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Piling & foundations spotlight: Tube squeezers

Crossrail advance works at London’s Tottenham Court Road Underground station have begun close to live Tube tunnels. Gemma Goldfingle investigates.

Advance work for London’s £15.9bn east-west Crossrail project has begun ahead of the start of full scale construction next year. Tottenham Court Road will be a major station on the route.

Already an interchange between the Northern and Central lines it is used by passengers travelling between London’s Theatreland and its main shopping precincts.

The station is currently bursting at the seams, and traffic is forecast to increase after Crossrail opens so underground expansion is necessary.

Advance works for the new station began last year when a 30m long underground service bridge was built 2m underground to relocate cables and pipes away from the site of the future Crossrail station.

Subcontractor Abbey Pynford working for main contractor Birse Metro created the foundations for this bridge. A piled wall was also required to form the first part of the walls for the new Crossrail ticket hall.

“We were working on one of the busiest streets in London. There was no option to close roads so we had a 3.5m wide section of pavement to work on.”

Paul Creswell, Abbey Pynford

“We were working on one of the busiest streets in London. There was no option to close roads so we had a 3.5m wide section of pavement to work on,” says Abbey Pynford business development manager Paul Creswell. “We had to use the smallest rigs possible to minimise disruption.”

A Hutte 203 rig was used as the machine is only 1.4m wide and weighs just 5t. The rig installs a secant piled wall, driving 350mm diameter piles to depths of 16m in London Clay.

Piles like these are traditionally installed using a 50t to 70t rig, but because of site restrictions a larger rig could not be used.

No room for error

Verticality can be affected when using smaller rigs, and,given the proximity of the piles to London Underground (LUL) tunnels, there was no room for error. “At one stage we were piling within 250mm to the side of a passenger crossover tunnel, well under LUL’s 3m exclusion zone,” says Creswell. With this proximity, using standard technical guidance for piling walls was not sufficient.

The Institution of Civil Engineers’ Specification for Piling and Embedded Retaining Walls allows for a 1:75 displacement on verticality while piling. Drilling to 16m depth could bring the piles perilously close to one of the Tube tunnels, possibly undermining the structure.

The contractor had to go beyond conventional piling tolerances to ensure this did not happen. This meant there was a need for careful guiding and frequent verticality checks using borehole inclinometers and specialist logging equipment usually used in the quarry industry.

Under pressure

Every 2m a real-time reading had to be taken to measure the inclination and ensure it was perfectly straight.

The small rig could only drill 1m at a time, so the pressure on site operatives to maintain verticality was even greater. “As sophisticated as the monitoring equipment is, it will only tell you when something goes wrong. It is the accuracy of the team, drilling and guiding the pile down that gets the results,” says Creswell.

“It is the accuracy of the team, drilling and guiding the pile down that gets the results.”

Paul Creswell, Abbey Pynford

The site could just about accommodate a silo that could hold 35t of material. When it had to be refilled, one lane of Charing Cross Road, the main road above the site, had to be cordoned off for the deliveries to be made. This was done at 5am to minimise disruption to road-users.

Recent tests revealed that the piles were found to have achieved verticality ratio of better than 1:200. More impressively, the piling was completed under budget and in only four weeks, seven days ahead of schedule and under budget.

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