Geo-engineering along the A55 North Wales Coast Road

The A55 Chester to Holyhead trunk road comprises 145 km of modern dual carriageway along the coast of North Wales. As part of Euroroute E22 (Sassnitz to Dublin) it forms a main artery to the British and European motorway network. Construction (1969–2001) entailed 30 separate schemes crossing diverse geological terrains that posed a wide variety of ground problems of considerable engineering interest. This paper describes the diverse geo-engineering features associated with the project and highlights certain aspects of the design and ground engineering methods used to mitigate or resolve some of the geotechnical concerns along the route. On the easternmost schemes, uneven settlements resulted from kettle holes in till, backfilled opencast coal mining areas and pockets of postglacial peat. At Halkyn Mountain, the construction corridor crossed a zone of abandoned lead-zinc mine workings and through the Clwydian Range it traversed a cluster of sinkholes. On Rhuallt Hill, adverse discontinuity conditions in Silurian siltstones created slope instability in a rock cutting. In the Vale of Clwyd, the till plain provided satisfactory foundation conditions but further west, soft alluvium and peat beneath the coastal plain caused major ground engineering difficulties. The Conwy Crossing was the first immersed-tube road tunnel in the UK and the mountainous headlands of Penmaenbach and Pen-y-Clip, composed of Ordovician igneous rocks, required the construction of hard-rock road tunnels. The westernmost schemes on the mainland traversed Cambrian and Ordovician strata mantled with glacial deposits. The final extension across the island of Anglesey was constructed over schist and gneiss of the Precambrian crystalline basement.