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Ground work - crossrail

Deep in the bowels of London's newest office block, the first piece of Crossrail infrastructure is nearing completion. Ruby Kitching reports.

Taking a gamble that construction of Crossrail will go ahead, a key piece of work for the project is being carried out. As underground trains rush from east to west across the capital through Crossrail's tunnelled central section they will push a pressure wave of air before them. To provide a relief valve, a 40m deep, 8.2m diameter shaft is being sunk beneath one of the capital's sleekest new office blocks at Moorgate.

Mining the £1M shaft in between the deep, piled foundations of Moor House (NCE 4 September 2003) has proved surprisingly difficult.

Although there have been challenges building the shaft and office block simultaneously, and with site logistics on the tightly constrained City site, the principal headache has been ground conditions. Designer Arup and contractor Skanska had to safeguard against movement of the building as the shaft was sunk, and allow for deflection in the shaft when work on the main Crossrail tunnels is eventually carried out.

'When you dig the shaft, 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, ' explains Arup project director Duncan Nicholson.

The closest of the building's piles are less than 2m from the shaft excavation.

To prevent movement of the piles they have been partially sleeved - the sleeves will move rather than the foundations. But sleeving reduced skin friction on On the water front The shaft is constructed through 26m of London Clay, 18m of mixed Lambeth Group gravels, clay, sand and pockets of water, and a few metres of Thanet Sands. Water below the London Clay had to be reduced to prevent flooding of the shaft during excavation. This was achieved with pumped wells installed by groundwater specialist WJ Groundwater five months before excavation.

'Our biggest fear would have been a power cut and all the dewatering pumps turning off, ' admits Cova. 'It takes just two hours for the water level to recover 2m.'

As a precaution against hitting undrained pockets of water in the Lambeth Group layer trial holes were sunk in advance of the shaft. In the event of hitting water, secondary well points would have been installed through the existing shaft lining.

the piles, forcing deeper piling.

'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, ' notes Skanska geotechnical engineer Andy McNamara. The compromise was to sleeve half the length of the 43m long piles and design the unsleeved portion to withstand a 20% reduction in skin friction.

Arup's finite element modelling revealed that 20% stress relief equates to 20mm of ground movement. So a strict regime was developed where basement and street levels have been closely monitored to help predict when contingency plans should be implemented. These consist of compensation grouting behind the shaft lining to reinstate original earth pressure. Insitu horizontal and vertical ground movements have also been measured using inclinometers and extensometers.

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

Future tunnelling and construction of an underground Crossrail ticket hall, measuring 40m deep by 30m wide and 240m long, next to London Underground's existing Moorgate station, was another cause for concern. Anticipated horizontal ground movement of up to 40mm could significantly affect the finished shaft.

Accordingly, the shaft has been designed to provide greater flexibility than would be normal:

The shaft lining is made up of 12, 350mm thick, 1m deep precast segments. These are reinforced using steel fibre rather than traditional steel bars to allow easier post-construction drilling, so that compensation grouting can be carried out if needed.

Specialist concrete contractor Charcon also designed the segments with PTFE 'slip surface' coatings between lining rings. This allows each ring to move about 1mm while still remaining watertight.

Rising groundwater has been an additional concern. 'Groundwater is recovering by 1m per year in London, causing the ground to swell, ' says McNamara. It also creates the problem of buoyancy in structures below the water table.

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, while the shaft's base slab has been 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. Shaft base level was reached earlier this month.

For all this bother, the shaft lining is only temporary. If or 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 sighs.

Digging deep Because space is tight in and around the shaft most of the construction has been carried out mechanically. An excavator has been used to dig the shaft. Spoil has been skipped out and dumped into a muck bin with the aid of a hoist mounted on a 20m long runway beam slung beneath Moor House's ground floor slab.

This system has also been used to carry and lower lining segments into the shaft, where a second hoist has been used to position them. Segments were bolted together and only levelled up when a full ring was complete. The annulus behind each ring was grouted before the next bench was excavated. Excavation and lining was carried out as a continuous, 24 hours a day, five days a week process to prevent ground collapse.

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