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The North Wales Coast Road has long been a proving ground for engineering innovation, particularly during the 25 year period that saw the Welsh Office dual the 100km from Chester to Bangor. Centrepiece of this was the Conwy Crossing, including Britain's first immersed tube road tunnel, but this is flanked by other great feats of engineering (A55 project special August 1994).

There would have been challenge enough in finding a route for the A55 dual carriageway along the narrow coastal plain, across the Conwy Estuary and through rocky headlands. Added to this, the geology is complex, and to make matters even harder the best corridors had long ago been taken by Thomas Telford's road and Robert Stephenson's railway.

Nothing was easy in building these coastal sections through Colwyn Bay, Conwy, Penmaenbach, Penmaenmawr, Pen-y-Clip and Llanfairfechan in the late 1980s and early 1990s.The 102 working day public inquiry - the longest then known - finished in February 1976, and led to the decision to cross the Conwy Estuary by tunnel not bridge, with another tunnel replacing a proposed viaduct at Pen-y-Clip.

The £227M Conwy Crossing immersed tube elements make up just 710m of the 6km project. Cut and cover sections at either end lead into long lengths of exceptionally difficult open cut approaches. Fourteen bridges, including a 10 span viaduct on 40m deep piles, all helped make this the biggest and most challenging of the contracts.

Gournd conditions proved a challenge throughout the road's length. Tunnelling at Pen-y-Clip was far more difficult than even the boreholes showed, with the outer thirds passing through exceptionally variable and poor ground - wide open joints, infilled with clay and what appeared to be topsoil.

Only the centre was through rock. Nearby at Penmaenbach, tunnelling could not even start until some 40,000m3 of scree was removed from above the portal. Most of the tunnel was formed by drilling and blasting, which went smoothly, but the eastern 90m through alluvial drift took almost as long as the other 550m.

One of the first high strength concrete carriageways and an award winning masonry wall are also along the route, as is a reinforced concrete structure that could be thought of as a tunnel or a bridge, carrying the railway through the road embankment.

There is also a series of innovative bridges with slender continuous decks to minimise maintenance. And as befits a coastal road, construction has also included sea defences, promenades and a marina.

Two organisations dominated the consultancy work - Travers Morgan, now Symonds, and Clwyd County Council - while many of the big names in contracting carried out sections of the work. In particular, a JV of Costain and Tarmac built the Conwy section, and Laing won several of the major contracts.

Just as work was finishing on the mainland, thoughts were turning to the other side of the Menai Straights. Dualling to create the A55 across Anglesey is now under way by UK Highways, and this too is an innovative contract, the first DBFO in Wales (NCE Roads June 1999).

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