DESIGN AND CONSTRUCTION OF CONWY'S 710M IMMERSED TUBE TUNNEL

A six element immersed tube tunnel design was used for the improvement of the A55 coastal route in North Wales because a bored tunnel would have been much longer and deeper and would have been located in very disturbed ground. A tunnel was chosen for the crossing of the river for environmental reasons even though there was a premium of U15M over the original bridge scheme. The project comprised a junction to an existing roundabout by a new road on a 10m high embankment, an interchange on reclaimed land followed by an underpass beneath the existing A55 and railway to the tunnel of total length 1080m. Three large excavations were required, one for each of the cut-and-cover approach sections of length 260m and 120m and one exacavation for a casting basin. Strict temperature control of the concrete was needed and so liquid nitrogen injection was used. A trench up to 20m deep was dredged across the river bed into which the tunnel segments were sunk. The space between the ends of the segment was evacuated and natural hydrolastic pressure used to join the elements, a rubber gasket making the seal between them. Pontoons were used to position and sink the tunnel elements; they went down with the tide, were secured, and the rising tide then lifted the element. Fill was placed over the top of the completed tunnel so that the river regime was not changed. Standard surfacing was applied within the tunnel.