Conwy Tunnel, A55 Expressway
Between Llandudno Junction and Deganwy, the Wales Coast Path crosses the A55 Expressway near the eastern portal of Conwy Tunnel. The land in this vicinity was created by the “cut and cover” tunneling method, which involves digging a trench and then placing a roof over the top.
The “immersed tube” method was used, for the first time in Britain, for most of the underwater part of the tunnel. Six sections of concrete tunnel tube were cast in a basin excavated near Morfa Conwy. Each section weighed c.30,000 tons and measured 118 metres long, 24 metres wide and 10.5 metres high. The cross-section was large enough for two lanes of traffic to run easily and safely through the tubes. The tunnel was designed by Travers Morgan & Partners, with the assistance of Christiani & Nielsen.
The contractor, a Costain and Tarmac joint venture, used buoyancy devices to float the tubular sections out into the estuary. They were then lowered into their final positions, in a trench across the bed of the estuary. Digging and reinforcing that trench was one of the most challenging aspects of the contract. There was also significant work to create the approach ramps down to the tunnel at each end, and the cut and cover sections at each portal.
The tunnel cost more than £140m to construct. Queen Elizabeth II officially opened it on 25 October 1991. The basin where the tunnel sections had been cast subsequently became Conwy Marina.