Geo-engineering along the A55 North Wales Coast Road

Abstract

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.

Chapter 9 Peat hazards: compression and failure

“…Experience in Ireland is more widespread given the more common occurrence of peat across both upland and lowland environments (Long and Boylan, 2013). However, wherever peat occurs in small pockets or over entire regions engineers must be aware of its hazardous consequences (Nichol, 2001).…”

Chapter 6 Material properties and geohazards

“…Examples from Canada (Bishop et al 2008, Evans 1982, Fletcher et al 2002, Geertsema et al 2006, Ito & Azam 2009, France (Van Asch 1996, Giroud et al 1991, Jongmans et al 2009, Bièvre 2011 and Estonia (Kohv et al 2009(Kohv et al , 2010a(Kohv et al , 2010b illustrate the highly problematic nature of these deposits. In the UK, their distribution is equally widespread and examples of their tendency to failure have been documented in Yorkshire: former Glacial Lake Humber, (Taylor et al 1976), in Scotland (Rowe 1995, Cochrane & Carter 1991, in Northern England (Hughes et al 1998), in Cleveland and Teesside: Glacial Lake Tees (Phipps 2001, Bell 1998, in North Wales (Nichol 2001) and in south west Wales with instability in deposits from Glacial Lake Teifi (Fletcher & Siddle 1998, Maddison 2000, Gibson et al 2013. Murton & Murton (2012) provided a detailed overview of the former glacial lakes of lowland Britain, areas where glaciolacustrine deposits should be expected in predictive ground models.…”