The contract was awarded to Sir Alfred McAlpine (Northern) Ltd in the sum of £1.85 million on 3^rd November 1975. The contract period was 14 months and the works consisted of the construction of the approach embankments for the proposed bridge carrying the M180 South Humberside Motorway over the River Trent.

The site of the works overlies alluvial deposits and was at one time a plant covered marsh which has been reclaimed by controlled warping, resulting in highly fertile Grade I agricultural land. It has, however, a very low bearing capacity and controlled rates of filling were specified to prevent failure. In addition, extensive drainage was carried out to reduce the time for pore water dissipation and more than 500 instruments were installed within the ground and the embankment to monitor movement and pressure changes. Up to 2 metres of settlement were anticipated. A drainage layer consisting of rockfill sandwiches between two layers of a permeable membrane formed the base of the embankment. The main core of the embankment consists of lightweight fuel ash surrounded by a bund of heavier material. The works were divided into three stages and include a cessation of filling between stages one and two. Stage one was generally 6 metres high and 8 metres in the future bank seat area. Stages two and three were only executed when the analysis of the instrument readings indicated that it was safe to proceed.

Associated with the vertical settlement, lateral movement was expected, and it was necessary to induce this movement before allowing piling for the bridge bank seat foundation to commence.

The adjacent West Butterwick Road was also raised, in the vicinity of the bridge, to provide additional weight and stability at the end of the embankment.

Also included in the Contract was approximately 1 kilometre of shallow embankment, overlaying a peat area where high settlement was anticipated and also a 1½ kilometre access track from Burringham Road.

Considerable strengthening and widening was carried out by Humberside County council, on behalf of the Department of Environment, on certain local roads, to cope with the import of material required for this and other M180 Contracts, and special measures included in the Contract to reduce the nuisance caused by the works.

It is of note that throughout the duration of the pre-construction soil survey, the existing water table was monitored at ground level, and the design of the embankment and instrumentation scheme was based upon this information. However, the water table when construction commenced was monitored at three metres below ground level, a result of the dry summer and mild winter of 1975/76. This drop in the water table had the effect of reducing the anticipated ground movements, and considerably improved the overall stability of the works when construction commenced.

The M180 crosses the River Trent about 2 miles upstream of the existing Keadby Bridge west of Scunthorpe.

The Trent Bridge was designed to carry the dual three-lane motorway over the River Trent and over two side roads, one either side of the river. The river is approximately 150 m wide and navigable as far as Gainsborough, the main river traffic being barges and coasters. At the bridge site there is a tidal range of about 5 m with current flows of up to 5 knots.

It was necessary for the bridge to be in service and during its construction to provide sufficient headroom and waterway for shipping. However, the soil conditions either side of the river consist of soft silty river deposits which restricted the height of the approach embankments to about 9 m. The depth available for the deck construction was therefore limited.

Two designs for the bridge were prepared to meet these constraints, one with a deck consisting of steel plate girders and the other of continuous pre-cast concrete post-tensioned glued segmental box construction to be erected using an overhead launching girder. Each design was of four spans with a pier in the centre of the river.

On the advice of the Hydraulics Research Station regular surveys of the river bed by the British Transport Docks Board were instigated, which were to continue throughout the construction period.

On 30^th August 1976 tenders were invited for the main Trent Bridge Contract. Tenderers were required to price two alternative designs namely the steel plate girder design and the pre-cast prestressed concrete glued segmental design. The Instructions for Tenderers drew attention to the mobile nature of the river bed material and the likelihood of erosion around any temporary works placed in the river bed.

The lowest tender was from Cementation Construction Ltd for the concrete design for the sum of £3,335,332. The 2 year contract was awarded to Cementation. Work started on 4^th January 1977 and the due date for completion was 1^st January 1979.

The design provided for a symmetrical bridge of four spans of 48.5, 85, 85 and 48.5 metres. Each carriageway supported by an independent superstructure of twin concrete box girders varying in depth, from approximately 4.9 m at the piers to 2.1 m at mid centre spans and 1.7 m at the bank seats. Each box girder is made up of 91 pre-cast concrete units 3 m long and 4 m wide with 2 m deck slab contilevers. The units vary in weight from 35 to 75 tonnes.

The units to be constructed by counter casting one unit against another to form the faces to be joined. The joint faces have shear keys of corrugated profile on the webs to carry the shear force at the joints before the adhesive cures.

The box girders were designed to be built by the 'balanced' cantilever method of construction using a launching girder to position the units. The unit over a pier is first erected and then units added to each side in turn to complete the balanced cantilevers. The ends of these cantilevers are then joined at mid centre spans and extended at the bank seats onto the bearings to form a continuous box girder.

The units are joined using epoxy resin adhesive and are stressed together first by temporary prestress consisting of 6 or 8 No. 36 mm diameter Dividag bars in the top and bottom flanges and then by permanent prestress following some 2 units beyond the advancing ends of the balanced cantilevers. The permanent prestress consists of two 19/15 dyform strand tendons in the top flanges of all except the last cantilevered units. There are a total of 26 permanent prestress tendons per balanced cantilever. The temporary prestress is removed in stages.

The epoxy adhesive is spread on both faces of the joints to a thickness of 1 to 2 mm. Whilst initially performing the function of a lubricant to assist in the mating of the units it eventually cures to provide a water and grout tight gasket at the joints and to develop sufficient strength to transmit shear and comprehensive forces across the joint in conjunction with the permanent prestressing.

The ends of the balanced cantilevers at the mid centre span positions are joined by in situ mass concrete nominally 200 mm long and continuity is achieved by 10 top and 4 bottom permanent prestress tendons. At the bank seat two units extend the balanced cantilever onto the bearings and continuity is formed by 6 top and 6 bottom permanent prestress tendons.

Adjacent box girders forming the superstructure to one carriageway are joined by in situ concrete bonding the reinforcement projecting from the ends of the deck slab cantilevers.

In situ reinforced concrete at parapets and central reserve complete the deck structure.

The substructure consists of reinforced concrete piers and bank seats founded on steel piles driven to set in the marl. Steel H-piles support the three piers and east bank seat.

At the west bank where horizontal ground movements was anticipated in the silt and peat layers the bank seat is supported on vertical tubular steel piles, which give uniform horizontal strength in all directions, and is restrained from horizontal movements by an anchor block cast in the embankment and coupled to it by 7/13 dyform strand tendons. The centre river pier has a specially shaped base and sheet pile skirt to minimise the effects of scour and is designed for vessel impact. The shape was determined on the basis of tests carried out at the Hydraulics Research Station.

The carriageway surfacings were to consist of a 40 mm thick hot rolled asphalt wearing wearing course with 20 mm coated chippings on a hot rolled asphalt regulating base course nominally 40 mm thick, all on a proprietary approved waterproof membrane.

The drainage was by continuous entry hollow cast iron kerb units. Navigation lights were to be fixed to the centre and west piers and over the navigation channel.

There were a substantial number of problems encountered with the centre pier cofferdam which lead to claims from the contractor and delays to the programme.

Meanwhile the first five pre-cast units had been cast at Dowmac's works although remedial works to them was necessary. Dowmac Concrete which was awarded the £1 million contract for the supply of the 364 deck units had to devote a considerable area of its prefabrication yard to storage of the finished units and the two precasting moulds.

The launching girder had been delivered to site in late September 1977 and assembled. However the Contractor experienced considerable difficulties in commissioning the girder and it was not until 28^th February 1978 that the first concrete deck unit was erected i.e. 19 weeks behind programme. Further problems with the process of erection, and Industrial action both Nationally and locally caused further delays and by mid November 1978 the work was 35 weeks behind programme and progress was bedevilled by mechanical failures and strikes. Throughout the winter there were further delays owing to the abnormally bad weather.

By the end of June 1979 all 364 No. units had been erected. Completion of the finishing works allowed the westbound carriageway to be opened to traffic on 20^th July 1979 by Kenneth Clarke MP Parliamentary Secretary for Transport. This was followed by the completion of the eastbound carriageway on 26^th October 1979 and the opening to traffic on 31^st October 1979, some 43 weeks late.

Despite the trials and tribulations of both Contractor and Engineer this splendid bridge enables a quick crossing of the River Trent and the tedium to motorists of the bottleneck at the old Keadby Bridge long forgotten.