Your browser is no longer supported

For the best possible experience using our website we recommend you upgrade to a newer version or another browser.

Your browser appears to have cookies disabled. For the best experience of this website, please enable cookies in your browser

We'll assume we have your consent to use cookies, for example so you won't need to log in each time you visit our site.
Learn more

Cleared for access

Airports Heathrow T5

As London Heathrow approaches its summer peak David Hayward discovers that most action is not on the runways but between them.

Construction activity at London's Heathrow Airport T5 site is now vying with the surrounding runways for frequency of arrivals and departures. As the twin runways boast take-offs and landings every few minutes, the new terminal site sandwiched between them will claim at peak a delivery or departure through its single gate entry every 36 seconds.

The logistics of keeping up to 250 contractors supplied every day with materials, plant, a 5,000-strong workforce and all the canteen food they can consume, on a congested site with negligible storage capacity, is seen by engineers as more challenging than the purely technical task of constructing BAA's ú3.7bn ($5.2bn) complex.

If left unchecked, several hundred vehicles could demand entry every hour. But a strategy has been developed, paring back this hourly flow to a manageable 100, based in part on the conversion of nearby wasteland into an advance storage area and delivering materials through the gate on a 'just in time' basis.

One of the first teams to enter the now cleared 260ha site of a one-time sludge treatment plant was foundations contractor Bachy Soletanche operating, in BAA parlance, as a second tier supplier to main civils contractor Laing O'Rourke. Now working flat out on one of the UK's largest geotechnical contracts, the company is employing virtually its full arsenal of foundation techniques, installing $56M worth of bored, contiguous and underream piles, plus ground anchors, slurry and diaphragm walls.

'Pile installation into the underlying London Clay is not technically demanding, ' says project manager Rob Jackson.

'But sinking 16 piles every day on a confined site where storage of literally anything is strongly discouraged, turns delivery of the right materials to the right place at the right time into a major logistics exercise.'

The scheme, for which Jackson's team must provide over 4,000 piles, centres on a main 384m long, eight-storey core terminal. This will be supported by two satellite buildings, though only the first is being built during the initial phase, which is due to open in spring 2008.

Planning constraints dictated a single site entry point which later this year will be upgraded from a narrow road into a ninelane motorway style toll gate, to be drip fed by the offsite storage area. This 'logistics centre' at Colnbrook, 2km away, will have its own railhead receiving seven materials supply trains every day.

The centre will store everything the site needs for the following 24 hours of construction.

As that day's site supply is routed from Colnbrook, the following day's arrives at the centre. The store is always full, while the site remains free of anything that will not be used immediately. Supplies arrive, say site engineers, 'at the latest responsible moment'.

Supporting Colnbrook and already operational is a state-ofthe-art $56M rebar assembly factory 6km from the site. Here computer guided robots assemble, cut and bend 100t of rebar every day, most of it being formed into pile cages.

With Bachy Soletanche alone estimating daily demands of 25 lorry loads of rebar cages and 125 concrete trucks, all contractors will be forced to carry out extensive forward planning to satisfy demand for materials.

'We must list our exact requirements for rebar and pile concrete six weeks in advance, with concrete pour times finalised a week early, ' says the company's resources controller Niall Hardie. 'This ensures vehicle movements are minimised and programmed into the site working day.'

The foundation specialist's first task was to seal the site with a 3km perimeter slurry wall, dug 6.5m deep to key into the clay.

Completed earlier this year, the cut-off trench allowed overlying gravels to be dewatered and signalled the site's invasion by around half a dozen other contractors.

Earthworks contractor Blackwell is half way through a 6.3Mm 3muckshift to remove an average 5m of gravel and create a network of five different foundation levels for the terminal buildings. These areas are now being used as piling platforms from which around 2,000 mainly large diameter piles for the core terminal and satellite are being installed.

But the buildings' internal layouts must remain flexible with sizeable column grids demanding single pile foundation loads of up to 2,900t. Original foundation designs to accommodate such high loadings were based on 500 large, 35m deep underream bored piles with up to 6.3m wide belled bases.

The time between original design of T5 and arrival of piling rigs has been inordinately protracted. Yet the years of enforced delay have not been wasted.

They allowed design teams the rare opportunity to sit back and repeatedly reanalyse options.

Jackson recalls that he first started looking at conceptual costings for T5 foundations in 1997. And the company 'enjoyed' an unprecedented two-year detailed planning period. The most positive outcome of this has been an innovative year-long pile test reducing the number of expensive under-ream piles by over 90%.

The test's aim was to evaluate the long-term effects of loading on pile capacity. Engineers sank four large diameter piles into the site's London Clay and, at intervals over the year, loaded them to the point of failure.

'No one in the UK has attempted full-scale testing of how such piles perform in this type of ground with repeated loading over time, ' claims Bachy Soletanche senior design engineer Hugh Unwin. 'The assumption, though untested, has been that pile loading capacities gradually increase.'

The test results were surprising and significant. 'We found that the exact opposite occurs, ' says Unwin. 'Changing water pressures in the clay resulting from pile installation reduced effective horizontal pressure, so lowering bearing capacity.'

Such knowledge, especially the observation that capacity was reached and remained static after around eight months, allowed the contractor and BAA first tier supplier, consultant Mott MacDonald, to amend T5's high-load pile design, basing it more on performance than just recommended safety factors, resulting in a costsaving of over ú2M.

Similar savings are predicted from a further value engineering exercise, headed by Laing O'Rourke and involving segregation of the site's differing height work areas. An original plan to create up to 8m high battered slopes between areas would have required a five-stage excavation process before piling or construction could start on adjacent platforms.

Now Bachy Soletanche is sinking vertical contiguous piled retaining walls instead of forming slopes, allowing both areas to be excavated then used as work platforms at the same time.

The additional cost of the walls is more than compensated for by an estimated eight-week time saving on the project's critical path.

Have your say

You must sign in to make a comment

Please remember that the submission of any material is governed by our Terms and Conditions and by submitting material you confirm your agreement to these Terms and Conditions. Please note comments made online may also be published in the print edition of New Civil Engineer. Links may be included in your comments but HTML is not permitted.