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Cause for celebration

Rail Christmas possessions

Contractors on the £107M Channel Tunnel Rail Link Contract 103 are steeling themselves for a major push at Christmas.

Alan Sparks visits St Pancras.

Work at CTRL's final destination, St Pancras Station in central London, is centred on preparation for a 75m steel bridgeslide over the East Coast Main Line (ECML). The structure is due to be jacked on Christmas Day.

Last year, the same team missed its turkey when another bridge slide operation was carried out over the festive period. While most people would be reluctant to work Christmas Day, the team here is happy to delay celebrations until after the 55-hour line possession.

'Everybody involved has lived and breathed this part of the project for most of the year.

With the level of quality and dedication that the people on this site have displayed, you want to be here when it all comes together. This is the summation of all our hard work and nobody wants to miss it, ' says contractor Kier-Nuttall joint venture project manager Russell Lang.

Twelve months ago, a pair of smaller trusses were launched at another part of the site. This time round the span is longer and, at 2,065t, the weight to be manoeuvred is almost twice that of each of last year's spans.

Earlier this week, engineers began installation of sliding tracks beneath the throughtruss. These have a channel section, along which will slide pairs of 'skates' fitted to the underside of the truss's support girders. To fit the skates the truss must be lifted 1.8m. The skates will slip along the tracks on PTFE pads, lubricated with oil.

One pair of skates will support the rear of the truss while another set will be positioned around mid-span. To counter the cantilever's overturning moment, concrete decking within the truss has only been completed to two-thirds of its length.

Although the bridge has been fabricated by Watson Steel in line with its final position, the push will be far from one dimensional. 'The deck tapers along its length and we have had to construct an extra support girder under its right-hand side.

The left-hand skate will run in a straight line, but the sliding tracks on the other side must handle a lateral change in support position as the deck stretches out, ' explains design manager Adrian St John.

The truss is fitted with a 27m long temporary nosing, which must marry up with a temporary support trestle erected between the ECML fast and slow tracks to prevent the truss toppling onto the railway. It will cantilever out across the ECML until the mid-span skate reaches the west abutment and the nosing rests on the trestle. The mid-span skates will then be removed so that, instead of acting as a cantilever, the truss will be simply supported.

'To ensure the nosing lands in position there are vertical jacks to manipulate the structure's height and by monitoring the pressure in each jack we can keep the bridge on line during the process, ' says designer Rail Link Engineering senior construction engineer Paul Garth.

The rear skates will remain in play until the bridge reaches the end of its run. These, the temporary trestle and the nosing will then be removed and the bridge dropped into its final position.

Before the push, further possessions are needed to finish the intermediate trestle. Large struts must be installed to resist horizontal force exerted during the push.

Launching will take place in 1m increments, with hydraulic rams pushing from specially welded cleats along the outer edge of each track. Once extended, the rams will retract and pull away from their support cleats. They will then be clipped into position ready for the next shunt. Performing the tricky operation will be Dutch specialist Lastra.

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