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Gerrards Cross air rights Shopping for space

Foundations - When supermarket giant Tesco found there was no space to build in a small Buckinghamshire town it created its own prime town centre site. Andrew Mylius discovers how.

The small Chilterns town of Gerrards Cross, west of London, looks fairly unassuming. But check out the wheels in the station carpark or the cut of the townsfolk's clothes, and it is clear that Gerrards Cross is minted.

Yet the major food retailers have been frustrated in their ambitions to tap into what ought to be a lucrative market by a shortage of developable space.

The town centre is packed with houses and small businesses leaving no space for aisles of tinned, frozen and fresh produce.

Supermarket giant Tesco is about to impose its presence, though, by creating a new site for itself from scratch. It is spending £20.3M on a pioneering 'air rights' project over Network Rail's Chilterns Line, right next to Gerrards Cross station. The line runs in a deep cutting through the town centre, and the scheme involves erecting a reinforced concrete vault over a 320m stretch of track and backfilling up to original ground level.

By September next year there will be 1.82ha of new land complete with a store styled in Tesco's brand of 'farmyard vernacular' and parking space for 300 cars.

Main contractor for the job, Jackson, is 20 months into a two and a half year contract.

But it is only in the last couple of months that the site has really begun its transformation, with erection of vault elements.

Jackson has put most of its effort over the last year into the ground, installing 2,200, 450mm diameter cast in situ concrete piles capped by continuous beams to support the edges of the concrete tunnel vault. The 6.9m high, 20m wide, three pin vault has been designed to allow for future four-tracking of the twin track line, and for electrification.

Four tracking will require a lowering of the track bed, exposing the capping beam and a short stretch of the piles themselves, which consultant White Young Green has designed to cope with the resulting reduction in friction. They are slightly over length, toed into clay 8m to 10m down, overlying chalk strata.

'We stopped the piles in the clay to avoid creating too rigid a structure, ' notes White Young Green principal engineer Chris Chilver. The vault will deflect 40mm during the backfilling process.

Technically the piling work has been nothing out of the ordinary, reports Jackson project manager Stephen Christian.

Logistically the job has been 'full of twists and turns', though.

Christian's site is long and narrow with only one point of access, on the northern side of the railway tracks. Jackson has been forced to wait for nighttime possessions to lift plant, reinforcement cages, and any other materials or equipment across the tracks - although during the day culverts tunnelled underneath the track bed have been used to pump concrete across.

Meanwhile, Jackson has been forced to use mainly minipiling rigs in order to comply with Network Rail's height restrictions next to the railway. Maximum mast height has been 5m, and some locations is restricted to half that height.

In the main it has been possible to carry out piling and placement of reinforcement cages during the day. However, 160 piles have had to be bored at night at locations where there was a risk of the rig toppling.

That is all history, though, and Christian is now grappling with what he admits is an engaging technical challenge.

The air rights site is sandwiched between Gerrards Cross station at its western end and Marsham Lane Bridge to the east. It incorporates Packhorse Road Bridge, a late 19th century three span brick structure which will be entirely buried during the fill operation, its deck marrying in with final ground level.

Packhorse Road Bridge clashes with the concrete vault, but because it carries a major road it cannot be demolished.

The new reinforced concrete vault fits underneath the crown of the central arch easily enough.

But it cannot accommodate the vault's 20m span. Accordingly, vault supplier the Reinforced Earth Company has manufactured a series of crown only vault elements that will be supported by beams running on the inside faces of Packhorse Road Bridge's piers.

This sounds simple enough, but to enable the supporting beams to act independently of the existing brick structure huge reinforced concrete cantilevers must be constructed, taking loads directly to the ground.

There will be four of these cantilevers either side of the railway line. On each side, two will flank Packhorse Road Bridge's piers, and two will be threaded through the piers themselves. Abbey Pynford is cutting the 2m deep by 1.5m wide holes needed to accept the new structure. Specialist piling contractor Brawnlow is installing a mat of piles behind the piers to take thrust forces.

Casting of cantilevers through the southern pier is advancing well, with piling behind the northern pier nearing completion.

When cantilevers are complete and the new vault elements have been slid into place underneath Packhorse Road Bridge the annulus will be closed with pulverised fuel ash (PFA) grout as part of the backfilling operation. Backfill will be 95,000m 3of limestone overburden discarded by quarrying operations in the Mendips.

Finally, the old structure's arches will be hacked through, destroying its structural integrity and effectively turning it into more fill. 'We're breaking the backs of brick arches to reduce Network Rail's maintenance burden, ' explains Chilver.

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