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Gerrards Cross collapse:doubts over arch positioning

DIFFICULTIES WITH positioning concrete arch sections could have been a major contributor to last June's catastrophic Gerrards Cross tunnel collapse it emerged last month.

Sources close to the project have also suggested that some concrete arches used for the tunnel were outside design tolerances.

At Gerrards Cross, south east England, a major rail accident was narrowly avoided when a 30m section of new tunnel collapsed onto the live London to Birmingham railway line.

At the time contractor Jackson was creating a 320m length of tunnel over the railway using a precast concrete arch system, to cover the railway as it ran through Gerrards Cross in deep cutting.

The aim was to fill in the cutting and build up the ground above the tunnel to create the site for a Tesco supermarket.

nitial investigations concluded that overloading of the tunnel crown was to blame for the collapse.

Contractor Costain replaced Jackson Civil Engineering on the job in March and was charged with carrying out a three-month review.

Its report was due to go to client Tesco last month and was to be forwarded to Network Rail for approval before Costain could continue with the job.

Costain declined to comment.

But a source close to the project said that the contractor's report would expose problems with installing the arches within the constraints of night-time railway possessions.

'The design, while working fi ne in theory, is a major risk when working within the constraints of a UK railway, ' said the source.

'Because there is no fixed connection at the crown, the design relies on a perfect fit that demands perfect alignment. If it's not a perfect fit then rather than having the load distributed uniformly along the arches you get point loads where the arches meet.' It is thought that this problem may have been exacerbated by the fact that some of the arches had shrunk during construction or transportation and were not checked on site before installation, said the source.

Jackson Civils chief executive Richard Neale said that he had not seen Costain's report.

'We were involved in the project for a long time and looked at all aspects of construction, ' he said.

'We have had no access to any additional investigations carried out by Costain or others and have no way of checking its validity, ' he said.

On the issue of segment positioning and quality control Neale said that he had no idea whether the claims were true or not: 'It is a complex issue and I will be surprised if anyone has come up with anything concrete yet.' A draft Arup report on precast concrete tunnel commissioned by the Highways Agency stresses the importance of inspecting segments for damage before installation.

It also highlights the need to align segments correctly. The report was commissioned following the Gerrards Cross accident.

The Agency is considering the use of a similar tunnel design on its $61.2M Dobwalls bypass in Cornwall. Contractor Interserve plans to build the 60m long, 15m span tunnel next June.

'We are already in discussion with Network Rail, ' said a Highways Agency spokesman.

'There are other options.

Whatever design is chosen, it will be subject to the Highways Agency's rigorous approval process.'

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