the design team wanted to install the piles at the extremes of the grey zones to optimise the spans.

The resulting design is very similar to the client’s reference design, with one main exception: the deck is constructed from post-tensioned insitu reinforced concrete rather than steel. “The advantage of concrete is the repeat construction methods,” explains Moir. “The contractors have a lot of experience and knowledge of these systems.

Cofferdam filled with sand and gravel to protect innner work

Cofferdam filled with sand and gravel to protect innner work

One of the pylon cofferdams

“We made a conscious decision to go for reinforced concrete for a number of reasons,” he adds. “One was price: we were tendering at a time when there was a lot of fluctuation in the steel price. Another key requirement for the design was – because it is a 30-year concession – to reduce the maintenance impact.

“When you look at the overall cost, there is a massive difference in the maintenance cost of steel compared to concrete. And there was the issue of where the joints are, and trying to reduce the number of joints. We tried to look for the lowest maintenance impact on the structure.”

The Mersey Gateway Bridge deck is lower than other similar bridges around the British coast, as larger ships do not use this section of the River Mersey.

Clearance restriction

In fact, the height of the bridge deck is not dictated by clearance over the river, but by a requirement to provide a clearance of 24.8m over the Manchester Ship Canal at the southern end of the structure.

An additional requirement of a 10m clearance over the St Helens Canal at the north end results in the main river spans being around 23m above water level. However, the estuary is exposed to strong winds, so despite the relatively low height of the bridge deck, wind shielding is being provided to ensure that the new road is a robust transport link that will reduce overall traffic congestion in the region.

The Thelwall Viaduct, which carries the M6 motorway over the Mersey further inland, was built without wind shielding, and has to be closed to traffic in high winds. “When it does, the Silver Jubilee Bridge is jammed solid because we don’t have that network resilience at the moment,” says Moir.

Central cofferdam

Central cofferdam

A 1km trestle bridge gives access to the main pylon foundations

The construction team took an early decision to install a 1km long temporary trestle bridge right across the river, from which all the concrete and reinforcement could be delivered, and to act as a platform for the main construction work.

The trestle bridge was built simultaneously from each side of the river, and consists of 12m deep piles that support pre-assembled steel trestle deck sections topped with precast concrete planks.

“We have tried to minimise the amount of work being done in a marine environment,” explains Moir. “The temporary trestle allows us to get out of the marine environment as much as possible. For example, we could use land-based plant for the cofferdams.”

Cofferdams for pylons

The cofferdams were needed for the installation of the main pylon foundations, which consist of 20m diameter circular spread footings. “We are fortunate to have quite a high sandstone horizon, which means there is no need for long piles [for the pylons],” explains Moir.

“The sandstone is around 16m below river level, so there is quite a substantial cofferdam arrangement. The dictating factor was the geology: we had to go down to minus 5m to be below the final predicted scour level, so we always knew we would have to dig a massive hole.”

The cofferdams are made up of two rings of sheet piles, one of 40m diameter with an inner ring of 20m diameter. The space between the two rings was filled with stone to create a working platform so that the material inside the inner piles could be excavated down to the sandstone bedrock.

26 hour concrete pour

At this level, the circular footing could be cast, with the largest of these requiring 1,400m³ of concrete to be poured in a 26 hour continuous pour.

The pylons themselves were cast using self-propelled jump-form rigs, with all three towers built simultaneously.

At the base of each pylon the construction JV installed the formwork traveller systems used to cast the post tensioned reinforced concrete main bridge deck sections.

Balanced cantilever deck

The deck has a trapezoidal cross section with cantilever ribs every 6m. It is being built by balanced cantilever, with one 6m long section cast from both sides of each pylon every week – giving a total of 36m of deck construction per week. The deck sections are connected to the pylons by the stay cables, which range in length from 41m to 226m. They are made up of between 41 and 91 strands per cable.

The approach viaducts at each end of the main bridge are made up of multiple spans of 64m on the north side and 70m on the south. Again, they have cast concrete decks, and the JV has once again adopted formwork technology used elsewhere around the world in the form of a movable scaffold system (MSS).

“The driver behind the construction methodology was that the partner companies had quite a lot of experience of using formwork travellers and MSSs,” he adds. “It also brings cost and programme benefits, and the cyclical nature of the process allows you to optimise.”

The approach structures are almost finished, and the main bridge deck sections are on schedule, having passed the half way mark in March. The entire structure and the associated landside road improvements are due to be completed in time for an Autumn opening.

Landside works

 

While the river bridge may be the centrepiece of the Mersey Gateway project, the landside works to link it to the existing road network should not be underestimated.

They have been split into nine individual projects, many of which would be major projects on their own.

A total of 7km of new roads are being built as the route runs from the A562 Speke Road in Widnes in the north to Junction 12 of the M56 in the south. The first new major junction at the north end occurs as the route heads eastwards: the Ditton Junction linking the Speke Road with the remodelled A533 (Queensway).

“It used to be a grade separated junction with two bridges; we are replacing that with a through junction with one bridge in the middle,” explains senior operations manager Jim Rice. “The old approach road is gone, and the excavated material is being used in the new embankment to the north abutment of the crossing.

“It has been a big challenge traffic management-wise,” he adds. “We have had to keep 90,000 vehicles a day moving.”

 

Astmoor bridgewater viaduct march 2017

Astmoor bridgewater viaduct march 2017

The Bridewater viaduct is a major project on its own

 

After the Ditton Junction, the route heads across land previously occupied by industrial units, requiring considerable demolition and a lot of remediation of excavated contaminated soil. “Widnes is the birthplace of the chemicals and salt industries,” explains Rice. “Everything we’ve excavated is being re-used, so it has been mixed with PFA and cement to ensure the hydrocarbons are bound in. Then we can use it high up in the embankments under the pavement.”

The route heads over a rail freight line, before crossing the A557 Widnes Eastern Bypass, and more industrial areas to reach a new roundabout, the Widnes Loop. This section requires six major junction realignments and the construction of a seven-span bridge structure.

On the south side of the river, the route follows the Central Expressway, which was built as part of Runcorn’s original road network. The first major junction on this side is the Bridgewater Junction, which is being remodelled to allow traffic to run freely between the new Mersey Gateway Bridge and the Central Expressway. A new flyover will carry this traffic, while traffic using the existing Bridgewater and Daresbury Expressways will travel underneath the flyover and through the new junction.

“We came up with a dumbbell arrangement that saved a lot of imported material and saved a big retaining wall down the side of the Bridgewater Canal,” explains Rice.

 “That value engineering exercise also enabled us to retain two existing canal bridges.”

He adds: “The high point of the whole bridge is where it goes over the Manchester Ship Canal. The Bridgewater Viaduct links to this: it is a 650m long structure carrying three lanes. It is elevated by 22m, with two diverging roads to dumbbells. It is a massive job in its own right, built with 40m long precast beams.”

“Various other improvements have been designed to ensure that the Central Expressway is the preferred route for traffic to cross the estuary, before it links into an improved Junction 12 of the M56 to the south west of Runcorn.”

 

 

 

 

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