Engineers are using lightweight concrete as part of the work to repair bearings on an iconic bridge in Peterborough.
The £5M job to restore the Nene Bridge in Peterborough has produced an innovative solution for the repair of four of its eight V-shaped piers while preserving its distinctive appearance.
Indeed, it is claimed the project is the first in the UK to have reinforced concrete jackets constructed around V-shaped piers to preserve the aesthetics of a structure.
Skanska, the contractor responsible for undertaking the bridge repair for client Peterborough City Council, is also using a concrete mix that is strong enough to strengthen piers and carry jacking loads but light enough to minimise the additional load on the structure’s foundations.
Repairs to the road bridge, which was built in the 1970s, have been triggered by recent inspections which have shown signs of structural distress to the bearings and cracking in the saddles of the piers. In addition, some of the piers have begun to show signs of strain.
This posed a major problem for the council since the bridge provides a crucial connection for vehicles travelling between the A1 and the A47. It is also an important link for pedestrians and cyclists travelling between the north and south of Peterborough.
“This bridge is a key part of the city’s major route network, carrying in excess of 60,000 vehicles each day and it is vital this iconic structure is strengthened, so it can continue to be used without any restrictions for decades to come,” says head of Peterborough Highway Services Andy Tatt.
Four of the 155m long bridge’s piers are in the River Nene while the other four on land located either side of a railway. Work is being carried out by Skanska on all four of the land piers and two of the river piers. The four land piers are each constructed on capped piled foundations while the two river piers being worked on are on spread footings.
Replacing the bearings is a major challenge. The piers do not have enough space at their tops for positioning temporary jacks to lift the 9,200t deck so that the bearings can be replaced.
In addition, the steel box girders forming the bridge spans have no jacking points or are unable to withstand temporary forces exerted by jacking.
“There was no jacking stiffness within the existing box girders and also no jacking points on the existing piers,” explains Skanska site agent Dan Wood. “There is no solution in the original structure to replace the bearings,” he adds.
This is where the reinforced concrete jackets come in. The idea started with Skanska’s design team developing a plan to encase the piers in reinforced concrete jackets, strengthening and broadening them and providing jacking platforms from which to raise the structure, and allow the bearings to be replaced.
For the solution to work, the concrete used in the jackets must be strong enough for the piers to carry the jacking loads, but light enough to minimise the additional load on the foundations.
“It would have been a lot easier just to wrap the whole thing in a big block of concrete,” says Skanska project design team leader Stuart Watkins.
But he says this solution would have been untenable if the bridge’s unique design was to be respected. And if standard concrete was specified it would have led to the reinforced concrete jackets adding 100t to each of the bridge piers – too much for the foundations to bear.
“We did some analysis and worked out what level of additional load we felt comfortable adding to the piers. It works out to be about 10% of additional weight on top of the piers without having to start modifying the existing foundations and making them stronger. We did not want to do that because some of these foundations are 6m or 7m underneath the riverbed,” adds Watkins.
So, Skanska turned to materials specialist Aggregate Industries. It suggested the use of Lytacrete, a concrete mix using Lytag, a lightweight secondary aggregate. The mix was refined following trial pours to tailor its use at Nene Bridge. The design strength of the concrete is 50N/mm², which is the strength required to take the anticipated loads from the jacking procedure. Lytag is a synthetic lightweight aggregate, produced by sintering pulverised fuel ash at roughly 1,200°C to 1,300°C to create spherical, chemically inert pellets with a lighter weight porous structure.
Using Lytag instead of traditional aggregate reduces the dead load of the jackets by 25%, while offering the same level of structural performance. The lightweight, flowable, self-compacting concrete reduced the weight added to the bridge piers from 100t to 60t.
“The river pier foundations are deep. So that was one the reasons we manipulated the design to make the jackets the size they are, to try and ensure we stayed within that 10% additional weight. Otherwise the risk would be that the piers would start to settle, meaning they would lower and sink into the ground slightly,” says Watkins.
Work began in April last year when steel reinforcement cages were wrapped around the piers. The distinctive V-shaped appearance of the original piers has to be retained and this had to be accommodated by the complex formwork to be built up around each pier. As two of the supporting piers are within the river bed, a temporary coffer dam has also been built so that they can be accessed below water level. The steel reinforcement varies from 16mm diameter to 25mm depending on its position within the structure.
Concrete was installed using an M20 pump.
In addition, Skanska has externally strengthened the bridge’s four steel box girders to accommodate jacking by fixing a fabricated steel loading bracket to its outside face. The brackets have been positioned to suit the existing stiffener locations within the steel box beams to ensure it is strong enough when completing the jacking of the bridge. This method was selected to avoid the need to complete fabrication and welding within the confined space of the box girders, which site workers say poses significant health and safety risks.
A key aspect of the project is extensive use of 3D modelling software.
“We came with these SketchUp [software] drawings to try to put a series of phases together for the project, and then from there we were able to get more of an idea on pricing, the programme, how long the programme was going to take and what contractors we would need to get involved,” says Skanska project manager Scott Blackburn.
So far, Skanska has fully encased the four land piers with concrete jackets but has yet to complete the work on the two river piers to be strengthened.
Bearing replacement has not yet begun. Work is on schedule to be completed by October 2019. Once complete, the reinforced piers are expected to last for approximately 50 years.