UNDER COVER JOB

This article describes the construction of the Conwy crossing, Britain's first immersed tube tunnel which was eventually opened in November 1991. The 1km long tunnel is a crucial link of the A55 North Wales coast road, bypassing the town of Conwy. A tunnel was chosen rather than a cheaper bridge, to preserve the view of Conwy castle and town, which is designated a world heritage site. As both sides of the Conwy river are heavily populated, the tunnel had to be built under the estuary. Approaches and cut-and-cover lengths were formed inside large areas of the riverbank mudflats at the eastern end and salt marshes at the western end. The original four year, three months contract had to be extended by 35 weeks. Although the greatest challenge was to lay the immersed tube, the worst problems occurred in the ground on both sides of the estuary. Very soft alluvial clays were found below the planned road embankment on the eastern bank, so that the embankment took about two years to complete. Diaphragm walls were sunk 45m to form abutments and foundations for four new overbridges at the lowest part of the road. Unexpected ground conditions forced a revised design for part of the cut-and-cover concrete box section. Elaborate operations were needed to cast and place the concrete box tunnel elements.