A highly complex structure, limited access, working over water and keeping the crossing operational is making restoration of Blackfriars railway bridge in central London a mighty challenge. Declan Lynch kicks off NCE’s
plant special with a report on a key section of the Thameslink rail upgrade.
London’s Blackfriars railway bridge is being completely refurbished as a key £350M component of the £5.5bn Thameslink programme designed to increase north-south passenger capacity across the capital by 50%. The Victorian structure is not proving an easy one to restore as the crossing is structurally intricate and repair and replacement work has to be carried out around a live railway.
A rake of cranes, operating in close proximity and concurrently, is needed to do the job.Adding to the contractor’s difficulties is Old Father Thames flowing just below.
The existing five-span bridge comprises a wrought iron deck supported by spandrels rising from longitudinal rib arches springing between piers in the river. Each span has up to 12 rib arches. Refurbishment involves strengthening everything from the rib arches down and replacing everything from the bottom of the spandrels up.
In addition, extra rib arches are being installed - some of them supported on previously redundant piers alongside the existing structure - in order to widen the crossing to suit the planned increased in capacity on the line.
“Each span must be restored almost independently but worked on at the same time”
Ultimately, the bridge will “house” the new Blackfriars station which will span the Thames. Balfour Beatty is the lucky contractor charged with doing the work.
“What is unique about the Blackfriars bridge restoration is that it must be completed while maintaining an operational track during the entire length of the project, say, for a handful of three day possessions,” says Network Rail programme manager Laurence Whitbourne.
“In order to achieve this, it requires each span to be restored almost independently but worked on at the same time,” he adds. The cranes are instrumental in helping Network Rail achieve this.
Five 70t Kobelco crawler cranes are in operation on each of the spans to assist with the restoration work. A further two 25t Hitachi cranes sit on the bridge deck performing general duties. All seven cranes were lifted onto the bridge with a 135t Kobelco situated on the south bank. A 120t Hitachi crane sits on the north bank loading rib arch segments into position and loading waterborn deliveries onto site.
Another three tower cranes sit on the north and south banks to help construct the station, making a total of 12 cranes onsite.
As the existing bridge deck is wider at its northern end, additional supporting rib arches and deck extensions east and west are being constructed to “thicken” the deck and thus achieve a consistent width.
To the east, three rib arches have been added to the spans at the south end of the structure. Each new arch comprises three sections weighing 45t delivered to site by barge, taking traffic off the busy streets of London.
“The deliveries are tide dependent, so this must be reflected in the programme,” says Network Rail senior programme manager Dan Athol. A safety boat is on duty during the whole operation.
On the west side, 15 rib arches are being added to the structure, using the redundant piers from the previously demolished bridge.
Simultaneously, the existing rib arches on the bridge superstructure have been checked and 95% have been strengthened using rib-end flange repairs. In all, 1,800t of new steel is required for the project, and 37,000 rivets have been checked on the bridge.
“One of the key challenges with the whole project is the amount of stakeholders involved”
With the additional east side rib arches installed and existing arches strengthened, Balfour Beatty is currently replacing the east side bridge deck. The deck is cut transversely in approximately 3m wide sections and the deck plate is removed by crane. Existing spandrel posts - small columns which span from the top of the arch to support the bridge deck - are then being removed. The existing rib arch will be cleaned up, ready for the new spandrel post.
These new posts and deck will then be fitted using bolted connections.
The whole process is taking around 10 days per section from start to finish and is being carried out by steel subcontractor Taylor Watson. It is a delicate and tightly run programme with very precise tolerances of 10mm on the steel work.
“We need reliable and accurate cranes so Balfour Beatty has purchased them new for the job,” says Whitbourne.
Once the new deck plate has been completed the contractor will install a waterproof membrane, collecting rain water that will be re-used in the station. The east side of the bridge will be complete in November when the ballast and track is put in place ready for the track switch. After the track switch, the west side of the bridge will be rebuilt incorporating the existing piers.
“The existing piers are being strengthened by breaking down existing structure and installing the precast concrete rings. It is a relatively straightforward job, the only difficulty being that the existing columns were a lot stronger than anticipated and therefore it took longer to break them down,” says Athol.
Along with the complete restoration of the bridge, a brand new Network Rail and London Underground (LU) shared ticketing hall on the north bank is being built above London Underground’s live Blackfriars Circle and District Line station.
This station is currently sheltered beneath a track protection shield which was built ahead of the demolition of the original Blackfriars railway station at the start of the project.
Installing the shield was a delicate operation, again highlighting the need for cranes. It consists of 39 individual steel plates weighing up to 7.4t. The shield elements were delivered to a site next door but one to the station. The units were then lifted over the five storey office block next to the site using one of the tower cranes mounted on at the north end of the bridge. They were then lowered into the Tube station via an LU vent.
Once in the station the elements were postioned vertically on either platform to support other sections running horizontally above the track.
Pre-installed spreading beams on the existing platforms were used as tracks and the protection units were winched along them to their positions.
Once in position, the units were bolted together. The shield covers the length of Blackfriars Tube station, which is closed until late 2011.
Groundwork for the north bank station requires 484 contiguous and secant piles and 112 bearing piles. “The piling operation is now underway. Once complete we bring the structure up to level, install a reinforced concrete slab and above this will be the steel frame structure for the ticketing hall,” says Athol.
“One of the key challenges with the whole project is the amount of stakeholders involved with the job,” he adds. “There are two track operators, Network Rail and Transport for London, both of whom need their lines to remain open throughout the project. There are two local authorities, the City of London on the North bank and Southwark on the south bank. Not to mention London Port Authority, train operating companies, local businesses and residents.”
Progress is on schedule for the whole project and the new station over the Thames is due to open in 2012 in time for the Olympic Games.
Crawler cranes used on site
Five Kobelco CKE700s (bridge)
Maximum lifting capacity: 70t Working radius: 3.1m
Maximum boom length: 54.9m
Total operating weight: 67.1t
One 135t Kobelco crane, Southwark Street (south bank)
Maximum lifting capacity: 135t
Used for lifting all other cranes up to track/bridge level
One 120t Hitachi crane, Thames Wharf (north bank)
Maximum boom length: 30.4m
Two 25t Hitachi cranes (general duties, Blackfriars Bridge)
Two Terex CTL630-32
Capacity at max length: 9t
Max Jib length: 60m
One Terex CTL430-42
Capacity at max length: 6t
Max jib length: 60m
Blackfriars bridge team
Client Network Rail
for London, London Port Authority, City of London (north bank),
Southwark (south bank),
Train Operating Companies,
Local businesses and residenst
Consultants Jacobs (lead) plus
Tony Gee & Partners
Atkins Station Information & Security Systems
Principal contractor Balfour Beatty Civil Engineering