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Risk of multiple fatalities shuts London-Wales route

A potential catastrophic collapse caused by shear failure of the rock anchors, forced part of the main route between London and Hollyhead on the A5 to close last week.
On Friday Welsh Assembly officials, responsible for maintaining the route, ordered a 20-mile diversion to be implemented after a Mott MacDonald risk assessment branded the 640m cutting on the A5 between Ty Nant and Conwy, unstable.

'There could have been a catastrophic failure at any time,' said Clwyd West Assembly Member Alun Pugh, speaking at emergency meeting of local community member last Saturday.

'You would literally get hundreds of tonnes of rock coming all over the road and there could have been multiple fatalities,' he said.

Mott MacDonald was called in to investigate why steel plates sitting on the face of the rock in front of the rock bolts, had sheared off at the cutting, which was only completed in July 1996 for £18M (NCE 14 December 1995).

Conwy County Borough Council engineers spoken to by NCE at the site, blamed swelling of the clay beneath the overlying slate for the failure.

The north face, on which the majority of failures have occurred, is a tiered 24m tall slope that rests on soft clay on an inclined bedding plane.

Engineers believe that water infiltrated the slope from farmland above, causing the clay strata to swell. This then pushed against the rocks and pressure was released at the nut and plate, the rock anchor's weakest point.

However Welsh Assembly engineers have suggested that the failure could be due to corrosion of the anchors. Small pitting of the high tensile steel around the face of the rock caused stress fractures in the bolt, plate and nut they said.

The first rock anchor plate sheared in 1999, with more incidents recorded upon inspection two years later. But in May 2005 Mott Macdonald was called in after the number of failures escalated.

Despite having a 100 year design life, Pugh said that the cutting would need to be rebuilt. He said that so many anchors has failed that the only safe solution was to level off the north face from its current 70 degrees to horizontal gradient to a shallower 35 degrees.

'It's not a question of money. It will be re-profiled, but this will take months as it requires land acquisition and the blasting of over 100,000 tonnes of rock. Our current priority, therefore, is to open the old A5 as soon as possible,' he said

Rock anchors have also reportedly failed on the south face, but the incidence rate is far less and re-profiling was not required, added Pugh.

Remedial works are expected to take up to a year.

Original designers for the scheme were Rust Consulting, now owned by Parsons Brinckerhoff. Contractor on the project was Miller

John McKenna in North Wales

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