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Costain finishes Gerrards Cross tunnel rebuild

Costain has reached a major milestone in its work on the Tesco Gerrards Cross tunnel project with completion of the reinforced concrete over-build above the busy Chiltern Main Line railway between London Marylebone and Birmingham Snow Hill.

A 30m stretch of the 320m long tunnel, which was developed by Tesco to enable a new supermarket and car park to be built atop it, collapsed in June 2005 as it was being constructed by Jackson Civil Engineering. In the subsequent proceedings Costain as main contractor and engineering consultancy Scott Wilson as lead designer were brought in to complete the project.

While Costain has kept - and replaced where necessary - the original precast concrete tunnel arch units, it has effectively used these as structurally redundant but permanent formwork, and constructed a new, contiguous reinforced concrete arch above them. Whereas the original opposed precast units met in the middle at a pivoted crown joint, the new reinforced concrete arch is seamless, adding greater strength to allow it to securely bear the weight of the backfill that will be laid over it and on which the Tesco store and car park will be constructed.

The works to backfill above the reinforced arch with a combination of conventional aggregate and innovative, lightweight foam concrete are due to be completed in July this year, with work on the Tesco store itself starting immediately thereafter. The store is due to be open for business, on programme, by the end of 2010.

“Obviously after the collapse, everyone had a heightened awareness of the difficulty of this site,” said Costain project manager Martin Baughurst. As a result, “the levels of review are massive and to get everyone’s buy-in took time”.

The biggest challenge, he says, was to get Network Rail’s agreement that Costain could work above a live railway in standard daylight hours, minimising the requirement for costly and publically intrusive rail possession work.

To gain approval from Network Rail and reassure other stakeholders, Costain commissioned and installed a pair of goliath cranes specifically engineered for the job, allowing for the safe and controlled transfer of materials and equipment.

This was necessary as there were stringent limits on the impact loads that the precast concrete tunnel arch units could withstand if a conventional crane accidentally dropped a load on them at height.

The project is anticipated to be completed on schedule later this year.

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