Over the 2.5km course of the East London Line Extension (ELLX), from Shoreditch heading north to Dalston, every other step seems to fall on a bridge.
Over almost its entire length, the rail project is piggybacking on the Victorian Kingsland Viaduct.
There are 170 brick arches in the structure and 21 road crossings.
Built by the North London Railway in 1865 the viaduct runs from Dalston to Broad Street Station (now closed), next to Liverpool Street Station. It was in use until 1986. 'It had been out of operation for a very little time so the structure was in a reasonable condition, ' says Barry Noble, client Transport for London's (TfL) ELLX project director.
New rail will be laid along the viaduct from a new station at Dalston, linking into the existing East London Line at Shoreditch, where it currently terminates.
The line will also be extended south from New Cross to Crystal Palace. TfL Rail head of stakeholder management Peter Boxell says the government will be asked to fund phase two of the ELLX next year. This will take trains to Highbury & Islington Station by connecting into the North London Line and involve an extension south-west to Clapham Junction.
To prepare for track construction and the installation of electried third rails and signalling 'we realised it would be advantageous if we replaced bridges where necessary and refurbished where we could, and brought the viaduct and drainage up to scratch', says Noble.
'This will enable the main works contractor to concentrate entirely on track, electrical and signalling packages, which typically get pushed to the back of the schedule when heavy civils works are involved as well.'
TfL is anxious that there should be no delay or cost overrun on the ELLX as credit rating agency Standard & Poor's is monitoring the project to gauge how safe TfL's financial hands are.
The work is being carried out under a £36M, 19-month design and build enabling works contract, let to Taylor Woodrow with consultant Faber Maunsell.
Bridges along the route are short span - 10m-15m - and are of two types: iron plate girder, and brick jack arch structures.
The 11 jack arch bridges are in reasonable condition and are being refurbished, says Taylor Woodrow project manager Mark Howard. Their cast iron beams are being grit blasted and repainted. Repointing and patching of brickwork is being carried out where needed.
Ten plate girder structures are being replaced, however.
A standardised design of composite decks on longitudinal and transverse I-section steel beams is being used.
'They're not remarkable bridges, but they're sympathetic with what was there originally, ' says Faber Maunsell associate director Richard Sutherden. He adds that differences in span and the fact that many of the bridges cross roads at a skew means each structure has required unique analysis.
Construction using small, light elements has been essential for agile, speedy erection, notes Howard. Hard up against the viaduct there is limited space for cranage, and road possessions have to be short to minimise traffic disruption.
Sutherden says that another guiding principle has been to modify the bridges' existing abutments as little as possible.
In the main this is not a problem.
Only two tracks will run over the viaduct when the extension is complete, compared with four in the 19th and 20th centuries.
Maximum live loading on the bridges will be lower, even though trains have become heavier, and exert larger traction and braking forces.
The bridges themselves are also lighter than those they replace.
'However, you're never sure what voids you'll nd [within the brickwork] when the old bridges are removed, ' says Howard. Voids - the space within viaduct piers - have had to be filled with foamed concrete at three locations to support the abutment wall and combat potential horizontal forces from live loading of the new bridges.
This has been particularly important for the abutments that will support a 45m span bowstring arch bridge across the Regent's Canal. The 14,000t structure replaces a double span bridge; it is the only place on the northern ELLX where loading on the abutments exceeds that of the original structure.
Foundations for the bridge's northern abutment have been strengthened by installing 21, 600mm diameter bored concrete piles to 18m below ground level.
The piles were installed directly through the abutment's 1.2m thick brickwork using an 80t rig working from a temporary platform, constructed using lightweight aggregate.
Voids behind the southern abutment were lled with foamed concrete to provide lateral support. The bowstring arch itself, though of modest span, is a UK first, claims Noble. 'It's a type of bridge that has not been used on the railway before.' 'We have had to work hard to convince TfL and London Underground that it will take railway loading, ' says Howard.
'We needed to prov e that the hangers could resist fatigue loading. Conventionally we would have used Macalloy bar hangers, but they're more prone to fatigue. Instead we've gone for square hollow sections attached with a bolted split-plate connection, top and bottom.' Bridge steelwork is due to be erected in mid-September.
Concrete Omnia planks will be placed on the bridge's steel transverse beams, allowing a cast-insitu concrete topping to be poured.
Taylor Woodrow is well over half way through viaduct arch repair following an intensive period of inspection and analysis. Each arch consists of ve concentric brickwork rings.
Over the past 140 years bricks have spalled, there has been mortar loss and cracking, and rings have debonded from those behind. Coring, radar scanning, removal of 1m 2 patches of brickwork and computer analysis have been used to determine their structural condition.
Repairs include underpinning, repointing, patching and grout injection. Taylor Woodrow is scheduled to complete its enabling works contract in December, ready for the £450M main works contract to start.