The Mersey Gateway bridge project continues on schedule
First publishedin World Highways
Work continues on the 2.3km Mersey Gateway signature bridge project close to Liverpool in the UK. David Arminas reports on some of the construction highlights
Under construction is a cable-stayed structure with three towers that will span the Mersey River’s expansive mud flats between the towns of Runcorn and Widnes near Liverpool.
Including the approach viaducts on each side, it will be 2.3km long with a river span of 1km. The main bridge deck will be reinforced concrete. The 80m-high central tower will be shorter than the two outer towers, where the north tower will be 110m high and the south tower will reach 125m.
It is estimated that up to 80% of traffic crossing the estuary will use the new bridge. However, the old Silver Jubilee Bridge, opened in 1961, will be far from redundant when the Mersey Gateway opens in late 2017. At that time, the old bridge will be closed for extensive refurbishment, although it will remain open for pedestrians and cyclists.
Work started on the new bridge in mid-2014 with two temporary access roads being built across the salt-marsh. A1km temporary trestle bridge with a concrete slab surface around 9m wide was built across the estuary as an access platform for teams constructing the three pylons. Upwards of 140 steel piles were driven down around 16m to support the temporary access bridge which was completed in May last year.
The bridge’s span is around 1km, but 20 reinforced supporting piers will to carry 1.3km of approach roads. Cofferdams were built to allow the driving of around 300 steel piles into the riverbed for pylon construction. Into each cofferdam was laid a concrete base onto which a cage consisting of 190tonnes steel reinforcing bars was assembled. Into the mix went 1,400m3 concrete to form the pylon foundations.
Main bridge deck Only a few weeks ago, the landscape was irrevocably changed when two of the six giant main bridge deck building machines at the south pylon moved apart for the first time. The two machines – form
travellers – operate in tandem to enable construction of the 1km-long reinforced concrete bridge deck across the Mersey.
The 270tonne machines act as movable concrete molds, operating in a similar way to the Movable Scaffold System (MSS) that is building the elevated approach viaducts.
The form travellers were assembled at the south pylon earlier this year before being lifted to their starting position around 25m above the riverbed. Construction teams then cast a pier table – a rectangular shaped platform – around the bridge pylon before preparing to start work on the main bridge deck.
“Each segment of the bridge deck is made in the same way,” explains Merseylink’s Kyuyoung Choi, operations manager for the main crossing. “Reinforced steel is placed into the mold and we then pour around 130m³ of concrete inside to create each segment.
1. Mersey Gateway Bridge seen in the background will handle most of the traffic using the Jubilee Bridge 2. Peri's Automatic Climbing System works its way up a pylon 3. Form travellers on south pylon ready to move apart 4. Concrete pour for north pylon base, July 2015 5. The Silver Jubilee Bridge rehabilitiation
“From the third segment onwards, we install the connection boxes, which are called ‘delta frames’, for the steel stay cables, which in turn are then attached to the upper pylon. The form travelers powered by a hydraulic system move forward on a set of rails to the next position and the process is repeated,” says Choi.
Deck segments are cast simultaneously. This allows the bridge deck to “grow” from either side of the pylons until it meets the connecting bridge deck and the structure is complete.
Another pair of machines will be launched from the north pylon later this month and the third pair will start from the central pylon during October.
Work on the bridge deck is expected to finish in summer 2017. During this time the form travellers will have cast a total of 154 deck segments. It takes a week to construct a segment that measures around 33m wide and 6m long.
The largest number of deck segments - 33 - will be cast from the south pylon leading to the south elevated approach viaduct and 31 will be cast from the south pylon in the other direction towards the central pylon.
The north pylon will see 29 deck segments cast on one side and 27 on the other, while 17 will be cast from either side of the lower central pylon.
When the bridge is complete, the form travelers will be dismantled and recycled.
High-level action Peri, a German-based formwork specialist, has been involved with the construction of the three pylons. It started with building the lower pylon up to the hammerhead where the central pier table that will support the roadbed is formed. Peri is now finishing the three different-sized upper sections of the pylons.
“Completion of the lower pylons was done with our VARIO equipment, standard forms with a ply face and poured in four lifts. This is standard for any formwork company," says Wayne Fisher, central and Wales regional director for Peri.
“But then we moved onto the hammerheads followed by the pier table, much more interesting from an engineering perspective. The hammerheads were special shapes that challenged the carpenters within Peri’s fabrication team working offsite.
“We made the shutters to exact measurements, used even CNC machines to ensure accuracy for putting the formwork together to make the hammerhead and pier shapes. We put together IKEA-like flatpacks for shipping to the site. It was very efficient, with assembly drawings included.”
For the construction of the hammerheads, Peri used its VARIO GT24 formwork with its SB Frames and VARIOKIT system. Unlike a usual application, which would see the SB Frames system used for single-face concreting, the Mersey Gateway saw Peri employ the frames as a platform, which meant a unique design was required. The intricate shape of the hammerhead, which included lots of faceted surfaces, meant that cleat boxes - or make up boxes - had to be attached to the flat face of the formwork to form the desired shape.
Even so, formwork for hammerhead construction also used standard equipment, says Fisher.
Once the central hammerhead is complete, the formwork is removed and Peri’s SB platform is reused for construction of the pier tables. The pier tables accommodate the installation of the form travellers, meaning construction of the main bridge deck can commence. Peri’s adaptable scaffolding system, PERI UP, will be supported by the SB platform to create a secondary deck at the pier table level.
About 12 sections of the pier table are built offsite and trucked onto site, lifted onto the hammerhead by crane and then have concrete poured into them. Together the lot makes up the pier table.
6. Hammerhead at central pylon, July 2016 7. Peri's ACS on a pylon and ready for a concrete pour 8. North approach viaduct in July 9. Wing traveller on north approach viaduct
A significant challenge for the designers of the pier table formwork was the requirement to transfer the high vertical loads into embedded anchors within the lower pylon. As the lower pylon geometry dictated the maximum possible number of anchors, Peri suggested a sequenced construction in layers to avoid the accumulation of vertical forces, ensuring that the loads can be safely supported, making the process more manageable.
“There is 1,000tonnes of concrete to support. More often than not, the forming of that central pier table is done with bespoke formwork, purpose-made. So it is supplied on a sale-only basis. What we have done all along is to deliver standard but higher quality equipment. To this end, we worked with the consortium to change, only slightly, the amount of pour sequences that we did and the amounts of different types of pour in this area so we could use standard equipment.”
Peri is now constructing the upper pylons, everything above road deck level. Concrete for each upper pylon is poured into the surrounding formwork, 5m depth at a time. Normally the formwork is lifted upwards for the next pour by using a crane.
One of the challenges common to all bridge construction projects is high wind speeds. Peri’s Automatic Climbing System (ACS), which supports and provides access to formwork systems, can be raised hydraulically without the need of a crane and is supported upon a rail fixed to anchors that are cast into the concrete. The advantage of the hydraulic system for the Mersey Gateway is that it can withstand higher wind speeds than standard cranes, meaning windy conditions cause fewer disruptions. Additionally, as crane use is restricted on-site, using this system means that other areas of the construction project can fully make use of crane-time.
Each of the four sides of ACS formwork around the pylon are temporarily bolted onto the side of the pylon. They also rest on vertical guide rails that are connected to the side of the pylon by climbing bolts, all of which are embedded into the side of the pylon during the concrete pour.
“The beauty of this system is that all four formwork platforms can be raised those 5m by way of two hydraulic rams sitting at about second level within the formwork,” says Fisher. When the concrete pour is set enough - at 25N/mm² - the vertical guide rails are first moved upwards 5m and then secured by huge doorknob-like anchors screwed into pylon.
The formwork is unbolted from the side of the pylon and hydraulic rams, one pushing and one pulling, slide the formwork up 5m on the vertical guide rails. It is again bolted onto the pylon for security.
To complete an upper pylon, it takes around 21 lifts. Once the pylon is completely poured, only then will a crane be called up to take the formwork off its anchors and lower the sections back to ground level where it can be fully dismantled either on site or taken off-site. The higher the crane’s lift capacity, possibly using a big jib to gain height, the fewer sections into which the formwork needs to be broken down. The formwork comes back to our yard were every nut, bolt and washer is counted.
“The contractor thought that it would be our formwork process that would dictate the sequencing. In fact, our ACS can move up those 5m in less than a day.
The critical elements are how quickly you get your concrete there, how fast you get your steel into position and how quickly the concrete cures to a certain strength. A crush test is done on a sample of concrete to confirm its strength. “Sometimes the formwork is taken back from the pylon and we wait maybe a few days for it to attain the right strength.”
TOP: Fixing steel reinforcement, north cofferdam, July 2015; BOTTOM LEFT: Form travellers on south pylon; BOTTOM RIGHT: Centre sections of MSS Lift being dropped into postion
Viaduct construction It was the biggest lift for the entire Mersey Gateway Project and it took careful manoeuvring over two days in July. It also took two huge cranes – one 700tonnes and one 750tonnes – working in tandem to hoist into place the 77m-long centre section of the Movable Scaffold System.
The MSS is essentially a bridge building system to help construct the bridge’s south elevated approach viaduct that will connect the industrial port of Runcorn’s main road network to the bridge.
When assembled, the MSS measures 157m long, 8m high and 22m across at its widest point. It weighs in at 1,700tonnes. Most machines of this kind build bridge spans only of up to 60m. However, the MSS for the Mersey Gateway Bridge was specially made so it is able to cast spans of up to 70m.
The two giant cranes lifted the MSS’s two 240tonne main girder sections over 12m high. The cranes then gingerly swung the girders towards and placed them onto landing skids on the first pier of the approach viaduct being built.
Each lift took around three hours and the construction team is now assembling the MSS, using a self-erect tower crane to complete the work. Once assembled, it will begin its task of building the viaduct sections northwards over the Manchester Ship Canal, towards the River Mersey. The MSS will eventually meet the bridge’s suspended road deck section being constructed southwards from the south pylon.
Merseylink decided to deploy two MSS machines, both built in China, to save valuable time that otherwise would have been taken to dismantle the other MSS- called Trinity - and transport it south across the for reassembling.
Trinity is currently in place on the north shore of the Mersey River and building the bridge’s other approach viaduct, near the town of Widnes. The machine is casting 11 spans of deck for the elevated approach towards the bridge’s first supporting pier.
The first span that Trinity completed involved pouring 1,146m³ of concrete into the MSS non-stop over 34 hours. When finished, it measured around 60m long and 18m wide and is angled at 5° to allow vehicles to travel safely around the curve of the approach road.
Winging it Work started in June this year on each of the outer road lane decks of the north approach viaduct and by early September more than 20 of the 62 sections had been completed.
Construction is being done by a 280tonne ‘wing traveller’ that follows the movable scaffold system (MSS) which is constructing the central part of the carriageway.
The traveller - sometimes called the MSS Construction Cantilever Traveller – is 4m wide and 20m tall. It works in a similar way to the MSS. It acts as a movable concrete mould to complete the full deck width, which, at just over 43.5m at its widest point, will carry six lanes of traffic.
The wing traveller is fixed onto two railway tracks that sit on top of the deck section that has already been cast by the MSS.
Concrete is poured into both sides of the machine at the same time, enabling workers to cast 12m-long sections of the outer deck on each side of the viaduct. Each pour will consist of around 80m³ of concrete - 40m³ each side. At least 62 concrete pours are needed to create the entire outer deck length of the north approach viaduct.
Once the concrete has set, hydraulic jacks push the machine forward to the next position and the cycle is repeated. The main structure of the north approach viaduct is expected to be complete by the end of this year.
Again, similar to the using two MSS - one for each viaduct - there will be two wing travellers. The second wing traveller machine, for the south viaduct, will be installed in November. Its work will be finished in the spring of next year.
Mersey Gateway: who’s who
• Contract type: 30-year design, build, finance and operate;
• Project delivery: Mersey Gateway Crossings Board, set up by client Halton Borough Council;
• Project company: Merseylink Consortium, appointed by Halton Borough Council in March 2014;
• Equity partners: Macquarie Capital Group, BBGI which is a global infrastructure company and FCC Construcción, part of FCC Group;
• Construction joint venture: Kier Infrastructure and Overseas, Samsung C&T Corporation and FCC Construcción;
• Demarcation: L&R Roadlines;
• Tolling: Emovis, part of Abertis, a French global motorway operator;
• Dynniq: Traffic control and information systems.
• A 2.3km-long cable stay bridge of three pylons with a river span of 1km;
• An 80m-high central tower will be shorter than the two outer towers, of which the north one will be 110m and the south one 125m;
• The deck will be six lanes, three in each direction, with a speed limit of 95km;
• Up to 30 supporting piers will carry the approach viaducts.