The Wynhol Viaduct, carrying the M5 motorway over the southern slopes of the Gardano Valley near Bristol, is an example of well-prepared detailing and experienced construction combining to produce a neat, functional and economic structure which does not detract from its surroundings and has required little maintenance.
To minimise disturbance, the motorway was sited on the steeply cross- falling upper slopes of the valley. Rock excavation was minimised by splitting the carriageway levels, with the southbound up to 10m above the northbound. Most variations in ground level were accommodated with varying cut and fill.
Prestressed-concrete trapezoidal box decks 18m wide with 56m pans were developed. Initially, the design team thought that these might be segmentally precast, transported and erected in balanced cantilever construction with a specialist erection girder resting on the piers. Difficulties with transportation and availability of equipment discounted this approach. Limited access and space also ruled out precasting on-site.
Therefore, the decks were cast in-situ on temporary falsework. Spans typically 57m long were adopted - 10 for the upper southbound viaduct, nine for the lower northbound viaduct - with twin-cell 18m-wide 3m-deep box decks.
The resulting design produced a structure that is aesthetically pleasing, durable and, not least, economic. Its construction cost, including variations, of £80/m2 represented excellent value even in 1971.
OWNER Highways Agency
MAIN CONTRACTOR Kvaerner Construction, (formerly Cementation Construction)
CONSULTING ENGINEER Hyder Consulting, (formerly Freeman Fox & Partners)