NORTH WALES COASTROAD (A55)

This contract is one of a series to construct a continuous  grade-separated carriageway along the North Wales   coastline to improve the link between Ireland (by way of   Holyhead) to the Channel ports. 

This particular construction provides the approach to an   immersed tunnel crossing of the Conwy estuary. An 8m high   embankment to carry the carriageway is founded on soft   alluvial estuarine clay overlaid by a laver of refuse material,   1m to 4m thick.  

Where the alluvial clay was at its shallowest, after having  removed the refuse, a separator geotextile was placed to   provide a separation filter layer below a drainage blanket of   single-sized 37.5mm crushed rock. Over the deepest clay   area, a drainage blanket (0.4m) was placed directly onto the   stripped ground. Vertical band drains were then driven   through to a depth of 6-10m to shorten the period for   dissipating pore water pressure below the embankment. 

Paralink 300 was then placed before completing in   controlled lifts the remaining 2.2m of drainage stone.   Paralink was introduced to take up the stresses imposed by   the settlement of the embankment and thereby even out   any differential movements. Instrumentation has been installed under the embankment   to monitor the settlement profile, edge movements and pore   pressure. No significant displacement has been recorded   proving the effectiveness of the solution proposed.