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Gerrards Cross failure sparks precast arch tests


FEARS OF another arch collapse like the one at Gerrards Cross on 30 June have prompted the project's tunnel lining maker to test a new product under similar loading conditions.

The failed Gerrards Cross arch collapsed as it was being loaded with backfill which was to have created the site of a new supermarket.

As a result, precast specialist Macrete has devised a test to ensure the new arch can withstand uneven loading.

The new product will come as a series of flexibly connected voussoir segments which form an arch when lifted off the ground.

It is then lowered onto anchor blocks which lock it into place.

Testing will take place at the same precast concrete works in Northern Ireland that produced the reinforced concrete sections for the Gerrards Cross tunnel (NCE 7 July).

The 'flexible arch system' is very different structurally to the arches at Gerrards Cross. It is primarily intended for use in road bridges of up to 9m span.

Macrete had planned to carry out full scale load tests next spring on a 5m wide, 5m span bridge to be erected at a nearby quarry (see diagram. ) 'But after Gerrards Cross we felt we had to assess how the arch design would react to uneven backfilling, ' said Macrete project manager Jim Montgomery.

The new arch design is based on work carried out by Queen's University, Belfast, funded partly by the ICE and the Northern Ireland Roads Service.

For the backfi lling test a single arch ring with a span of 5m and rise of 2m will be used. The only steel in the construction will be a mesh inside a 50mm thick layer of sprayed concrete on top of the arch. The test should be completed in six weeks.

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