|Location Map ( geo)|
The tunnel is of twin box cross-section to accommodate dual two-lane carriageways. It is about 1090m (3,576ft) long, comprising an eastern cut-and-cover section of some 250m (820ft), a western cut-and-cover section of about 120m (394ft), and six immersed tube sections each of about 118m (387ft). The six sections were cast in a basin on the western bank of the Conwy estuary, which had been used in World War II to fabricate the Mulberry floating harbours. Each section once complete was closed off at each end by temporary bulkheads. When all were complete, the basin was flooded and the secions were floated out and lowered into place.
The tunnel was designed by R Travers Morgan & Partners, with the assistance of specialist consultants, and built by a joint venture of Costain and Tarmac for the Welsh Office between September 1986 and October 1991, somewhat overrunning the contract period of 4 years and 3 months. It was opened by the Queen in October 1991.
After the tunnel was completed, the casting basin was re-used to form the present Conwy Marina, which is now surrounded by buildings, all built since 1991.