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Suspect segments lifted out of collapsed tunnel

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CONTRACTORS WERE removing 14 precast concrete tunnel segments from the stricken Gerrards Cross air rights project this week, after a survey revealed they were not performing as designed since the collapse.

It is hoped segment removal will make the rest of the rail tunnel safe enough for Network Rail to repair damaged track and signalling.

A 30m length of the 20m wide tunnel collapsed onto the London-Birmingham railway line at 7.30pm on Thursday 30 June (NCE 7 July).

The £20.3M project involved erecting a 320m long segmental concrete vault over the twin track line and then backfilling up to original ground level.

Design and build contractor is Jackson Civil Engineering with consultant White Young Green.

The railway, which runs in cutting through Gerrards Cross, Buckinghamshire, was being 'tunnellised'. The aim was to create a new town centre site on top of it for supermarket giant Tesco.

Jackson chief executive Richard Neall confirmed through a spokeswoman that there was 'a slim chance they (the segments) may have been overstressed during the collapse. We have to employ the principle: 'If in doubt take it out'.' On Monday, Jackson presented its analysis of the collapse to Network Rail, as well as explaining what it had done to make the site safe.

A Network Rail spokesman said it would take a week for its engineers to digest the report and decide whether to accept Jackson's assessment.

If Jackson's report satisfies Network Rail it will take at least another week to carry out the repairs, he added.

The tunnel works as a three pin arch composed of 2m wide, 350mm thick half span segments leaning against each other.

Each element is footed in a trough running the length of the foundation beams. The crown joint is a simple male-female 'hip' joint.

The vault is inherently flexible. One arch supplier told NCE that up to 200mm of deflection, up and down, could be expected as fill was compacted first on either side of the structure and then over the crown.

Investigations into the collapse have focused on the fill operation.

Engineers contacted by NCE suggested that inadequate compaction of material to the sides of the tunnel could have allowed the arch to spread as load was placed on the crown. This could have led to loss of arching action.

To find out whether the remainder of the tunnel is safe, fi ll up to 3m deep has been removed from above the crown, exposing it over its entire length.

This has included excavating it from within the steel frame of the new supermarket. This operation was completed last week.

As load was removed, the crown of the vault should have sprung up.

Deflection of the crown was measured as fill was excavated.

A site source told NCE that 14 of the remaining segments had failed to behave as expected, and had not defl ected up when the fill was removed.

These were being removed as NCE went to press.

'We can't prove these segments are structurally sound so we can't just compact the fill [to either side of the tunnel], ' said the source.

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