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Piling into difficult ground

Foundations CTRL

Driven piling contractor Aarsleff believes extending the factory process into the field is key to the success of a contract to place a piled slab across Essex marshes for CTRL.

CTRL Contract 310 covers 13 km through the Thames marshes between the London tunnels and the Thames tunnel. Keller is installing foundations for the three big viaducts (see page 24) But around half of it, some 6.5km, is on a piled slab.

'Given the soft ground there were two approaches we could have taken to crossing the marshes, ' 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.'

Options available for this were either ground improvement or piling, 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 eastwards 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 conditions 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 the 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 was very important, in that it enabled the identification of the ultimate load before work started on site, ' Allwright says.

'If we hadn't had the trial we would have spent the last six months trying to figure out what was going on - this section has had a very long and complicated gestation.'

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. 'With big square piles, 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 £5M piling programme in June. It includes installation of around 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 quite modest, until you appreciate 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 - but RLE's approach anticipates and makes provision for isolated variations.

The ability to anticipate ground conditions accurately is therefore crucial: not only to confirm that driven piles can be used but also 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 pretty 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, but specially 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. Aarsleff is carrying out the limited headroom work under a separate £830k subcontract.

But combined with its main works, Aarlseff will install around 3,500 driven piles along the slab and the company anticipates using joints on about 20% of them.

At the western end the route passes through Ford's Dagenham plant. Here, and in other sections of the slab, vibration, noise and occasionally ground conditions preclude the use of driven piles.

The contract was initially tendered using continuous flight auger piles in these locations, but earlier this year there was much excitement over the potential of using the Belgian screw pile system. Extensive trials were favourable and a French contractor was awarded the work. However latest reports suggest that under production working conditions the screw piles are not performing adequately and RLE is seeking an alternative.

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