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Squeeze on the up and under

Foundations CTRL Contract 310

Keller's £8M foundation package on CTRL Contract 310 includes some of the biggest rotary bored piles the company has ever installed - and certainly some of the trickiest. Paul Wheeler reports.

Morgan Vinci's Andre Coudret is especially proud of the company's solution for getting the high speed Channel Tunnel Rail Link (CTRL) below the QE2 Bridge on Contract 310.

The Anglo-French joint venture's design and construct Thurrock Viaduct has the CTRL running over the top of the anticlockwise-M25 Dartford Tunnel approach viaducts and under the clockwise-M25 QE2 Bridge deck. It is a tight squeeze, but it fits and is buildable - just.

The key is Morgan Vinci's alternative design, which uses the incremental push launch system in place of the posttensioned glued concrete segmental structure originally envisaged.

It is, no doubt, the feature that will be highlighted when the project gets shortlisted for various awards - as it inevitably will - a few years down the line.

Right now, however, Coudret, as Thurrock section manager, is more focused on some pretty impressive foundation engineering. Keller Ground Engineering, working under a partnering agreement with Morgan Vinci, is providing deep piled foundations for the 22 viaduct piers and two abutments. This includes 99, 2m diameter rotary bored piles to depths of up to 43m.

But it is not just the dimensions that make the piling noteworthy, the project throws up some pretty tricky logistical challenges too. This is especially so at Pier 16, which is squeezed into a 3.2m gap between two of the Dartford Tunnel's approach viaducts. Spend half an hour on site, be it with Morgan Vinci, Keller or client Rail Link Engineering (RLE), and the conversation invariably comes back to the 'dreaded Pier 16.'

It also happens that ground conditions are worse and pile depths greatest at Pier 16.

This, explains Rod Allwright, RLE's lead geotechnical engineer on C310, is because 'the alignment cuts across an infilled river valley. The chalk surface dips down as the route gets closer to the Thames.'

Four large-diameter bored piles support all but two of the viaduct's 22 piers. Exceptions come about where there is an asymmetry between the deck and pier, resulting from third party interfaces.

Piles are generally deeper and of larger diameter than called for in the original tender, because the chalk quality - and stiffness - was found to be worse than anticipated. This was established from additional ground investigation works carried out by Morgan Vinci.

Along the viaduct's 1km length, the depth to the top of the chalk varies from a few metres to around 18m below ground level. 'Typically the river terrace gravel is 5m to 8m thick, ' says Charles McAnally, Keller's director of special projects.

Keller first investigated using short casings terminating in the gravels, and forming the pile shafts under bentonite or polymer support.

However, the project team took the view that where the new viaduct is close to existing structures - especially the M25 crossings - it was too risky to excavate the gravels supported only under bentonite. To ensure the integrity of these existing foundations, RLE stipulated use of casing penetrating down into the chalk, including at the critical Pier 16.

Keller felt it was best to use deep casings throughout, which would avoid switching to a new technique for the most challenging part of the project. 'It was better to get the experience, go through the learning curve and have the technique perfected before attempting the most difficult sections, ' reports McAnally.

Its methodology is to vibrate the casing through the gravel and penetrate the chalk by between 3m and 5m. Once below the casing, the pile shaft is supported by maintaining a high water head within the shaft as it is bored to full depth.

Casing depths are determined using pier-specific supplementary ground investigation, which to date, says McAnally, has tied in well. 'We are looking to sink the casing into chalk with an SPT N-value of the order of 15 to 20. 'Resulting casing lengths are typically 20m. Piles next to sensitive locations, including Pier 16, will remain permanently cased, as vibrations and ground movements associated with casing withdrawal are expected to be greater than during installation.'

It is an approach that appears to be paying off. Providing Keller can vibrate the single-piece casings through the gravel and into the chalk, boring under water is more straightforward than boring under bentonite.

Keller experienced some initial difficulties vibrating in the first few casings, but after upsizing the plant, the project is rolling along nicely.

Rapid and continuous working is a key issue with such large piles. 'When you're going so deep in the chalk, speed is important because degradation and softening is a concern, particularly where the weak chalk is encountered, ' says McAnally. Once below the base of the casing the boring, cleaning and concreting must be undertaken in a continuous cycle and completion must be achieved within 12 hours.

This is quite a challenge when you consider that on the largest of the piles, concreting alone will take around six hours.

Concrete is supplied from an onsite batching plant, and for the largest piles Keller anticipates switching from truck delivery to a direct pumped supply.

Keller will soon move onto Piers 13 and 14, where it will monitor ground vibrations and movements with inclinometers during pile installation. This information should validate the approach and allay any concerns of Dartford River Crossing (which operates the M25 crossings) over vibration levels and movements well in advance of the installation of Pier 16.

Keller's programme is slightly behind the original plan.

McAnally puts this down to the considerable increase in pile lengths resulting from the low-grade chalk and teething troubles encountered installing the casings. Additionally, changing the sequencing of work due to access restrictions has contributed to the delay.

Nevertheless, in the true spirit of partnering, Morgan Vinci is untroubled by the programme slippage. Apparently it has created no significant overall programme issues.

Keller mobilised to site in May, and following test piling started the main Thurrock Viaduct works in mid June. It anticipates moving on to Pier 16 in mid January.

Under current proposals the work will be carried out under live traffic conditions. Keller will track in its Soilmec R622 rig, carefully lifting the mast up through the narrow gap between the viaducts.

Building up the heavy sector

Keller has been progressively developing a large bored piling capacity for the last four to five years, principally through its on-going involvement at Canary Wharf. This is continuing on CTRL Contract 310, where Keller is providing heavy foundations for the main structures in an £8M partnering agreement with Morgan Vinci.

Within Keller's work on CTRL 310, the 1km long Thurrock viaduct is probably the most complicated. Piles are up to 45m long, 2m in diameter, cased into the chalk and then excavated under water.

The 520m-long Rainham Viaduct includes 82 piles, 1.5m in diameter, and up to 30m long, sunk through alluvial sands and gravels under bentonite. This section also includes 260 continuous flight auger piles up to 750mm diameter and 16m deep.

Tank Hill comprises three structures, and with 60 piles complete Keller is two thirds of the way into the job and well ahead of programme. Piles range in diameter from 750mm to 1.2m and are typically 20m deep. Chalk has proven to be of good quality, which means piles are cased to the top of the chalk and then open-holed.

The 600m-long Aveley Viaduct includes around 250 piles, 25-30m in depth and split equally between 1.5m and 900mm in diameter.

Keller tendered this section with bored piles under bentonite, but as an alternative is looking at large diameter continuous flight auger.

Problems here include groundwater at ground level and the possibility of solutions features in the chalk.

Keller's C310 work is scheduled for completion in June next year - 12 months after starting on site. At the peak of activity Keller had five piling rigs and eight service cranes on site. This includes a new Soilmec R622, an investment that suggests the company may well be hoping to increase its activity in the heavy foundation sector - territory currently dominated by a relatively small number of players.

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