A2/A282 Dartford Improvement Scheme

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Architect

JACOBS

Steelwork Contractor

FAIRFIELD-MABEY LTD

Main Contractor

COSTAIN

Client

HIGHWAYS AGENCY

The Highways Agency identified the need for a major road improvement programme at the A2/A282 intersection in north Kent. The aim of the project was to divert traffic away from the roundabout sitting below the M25 and above the A2. This was achieved by constructing a link to carry traffic from the A2 westbound onto the A282 northbound, and another from the A282 southbound to the A2 eastbound. A third dedicated lane on the A2 westbound funnels traffic to the southbound M25. These link roads required four new bridges to carry traffic over the existing highways.

More than 4,000t of weather resistant steel was used for the construction of the four new structures. One of the main considerations was that the bridges would need as little maintenance as possible due to access constraints. With weather resistant steel the rust ‘patina’ develops under conditions of alternate wetting and drying to produce a protective barrier, which impedes further access of oxygen and moisture and eliminates the need for it to be painted with safety, environmental and cost benefits.

The four structures consist of the main east to north link flyover, which is a 420m long nine span viaduct with more than 1,800t of steel. Two parallel five span bridges of 1,000t of steel each are both 250m long and run on either side of the existing A2. The fourth and final structure that was erected required 230t of steel and carries southbound A282 traffic over a small slip road before linking up with the A2.

Work on site for the east to north link flyover began in January 2007, with erection in February. Some sections were assembled on site with the majority of steelwork being lifted using a 1,000t telescopic crane. In May, a critical milestone was reached with the final section of the structure lifted in overnight to link the A2/A282 east-north viaduct. The middle section of the bridge was erected first, followed by the remaining spans toward the south abutment, completing six of the nine spans in one sequence. The remaining spans toward the north abutment were erected thereafter. The nine bridge spans carry the structure over eight piers. Five of the concrete formed piers have four columns while the three middle piers only have two columns. The middle three spans traverse open ground so there was enough room for temporary trestles to support the steelwork during erection.

All nine spans of the east to north link flyover vary in length from the longest at 59m to the shortest at 38m. The structure is curved in both plan and elevation. Steelwork for the bridge deck consists of two braced pairs of girders for each span which were paired on site and lifted into position using the AK680 crane. Cross members are situated at 8m intervals. Each pair of girders consists of four steel sections which were bolted together on site and prepared for erection.

At the design stage it was decided to fit the GRP permanent formwork onto the preassembled steelwork at ground level, which reduced the amount of working at height and meant fewer road closures. Seven optimum positions for one crane to lift all of the bridge decks into place during the steel erection process were worked out. The concrete deck was then poured following on three spans behind the steelwork.

The A2/A282 scheme was completed in December 2007, five months earlier than expected.

Judges’ Comment

s:

A large, busy interchange with one large bridge and three smaller ones. The composition of the bridges, with consistent architectural language, works well in the gently rolling landscape. The plate girder spans were pre-assembled with the decks, and exceptionally heavy lifts minimised traffic disruption.

The innovative use of weathering steel on this scale is practical and appropriate.