FAILURE OF rock anchors in a cutting on the A5 in north Wales last month forced traffic back onto the disused road it replaced in the mid 1990s.
The Welsh Assembly, which is responsible for maintaining the route, ordered a 32km diversion after a Mott MacDonald assessment said the 640m long cutting between Ty Nant and Dinmael was unstable.
Mott MacDonald was called in at the end of May to investigate why the anchors' steel face plates had sheared off in the north face of the cutting, which was only completed in July 1996 by Miller Civil Engineering (now part of Morgan Est).
The original designer for the scheme was Rust Consulting, now owned by Parsons Brinckerhoff.
Traffic was moved from the original winding road, along the side of Ceirw Gorge, because of high accident rates and a lack of verges.
A combination of rock anchors and bolts, dowels and protective netting was used to stabilise the 24m deep cut, excavated by drill and blast through the complex geology.
This comprises Upper Ordovician deposits, predominantly a fine grained siltstone which has undergone low grade metamorphism.
Slope stability is significantly hampered by the presence of weak, soft volcanic tuff on inclined bedding planes.
The first rock anchor plate sheared off in 1999, with more incidents recorded during a 2001 inspection.
Mott MacDonald was called in after the number of failures escalated.
It is believed water infiltrated the slope, causing the clay-like tuff to swell. This increased stress on the anchors.
John Harrison, senior lecturer in rock mechanics at Imperial College, said weathering of the tuff could create clay minerals, including montmorillonite, that develop large swelling pressures if constrained - as in bedding planes.
'Additionally, as the tuff weathers its friction angle would reduce, lowering the frictional strength of the bedding planes to almost zero, which in turns means that any rock reinforcement would need to be substantially stronger, ' he said.
Any movement in the rock face from swollen ground conditions could be due to blocked drainage, Harrison added. 'The original design incorporated drainage, which suggests that the water pressure was actively being reduced.' An increase in water pressure could exceed the design strength of the reinforcement elements, leading to failure, he said.
It is possible the face plates and bolts sheared off with the increased tension because they are the rock anchors' weakest points.
But Welsh Assembly engineers suggested the failure could be due to corrosion. Small pitting of the high tensile steel around the face of the rock caused stress fractures in the bolt, plate and nut they said.
'There could have been a catastrophic failure at any time, ' said Clwyd West Assembly Member Alun Pugh. '[There could have been] hundreds of tonnes of rock all over the road and there could have been multiple fatalities, ' he said.
Pugh said so many anchors had failed, despite a 100-year design life, that the only safe solution was to repro. le the cutting's north face from 70° to 35°.
'It's not a question of money. It will be repro. led, 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 repro. ling was not required, Pugh added.
The remedial work was awarded to contractor Jones Brothers in midJune. Detailed design, planning and preparation works were due to start this month, the Welsh Assembly said, with work 'likely to take six months or more'.