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AHEAD OF ITS TIME

UNDERGROUND CONSTRUCTION - Construction of office block in central London has been complicated by the the need to sink a shaft for the proposed Crossrailscheme beneath it. Ruby Kitching reports.

In a gamble that construction of London's Crossrail will go ahead, a key piece of work for the proposed east-west link across the capital has just been completed.

Construction of a £1M draught relief shaft was finished in July in the basement of the Moor House office building, which is being built next to Moorgate Underground Station in the City. The shaft will allow escape of air pressure waves pushed by trains running through Crossrail's tunnels.

The project is due to be debated in the next session of parliament. If it gets the go-ahead, tunnelling could begin in October 2008 (GE August 04).

Developing the design and a suitable construction method for the shaft has been a threeyear headache for consultant Arup and main contractor Skanska. The problem was how to sink an 8.2m diameter, 40m deep shaft through some of the most unpredictable ground conditions in London while building an office block above it, all in a busy financial district.

The solution was to strictly control construction methods and to develop contingency plans beyond contingency plans, says Arup project director Duncan Nicholson.

Design constraints fall into two categories:

the way the building will react when the shaft is built and the way the shaft will react during Crossrail tunnelling.

'When a shaft is dug, the ground will naturally want to move to fill the excavation.This has the effect of reducing stresses in the ground and the skin friction on adjacent piles, ' Nicholson says. The piles are less than 2m from the excavation. If they failed, the consequences for the superstructure would be catastrophic, he adds.

The solution was to sleeve the piles so that any ground movement would not affect the foundations (GE November 2002).

'But we couldn't sleeve the whole pile as they'd have gone on forever - and we're already on the limit of how far we can pile down to, ' adds Skanska's geotechnical engineer Andy McNamara.

The compromise was to sleeve half the length of the 43m piles and design the unsleeved portion to withstand a 20% reduction in skin friction.

Most importantly, Arup and Skanska needed to be forewarned of any stress relief so that contingency plans could be mobilised.

Finite element modelling revealed that 20% stress relief equated to 20mm of ground movement.Monitoring for basement and street levels helped predict when contingency plans should be implemented. Ground movements were also measured using inclinometers and extensometers.

'We reached the warning trigger level of 10mm for horizontal movement, ' admits Nicholson, 'but no action was required.'

Future construction of the 30m wide, 240m long and 40m deep ticket hall for Crossrail's Moorgate station was another cause for concern. Anticipated horizontal ground movement of up to 40mm could significantly affect the shaft.

The shaft lining is made up of 12, 350mm thick 1m deep precast segments. These are reinforced using fibres rather than traditional steel bars to allow easier drilling for compensation grouting if ground movements call for remedial work.

The segments have PTFE slip surfaces which allow each segment ring to accommodate a net displacement of 40mm, about 1mm per ring, while remaining watertight.

Rising groundwater was another concern.

'Groundwater is recovering by 1m per year in London, causing the ground to swell, ' McNamara says. To combat growing ground pressure and uplift forces at the base of the shaft, joints between the lining segments are designed to cope with some flexure and the shaft's base slab is heavily reinforced.

'We've got about 40t of 50mm diameter bars to go into the bottom of the shaft to resist those stresses, ' says Skanska project director Matt Cova. The shaft base level was reached in July. But the lining is only a temporary; when Crossrail is eventually built, a permanent lining will be installed.

'The design life of the shaft lining is only 21 years. If Crossrail isn't built by then, it might have to be backfilled, ' Nicholson says.

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