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M25 spans are flights of fancy for Balfour

ROADS & BRIDGES

As well as complex logistics, M25 widening works near Heathrow have included some technically challenging bridge construction, using 'flexitop' piles and shaped girders.

Adrian Greeman reports

FOR BALFOUR Beatty project director Peter Andersen, the latest M25 widening work has been primarily about planning, logistics and organisation.

'It is all about getting the men, machines and materials to the workface while keeping Europe's busiest motorway operating a metre or two away, ' he says. That means thinking not only about the construction itself, which in the main is relatively straightforward, but programming, public relations, vehicle recovery, traffic control and direction, and co-ordination with police and fire services.

Balfour Beatty's contract includes extra lanes for a 13 km stretch of the M25 on the west side of London as well as a complete new junct ion and spur road link into Heathrow Airport. This will be the primary road access into the new Terminal 5 when it opens in 2008. In the interim it will also serve for better T5 construction and fit-out access from the end of this year.

The 'gateway' road needs five bridges. Three of these are small, one already complete and virtually unnoticeable, and two linking the T5 road into the interchange. The other two are the 'sexy ones', says Mr Anderson, that are larger and more complex to build.

First is a crossing of the M25, which, in its newly widened form, means a clear span in the centre over eight lanes and side spans extending over a further two lanes and hard shoulder on the inside.

'The illust rat ive design in the tender was for a six-span viaduct but we changed that to a three-span design, 34 m, 44 m and 32.5 m, ' says Mr Anderson. Width was a generous 27.5 m to cope with three lanes each way.

Balfour Beatty's contract is primarily a design and build so it had the opt ion to subm it alternat ives for approval. A design was suggested by Balfour's own designer, a double team of consultants Gifford and WSP, and approved by Atkins, consultant to the joint client of the Highways Agency and BAA.

The main reason for the change was the value engineering of the multi-span bridge, which allowed the construction to fit better into the complex logistics of the widening work. There were also savings achieved on building a central pier support and foundations, though clearly the longer steel beams required meant more cost.

But without the need for work on a pier in the middle of the road the possessions required for the br idge erect ion could be greatly reduced.

Piling was still needed on the two side piers, which sit in the original position of the M25 hard shoulder but work for that could more easily fit into the overall sequencing.

Foundations are 750 mm-diameter bored piles sunk into the London clay beneath made ground and some gravel. Steel for the side spans was brought by subcontractor Cleveland Bridge by truck from Middlesbrough and erected some months ago.

There was then an extended wait to complete the bridge superstructure because night possessions were needed to lift in the remaining steel and these did not fit the overall programme for some time.

A series of three-night possessions allowed Cleveland to have the main central beam sect ions of the br idge lif ted in.

'These were 30 m-long sections because the beams on the side spans had been fabricated overlength and projected an extra 7 m on each, ' says Paul French from Atkins. Fixing was by a bolted splice.

Deck concreting over omniplanks is under way at present. Concret ing is also ongoing on the second large bridge, the Colne River valley crossing. But this required more complex work to get there, both in design and construction.

'The public inquiry for Heathrow set very specific requirements for the bridge, ' says Paul Luft from Atkins, whose design was followed for the bridge.

'The bridge had to allow as clear a vista along the river valley as possible, which is an amenity area and also close to a site of special scientific interest. The river, more a large stream, is a key ecological feature.' An architectural input was made for the bridge, which has tapered piers and a complex 'wing' shape for the steel beams which deepen on a curve over a central pier. The beams also widen and diverge because the end of the bridge fans out to where the small slip road bridges begin.

'And it is on a 10 degree skew, ' says Mr Luft.

All of this made the bridge fairly difficult to analyse and design, he says. On top of that the bridge is an integral design; the abutments are fixed and all expansion effects are taken up in the bridge itself.

There is a small slip bearing on the pier for movement but the piles must also allow for the bridge stresses.

This was the first complication for Balfour Beatty, says Chris Till, its T5 structures agent. 'We have used flexitop piles for the foundations which are complicated to install.' Stent did the work.

The piles have a double skin allowing the inner part to move relat ive to the ground around it and thereby respond to the bridge movements above in hot sun or cold nights. 'There is an inner lining which contains the main concrete pile and then an outer lining with an air-filled annulus between them which allows for the movement.' It is more complicated because a third temporary casing must be used for the installation.

'Pushing in the casing is not accurate enough so you make a working space with this one, augur it out, and then install the main casing, ' says Mr Till. Both are pushed in far enough to toe into the clay layer about 15 m down.

Once the main casing is in it is grouted up then the temporary casing is removed. Inside the now accu rate pile hole a smaller casing is inser ted and again toed to the clay; this inner part is then augured to full depth on down through the clay which does not require any casing. Reinforcement for the piles, up to 39 m deep, was then installed and concreted.

But that is not all. On the west side of the bridge the piles have yet another layer, meaning four casings in all were inserted like Russian dolls, one inside the other.

The extra case is needed to cope with made ground which could settle and move in the future. So an outer permanent casing creates another 80 mm wide annulus around the flexitop annulus, th is time filled with a deformable bentonite. 'That allows for any distortions caused by settlement movements, ' says Mr Till.

There were further complications during installation. Because the bridge is close to the runway ends at Heathrow there is a height restriction on equipment. To keep rigs within a 24 m envelope, the piles had to be installed at ground level before the 8 m high embankments were completed at the abutments. Addit ional sect ions of tubing were welded on for each of the mult iple layers of the piles, after which the backfill could be completed.

Concreting the piers was relatively straightforward but then the height restriction again affected work when the main steelwork was installed. This was done in three sections for each of the four main beams.

A first section of beam 17.8 m long and up to 2.4 m deep and weighing 331 tonnes was then lifted in over the piers. For this a heavy RMD frame had been built up from the pilecap in a Y shape to support it, or rather two of the centre sections together, which are linked. Once this and its partner were in, on either side of the pier, the back span sections were lifted in and welded to the centre pieces.

These were 24.4 m long and weighed 38 tonnes.

The end of the side span rested on a plinth at the abutment. The beam could not be concreted in until the whole bridge was in place and the deck installed, when the loads on the beam would give it a final shape. 'Otherwise the steel sits at the wrong angle transferring the stresses at the wrong angle down into the abutment piles, ' says Mr Till.

The final lift was the longest at 35.6 m for the beam across the river. The sections weighed 65 tonnes and were bolted.

As with the piling, the height restriction was a problem during the lifting. It meant using a bigger crane because the more powerful jib could be used at a shallower angle to lift the same weight, and therefore a lower overall height. 'It was a Liebherr mobile with an 800 tonne capacity and we used it with just two of the telescopic sect ions extended , ' says Mr Till. Sections were a maximum 62 tonnes.

A smaller 200 tonne crane was also used by Cleveland Bridge for the bulk of the steel such as the ladder beam sections between the main girders.

Concreting is currently in progress for the deck, which will be 27 m wide and carry three lanes either side into T5, whose giant structure now stands highly visible and almost ready for boarding just 300 m away.

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