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Subsurface structures

Keeping the muckshift on plan was essential for the construction of the vast subsurface works. This is where Bachy Soletanche picked up the gauntlet.

'Our works have covered foundations and geotechnics, ' says Bachy project manager Rob Jackson. 'This has included piling, diaphragm walls and ground anchors in the largest foundations contract in the UK for many years.'

On the two main terminal buildings alone, Bachy has installed over 2,000 piles. These have ranged from raking piles, secant piles for the airside road tunnel and large diameter auger piles. But most impressive of the piling works is the permanent contiguous retaining wall built beneath the main terminal building.

Jackson picks up the story: 'In the original design we were to install piles half way down a battered clay slope that would pick up a floor slab at a higher level.

But the E&E team needed to improve the programme schedule and following a 'challenge review', a new solution was found. It involved opting for 297 permanent 1,200mm diameter, 25m deep piles, which meant that Bachy was able to cut back the slope, reduce cut and fill, and free up valuable space. This was our first main piling on the site and enabled concreting works to get to the lower levels quicker - saving six weeks on programme. This made us very popular.'

Further savings were made by Bachy's promotion of its TMD anchor system for use on sheet piled retaining walls. This differs from traditional anchors as grout is injected under pressure through a tube a manchette to enhance the anchor/soil bond.

'We witnessed a two-fold improvement in capacity using this system and that meant we were able to use much shorter anchors taking more load, ' adds Jackson.

'We have an 18m retained face using just three rows of anchors, with approximately half the drilled length and we were able to save three to four weeks compared with the conventional anchor system.'

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