Investigating rock anchorage failure at major road cut in Wales

Abstract

Rock anchorage failures occurring only 6 years after installation to support a major cutting on the A5 trunk road west of Corwen in North Wales were exhaustively investigated, ultimately resulting in their removal and partial replacement. The failures in Glyn Bends rock cutting were detected when head assembly components were found on the ground during a routine inspection. The investigation of the 120-year design life double-corrosion-protected anchorages included the innovative use of endoscopy, selectively supported by subsequent physical dismantling and metallurgical examination. On the basis of the investigation and the subsequent recovery of some of the installed anchorages, it was confirmed that at least 24% of the anchorages originally installed had their factory corrosion protection fatally compromised by partial removal. This was aggravated by the omission of the caps, which allowed the protective grease to escape, and placing many anchorages in recesses which aggravated surface corrosion. Metallurgical investigation emphasised that corrosion protection is critical to very-high-yield steels. No records of the alterations made were ever produced. The trunk road had to be closed at short notice for 8 months on public safety grounds while the main cutting face was reprofiled to a safe unsupported angle on the north side and re-anchored on the south side.