Gaining access for deck maintenance work on the busy Silver Jubilee Bridge over the Mersey required some interesting lateral thought, Jessica Rowson discovers.
More from: Bridges special: Steeling the show
Tunnel access techniques − sort of − are being used during refurbishment of the iconic Silver Jubilee road bridge which spans the River Mersey between Runcorn and Widnes. An access shaft, narrow materials delivery portal and incremental horizontal progress across the mighty river are strangely reminiscent of tunnel working. But here the work is 24m up in the air, making further comparisons less relevant.
Refurbishment and corrosion protection of the crossing’s four-lane deck have required an access system that runs under the structure so work can be tackled from below deck level rather than above.
The unusual approach has been dictated by the need to keep disruption to heavy traffic flows across the structure to an absolute minimum.
Tying communities together
The Silver Jubilee Bridge forms the only highway link between Runcorn and Widnes − the principal towns of Halton Borough − and serves to tie the two communities together, not least by providing essential access to shared services.
It is also the only significant road crossing of the Mersey river for 32km, lying between the Mersey tunnels to the west and the M6 at Thelwall viaduct to the east. The bridge carries the busy A533 and serious traffic disruption can paralyse the region.
“It’s a very busy route, used by 85,000 vehicles a day, sometimes more.”
Mike Bennett, Halton Borough Council
“It’s a very busy route, used by 85,000 vehicles a day, sometimes more,” says Halton Borough Council bridge and highway maintenance manager Mike Bennett. The bridge − the largest such structure for which any local authority has responsibility − is vital to local residents and also to people from further afield travelling to and from central Liverpool and Liverpool John Lennon airport, for instance.
The Silver Jubilee Bridge comprises a steel compression arch from which 48 hangers suspend a reinforced concrete deck via steel lattice transoms.
Opened in 1961, the structure was seen in recent times to be suffering the usual problems of old age. In particular, corrosion of steelwork and concrete deterioration due to the marine atmosphere and penetration of deicing salts caused concern.
Local framework consultant for the maintenance of the crossing Mott Macdonald became involved in preparing a 10 year maintenance strategy for the bridge and its approach structures. Then earlier this year, the council appointed framework contractor and Balfour Beatty regional subsidiary Balvac to the early task of painting bridge deck beams and repairing the deck itself. It was Balvac which came up with the access scheme.
“We generally work from the top but this time we are working from underneath to minimise the interface with the public,” says Balvac site agent Tommy Leydon. “We have created a tunnel concept system to achieve this.”
“We generally work from the top but this time we are working from underneath.”
Tommy Leydon, Balvac
Expressed simply, the process is as follows. Balvac has built an access tower − like a shaft − under the Runcorn end of the bridge. Via this tower, men and materials travel upwards from ground level to a walkway slung below the bridge deck.
Narrow at only 1.2m wide, the walkway provides access to a scaffolding work platform, also suspended from the deck.
The platform has the same width as the bridge and is seven 12m bays long. A bay is the width of the longitudinal spacing between cable hangers.
The platform gives access to around 750m² of bridge deck soffit for steelwork painting or concrete remedial work. When work on the structure at bay one is complete, the scaffolding under this bay is dismantled and leap-frogged to bay eight while the access walkway is extended. When bay two is finished, the scaffolding is moved to bay nine and so on. There are 25 bays in all.
“One of the major challenges of this project is the logistics of the material handling − getting it all up and across. You don’t realise how little space you have until you start putting things into it,” says Leydon. “That said, the walkway is equipped with tunnel rails and bogies to allow efficient transport of materials.”
The game of scaffolding leapfrog has just begun. It is reckoned that each bay of scaffolding will take around a week to erect.
The scaffolding is enclosed with Envirowrap − the original paintwork had a red lead primer and this has to be safely contained when blasted off. Repair and repainting work started at the end of August.
Each bay is subject to a series of processes. First the steelwork is washed down and dry blasted to remove the existing paint. Wet blasting is often used for taking steelwork back to its parent metal as it cleans at the same time as stripping paint, but for this contract, the team decided to specify dry blasting for environmental reasons.
“We’ve moved away from wet blasting. We are proceeding on the basis of dry blasting alone. If we can avoid wet blasting, the job is shorter.”
Mike Bennett, Halton Borough Council
“We’ve moved away from wet blasting,” says Mike Bennett. “Traditionally wet blast was part of the specification: it takes out any salt on the surface. However the process creates humidity in the area [which can affect adherence of subsequently applied paint] and grit becomes ensludged and more difficult to remove.
So, subject to a rigorous regime of salt testing, we are proceeding on the basis of dry blasting alone. If we can avoid wet blasting, the job is shorter.” “It’s better to keep water and paint separate,” agrees Leydon.
“With wet blasting you would need to suck water back once blasted to prevent it entering the river. From an environmental point of view you don’t want any runoff. It also creates sludge which couldn’t be vacuumed and would need manual handling.”
The condition of the steelwork becomes clear once the paint and layers of rust have been stripped and inspections are carried out to determine whether any steel repairs are needed. Metalwork is then repainted but this work is proving particularly time consuming due to the type of construction of the bridge.
“It’s a striking structure, iconic for the area, but a swine to maintain.”
Mike Bennett, Halton Borough Council
The open lattice steelwork made up of a number of angle sections, means that there are nooks and crannies which are vulnerable to corrosion and in need of special attention. Every edge of steelwork needs an additional layer of paint − what is known as a stripe coat.
“It would have been easier if it was fabricated from plain beams”, says Bennett. “It’s a striking structure, iconic for the area, but a swine to maintain.”
Deicing salts have leaked into the concrete bridge deck, causing deterioration in some areas. This will be addressed from underneath although some traffic management will be needed at weekends as the deck will be temporarily weakened while the repairs are carried out.
Cathodic protection is being installed to the deck at the same time as the other under deck works to hinder any further deterioration.
Maintenance work is also being carried out on the bridge’s hangers. Last summer, those on the east side of the bridge were wrapped in protective tape in a bid to protect them from airborne salts. “For the bottom 3m we have added a sheath for vandal protection, triggered by a vandal attack on a bridge on the M60,” adds Mott Macdonald bridge associate Ray Langley. “Then, at the point where each hanger hits the transom beam, we have a cowl to prevent water ingress into the joint.”
Bennett adds: “With modern cable stay and suspension bridges, there is a degree of redundancy. You can take one cable or hanger out and the bridge will be fine.”
“It’s not the case here. The structure is sensitive to each hanger being able to do its job. We’re wrapping the cables with a protective tape. The ones on the east side we did via rope access last summer and we will be doing the west side later on this year.”
Maintaining the Silver Jubilee Bridge will become a lot easier if the proposed £431M Mersey Gateway gets the go ahead.
This will provide a second crossing the River Mersey close to Runcorn and take around 80% of the traffic currently using the Silver Jubilee bridge. This will be mainly regional traffic − the Silver Jubilee Bridge will then handle predominantly local traffic.
Current plans are that the bridge would be reconfigured to one lane of traffic in either direction with dedicated space for cyclists and pedestrians.
The Mersey Gateway project has just been subject to a public inquiry and a decision is expected in early 2010. If approved, the crossing could be open by 2015.