All hands on deck

Why read this

Major strengthening of a motorway bridge

Difficult access calls for manual handling of 500t of materials

Extensive use of radar

Looking across the Medway valley, the view is spectacular. The whole area is a hive of construction activity. Both the A2/M2 widening and CTRL projects converge at the river and are now erecting new bridges. From 30m up, below the deck of the existing M2 Medway Bridge, plant and tower cranes can be seen in all directions.

The M2 Medway Bridge deck is undergoing £12M strengthening works - it will carry traffic bound for the coast, with vehicles headed for London crossing the Medway on the new motorway bridge now under construction. The Nuttall and Freyssinet teams carrying out the work are getting quite used to the view and the climbing involved. And the crawling.

Workers climb up a flight of ladders to the deck and then into the long balanced cantilever box sections.

Ninety-six prestressing tendons are being retrofitted to the bridge's 12 box section cells while traffic thunders overhead.

Anchor blocks and diaphragms are cast into the box sections to attach the tendons, using 1,000m 3of C50 low shrinkage concrete. The 168 anchor blocks are attached to the existing structure with 2,500 Macalloy tie bars.

Over 500t of materials, steel, formwork and scaffold have had to be manhandled up to 150m through these cramped box sections via 1m 2openings cast into the webs and diaphragms. Nuttall project manager Paul Fewtrell climbs up twice a day to inspect the work, and is clearly proud of his 120 strong team: 'It's been sheer physical toil - they've done a great job, ' he says.

Concerned by the vast scale of manual handling required, Fewtrell called in the Health & Safety Executive. Scrutiny of the project confirmed that there was no way round it, though. Happily, the tally of injuries so far comes to no more than a few trapped fingers.

Designed by Freeman Fox and built in 1963 by a Kier/Christiani & Nielsen joint venture, Medway Bridge was the longest cantilever structure of its kind at the time. It consists of two completely separate, twin balanced cantilever spans for the north and southbound carriageways, with a single viaduct at each end carrying both carriageways. The 61m cantilever and 93m anchor spans are balanced over two river piers; the cantilever spans connected by 30m drop-in suspended section.

The Maunsell designed project is about halfway through.

Strengthening is being carried out as part of a 15 year rehabilitation plan and will stiffen the deck to cope with increasing freight traffic on the busy Dover/Channel Tunnel to M25 route. Tendons are now in and stressed up to 300 tonnes on the first cell, which has caused the cantilever span to rise by 70mm.

Stitching the Macalloy tie bars through the existing structure has been a major technical challenge. Radar was used to detect the existing rebar and prestressing tendons before diamond drilling. Existing tendons were inspected for corrosion and voids by using an endoscope down the drill hole. 'Locating all the existing rebars has been technically difficult, ' says Fewtrell. 'We've cut one or two.'

Strengthening is due for completion at the end of the year, when another contract will start to stitch the two balanced cantilevers together, forming a single deck for the four-lane M2 northbound carriageway.