7B Rail Extension Project Under Way in London

In London, work is well under way on the $7.4-billion initiative to improve Thameslink, the city’s only north-south surface rail line. The ambitious project, which includes new railway infrastructure and renovated stations, will double the line’s capacity by 2018.

Remodeling the old Blackfriars Railway Bridge, which carries Thameslink over the River Thames, is the central element of the project’s first phase, due for substantial completion next year. Current work includes replacing the Blackfriars terminus and building a new station at the south abutment.

The 125-year-old, wrought-iron bridge is being widened with new steel arches and is getting a new steel roof. Moreover, to accommodate 50% longer trains, the bridge’s station platforms are being doubled in length from the north river bank to cross the full 300-meter-long bridge.

A few kilometers up the line, Farringdon Station is being rebuilt. Further, a new viaduct is taking shape to unblock a bottleneck on the south bank outside of the London Bridge terminus, one of the city’s busiest transportation hubs.

The London Bridge terminus will be rebuilt in a second, five-year phase at an estimated cost of $1.4 billion. It is planned to open in 2018, more or less coinciding with the completion of the larger, east-west Crossrail railroad tunnel project, which is under construction. But for now, Blackfriars Bridge is taking center stage.

The roughly 300-m-long, five-span bridge was built with deck girders supported on posts rising from 5-m-tall arches. All the metalwork was wrought iron. Spanning typically 60 m between stone and brick piers, the arches are set 1.7 m apart.

To widen the bridge to 30 m from 23.5 m, four sets of new steelwork arches are being added. The new design replicates the old structural form but replaces iron posts and girders with steelwork.

One new set of arches already has been added to the east side, partially cantilevering from the old structure. The west side is being extended by three arches, supported by the foundations of the old, red piers of a freight-train bridge, demolished in the 1970s.

New steelwork is being fabricated to allow for the large geometrical variations found in the old structure. “We are talking about 10 to 15 millimeters,” says Tony Westlake, project director with the structural designer Tony Gee and Partners, Esher. “A large amount [of the bridge] was built manually. No two arches were exactly the same.”

To ensure a good fit, the contactor surveyed each of the roughly 30,000 original rivet locations and matched them with bolt holes, drilled into the replacement steelwork, says Chris Evans, project director with the London-based main contractor, Balfour Beatty Civil Engineering Ltd.

Balfour Beatty is remodeling the bridge and building the new Blackfriars stations under a fee-based, target-price contract, valued at about $620 million. Its subcontractor, Watson Steel Structure Ltd., Bolton, is supplying and erecting 6,800 tonnes of steelwork, with 5,500 tonnes for the deck and the rest going into the roof.