A seemingly straightforward strengthening and repainting job has been complicated by extremely difficult access centred on the presence of nine neighbouring railway lines.
Mitre Bridge carries the West London Line over the Grand Union Canal and the Great Western Main Line into Paddington – a vital railway asset that is more than 100 years old.
The bridge is now showing its age, however, with structural deterioration leading Network Rail to impose speed and loading restrictions on passing traffic. This led to Bam Nuttall being brought on board to blast and paint the bridge as well as carry out the structural repairs.
But the process has not been as straightforward as it sounds, with access proving extremely challenging and the proximity of nine active railway lines underneath the bridge posing further difficulties.
Mitre Bridge is a single-span trussed girder bridge carrying two railway lines.
Bam Nuttall started on site in June 2016, with the project initially scheduled to have completed by now. However, problems in securing possessions of the tracks underneath have led to numerous delays.
The lines underneath include two Crossrail depot lines, which didn’t exist when Bam first arrived on site, and four tracks belonging to the Great Western Main Line with trains travelling at speed, as well as various other depot lines.
“Those depots are live all the time, so getting possession access underneath is extremely difficult,” says Bam Nuttall site agent Lauren Beacham. “The plan has always been to remove that interface with the railway as quickly as possible.”
“The depots are live all the time, so getting possession access underneath is extremely difficult”
Lauren Beacham, Bam Nuttall
To do this, the team built a 16 m-long ‘tunnel’ to separate site workers from the railways on top of the bridge, via an encapsulated scaffold structure that features a horizontal desk about halfway down its height, positioned above trains that continue to run across the bridge. The scaffold and exterior canopy then continues down the outside of the bridge, allowing for a second working platform, or soffit, below the bridge. Operatives can also gain access between the vertical canopy and the bridge exterior.The work is phased, with the bridge being painted and repaired in 16 m sections.
At the time of CN’s site visit in November 2017, the work was well into its second phase, with the encapsulation having already moved on once. “We’ve effectively built a screen with two sides and a lid, so everything on the outside of the structure is then accessible without any railway possessions,” Ms Beacham says.
The team has also installed a running track along the top of the bridge, with the encapsulation sitting on this and featuring small wheels so it can be rolled downhill along the bridge with ease, once one section of painting and strengthening is completed.
The encapsulation also provides access to the underside of the bridge via a soffit, which hangs from the sides of the scaffold structure. The team did consider going along the bridge and leaving the underside unpainted, but as that would have required a return possession, it was decided to find a way to do it in one go.
The issue with hanging a soffit underneath, however, was the presence of the overhead lines (OLEs) for the track below, with the structure needing to be at least 300 mm away from it at all times. “[With] some of the lines we’ve got miles of room, as they’re at different heights,” Ms Beacham explains. “But in two locations, including where we are now, it’s really tight.”
Bam had to go through extensive design approvals with Network Rail, bonding the structure in all sorts of different ways to make sure it didn’t get any arcing from the OLEs.
The soffit underneath also had to span 14 m – the width of the bridge – which proved difficult. Bam has used a Trad system scaffold with some additional bespoke beams that are “really narrow and really stiff”, Ms Beacham says. This allowed the soffit to span the 14 m with minimal contact points – the team has only had to tie it to the bottom of the bridge in two locations.
“There’s a rigid inspection regime making sure these ties are tight at all times, to make sure we don’t get any closer [to the OLEs]”
Lauren Beacham, Bam Nuttall
All of this in turn makes for a fairly unpleasant working environment inside the encapsulation, particularly underneath the bridge on the soffit. “It’s really tight in there,” Ms Beacham says. “There is a drain that runs down the centre of the bridge that we’re replacing which cuts off access, so you have to go in one side all the way up and over, and down the other side.”
Up between the girders of the bridge, workers on the soffit have around a metre of headroom – but it requires them to squeeze through a 300 mm-wide gap between the bottom of the girders and the soffit to get there. “There’s a rigid inspection regime making sure these ties are tight at all times, to make sure we don’t get any closer [to the OLEs],” Ms Beacham adds.
Each phase sees the team carry out grit blasting and repainting, as well as structural steel repairs, over six weeks. Once that time is up, the encapsulation should, in theory, roll on.
However, securing railway possessions has proved problematic, as each move of the encapsulation requires a possession of the lines underneath to avoid any potential issues with the OLEs.
“Phase one was great because at the time we built it these Crossrail depot lines weren’t in,” Ms Beacham says. “It was just a construction site, so we were able to build [the encapsulation] from the ground. We craned in the roof, then managed to build the sides and the soffit from access underneath.”
But moving from phase one to phase two, Bam needed to knock out power to the OLEs and isolate them – and as phase two is the closest point the bridge comes to any OLE, it proved especially difficult. “You can isolate the line up to the bridge really easily, but there’s an area under the bridge – just the width of the bridge – that sits in a no-man’s land, and the only way you can isolate it is to knock out everything to the west of it,” Ms Beacham explains.
“All in all it’s a pretty simple job, in that it’s grit, blast, paint and strengthen the bridge. But access has been the huge issue – and that’s what’s made it complicated”
Lauren Beacham, Bam Nuttall
This wasn’t an option. Instead, once a week there is a four-hour period where all of the lines underneath are knocked out, allowing the team one hour of working time by the time the line is handed over. “That isolated everything apart from this one line underneath the bridge,” Ms Beacham says.
After a three-month delay, Network Rail granted permission for the line to the west of the bridge, leading into and including a depot, to be knocked out in one night, allowing Bam to move over the tricky OLE.
At the time of CN’s visit, the team was back in the same situation. Phase two was approaching its conclusion, but Bam needed to negotiate a new possession to move into phase three, as the original delay meant the team was out of sync with its original planned possession windows. This was subsequently achieved in the following weeks, allowing work on phase three to get under way.
“We should be finished with the main work in April next year,” Ms Beacham says. “Then there’s just some small work and there’s lots of places where the scaffold is touching the structure.
“We can paint around it now but we can’t necessarily get to everything. So we’ll do a sweep through the bridge at the end, prepping and painting contact points. There’s also a few bits at track level, some ballast boards to replace and some mesh panels that need to go on. But we won’t need a full site presence at that point.”
The structure of the bridge isn’t actually in too bad a condition, with the only major strengthening applied to the end three bays on all four of its corners. Most of the other work has been overplating and replacing bits of rotten steel.
“All in all it’s a pretty simple job, in that it’s grit, blast, paint and strengthen the bridge,” Ms Beacham says. “But access has been the huge issue – and that’s what’s made it complicated.”