The £1.86bn Mersey Gateway bridge is designed to ease congestion on the ageing Silver Jubilee Bridge between Runcorn and Widnes. NCE reports on seven stages of the biggest project in the North West.
The Mersey Gateway Project is about so much more than building a new six-lane toll bridge over the River Mersey. Most of the press imagery for the project features artists’ impressions of the imposing cable-stayed bridge that will connect the towns of Runcorn and Widnes and relieve congestion on the ageing Silver Jubilee Bridge. But the construction team is actually building 12 new bridges along its alignment.
The £1.86bn project is being constructed in several overlapping stages by the Merseylink consortium (see box). It features numerous innovations including most notably, the use of a self-launching movable formwork system to build the elevated road viaducts which will connect the central span with the local motorway and road network.
In this special report, NCE looks at the most significant stages of the project.
1. Enabling works
In May 2014, construction of two access roads across the saltmarsh on either side of the river began. Roughly 200,000t of locally sourced stone was used to build these roads which will enable the construction teams to reach and work in the estuary. The work took 16 weeks to complete and was complicated by the fact that the saltmarsh is an area of environmental interest.
“We had to put geotextile down on top of [the saltmarshes] to start with, then we put a starter layer and we built our haul road up to the required height,” says Merseylink project director Richard Walker.
Where this access road crosses the disused St Helen’s Canal on the north bank of the Mersey, the construction team had to dam off the waterway, undertake a fish rescue, fill the section with aggregates and install a 900mm diameter pipe to allow free movement of water and aquatic species as work progresses.
Shortly after construction of the access road finished, work began on a temporary trestle bridge to provide an access platform for construction teams working on the new structures in the river. To build the trestle bridge, Walker explains that 140 piles were driven approximately 16m down into the silt, penetrating the rock below it.
“Essentially they are driven tubular piles positioned by a specially fabricated piling gate, bolted onto the end of the preceding section and that
determined accurate location,” he says. “Then you slot the pile into it and hammer it down.”
2. Bridge pylons
Construction of the temporary trestle bridge enabled work to begin on the foundations for the three bridge pylons that will support the main cable stayed bridge. Temporary cofferdams consisting of a 40m diameter outer circle and a 20m diameter inner circle were formed using approximately 300 steel sheet piles. Once the cofferdam was built and the water pumped out, the outer circle was filled with roughly 8,000t of locally sourced stone and sand to create a dry environment. This allowed workers and plant to access the inner circle to begin the excavation for the foundation works.
“The technique is basically to construct the outer [cofferdam], fill it with imported aggregates then install the inner one and progressively dig down between the outer and the inner to put on steel waling beams, then fill that back up again,” says Walker.
The Merseylink Consortium comprises Australia’s Macquarie Capital, Germany’s Bilfinger Project Investments Europe and Spain’s FCC. It has the concession to build, finance and operate the bridge.
Merseylink Consortium has awarded the construction contract to Merseylink Construction Joint Venture. This comprises FCC Construcción, Kier Infrastructure & Overseas Limited, Samsung C&T Corporation and its design team - a Flint & Neill /URS joint venture.
“Then you dig down on the inside [of the internal cofferdam], down to rockhead level, which is probably somewhere in the region of 12m to 14m below the sea level and about 10m to 12m below the river bed.”
In early August of this year, the project’s biggest single concrete pour took place when 1,400m3 of concrete was poured into the south cofferdam to form the base of the south pylon. This followed a pour of 1,200m3 in the preceding week to complete the north pylon foundations.
Speaking about the pour for the south pylon, Merseylink’s site agent, George Houston, says: “This is one of the biggest concrete pours that I’ve ever been involved with. We’re using a massive amount of concrete - over 600 truckloads just for the three pylon foundations. It’s an extremely technical part of the project.”
All three pylons should be visible in the estuary by the end of this month.
3. Approach viaduct supporting piers
The elevated approach viaducts to the central cable-stayed bridge cross an area of saltmarsh on either side of the estuary, spanning sections of the Manchester Ship Canal and the St Helen’s Canal. These viaducts will be supported a total of 20 piers.
“Each pier sits on six 1.5 metre-diameter piles which generally go down anything up to 55m - the last eight metres of which are embedded into rock,” says Walker.
Once the foundations for the 20 piers have been laid, a concrete pile cap 2.5m deep and measuring roughly 12m by 8m will be built for each of the main pier shafts. Work will then begin on the pier shaft which is approximately 3.5m by 2.5m. These are hollow and made out of reinforced concrete. “When these are finished, they will be topped off with the pier head which is octagonal in shape. Then you have bearings and the bridge deck on top of these,” says Walker.
The height of the shortest and tallest piers varies from 5m up to 25m for the highest point where the approach viaduct crosses the Manchester ship canal - this is so ships can pass underneath.
This work started in early 2015 and is scheduled for completion in early 2017.
4. Approach viaducts
The approach viaducts on both sides of the estuary will be constructed using a massive bridge building machine designed by Merseylink engineers with the help of Norwegian company NRS. The 157m long, self-launching, movable scaffolding system (MSS) supports the formwork for the approach viaduct decks. These will be constructed in spans approximately 70m in length and over 18m wide.
Part of the system has to be locked to the bridge pier and then the concrete is poured into the mould to create the 70m deck span.
“This steel girder MSS is effectively a self-propelled gantry - it’s 157m long, weighs about 1,500t and what it does is it supports the formwork system that allows us to cast the bridge deck girder insitu,” says Flint & Neill director Paul Sanders, whose firm is lead consultant to Merseylink.
“Once the concrete has gained strength, we post tension the first stage and once the post tensioning has been installed, then we’re able to release the MSS and launch it forward again.”
He explains that the decision to use the system was dictated by the need to minimise the impact on the saltmarshes beneath the approach viaducts.
“It not only allows rapid construction of girder-type decks but it does also mean that you’re not putting your feet on the ground when it comes to building the deck, it permits a very high rate of output because what you’re doing is providing a very large self-launching platform that supports the formwork system.”
This process will be repeated across all of the 11 bridge piers on the north side of the estuary. Then the machine will be dismantled and taken to Runcorn to begin work on the nine piers for the southern approach to the bridge.
Merseylink says that around 23,000m3 of concrete will be used to build the 19 spans - enough to fill nine Olympic sized swimming pools.
5. Main cable-stayed bridge
The Mersey Gateway Crossings Board is an arm’s length arm’s length body set up by Halton Borough Council to oversee the project. Paul Fenwick, its construction director explains that the decision to design the central cable-stayed bridge with three and not two pylons, was driven by height constraints imposed by flight paths into the nearby John Lennon Airport.
“The height of the structure was dictated by the proximity of the local airport,” he says. “This always meant we had to have a central pylon in the river because we couldn’t go high enough with the longer span of a two-pylon structure.”
For the cable stayed section the pylons are built first and then deck sections are built out from each one, in a balanced sequence, explains Sanders.
“We build the pylons and then we build the deck. Starting from any given pylon, we build it out equally on each side and we typically build it out with 6m long segments and then, for every segment we cast, we install a stay cable which supports that segment,” he says.
The stay cable spans from the pylon down to the segment that has just been built and this process is repeated on both sides of the structure until the cantilevers meet in mid span and a closing segment is put in place.
Travelling formwork will be used to cast the 6m long cable stayed deck segments. Using a similar principle to the MSS, these act as moveable concrete moulds for the deck spans. The travelling formwork systems will be assembled on the cofferdams surrounding the piers and then lifted to deck level where they will be fixed in place.
Once a segment has been cast, the form travellers are separated and moved to the next position.
“It’s a much more modest system than the MSS,” says Sanders. “It’s just a short cantilever section - you’re only casting a 6m long segment so it’s a much smaller bit of equipment.”
6. Reconfiguring the road network
As work continues on the approach viaducts and central bridge, eight major road junctions in Runcorn and Widnes are also being reconfigured so that traffic can be directed over the new bridge and away from the Silver Jubilee Bridge.
This includes upgrading 7km of highways to the north and south of the river and changing traffic flow in the area so that most local users will use the new bridge rather than the existing Silver Jubilee Bridge.
This work is split into eight sections. Section one and two of the project consist of relatively complicated elevated interchanges to connect the new road as it come off the bridge on the north side of the estuary.
“There are a huge amount of utility services in there that need diverting to make way for our new construction,”
Richard Walker, Merseylink
“There are a huge amount of utility services in there that need diverting to make way for our new construction,” says Walker.
Section 3 of the project is the construction of the approach viaducts and the central cable-stayed bridge. Section 4 consists of a £40M, 20m high precast concrete elevated viaduct that will pass over existing highways and the Bridgewater canal on the south bank of the Mersey.
“Section five, six and seven are about widening, refurbishing and improving the junctions along the existing central expressway,” adds Walker.
“In total we’re building 12 new bridges and five existing ones are being refurbished.”
7. Silver Jubilee Bridge
When work on the Mersey Gateway Bridge finishes, the Silver Jubilee Bridge will close for refurbishment.
Walker explains that the Mersey Gateway project will reduce congestion on the 54-year old bridge. The Silver Jubilee bridge was originally designed to take approximately 40,000 cars a day but now takes an estimated 90,000 vehicles a day.
“We don’t do anything until new bridge is open,” he says. “That is currently the primary route and [after the Mersey Gateway Bridge opens] we will be modifying it down from dual carriageway down to single carriageway and also installing a tolling system.”
This means the junctions where the bridge connects with the existing network in Runcorn and Widnes will have to be modified. “It’s not generally going to become a public thoroughfare,” says Walker.
“It will be used by public transport in the main.”
The final completion date for the Mersey Gateway Bridge is September 2017 and the Silver Jubilee Bridge modification will be completed in March 2018.