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Breaking away from the herd

A novel bridge design using steel piles integrated with the piers also showcased a new quiet and low vibration pile pressing system.Max Soudain reports.

Milton Keynes is synonymous with concrete. Not just its famous herd of concrete cows; the stuff is used in its office buildings, houses and roads.

But in the bridge building world at least, concrete's supremacy is being challenged with a new design using steel piles extended out of the ground to form the bridge piers.

Construction of Whaddon Road bridge has been so successful that Milton Keynes council is considering specifying the design for all new road bridges in its area.

The contract also saw first use of Dawson Construction Plant's push-pull system for installing high modulus steel piles.

Consultant Pell Frischmann developed the design for the Whaddon Road bridge, incorporating piled foundations reaching up to the bridge deck.

Ground conditions were stiff to very stiff boulder clay with some lenses of silty sand.

A high, fluctuating groundwater table meant there were potential problems using rotary bored or CFA piles, so driving the piles was the preferred option.

However, as the bridge is only 50m away from a residential area, noise and vibration had to be kept to a minimum.

The solution was to use Dawson's push-pull system, which employs up to six, 200t capacity hydraulic cylinders to drive standard profile sheet piles in a number of configurations, from walls to box piles. Each cylinder grips a pile and acts independently from the others, with piles pushed in turn.

Initial reaction is provided by the weight of the press (a four cylinder arrangement weighs about 12t) and by the rope crowd system on the rig - the Liebherr LRB255 leader rig used on the Milton Keynes contract can generate 40t. Further reaction for each push of up to 600mm is provided by the other piles.

Vibration and noise generated is similar to that generated by the crawler rig, says Dawson Construction Plant engineering director Mark Lee, adding that the technique is 'several times faster than other pile pressing systems'.

Final design of the Whaddon Road bridge, approved after lengthy discussions with client English Partnerships, consists of a slender deck supported by 16 high capacity steel box piles, eight for the abutments and eight forming the combined piles and piers. Each pile is formed from four Hoesch Larssen 43 sheet piles clutched together.

As this was the first time the system had been used, Dawson was brought in as subcontractor to main contractor Weldon Plant to install the piles.

Abutment piles were installed down to 9m. Pier piles were pushed 14m into the ground and rise about 5.5m to meet the bridge deck.

The piers were filled with concrete from 2m to 3m below ground level up to the deck - mainly for impact protection - and then cast directly into the deck, providing a moment connection. No bearings are needed, Lee explains.

'Normally, bearings on top of the piers would cope with lateral and dynamic loads. On this bridge there are expansion joints in the deck at each end, and the steel has enough flexibility - even though it is very small - to cope with any movement.'

A piling frame was used to ensure tolerances of -25mm and 1:200 verticality were met. Dawson also carried out vertical and lateral load tests, with abutment piles vertically tested to 1,440kN and pier piles to 2,760kN.

The steel piers were painted grey - purely for aesthetic reasons and not to combat corrosion, Lee stresses.

He estimates that replacing a lot of parts with just one element saved at least 30% on costs and cut six weeks off the programme.

The simpler structure is also much easier to maintain, a big benefit for the council, which is responsible for 700 bridges and the reason why it is considering specifying the design for future projects.

Dawson principal geotechnical engineer Ray Filip believes the push-pull system has great potential for a variety of applications, particularly in urban areas, where it can be used to install piles very close to adjacent buildings. He says the firm is even entering the diaphragm wall market, with a wall of H-piles separated by sheet piles.

'If it can compete in this market, it can compete with virtually anything in concrete, ' Filip says.

But he didn't mention cows. . .

Steel vs concrete Mark Lee of Dawson Construction Plant says the push-pull system was initially marketed on its low noise and vibration but as interest grew, its 'eco-friendly' aspects became increasingly important.

'Little is said about recycling foundations, especially in urban areas, ' he says. 'But where there are many old concrete foundations that cannot be reused and are difficult to remove, sites can become 'sterile'.'

Steel piles, on the other hand, can be pulled out of the ground once the structure has reached the end of its design life and either re-used or the steel recycled, he says. 'And there is no time limit for pulling - even though the ground grips the piles, there is always a good chance they can be pulled out.'

Other benefits of steel over concrete are that little or no spoil is created during installation - 'so there is no tipping tax and no lorries are needed to take spoil away, ' Lee says - and there are no holes to backfill once the piles are removed.

Material costs are reduced and can be controlled better. 'Often it is difficult to gauge how much concrete is needed.'This helps reduce the number of deliveries and allows them to be better timed, which is important in inner city areas where access is often restricted to night-time.

'Steel can be delivered before it is needed, whereas concrete has to be delivered when the pile is ready and casting is dependent on weather, ' Lee says. 'At Whaddon Road Bridge, all the steel for the 16 piles and columns was delivered on just one lorry.'

Concrete piles often need testing after installation, he says. While the box piles at Milton Keynes were tested, steel bearing piles can be tested during driving. In the push-pull system, this is achieved by measuring the pressure applied by the cylinder to move the pile.

'Interlock friction has to be deducted - this can be calculated by pressing a single sheet pile, ' Lee says.

Universal appeal Another recent innovation from Dawson Construction Plant is its 10,000kN universal pile extractor (LEFT).

A development of the firm's, smaller 4,000kN hydraulic extractor, the system uses the ground as a reaction force, is quiet and vibrationless and fits most H piles, U and Z profile sheet piles and can be modified to take small diameter tubular piles.

Mark Lee says pulling piles using a rig mounted vibrator or simply by pulling with a crane is potentially dangerous, especially when piles fail during extraction. The Dawson system is operated remotely and as it is low to the ground, the method is much safer.

The equipment has already been used to pull sheet piles on Heathrow Terminal 5 and on the Mass Rapid Transit project in Singapore.

'The 10,000kN extractor will pull the pile until it fails - if it can't get the pile out, it's not coming out, ' Lee says.

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