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Piles pass the test

Foundations CTRL

Extending the factory process into the field is key to the success of placing a piled slab across Essex marshes as part of the UK's high speed rail link to the Channel Tunnel.

CTRL Contract 310 covers 13km through the Thames marshes between the London tunnels and the Thames tunnel. Around half of it, some 6.5km, is on a piled slab.

'Given the soft ground, there were two approaches we could have taken, ' says Rail Link Engineering (RLE) geotechnical engineer Rod Allwright. The first was to form a high embankment, allowing the high inertia of its mass to stop resonance, he says, while the second was a stiff structural solution. 'Given the environmental issues associated with a high embankment, it was pretty clear from the start that RLE should go down the structural line.'

And with little precedent for ground improvement in this context, RLE decided to explore piling as the preferred option.

The resulting piled slab is effectively a lightweight bridge sitting on the ground, with rows of piles every 5m. The 10m wide, 450mm deep reinforced concrete slab was built in 60m long bays connected by movement joints.

It sits on 6,500 piles, averaging one per linear metre.

'Technically it's a very lightweight structure - rather like a table, ' says Allwright: 'The live load component is very heavy compared to the dead weight.

Live loads dominate the performances and in particular cyclic loading over the life of the structure governs the design criteria.'

RLE's understanding of the behaviour of the slab benefited greatly from some preliminary cyclic pile load tests commissioned at the tail end of 2000 - undertaken as Contract 305.

One of the limitations of the trial was the variation in the ground across the marsh. At the London end the alluvium thins to 4m, increasing eastward to 16m. In the west this alluvium overlies the dense granular/silty materials of the Lambeth Group.

In the middle it is underlain directly by London Clay and in the east by Thanet Sands overlying Chalk.

'We couldn't test every condition so we tested those that represented the largest proportion of the job, ' says Allwright.

Initially, test results suggested the need for extraordinarily long foundations to deal with the long term settlement caused by the repetitive cyclic loading of the passing trains. However, RLE also knew that that if it had a good dynamic analysis model, it could refine the design to be less conservative.

A critical breakthrough came when RLE realised it could apply the dynamic analysis expertise developed by Arup in its back analysis of London's imfamous Millennium Bridge's wobble.

This showed dynamic loads were less than those predicted by conventional dynamic load factors, and that a more economic design could be applied safely.

'The trial dynamic pile testing enabled the identification of the ultimate load before work started on site, ' Allwright says.

Now, some six months into the site operations, the preferred piling method is remarkably close to the piles tested during the trial. Where environmental and ground conditions allow - and that is more than half the piles - driven piling is the method of choice.

Like the C305 trial, contractor Aarsleff Piling is using 600mm square sections, believed to be the largest precast piles ever used in Europe.

RLE was initially attracted to the large precast piles because of a potential cost advantage over conventional sections. It also liked the greater level of quality control that is inherent in their factory-based manufacture.

The subsequent dynamic analysis suggests large section precast piles offer a performance advantage too. 'The foundation is 50% of the structure's weight, ' Allwright explains. 'The dynamic analysis showed that with the smaller piles you don't get the lateral resistance to achieve the braking and accelerating tolerances.' In contrast, he adds, the rows of 600mm square section piles at 2m centres are behaving more like a wall.

Aarsleff started its mammoth US$8M piling programme in June 2002. It includes installation of 3,500 driven piles.

Aarsleff's fleet has been bolstered by equipment from Danish parent Per Aarsleff, including a 105t self-erecting Hitachi 180 piling rig with a 28m high fixed leader.

This giant rig makes the huge pile sections look modest, but each weighs up to 12.5t. The 9t hammer has a maximum drop height of 1.1m and in practice taps through the alluvium with a drop height of just 0.3m, extending to the maximum to achieve the set in the gravel.

Driven piles can only be used where there is at least 500mm of gravel. The ability to anticipate ground conditions accurately is therefore crucial: not least so that piles can be ordered and manufactured well in advance of delivery to site.

'The secret to a successful driven piling contract is to extend a factory manufacturing process to the field, ' says Aarsleff Piling managing director Terry Bolsher. This means simplifying and standardising the installation as much as possible. Ironically though, simplifying a job of this scale is a complex process.

Difficult access during the ground investigation means there is a need to infill soil information along the slab's length.

And in this respect the programme is governed by the progress of the main contractor in laying down the working platform.

Soil conditions are confirmed using CPTs and sacrificial probe piles installed along the route in advance of the production piling. So far, Morgan Vinci's working platform has been sensibly ahead of the production piling - enabling Aarsleff to get the probe piling done in good time and achieve a continuous flow of production piling down the track. Realistically Aarsleff needs four weeks between probe piling and delivery of production piles to site.

In fact Aarsleff finds the pile lengths are generally within 10% of those predicted by the initial site investigation.

Piles to date have ranged in depth from 10.5m to 14.3m. A 14.3m long pile weighs 12.5t, which means two piles can just be transported on a wagon. If longer sections were to be used, Aarsleff would have to transport one at a time, which would be very expensive.

As the project moves eastwards and the alluvium thickens, deeper, jointed piles will be needed. The eight-pin mechanical tension joints are based on Aarsleff's standard pile joint, adapted to cater for the big pile sections. As Bolsher points out: 'The joints are in fact stronger than the piles.' They are nevertheless expensive to manufacture and take longer to install than single-length sections.

Jointed piles will also be used for low headroom work below bridges and where high voltage electricity cables cross the slab. These piles, which may be as long as 15m, will be made of 2.5m sections installed using a Banut 5t drop hammer attached to a specially adapted Banut piling rig with a shortened leader.

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