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Backfilling thought to be culprit in Gerrards Cross rail tunnel collapse

CONSULTANTS AND health and safety officials were last month considering whether surviving sections of the collapsed Gerrards Cross rail tunnel, north west of London, would have to be demolished.

Contractor Jackson Civil Engineering was building the tunnel over the Chiltern railway line as part of a £20.3M project for Tesco (GE December 2004). Work was being done under a design and build contract with precast arch supplier the Reinforced Earth Company.

A Health and Safety Executive spokesman said a site investigation by consultant White Young Green would determine whether the rest of the tunnel was kept or dismantled.

A 30m section collapsed on to the busy London to Birmingham line at 7.30pm on 30 June. The alarm was sounded by the driver of a train stopped in Gerrards Cross station and about to enter the tunnel.

The project involved installing a concrete vault over the twin tracks, which run through a cutting. The vault was to be covered with fill to reinstate the original ground level.

Network Rail operations director Robin Gisby said the backfill operation was the primary cause of concern. 'Something about the method of construction on this section is not right, ' he said.

Jackson chief executive Richard Neall insisted backfill operations were 'in line with what the design allowed'. 'There was a strict loading regime in place, ' he said.

The tunnel works as a three-pin arch, composed of half span, 2m wide segments leaning against one another under self-weight.

The design allowed 200mm of vertical movement, and relied on earth pressure provided by backfill for much of its long term stability.

It is thought the collapse was triggered by an imbalance in the placement and compaction of fill either side of the tunnel, combined with a surcharge of fill over the crown.

Geotechnical engineers expressed surprise at the apparent height differential between fi ll at the sides of the tunnel and the quantity of fill placed above the collapse zone.

'The collapse was probably due to too much load on the crown of the arch and not enough fill on the sides, ' said the head of one specialist consulting firm.

'Units [appear to] have failed by creating a hinge in the concrete section. The hinge has rotated downwards, which is consistent with the crown of the arch moving down under excessive load and/or the sides of the arch moving outwards under too little lateral restraint.

'This problem would have been made worse if there was a significant difference in the level of the fill on the two sides, ' he added. 'At the section that is still standing the central hinge has bent down, indicating an imbalance between the vertical and horizontal loads and a failure by outward spreading of the arch.' Engineers close to the project suggested the fill operation had departed from the approved method statement.

'There was a significant surcharge on top of the arch prior to adequate formation of the sides, ' said one.

'I believe there was a departure from the engineer's drawings. We're not talking about a couple of hundred millimetres differential, more like several metres.'

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