ORWELL BRIDGE

Nacton, Ipswich IP10 0NZ

The high-level Orwell Bridge carries the A14 dual carriageway (the Ipswich by-pass) across the Orwell valley. The bridge has a length of 1287 metres (about four-fifths of a mile) and is 24m (80 feet) wide. At 190m (623ft) its central span is the longest single span of prestressed concrete in the UK. There are more than 100,000 cubic metres of concrete in the bridge and its foundations. The superstructure consists essentially of two prestressed concrete box girders, each of which carries one carriageway of the road. The girders are continuous throughout the length of the bridge, and the only expansion joints are at each end of the bridge. The superstructure is supported by nineteen sets of reinforced concrete piers which range in height from 20m at the ends of the bridge to 45m at the central span. The piers rise from groups of concrete piles 1 m in diameter formed in the valley bed to a maximum depth of 40m.

A bridge was selected for the crossing because it would cost less than a tunnel and could be built more quickly. It had the further advantage that it could be constructed in a way which allowed the shipping activity of the port to go ahead unhindered while it was being built.

In order to make the least intrusion on the landscape of the Orwell estuary, while achieving the highest transport benefits, the route of the by-pass (the A14) was located as close as possible to the southern edge of Ipswich, so that it adjoins the industrial development of the port, container terminal and power station. The actual line of the bridge was set at an angle to the river so as to gain the best relationship to the surrounding terrain.

Two other basic design factors were the height and width of the central span of the bridge above the main navigation channel into the port of Ipswich. The dimensions of this span had to give adequate clearance for the largest vessels to use the port, both now and in the future. The higher the bridge, the more emphatic would be its impact on the estuary environment. Following discussions with the Port of Ipswich Authority and other relevant interests, the lowest height consistent with navigational requirements was adopted. As a result, the central span provides a clearance of 39m (128ft) above high water spring tides. The width of the span (190m) also was determined by shipping requirements.

Since it was essential that port activities should not be disrupted, the engineers recommended a form of construction which would enable the part of the bridge above the river to be built at high level and without interfering with shipping movements. The method involved constructing the navigation span and adjacent anchor spans by a system of balanced cantilevers, so as to build them out progressively in corresponding sections on either side of a pier. The piers and the spans were to be formed from concrete cast on the spot. This form of construction, using concrete box girders, secured a high degree of rigidity at an economic cost.

To ensure that the bridge could be built as cost-effectively as possible, its design was based on modular principles which standardised the size and shape of the largest structural elements and allowed construction to proceed in a regular cycle of operations.

As well as possessing an efficient and functional structure, the bridge had to be designed with an eye to its setting within the landscape of the Orwell valley, and particularly its effect on views along the estuary. The aim was to achieve a form that was both simple in its outlines and sympathetic to the valley contours. Frederick Gibberd Partners were commissioned to provide the necessary specialist advice on the architecture of the bridge and the landscaping of its approach roads. The resulting design received the approval of the Royal Fine Art Commission.

Sir William Halcrow & Partners were the consulting engineers responsible for the Orwell crossing; moreover, Thomas Telford was the Resident Engineer. Thomas was the great-great-grandson of another (somewhat more famous) Thomas Telford, the Scottish civil engineer (1757-1834).

The contract to build the bridge was won by Stevin Construction BV, who started work in October 1979. The project took 38 months: the first year involved work on the bridge foundations; in the second year, the foundations were completed and the piers and western part of the bridge were built; the third year saw the construction of the longer eastern part of the bridge and the completion of the detailed superstructure. At the peak of the construction work, the contractor's labour force totalled about 300, almost all of whom were recruited locally. The bridge was completed in 1982.

All parts of the bridge structure have been designed to ensure easy access for routine inspection. The box girders, which are hollow, contain works utilities such as power, telephone and lighting. The girder which carries the westbound carriageway also conveys a 711 mm water main of the Anglian Water Authority across the Orwell valley.

Like any large civil engineering project the bridge was contentious at the time.  However, the Orwell Bridge has become an icon of Ipswich – even though the western half is outside the Ipswich Borough boundary.. The gently curving bridge symbol has been used by many local organisations and companies.

All photographs are taken from The Ipswich Society Image Archive.

[Source: The Motorway Archive]

Links: The Motorway Archive; Ipswich Society Image Archive