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Three way stretch

Bridges Medway

Motorway traffic flowed across the new Medway Bridge this month. Is this the end of the story? Far from it, as Dave Parker reports.

There could have been just one new superbridge across the navigable River Medway upstream of Rochester in Kent.

Or one new and one old, according to the Highways Agency's structural engineer FaberMaunsell associate Alan Toms. 'When it became obvious that the widening of the A2/M2 meant increasing the capacity of the crossing to four lanes in each direction, we looked at a number of options, starting with straightforward widening of the original Medway Bridge.

'But what made this project unique was the fact that the Channel Tunnel Rail Link would be crossing the valley on almost the same alignment.'

Widening the old bridge was soon rejected on the grounds of foundation inadequacy. Several options involving a combined motorway/CTRL crossing were also considered, including a massive double deck structure with eight lanes of motorway above the high speed rail and a design which featured four lanes of motorway alongside the rail line on a single new structure.

In the end a triple crossing won the day, featuring a strengthened and remodelled old bridge, a new bridge for the CTRL and, slotted in between them with just 20m each side to spare, a new road crossing carrying four lanes of London-bound traffic.

Both the new crossings would be essentially similar to the first, box girders with related pier spacings and decks on virtually the same level. The illustrative design prepared by FaberMaunsell for the 950m long bridge featured post-tensioned concrete construction with 12 spans, the main river span measuring just over 152m. Atkins, structural engineer for winning JV contractor Costain-Skanska-Mowlem, produced a final design which was very similar if somewhat simpler, says CSM project manager Tony Scutt.

'We went for a very straightforward single twin cell box girder right through, with minimal haunches and chamfers to simplify formwork. At 20m wide, this is a pretty large box girder, which is one reason for choosing insitu construction.'

Another was logistics. By the time the CSM team arrived on site in January 2000, Nuttall was well advanced with the CTRL crossing alongside. Two railway lines ran along the valley side, one a busy commuter route into Victoria. Building the viaduct above these would pose the usual challenges to the site team.

For environmental reasons foundations for the river piers had to be bored, not driven.

These were massive; 2.4m diameter and going down 25m to the underlying chalk. On top were equally massive pilecaps, each containing 2,000m 3of high strength concrete.

'We asked a lot of the concrete mixes on this job, ' reports CSM construction manager Charlie Ball. 'The foundation mix has 80% OPC replacement by ground granulated blastfurnace slag (ggbfs).

'On the superstructure we were generally looking for a compressive strength of 38MPa at 36 hours from a C50 mix - which meant a 'summer mix' with 60% ggbfs replacement and a 'winter mix' with 50%. And as we were pumping up to 150m horizontally and 40m vertically there was lots of water reducing admixture in there as well.'

Balanced cantilever construction was again chosen for the main river spans, with pours of 80m 3occurring simultaneously at each end of the growing cantilever. One special design feature here was the encastré joint formed by continuation of the river piers right up to deck level, creating a 'zero section' from which the cantilevered boxes could spring.

Viaducts were largely constructed on falsework - except above the two rail lines. Here the working platform was provided by two enormous trusses weighing up to 1,000t each, which had to climb up the piers each side.

The inevitable deflection of the trusses under load was to cause problems, as Scutt reports. 'We were only allowed 1MPa of tensile force in the box girder before stressing, which was hard to achieve. The trusses had to be set with 140mm precamber to keep the crack width below a design value of 0.1mm'.

Given a free choice, CSM might not have opted for external prestressing tendons (see box), but made the best job of it possible. Hendy says the key element in the prestressing was the design of the deviator pipes, some 1600 in total.

'There's always a risk of spalling around the deviator pipes. We minimised this by using medium density polyethylene pipes, which are cheaper, corrosion resistant and easily bent.'

The result is the UK's - and very nearly the world's - longest post-tensioned bridge with external tendons. With all but surfacing work effectively complete on the new crossing the CSM team is now gearing up for the next stage - which could be equally challenging (see box).

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