Striding over the urban obstacle course that lies between the A13 and the Thames near Dagenham, snaking through the grounds of the Ford car plant, the 25.5M, 1.75km Dagenham Dock viaduct has just passed a crucial milestone in its 110 week construction programme. Deck precaster and erection contractor DMD has cautiously edged its massive yellow launching gantry beneath high voltage power lines only a few metres above and set it up again further to the east.
Soon DMD will continue launching precast deck segments, at a rate of at least 18 a week. Main contractor Tarmac has already completed more than 70 elegant 'wineglass' piers in advance of the gantry's progress. With several road and rail crossings already negotiated and several more lying ahead, the viaduct is progressing at the programmed rate - although work started 20 weeks late due to delays in service diversions.
The rest of Tarmac's 51M, 2.9km Heathway to Thames Avenue contract, the third in the current upgrading of the A13 between the M25 and the dual three lane section west of Dagenham, is also going well, with completion due in May next year.
But the viaduct has one fundamental difference, as Tarmac senior project manager John Franklin explains. 'The viaduct is built to our own alternative design prepared by Robert Benaim & Partners. Apart from some retaining walls, everything else is being built to the fully detailed conforming design prepared for the Highways Agency by Hyder. This includes a major interchange at the western end of the contract. But the viaduct is quite different from Hyder's original.'
Confronted with obstacles including the London to Southend railway line, the route for the projected Channel Tunnel Rail Link and the Dagenham Breach - a small lake popular with local anglers - Hyder opted for a multi- span viaduct with relatively short spans of around 30m. An insitu concrete deck was supported by plate girders spanning between slender concrete columns. In some places, however, particularly where the viaduct bridged the railway line at a 45degrees skew, longer spans of up to 60m were needed. In all there were 57 sets of columns along the route.
'Our tender for the project based on this design was the lowest submitted,' Franklin reports. 'But we also put forward a couple of alternatives which were significantly cheaper.'
These had been prepared by Tarmac's structural engineer Robert Benaim & Associates, and included a glued segmental precast post-tensioned concrete box girder design. Benaim associate David Collings explains:
'Our reservations on the conforming design were focused on the very large insitu concrete deck. We did put forward an alternative composite design with fewer, larger, plate girders, but the problem was still the insitu deck.'
Benaim, and Tarmac, preferred a largely precast solution, believing this would lead to higher quality and greater potential long term durability. Benaim's first version stuck to the conforming design's pier spacings, but then a longer span alternative was preferred.
Says Franklin: 'This suggestion came from DMD, who pointed out that the large range of spans was not really favourable to precast construction. By going for spans generally twice as long, we obtained significant advantages in terms of repetition and substructure costs.'
With Tarmac and DMD forming what Franklin describes as a 'quasi joint venture' on the viaduct section, Benaim acted as consultant for both, carrying out the detail design of the deck for DMD and the substructure for Tarmac. 'Ground conditions along the route are very variable,' Franklin goes on. 'Benaim was able to achieve significant savings by designing each pier foundation individually. And cutting the number of piers founded actually within the Breach was another big saving.'
In the final twin box girder design, the 31 spans still range from 30m to 66m, but for the vast majority the variability is between 55m and 65m. Individual segments, however, come in only two lengths, either 3.5m or 3.8m long. In all, more than 1,000 individual sections will be produced - including those for the triple box sections needed at each end of the viaduct to accommodate sliproads.
Benaim would have preferred to use bonded internal tendons to prestress the box, Collings says. 'Most similar structures elsewhere in the world use bonded tendons, but the Highways Agency still doesn't permit them for precast decks because of a perceived risk of corrosion.
'This reduces the efficiency of the post-tensioning, particularly on relatively shallow 2.5m deep sections like we have here, because you can't have tendons inside the flanges.'
External tendons enclosed in grouted plastic sleeves running inside the box girders were used successfully on the Second Severn Crossing, the only other comparable structure in the UK. 'But that had a much deeper deck section, so the percentage reduction in effective depth was much less,' Collings points out.
The real innovation at Dagenham, however, is the elimination of expansion joints along the entire length of the viaduct. 'We believe it could be the longest structure of its type in the world without movement joints,' Collings reports.
'Doing without them obviously saves on long term maintenance costs, and the disruption that replacing joints would cause.'
Instead, most of the piers have either free or guided bearings which allow the curving box to expand and contract in such a way that movement at the abutments is at right angles to the conventional movement joints between the deck and the abutments. Here, up to 800mm of thermal movement is possible. Out near the midpoint, the minimal movement which does occur is taken up by the flexibility of four sets of piers with fixed bearings.
Benaim achieved further useful savings by stressing the post-tensioning tendons to the higher limits permitted by a new Eurocode - more than 5% higher than in the old British Standard. Specified concrete grade is C50, and construction is by the balanced cantilever technique, using a launching gantry bought 'off the shelf' from Canada.
Insitu concrete had to be specified for the 'string courses' - edge beams - along the viaduct's outer edges, and more insitu concrete stitched the boxes together longitudinally, but overall, the long span alternative design offered a 2.5M saving over the complying original. After checking by Hyder, the consultant adopted Benaim's design, taking on design risks and producing maintenance manuals and so on.
By February 1997 DMD had set up a complete precasting factory close to the site, fed by the same Tarmac Topmix concrete plant that supplies insitu concrete to the rest of the project. Sea-dredged aggregates were chosen, and after a few early problems with concrete finishes, segment production has been going well, says DMD site agent Gerry van der Wal.
'We're achieving strengths of 12N/mm2 at 12 hours and 65N/mm2-75N/mm2 after 28 days. Segment design is quite complex, driven by the need to keep segment weight down below 100t, but we're now turning out units at a rate of around 16 every four day 'casting week'.'
Although 10% of the high Ordinary Portland Cement content has been replaced with ground granulated blast furnace slag, the use of heated mixing water means shutter striking times have been kept down to 14 or 15 hours even during the coldest weather.
Even after DMD hands the completed viaduct structure over to Tarmac, the main contractor will still have around 5M of work completing the insitu concrete, installing barriers and lighting and surfacing. Franklin reports that Tarmac has put forward its own high performance Masterpave as an alternative to the hot rolled asphalt wearing course originally specified for the whole length of the contract.
He says: 'We believe it's a better product anyway, but it has particular advantages on the viaduct as there is no messing about with chippings in a restricted space.
'And of course in an urban situation like this it has the important advantage of being a much lower noise surface than HRA.'
Retaining walls at the viaduct's western abutment have been built in reinforced earth rather than reinforced concrete. 'We did consider redesigning the western interchange as well, mainly to cut down on the falsework needed,' says Franklin. 'But in the end we decided the design costs would probably outweigh any savings we could achieve.'