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River Dee Viaduct
River Dee, north of Halton, Wrexham, Wales, UK
associated engineer
Travers Morgan & Partners
date  1987 - 1990, opened 8th June 1990
era  Modern  |  category  Road Viaduct  |  reference  SJ298409
ICE reference number  HEW 1782
It is claimed that this modern reinforced concrete structure is the highest road bridge ever built over a British river. The five-span viaduct carries the A483 Newbridge bypass over the River Dee in north Wales. Its graceful form gained Royal Fine Art Commission approval and won a 1991 Concrete Society award.
Almost 20 years elapsed between the initial studies for the viaduct and its completion. In August 1970, Travers Morgan was appointed by the Welsh Office Highways Directorate to undertake a feasibility study for improving the A5 and A483 south of Wrexham. In July 1972, the consultant reported on 34 different routes and configurations. Public consultation in June 1974 endorsed the present route of the A483, including the River Dee Viaduct.
The viaduct crosses a deep gorge flanked by thick woodland. Its inaccessible location forms part of the River Dee & Bala Lake Site of Special Scientific Interest. In addition, site investigations and geological mapping revealed complex geology and geomorphology, with faulting and the potential for slip circle failures. Piezometers and inclinometers were installed over the site to monitor conditions during excavation and after construction.
The constraints of the site led to an economic and durable structure consisting of a continuous post-tensioned reinforced concrete deck of varying depth supported by slender reinforced concrete columns. The geotechnical issues, difficulty of access for construction machinery and aesthetic considerations led to the choice of a viaduct with five spans and four piers.
The viaduct is 348m long and 57.3m above river level, with spans of 55.5m, 77m, 83m, 77m and 55.5m. The largest (central span) is over the river. Overall deck width is 12.3m, with aluminium parapets and 1m hard strips on either side of a single 7.3m wide carriageway.
Excavations up to 20m deep were required for the footings carrying the pier loads down to competent bedrock. At each pier, reinforced concrete pad foundations 4m thick sit above mass concrete bedding up to 6m thick. Before pier excavation could begin on the south side, probe drilling and pressure grouting of voids was undertaken in the shallower underlying coal seams. A total of 100 ground anchors, each of 50 tonne capacity, were installed uphill of Pier 1 (the northernmost column) and Pier 3 (south bank of the River Dee) to increase the long term stability of the gorge sides.
The north abutmentís pad foundation is similar to those on the piers, though shallower. The south abutment has a piled foundation of 1m diameter bored concrete piles cast in situ, which carry the loads 30m down to a level below potential slip planes.
Pier 2 (north bank of the river) is the tallest of the four rectangular hollow reinforced concrete columns at 46.7 m high. Piers 2 and 3 are integral with the viaduct deckís river span and act together as a portal frame ó slight flexing of the columns absorbs any thermal movements.
The columns were cast in 4m high lifts using a purpose-built climbing shutter. The concrete incorporates a 30 percent pulverised fuel ash cement replacement.
The superstructure is of post-tensioned prestressed concrete, cast in situ as balanced cantilevers. In cross section the deck is a single-cell box girder, 6m wide and of web depth varying from 3m to 5.7m. In elevation, it has straight haunches above the pier columns.
Travers Morganís original design envisaged construction of the viaductís deck using the balanced cantilever method, with a maximum cantilever of 35m. On the three main spans, this would have required inserting reinforced concrete infill segments (7m and 13m long) mid span to complete the superstructure. Contractor Edmund Nuttall Ltd commissioned engineer Robert Benaim & Associates to design an alternative, with a view to simplifying the construction and achieving overall cost savings.
Between December 1987 and the end of February 1988, Benaim completed a superstructure redesign. Travers Morgan checked and approved the alternative design, which was then accepted by the client.
The first stage of deck construction was to form a section of deck above each pier. Over the outer piers, where pairs of 2000 tonne capacity sliding bearings accommodate movements, the sections were fixed temporarily to the pier columns by high-tensile anchorages to prevent rotation during cantilevering.
The remainder of the deck was constructed in pairs of equal segments 3.5m long extending outwards from the pier sections. Each segment was cast in one continuous pour. Once the concrete had reached half the design strength, the two segments were tied together through the pier section by tensioning two multi-strand tendons and the formwork moved outwards for the next pour.
The cycle was repeated until almost mid span, where T-shaped bridging elements were used to link the two cantilevers with 3m long in situ concrete stitches. Stressed continuity tendons ensure the deck's structural integrity. Horizontal movements are accommodated by 200mm sliding joints in the deck at each abutment.
The design also takes account of future maintenance and repairs. More than 1.2km of stainless steel rolled channel sections were cast into the underside of the deck, providing a fixing for rails to carry a travelling gantry for inspection and working.
On 8th June 1990, the River Dee Viaduct was opened by the Minister of State for Wales, Wyn Roberts (1930-2013), the Member of Parliament for Conwy 1970-97. It contains some 30,000 tonnes of concrete, 2,000 tonnes of steel reinforcement and 125km of 12mm diameter prestressing strands.
In 1991, the viaduct won the Concrete Society Award in the civil engineering category. The judges commented that the project was "carefully conceived, detailed and executed".
Testing engineer: Harry Stranger Ltd
Resident engineer: D.M. Price
Contractor: Edmund Nuttall Ltd
Contractor's engineer: Robert Benaim & Associates (superstructure alternative)
Bored piling: DMD (Piling) Ltd
Stressing system and bearings: PSC Freyssinet Ltd
Concrete supply: Steetley Construction Materials Ltd
Stainless steel channel: Halfen
Gantry formwork: Newton Formwork Design Ltd
Research: ECPK
bibliography
"River Dee Viaduct" by David Price, in Concrete Quarterly, British Cement Association, Slough, pp.5-7, Autumn 1990
http://jncc.defra.gov.uk
www.ciht.org.uk
www.concrete-awards.org.uk
www.ice.org.uk
www.icevirtuallibrary.com
reference sources   CEH Wales
Location

River Dee Viaduct