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Blackfriars: Spanning the Thames

Work at Blackfriars is cleverly using abandoned bridge piers to create a wide new cross river station.

Blackfriars is one of half a dozen Victorian railway bridges over the Thames, no more noticeable than most among London’s multiple crossings.

Its five span, wrought iron, shallow arches are pleasing enough, and match the road bridge nearby, but the station, which sits on the north bank end, is an unassuming structure.

Not any more. When work on the second of Thameslink’s three phases is completed next year, the bridge will be a landmark sight on the river, transformed into a spectacular transparent steel and glass rectangle stretching from bank to bank.

The bridge has always been part station, with platforms stretching from the north bank across the embankment road and river shore onto the first couple of spans. Three lines terminate there and since the Snow Hill tunnel was reinstated in the 1980s, two are for through trains north to Farringdon.

Platforms on the bridge are being completely reconfigured to enable the operation of new 12-car trains across its five spans by the end of 2011. The whole structure will be enclosed by 50 repeating north light roof panels set in a saw tooth arrangement, with glass side-walls.

Platforms will be completely remodelled

Platforms will be completely remodelled

A modernistic entrance building, shared with the London Underground station below, will form the north end, and a new “pavilion “ entrance on the south embankment will give access from this side of the river for the first time.

To cope with the new dead loads, and new trains, the main bridge has to be strengthened and renovated. At the same time it is being widened by 6m along most of its length. The work is also dealing with the general decay and corrosion of the 1886 ironwork.

The existing five span bridge comprises a wrought iron deck supported by spandrels rising from longitudinal rib arches springing between piers in the river.

Lead designer is Jacobs Engineering, supported by Tony Gee & Partners for the main bridge works. Work is being overseen by Blackfriars project director Lindsay Vamplew.

The work entails a partial rebuild, with each of the five main spans stripped down to 57m long supporting
wrought iron arches, which spring from the main piers in the river.

“In three years of rebuilding the station it will only have to be shut to weekday commuters for eight weeks”

Laurence Whitbourn, Network Rail

First, old track work and platforms are removed and then the wrought iron deck plates are taken out. Spandrel support posts from the arches are removed and discarded, which involves slicing the heads off their old riveted foot connections.

The tightly set shank of each rivet has then to be punched out of its hole to clean up the ironwork. “Altogether that means taking out some 30,000 rivets,” says Network Rail senior programme manager Laurence Whitbourn.

Around 8,000t of scrap is being craned out to be taken away by river barge, and 14,000t of new material is delivered in the same way, an environmental benefit saving the congestion of 2,000 lorry journeys in the busy local City streets. New steel is being fabricated by Watson Steel and barge loaded downstream at Greenwich.

Arches are carefully inspected for corrosion, especially where the ironwork has been inaccessible in the past.Tony Gee has specified four generic repair types for different levels of damage, mostly involving bolting on additional steel plate. Engineers make inspections to decide which option contractor Balfour Beatty should adopt.

Finally, new post supports are attached with modern tension controlled bolts in the old rivet holes, and the new deck fitted.

Materials are brought in by river barge

Materials are brought in by river barge

Additional arches for the widening are being added alongside the originals in most of the spans. There will be one additional arch on the east side and three additional arches on the western side of the bridge in four out of the five spans. These will be fabricated from modern steel to the same shape.

To fit these in means extending the bridge substructure, which on the downstream, eastern, side of the bridge has required additions to the original piers.

The 6.2m wide piers were extended using precast concrete collars with a concrete infill, built up from the original and cantilevering outwards 45° to add an extra 1.5m. A special floating access platform was devised to install the units, which were cast on the bridge deck above.

There is strength to spare in the old granite and gritstone faced piers. In fact, breaking down the Victorian masonry and an early form of concrete infill, to fit the new collars, took far longer than expected.

“It was exceptionally hard,” says Whitbourn, adding that the contractor had to diamond drill cores from some of the piers to do it.

During a five-day procession the four sections of the old bridge were taken out and replaced by a 22m span 350t concrete and steel bridge section. It was slid into position on Christmas Day.

Upstream, the project makes imaginative use of piers from an old freight rail bridge, the deck of which was taken off in the 1980s to save on maintenance. Triplets of red painted iron columns have remained in the river for years, like some esoteric sculpture from the Tate Modern gallery just downstream.

These match the positions of the station bridge and allow the nearest of each triple group, 5m away to be incorporated into the new station. The old ironwork will be removed and replaced with concrete columns and the pier bases built up to carry new arch members.

That activity comes later, when the second of two activity phases begins early next year. Renovation is being done two tracks at a time, so that the station can remain operational.

Working just metres from the live tracks is one of the challenges of the job for the contractor, though with no night trains there can be overnight track possessions.

In November this year, an eight-week station closure will take place when the two lines for the through trains, which are currently on the west side of the bridge, will be slewed to the east side of the bridge, freeing up the west side of the bridge for the widening works when the station comes back into public use in January 2011.

Rib arches are used to widen the bridge

Rib arches are used to widen the bridge

This activity represents the revised and final track layout for the new station. “In three years of rebuilding the station it will only have been shut to weekday commuters for eight weeks,” says Whitbourn.

Although the station will be closed to passengers during this time, trains will continue to run through Blackfriars to their onward destinations.

Preparing for the track switch has been a significant part of works at either end of the bridge to date. On the north side, among demolition and clearing of the old station entrance building, there was a significant bridge slide that took place at Christmas 2009, a major project in its own right.

During a five-day possession the four sections of the old bridge were taken out and replaced by a 22m span 350t,concrete and steel bridge section constructed on site alongside the original and slid into position with hydraulic rams on Christmas Day.

All this had to fit into a bustle of 24/7 major works around it. The biggest part is reconstruction of the London Underground station for the District Line, which runs along the river’s north side. The station below is being completely rebuilt inside two large excavations either side of the LU tracks.

These 5m to 6m deep cavities are inside secant pile walls, and will make space for new office and station facilities for LU and for remodelled platforms, entrances and exits. Everything here takes place with trains still running, although the Tubes do not stop at Blackfriars at present.

How Blackfriars interior will look

How Blackfriars’ interior will look

To protect the Tube trains, a steel box enclosure has been erected around the tracks built from sections which were lowered in to the station through an existing vent shaft.

“We intend to take them out the same way,” says Whitbourn, “and are trying to decide how late to leave it.”
The later they go, the smaller the pieces they will have to be cut into.

An element of the work has been the demolition of part of a Network Rail office building, leaving a hair-raising partial void beneath upper floors. This will be restored eventually as the new common entrance building rises alongside. The building will have joint ticketing and concourse for the Tube and railway, stairs and escalators.

Further work has been underway on the south end where part of the brick viaduct approach is being demolished to make way for a new pavilion entrance. Piled walls will support a heavy concrete slab which will carry the re-routed track, while a ticket hall and stairs go below. Once trains shift over, the other half will be done.

English Heritage had to approve the south bank work, one of a dozen agencies and authorities involved on the project.

Coordinating works with others has been a critical part of the whole job, says Whitbourn. “We couldn’t have done it without help.”

This has included ensuring river clearance of the main channels for deliveries with the Port of London Authority, and arranging road diversions with Transport for London and the Department for Transport, to interactions with the London Underground, various historical bodies, utilities and the train operating companies.

Blackfriars in figures

124

age of the original wrought iron structure

8

week closure of Blackfriars during reconstruction

1

new rib for each east span

3

new ribs for each west span

12

car trains under cover

250m

length of longer 12 car trains

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