THE POTENTIAL for a catastrophic collapse forced part of the main route between London and Holyhead to close last week after the shear failure of rock anchors.
On Friday, Welsh Assembly officials responsible for maintaining the route ordered a 32km 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 an emergency meeting of local community members on 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 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 infitrated the slope from farmland above, causing the clay strata to swell. This then pushed against the rock, 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 fi st 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, the cutting will need to be rebuilt said Pugh. He added that so many anchors had failed that the only safe solution was to level 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, ' he said.
Rock anchors have also reportedly failed on the south face, but the incidence rate is far less and re-profi ing is not required, added Pugh.
Remedial works are expected to take up to a year.
Designer for the scheme was Rust Consulting, which is now owned by Parsons Brinckerhoff.
Contractor on the project was Miller, which is now part of Morgan Est.
John McKenna in North Wales The A5 is one of only two main trunk roads in North Wales (the other being the A55), and its closure at Ty Nant means local communities dependent on tourism are facing ruin, according to local Conservative MP David Jones.
Opening the abandoned 19th Century Thomas Telford-designed A5 has become the Welsh Assembly's main priority as the summer holiday season gets under way.
Work began on Saturday to level an earth embankment that would allow the new A5 to join the old road just before the cutting.
This work is expected to take two to three weeks but even when open, Telford's road will only be suitable for one lane of traffic and will not take heavy goods vehicles.