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Mind the gap

Rail DLR

Engineers working on a £300M extension to the Docklands Light Railway have spectacular views of London landmarks, but attention has been focused on buildings standing rather closer. Ian Lawrence reports.

Creating a 4.5km link between Canning Town and London City Airport takes Amec personnel perilously close to homes and business premises on one side and the terminal itself on the other.

When the extension opens in late 2005 driverless trains will run from the existing DLR at Canning Town along a section of embankment before crossing an initial 100m viaduct spanning the A1020.

Another embankment takes trains on to the 2.5km viaduct which passes over existing businesses and into the airport.

This stretch is approaching completion. The airport will be one of four stations on the route, which ends with a 900m trough that will contain King George V station when it is eventually built. This will be the starting point for two tunnels under the Thames that will take the DLR south to Woolwich.

The main viaduct will pass within 0.5m of the airport terminal and is only 1.8m away from a Carlsberg Tetley-owned building.

'From an engineering point of view, it is very difficult, ' admits project director Bob Goldring.

In response the team hit on a rarely used solution which not only speeds up construction but also minimises disturbance to the properties lining the route.

Twelve months ago a fourweek operation assembled a 100m long gantry at the western end of the viaduct site. Now with most of the 47 concrete spans erected, viaduct section engineer Mike Wall explains its operation.

He says the segments, each weighing between 30t and 85t, were cast in an on-site yard.

'They are delivered on low loaders, lifted off by winches and hung under the gantry on suspension bars.

'Once they are sitting together in the right alignment we install ducting inside the segments and then push the tendons through.

There are six tendons in each and there's between 14 and 32 strands depending on the span.

'Next we use a strand jack to stress the tendons. When we've done this we have the span on temporary jacks, it's self-supporting, and we release it from the gantry.

'After the suspension bars are gone, the gantry launches.

To start out it moves backward to where it picks up the front support. It then launches forward on to the next pier where it is post tensioned down.'

Goldring outlines a key advantage arising from this approach. 'One of the benefits with the gantry we are using is that we can get very close to the properties without causing much disruption, ' he says.

The gantry is heading for City Airport ready for the completion of this phase of the extension towards the end of January 2005. On reaching the airport, the team will use a technique similar to that which eased the construction as it crept alongside the Carlsberg Tetley site.

Wall explains: 'When we went past Carlsberg Tetley the rails the gantry slides on can be moved from side to side so we could push them right over to clear the building. We can work with a very low clearance and we'll be doing the same on the other side when we get to the airport.'

Perching above the winches 22m off the ground offers panoramic views of the London skyline, according to Wall.

'We've had some great views from the gantry of Canary Wharf and the docks because we are so high up above everything, ' he says.

But excellent views of the surroundings aside, he also highlights other more significant advantages of using this system.

'The whole thing is radio controlled by just one guy and it gives us the advantage of speed.'

Four segments can go up in a day using this method, compared with one every two days under a span-by-span approach. As well as keeping construction on target, interference with external groups is kept to minimum levels.

Wall says: 'We looked at it at the start and wondered if we would have to work at weekends to do it, but with minor control of traffic we are quite happy working over businesses. The further down the line we go the better it is, because businesses can take a look further up and find out how easy it was.'

Use of cranes is frowned upon next to live airports at the best of times, but avoiding their use along the extension means access quandaries are resolved.

'We don't have a problem with access because we only need to get a low loader in somewhere along the span, ' Wall confirms.

He adds: 'The gantry has limits on it for wind but it can still work in winds a crane would be stopped in.'

No falsework is required and the spans are both longer and twice as wide as they would be using precast beams.

Wall says this means a more efficient design and that fewer piers were necessary, which also cuts down on disruption for residents and means less work on traffic management issues during their construction, again saving time.

On the viaduct, Amec is working with Italian firm Rizzani de Echer. Rizzani handles the precast element, having had previous experience on Vancouver's Skytrain project.

The collaboration on the viaduct has seen upheaval to a number of airport-related businesses, including the essential relocation of a car hire firm, an in-flight catering operation and administration staff.

In total the project has necessitated taking 219 parcels of land, more than 90 third-party agreements and involved reinstating 27 residential gardens.

Despite this, relations with the community have remained good according to community relations officer Ian Thomas.

'We go to great lengths to make sure that everyone is kept informed well in advance of anything that could affect them. We also hold open evenings and publish a newsletter in the local papers.'

The project is financed through a PFI, with a joint venture between Amec and Royal Bank of Scotland creating City Airport Rail Enterprises (CARE) for the 30 year concession.

CARE general manager Harvey Pownall added that negotiations with Newham Borough Council had resulted in more stringent limits for noise, vibration and dust levels than national rail guidance requires.

He said noise barriers were installed along the route of the extension to mitigate potential problems for locals.

Completion of the 148 week project is due in December 2005.

Branches blossom

Additional extensions are in the pipeline to further boost DLR capacity.

Department of Transport approval has been granted for the 2.5km link to Woolwich, south east London.

The planned £145M extension will involve the construction of bored tunnels under the Thames and a new station at Woolwich Arsenal.

Four bidders, including the Amec/Royal Bank of Scotland consortium responsible for the London City Airport extension, have been shortlisted.

Direct services could be offered from Woolwich to the airport and on to Canary Wharf and the City, which Transport for London hopes will reinvigorate Woolwich and the Royal Arsenal site.

Current forecasts indicate a 2008 completion for this link.

Four new stations are also proposed along a northern extension to Stratford where the DLR will serve the Channel Tunnel Rail Link.

In converting the existing North London Line, DLR will increase frequency from two trains an hour to one every 10 minutes off-peak.

Trackwork and power supply will be upgraded, the stations constructed and rolling stock purchased as part of the £90M project.

The line could open in 2009.

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