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Box clever


Digging a big hole in Stratford for an international railway station does not sound like a difficult job. That is, until you realise that hole will be as long as ten football pitches.

Add that you have to build a concrete box in that hole, then put in a system that will permanently protect it from floating away thanks to the high water table and suddenly it sounds a lot less simple. But this is the immense task facing Rail Link Engineering (RLE) and contractor Skanska - against a pressing deadline.

Completion of the box ends are critical to provide 120m of completed structure at either extremity to form launch chambers for the London tunnel contracts Contract 220 and Contract 240. Without the box, the tunnels cannot happen.

'We've got to complete two thirds of construction in a third of our time period, and hand the ends over by May 2002, ' says RLE contract manager Bill Clowes. The central box section will then be dug out during 2003.

The Stratford box is being built in the town's 52ha railway lands to provide an area where Eurostar trains can cross over from one line to another and allow greater flexibility within the tunnels. An underground crossover (such as in the Channel Tunnel) would have been extremely onerous and costly and for this reason the box structure is very long to house these high speed crossings.

'It's a 1,070m long diaphragm wall subsurface structure, 50m wide and 16m-22m deep, ' says Skanska UK civil engineering project manager Ieuan Morgan.

Skanska, together with specialist piling arm Cementation Foundations, is building the box as part of Contract 230.

The box is also being built to provide an international and domestic station, which will sit at ground level giving access to the platforms below. The government hopes an international station will bring much needed regeneration back to the old railway town.

Advanced works are already well ahead, but the excavation will only begin after the official Section 2 start date on 2 July.

Excavation work will start with the deepest 22m western box end, where sections of diaphragm wall over 1m wide and up to 30m deep will be dug in series into the Thames gravels and Lambeth silts.

Once the concrete is cured, a concrete capping beam will be added and two rows of steel piles driven between the diaphragm walls. This is followed by a further 5m of excavation.

Following the excavation, concrete beam props 1m wide will be cast across the box at 10m spacings, employing reusable formwork. These will be left to cure for seven days to form the permanent props, before excavating down to 12m. At this level temporary steel props will be added, and the excavation completed down to the base unless further propping is needed.

'There is one level of permanent props at the west end, but two at the east, ' adds Morgan.

In September similar excavation will start in parallel at the eastern end. Props will not be used in the centre of the box, which is a straight cantilever structure.

Following completion of the walls, props and excavation, underslab drainage and a sump will be placed and the first base concrete pours can begin.

The pour of the generally unreinforced concrete base will be in two phases; the first down the centre of the hole, which will cure and take up shrinkage before stitches are added to join the walls to the base. Once the base is cured, the temporary steel props can be removed.

Close alliances are in operation at Stratford. Once the completed box ends are handed over to the Contract 220 and Contract 240 contractors, the Stratford site will serve as a working site for all three jobs.

'The tunnel contractors are going to manufacture the tunnel lining on our site, constructing a purpose built factory for the job, ' says Morgan. 'We're all inter-dependent. They provide us with spoil and we have to provide access.'

As part of Skanska's contract, the whole of the surrounding railway lands around the box must be raised an average of 6m in preparation for future development. And although 0.75Mm 3of spoil will come directly from the box excavation, the task needs another 1.5Mm 3from the tunnel boring.

The spoil placing will take place in stages. The first material will be used to create noise bunds around the north of the site, protecting the residents of Clays and Angel Lanes from construction and operational noise.

A further 7m of reinforced embankments will be built around the existing Temple Mills and High Mead freight lines that pass through the site. Rail bridges over the box must also be built for these lines at an early date. The bridges must be built to a tight schedule, as the team must aim to start work at the beginning of the agreed times for the temporary line closures. Two road bridges will also be constructed before handover, together with a viaduct to allow future possible movement of trains in and out of the box.

However spoil placing cannot be completed until tunnelling contractors have removed the tunnel lining factory. 'We're first in and last out, ' says Morgan.

Before any work can begin on site however, the first task is to clear the existing lands, while keeping the surrounding freight train lines operational.

'These are 140 year old railway lands, ' explains Clowes.

'They've been very highly industrialised and contaminated. It's pretty much a man made site.'

The site is covered with railway buildings and foundations, old engine sheds and rail waste.

'Every building is sheeted with asbestos, ' he says, 'and the area used to be an old refuelling area (for trains).'

One hundred and forty years' worth of fuel spillages can be expected, contaminating the land with phenols and poly-aromatic hydrocarbons.

A high water table also means that before any spoil can be placed, existing contaminants in the land must be stabilised to ensure the aquifer is protected.

Cleansing using a bio-remediation technique is likely.

The water level is already being monitored with a view to adding a sophisticated dewatering system for permanent use throughout the box's 120 year design life. The water must be pumped away or the box will suffer severe uplift.

'Without dewatering, the base would have had to be a monstrously heavy slab, ' explains Clowes, 'and we would have had to guarantee watertightness.'

Instead, 22 wells have been sunk around the box, 11 around each box end to lower water levels before digging. Permanent pumps will be added later.

When the box is complete, nine of the pumps will be in constant use at normal water table level, rising to 17 for a high level rise. The remaining five are for emergency use.

'We'll have a control system with alarms that monitors all of the wells, and operates and controls the pumps, ' says Clowes.

'The pumps will be in deep, clean water. They'll last a long time under those conditions.'

Contract 230 £105M Stratford box - Skanska UK Civil Engineering Piling by Cementation Foundations Contract 232 Stratford station - to be let Key points Building a 1km long subsurface box, large scale excavation, site redevelopment, permanent dewatering, tight programme

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