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Trial of the century

A full scale cyclic lateral pile test should help tenderers price one of the trickiest geotechnical challenges on phase 2 of the Channel Tunnel Rail Link.

Dr Nick O'Riordan, ground engineering manager for Rail Link Engineering, jokes that when he started work on CTRL he had a luxuriant head of hair. His now thinning pate is, he says, the result of the pressures of introducing value engineering within the tight deadlines of the project's first phase.

As preparation gears up for a summer start on section 2, O'Riordan is sure that the second time around will be a little less pressured.

'It's all about certainty, and trying to get ground engineering off the critical path, 'he says.'My aim is to make the ground engineering element a given, so that it is not considered a problem.'

His focus is surprisingly ungeotechnical but reassuringly practical.'We are trying to achieve low initial cost and economic maintenance, ' he says.Nowhere is this more relevant than for the 7km stretch of line that runs westward into London across the Thames marshes of Essex.

For the entire route, the track will run on a ballasted piled slab.

'Although we looked at alternatives this solution is very robust, 'he says.

There are obviously a number of issues to be resolved, not least the much talked of bow-wave effect - which O'Riordan says is a misleading analogy.'High-speed trains can generate resonance in which you get superposition of the stress pulse superimposed with the arrival of the train, ' he explains.

It may be an interesting phenomenon, but RLE's geotechnical team has been very much focused on the reality of situation.'We have one chance to get it right - and we didn't want to rely on theoretical models, 'he says.This is why RLE has undertaken extensive on-site full scale cyclical and lateral pile load testing to replicate transfer of forces into the foundations and soft ground during breaking and acceleration of the 270km/h Eurostar trains.

To appreciate the sensitivity and scale of the problem, one needs only to feel the ground movement created by lorries trundling down the nearby A13 trunk road. It leaves you in no doubt of the engineering challenge facing RLE and its contractors.

The real benefit of this complex field trial has been to provide practical information and a benchmark for main contract tenders. There are many ways to tackle the problem and RLE expects value engineering to be used fully on the section 2 contracts.

Conditions vary considerably across the marsh. The ground here is often portrayed as a consistent and relatively homogenous layer of soft peaty clay overlying gravel.In fact there is evidence of three or four north-south ancient river channels down through the marsh to the River Thames.

With CTRL's route cutting straight across these channels, contractors will encounter a widely varying, but broadly repeating, sequence of superficial gravels, clays and peats overlying Woolwich & Reading Beds and then Chalk.

For pile testing, RLE chose two sites that typify the extremes contractors are likely to encounter. These vary from 3m of soft clay overlying dense gravels to 10m of clay overlying chalk.Ground water is consistently high.

In the tests, RLE wanted to contrast the performance of continuous flight auger and equivalent section square piles.

The trial contract was let in a competitive tender and Amec, working with Aarsleff Piling 'put together a package that fitted'The sophisticated vertical and lateral pile testing programme was developed by PMC (Precision Monitoring Control).The resulting 600mm square precast concrete piles are believed to be the biggest of their type installed in Europe.

Given the variable depth of soft alluvium, and hence the lateral pile-soil stiffness, the aim of the trial was to establish a pile design with horizontal movements of less than 5mm under trains braking and accelerating.

Planning and designing the trial began a year ago, with pile installation in November last year and testing in the New Year.

In the spirit of its conception, the trial was 'open book' with foundation contractors from rival firms invited to observe installation and testing.Given the potential size of the prize for the lucky winner, it is no surprise that many turned up to watch. About 6,000 piles are expected to be required in this stretch of track when piling starts on site in 2002.

RLE believes the stiff piles and pile layout adopted in the trial are 'about as efficient as you can get'says O'Riordan.But value engineering is all about investigating the unexpected.

The tests were devised to produce data that will allow contractors to make their conforming bids. It is then up to the contractors to apply value engineering to produce greater efficiency. Contracts will be target cost with risk sharing, so it will be to the contractors'advantage to come in below target cost.

Of course the trial is heavily instrumented but, adds O'Riordan, 'We set out to record hard data that could be easily assimilated by tenders - this was not a research project.'

Nevertheless, he is clearly pleased with the quality of the data, particularly the robustness of the instrumentation, singling out strain gauges that suffered very low redundancy.'It has been a practical project, producing good quality, useful data.'

O'Riordan says it was important that the trial and installation of the trial piles were representative of the main contract works.This also allowed environmental auditing of the processes by local environmental groups who recorded noise and vibration levels during installation.

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