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Conwy Immersed Tube Tunnel
River Conwy, Morfa Drive, Conwy, Wales, UK
Conwy Immersed Tube Tunnel
associated engineer
Travers Morgan & Partners
Christiani & Nielsen A/S
date  November 1986 - 1991, opened 25th October 1991
UK era  Modern  |  category  Tunnel  |  reference  SH785782
ICE reference number  HEW 2069
photo  © G Laird and licensed for reuse under this Creative Commons Licence
Britain’s first immersed tube tunnel takes the A55 North Wales Expressway under the River Conwy. It was constructed as a bypass to the town of Conwy, reducing traffic on the upstream road bridge. The casting basins from its construction have been converted into a marina and a bird sanctuary. At the time, the crossing was part of the UK’s largest ever roadworks contract.
The tunnel is the fourth extant river crossing at Conwy. Its inception can be traced back to 1966, when the Welsh Office initiated a study to consider the problem of increasing congestion in Colwyn Bay and Conwy. The resulting 1968 report concluded that the existing A55 needed to be replaced by a major road traffic route.
In 1969, Travers Morgan was appointed to undertake a feasibility study of routes between Aber in Gwynedd and St Asaph in Denbighshire, a distance of some 45km. In July 1972, the preferred option was announced — a two-lane dual carriageway following a coastal route. A public enquiry followed, between May 1975 and February 1976.
The route for crossing the estuary was subject to a more detailed study, published in March 1980. Further consultation on the potential alternatives was undertaken, and a tunnel was promoted from July 1980. In spring 1982, an immersed tube tunnel was selected as causing less disturbance to the complicated estuarine geology than a bored tunnel, which would have had to be longer and deeper.
Though the more expensive option, a tunnel was chosen over a bridge for environmental reasons and to preserve views of the town's medieval castle, narrow streets and historic bridges. Conwy Tunnel bypasses the town, and is situated less than 1km downstream (north) of a trio of other bridges — Thomas Telford's (1757-1834) Conwy Suspension Bridge of 1826, Robert Stephenson's (1803-59) Conwy Tubular Bridge of 1848 and a modern road bridge constructed in 1958.
On 22nd March 1983, the Conwy Tunnel (Supplementary Powers) Bill was passed by the House of Lords. In September 1986, the tunnel tender was awarded to a joint venture between Costain and Tarmac as part of their 6km contract of dual two-lane expressway from Glan Conwy Interchange to Penmaenbach Head. Engineer for the project was Travers Morgan in association with Christiani & Nielsen. Construction commenced in November 1986.
The bedrock of the estuary is a mix of Silurian and Ordovician mudstones, siltstones and sandstones. Above the bedrock is stony north Wales till, overlain successively by glacial lake deposits, stiff Irish Sea boulder clay and alluvial granular deposits. The estuary’s tidal range is up to 6.5m at spring tides. Mathematical and physical modelling of the estuarine hydraulic regime was carried out to determine the optimum strategy for placing the immersed tube.
The 1.09km Conwy Tunnel carries two lanes of road traffic in each direction, separated by a full-height dividing wall. It consists of a 260m eastern approach, constructed using a cut-and-cover technique, a central 710m immersed tube and a 120m western approach, also constructed in cut-and-cover.
The immersed tube section is formed from six reinforced concrete units, precast inside a bunded cofferdam basin, 300m by 200m, at Morfa Conwy on the west side of the estuary. Each unit is 118m long, 24.1m wide and 10.5m high, and weighs 30,000 tonnes. In section, two chamfered rectangular eight-sided bores form the passageways for the roadway.
Steel plates 6mm thick, with integral shear studs, are fixed externally to the sides and bases of the units. The steel is coated with solvent-free tar paint and has zinc-anode cathodic protection. The roof of each unit is sealed externally with a polyester fibre-based bituminous membrane topped by 150mm of reinforced concrete.
Individual units are fitted with internal ballast tanks and made watertight by steel bulkheads at either end. The tanks were filled and the casting basin flooded to 1m above the roof of the units, allowing them to be checked for leaks.
The bund around the basin was then breached and the units floated into the estuary on a rising tide. They were towed into position by pontoons and sunk into the river bed on a falling tide. The bed had been pre-dredged to form a trench some 10-20m deep, and the units were settled onto concrete pads. Placement was carried out to a tolerance of only 10mm.
Once in their final positions, following a north west to south east curve along the river bed, the units were joined together underwater and the roadway through them completed. Sand was injected to fill the voids beneath the tube and graded backfill placed around its sides and top to fill the trench. The operation was completed with a protective covering of rock armouring.
The cut-and-cover sections and their associated approach structures, in the form of U-shaped articulated rafts, were constructed close to their final locations within bunded cofferdams over cut-off walls (grout diaphragms). They were placed using a method similar to that used for the immersed tube.
The casting basin bunds were formed from granular material dredged from the estuary, with their outer faces protected by rock armour. Open excavation for the approach to the east portal was protected from water ingress by a sheet-pile cut-off wall driven into boulder clay, which has been retained in the permanent works.
The finished tunnel is illuminated by 2,600 58W single and twin fluorescent lights and more than 1,100 luminaires of higher wattage, all with end access for maintenance. The computerised lighting system is designed to save energy and improve visibility. In addition, traffic movement is monitored from a control room, detecting stationary vehicles inside the tunnel.
On 25th October 1991, Queen Elizabeth II officially opened the Conwy Tunnel.
The overall contract was let at a tender price of £102m, and included a price fluctuation arrangement. The final cost of the project was £144m. The six immersed tube units contain some 64,000 cu m of concrete, with 26,200 cu m of concrete in the east cut-and-cover and 12,500 cu m in the west cut-and-cover.
Surplus spoil excavated from the casting basins and dredged from the tunnel trench was deposited south east of the modern road bridge (SH784775). It has been used to reclaim parts of a tidal area, now an bird sanctuary called Glan Conwy Nature Reserve (SH799771).
The basin where the tubes were prefabricated, to the north of the tunnel’s west portal, is now Conwy Marina (SH774787), which opened in 1992. The original cofferdam has been retained to form its walls. It is the largest marina in Wales, with 500 pontoon berths.
In January and February 2013, the tunnel’s lighting system was upgraded through the westbound bore. Work included installing 2,500 LED lamps, 4km of cabling and 3km of supporting steelwork. The traffic incident detection system was also upgraded. In January and February 2015, a similar programme was carried out in the tunnel’s eastbound bore. The new lighting reduces the crossing’s carbon footprint considerably.
Resident engineer: P.A. Stone
Contractor: Costain Tarmac Joint Venture
Contractor's agent: John F. McFadzean
Mechanical and electrical services: Mott MacDonald
Hydraulic model tests: Delft Hydraulics Laboratory
Dredging: Zanen UK
Piling and diaphragm walls: Bachy Bauer Consortium
Tunnel floating and immersion: HBM
Steel membrane: Pipeline & Energy Contracting
Lighting: Siemens Lighting Ltd
Research: ECPK
"Siemens light Conwy Tunnel", in The Journal of the Institution of Highways and Transportation & IHE, Vol.39, No.4, April 1992
"Immersed tunnel techniques", Proceedings of the conference organised by the Institution of Civil Engineers, Thomas Telford Ltd, London, 1990
"Costain Tarmac Joint Venture: A55 Conwy Crossing", in The Journal of the Institution of Highways and Transportation & IHE, A55 special issue, Vol.35, No.11, November 1988
reference sources   CEH Wales

Conwy Immersed Tube Tunnel