WIDESPREAD CORROSION of rock anchors on the A5 in Wales led to its closure last month, the Welsh Assembly confirmed this week.
The steel bolts, designed to support the rock face overlooking a 640m cutting, suffered damage ranging from minor surface attack to total failure, with two steel plates at the rock face failing and 'popping' off in 2002 (NCE 1 June).
Welsh Assembly officials said that the Mott Macdonald risk assessment, which branded the cutting unstable and led to the immediate closure of the highway, found that of 15 anchors tested 13 had corroded.
The Assembly refused to comment on the likely cause of the corrosion as it is locked in talks with the cutting's original designer Rust Consulting, now owned by Parsons Brinckerhoff, and contractor Miller, now Morgan Est.
Mott's study found that water was seeping into the anchor sockets, but experts said it is unlikely that corrosion would be caused by this alone. Former ICE president Sir Alan Muir Wood said: 'It would seem that there is some particular feature of the rock that [when met with water] is causing this to happen.' Designing against it is relatively simple, said Muir Wood:
'There are types of bolts and anchors encased in concrete and sometimes you have sheathed bolts.' A total of 400 anchors, ranging between 6.5M and 18m in length, are used to support the rock face either side of the cutting.
Lift tests were carried out on the anchors and a large number were found to be under significantly greater stresses than they were designed for, according to an Assembly spokesman.
Design solutions for remedial work are being explored by contractor Jones Bros and its designer Hyder Consulting.