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Less is more

Commuters using the M2 alongside the Channel Tunnel Rail Link site - which hugs the motorway for much of its length - must have got used to peering over their steering wheels at construction sites by now. After all, the CTRL Section 1 contracts snake all the way from Folkestone to Ebbsfleet - leaving spectators plenty of time to get bored with looking at chalky embankments and bevies of busy yellow excavators.

Or at least you would have thought that would be the case, but apparently people keep phoning up with questions. This is especially true for the combined £110M Contract 350/410, where callers seem particularly interested in construction of the piers for the Medway Bridge.

These are virtually complete bar the final hammerhead on the eastern splayed cantilever pier.

'Passers-by can't believe how slender the piers are, ' explains Alan Myers, contract manager for Rail Link Engineering, the consortium carrying out the design and project management of CTRL. 'But this bridge is designed to be robust. After all, it's eventually got to be able carry two high speed Eurostar trains at their top speed - that's 300km/hr - and to cope with their braking forces.'

Last week saw work by joint venture contractor Eurolink on the 1.3km bridge move into a new phase, with casting of the first 4.1m segment of the bridge's main central 152.4m cantilever span. The segments are being cast insitu, using a 100t travelling formwork gantry, currently sitting on top of the western 13m deep hammerhead. Each cantilever pier is designed to carry 33 segments - 16 centrally and 17 on the back span.

Also well over half complete are the post-tensioned viaducts that form the east and west approaches to the main span.

Viaduct decking is made up of reinforced box girder segments of three different types. Poured and cast on site before being push launched over the piers, standard segments take about 10 days to produce, but greater reinforcement and extra care is needed in the segments that will eventually sit on the piers.

Myers explains the most critical segments are those that sit either side of two movement joints in the bridge's deck: they will eventually bear the brunt of the bridge's post-tensioning forces.

A high tech operation, push launching is a popular option on bridge sites with difficult access such as Medway. The process relies on pushing large sections of deck over the existing piers without causing structural failure to pier or deck, or allowing the deck to slide out of control - as is suspected to have happened in a push launching failure at Injaka in South Africa (NCE 16 July 1998).

Injaka influenced Myers to set up a tight checking system and a plethora of instrumentation to ensure full control over the push launching process. So far nine launches have taken place on the west viaduct and seven on the east without serious problems.

Originally the western launches were to be completed before starting on the east side, to save on resources. But, Myers explains, initial problems on the western approach caused the programme to slip by six weeks - an unacceptable situation for the programme-critical bridge.

'First of all, we had access delays because the west site is between two railways lines, ' explains Myers. The Chatham rail line runs past the abutment and the Maidstone-Strood line directly underneath the bridge.

'Then our piling contractor Miller Engineering ran into difficulties because ground water levels were higher than previously thought during pile boring.'

Design issues also raised their heads when Eurolink formwork contractor Dumez GTM asked for the internal dimensions of the box girders to be made smaller to economise on shuttering. But in trying to optimise the design, the project experienced yet further delays and mitigation was no longer an option.

Myers continues: 'At this stage we decided we needed to reduce the risk and start launching work on the eastern side. We followed an option selection and cost estimation process, decided it was the best way and recommended the change to our client Union Railways, who agreed.'

The change meant bringing in secondary push launching equipment and other resources making the eastern push critical - but the move enabled the project to make up on lost time.

Before a push launch, four 10m long segments are added to the end of the previously launched section and post-tensioned together.

During the launch the front of the deck is counter-balanced by a steel nose that lowers down onto the next pier when it is reached. Once all the segments are in place the whole length of deck will be post-tensioned, the force of which is carried at the abutments and also either side of movement joints on the east and west approaches.

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