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Back on track

Dogged determination coupled with engineering expertise will soon bring the new Hungerford Bridge to reality. Diarmaid Fleming explains the structure's remarkable recovery.

Building bridges over the Thames last summer appeared to be a cursed art. Only weeks after Arup's reputation was dented on the Blade of Light, work on its cousin upstream, the Hungerford Bridge, also began to make headlines for the wrong reasons. Another Millennium project would fail to open in 2000.

Today, of course, the Blade remains shut and the contract to dampen its vibrational excitement has yet to begin. But at Hungerford work is at last steaming ahead on the reborn bridge project and while difficult challenges still lie ahead, the story of how this potentially doomed project was rescued is a remarkable one.

A walk up the junk-littered steps across the present footbridge connecting the South Bank complex to the north of the river graphically makes the case for the regeneration project. At night few relish this walk across its seedy spans, which hang like an afterthought on the side of the busy rail bridge.

Cross River Partnership (CRP) - a consortium of local authorities and companies including Railtrack - was set up to regenerate the Thames and its surrounding areas in central London. Erasure of the present Hungerford Bridge eyesore must have been first on their list.

A design competition for a new structure was won in 1996 by consultant WSP, architect Lifschutz Davidson and quantity surveyor Davis Langdon & Everest. The solution proposed two light symmetrical footbridges suspended a short distance either side of the rail bridge. The footbridges would consist of seven cable stayed spans with steel pylons founded on reinforced concrete bored and cast insitu piled foundations. Put out to tender in spring 1998, the contract was awarded to a CostainNorwest Holst joint venture in July 1999.

While the scalp of a prestigious Millennium prize project in the centre of London probably led to the clink of champagne glasses in the boardrooms and the CRP, dreams on paper soon succumbed to the cold reality of engineering in the Thames.

Apart from the obvious difficulty of working on a site based in the capital's heavily-trafficked river artery, the proximity to London Underground's (LUL) Bakerloo and Northern Line tunnels under the Thames would prove the greatest challenge.

'It was clear at early meetings that working close to Underground tunnels would be a major issue for the job, ' says project manager Gareth Hardwick of Westminster City Council.

In1996, LUL undertook a programme of strengthening works of the tunnels which run under the river parallel to the bridge.

Each was strengthened with a concrete shroud as protection against an explosion or other impact on the tunnel. Puncturing either bore could have flooded vast sections of the Tube, with potentially disastrous consequences.

Controlling this risk to LUL infrastructure was to prove more difficult than the most taxing of engineering challenges facing the site team, and almost scuppered the job. Of particular concern was the risk posed by unexploded WW2 bombs in the riverbed which, it was claimed, might be disturbed by vibrations from piling. A bomb was found nearby in the 1950s.

LUL identified two protection zones. The first stipulated that within 15m of the tunnels any work had to be carried out under Tube line possession with floodgates in the tunnels closed. A secondary area between 15m and 30m from the tunnels allowed some work to be carried out during engineering hours when the Tube is closed overnight. However, one third of the bridge's foundations lay within these two zones.

Westminster City Council carried out a sweep of the river bed to 3m and probed areas where the presence of metals was indicated. This was not enough to satisfy LUL. 'There was a theoretical possibility of a bomb being 15m deep into the riverbed, so we ended up probing to 15m depth, ' says Hardwick. Probing was carried out by specialist Bactec.

Despite uncovering nothing, detailed research into bombs dropped near the site itself uncovered new potential, if highly remote, risks. Research in Germany revealed that the Luftwaffe dropped bombs with timedelay fuses near the site. It was therefore possible for a blast to occur up to 96 hours after the bomb was disturbed, raising the possibility of an explosion when trains were running again. It seemed that the more the site team investigated, the more problems or possibilities were uncovered.

'We were nearly getting to the unbuildable stage. By late spring, early summer, I seriously wondered if the bridge would ever be built. The whole thing was lacking reality, ' says Hardwick.

Project costs, both from the delays and restrictions on construction, began to rocket. The scope of the works described in the original £21M contract based on the ICE design and construct form had changed beyond recognition, but sitting on the strict terms of the contract would have meant that the bridge probably would not have been built.

'We realised that the current contract would have made contractors very rich in this situation, and if we had carried on that route it would have become very expensive, positions would have become more entrenched and the job wouldn't have been built, ' says Hardwick. 'We needed to find a way forward.'

Brave decisions were required to turn the job around.

The first step to recovery was an agreement in August between client and contractor to co-operate on working towards a new contract. Extra money was found, with a crucial grant of £16.5M inspired by newly elected London mayor Ken Livingstone through Transport for London. The new Engineering Construction Contract form, option C, was signed in January for £39.5M. This now protects the contractor from unforeseen ground risk and third party requirements.

The new contract also required design changes, driven by cost and construction constraints. 'We established how much money could be found, and then tailored the contract to suit, ' says Hardwick.

Plans for two separate bridge links towards the Millennium Wheel and Royal Festival Hall were jettisoned, although piled supports in the river have been completed to allow their construction later. Specification details on other parts of the structure were also amended to cut costs.

Design changes also meant that bridge supports on the north side, which were to have been built in the river, will now be built in the dry. Piled foundations for these were nearest to the Tube tunnels - instead they will be founded on 3.6m diameter, 28m long shafts. This reduces working room, and avoids loading being passed to the tunnels, the river wall and to a sewer designed by Sir Joseph Bazalgette. However, the change increased deck span by 10m.

Another foundation on the upstream side of the bridge beside the southern Victoria Embankment was changed from bored piles to a single hand-dug shaft built within a cofferdam, again to satisfy LUL constraints.

The work involves shallow probing followed by digging to half the probed depth before restarting the process for the next dig.

Piled foundations to the north end of the bridge have also been redesigned as shaft piles while other foundations remain as 1.5m diameter bored and cast insitu piles up to 46m long.

Changes have also been made to the original bridge design after the JV contractor and its designer Gifford took on designs left by the competition winners.

The new contract gives complete design responsibility to the JV.

Consultant Gifford is completing the revised design.

The project has just seen successful installation of the first pair of massive 41m long, 250t ship impact beams supported on foundations in the river which will take impacts up to 30MN.

The contractor has also devised an alternative method of erecting the 315m long 4.75m wide, 600mm post-tensioned concrete decks to reduce cost and disruption to river traffic.

These will be constructed in 50m lengths in casting cells constructed over the southern span and slid progressively across the river. They will be supported on temporary works ahead of the construction of permanent steelwork pylons and cable stays.

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