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In a tight spot: London's new basement car park

The luxury cars expected to occupy a central London basement carpark are demanding
exceptional accuracy during the current excavation phase. David Hayward reports.

Marian Ailenei stands guard over a 14m deep basement excavation in the back garden of an elegant central London crescent of Victorian town houses.

No one is allowed in or out without his permission, and a large safety board alongside him records the movements of the several dozen construction personnel forming, what main contractor Walter Lilly affectionately calls, “the hole”.

Designated a confined space, this 30m long rectangular, four-level basement excavation demands constant gas monitoring, air changes every few minutes plus round the clock construction supervision by senior engineers.

As a carpark it will house 21 of the UK’s most expensive cars, all positioned remotely by a £700,000 state of the art computerised stacking machine, requiring just 75mm verticality tolerance for the excavation’s piled walls.

“I am able to stop the entire job if I do not know where everyone is within the excavation.”

Marian Ailenei, independent safety advisor

That it should fit exactly is testament to the skill of geotechnical subcontractor Expanded Piling ,which positioned 150 up to 26m long secant wall piles to a verticality tolerance of one in 200 - twice as tough as the norm.

The £37M conversion, for property developer Zegna III Holdings, of eight 1830s grade II-star listed terraced houses in Grosvenor Crescent is in itself a demanding operation.

“To be or not to be” a confined space, was an early question asked of this large but restricted access basement excavation. It was to be formed by a top-down excavation sequence adopting 10 controlled phases.

“The rules for defining a ‘confined space’ are somewhat open to interpretation” explains Walter Lilly chief engineer
Derek Brattle. “We called in an independent expert to advise us and, given the limited access and poor natural ventilation, decided we should designate it.”

Enter Marian Ailenei, the man with the ultimate authority to control access to the site.

“I am able to stop the entire job if I do not know where everyone is within the excavation,” he asserts. Supervisors, operatives and visitors are given different shaped tokens on entering the basement area. And a glance at the safety board highlights their location in the hole during the multi-level excavation phase.

Designated supervisors must always be at the work face, while daily air quality readings and constant gas monitoring - all controlled by Ailenei - ensure a safe working environment. Up to 10 air changes every hour, through a forced ventilation ducting network, plus a rigorous radio communications regime, complete the precautions.

“There is no point adopting half measures and we have to be 100% compliant.”

Derek Brattle

“There is no point adopting half measures and we have to be 100% compliant,” says Brattle “even though this has inevitably added significant cost to comply with the confined space precautions.”

Brattle must ensure design compliance and to sign-off the safety board at each of the 10 excavation phases, ensuring it is safe for operatives to proceed to the next stage.

These involve casting a floor slab, with a sequential 14m deep top-down excavation operation beneath it, while the building’s superstructure is built above.

An intermediary floor slab, to carry the plant room 4m below ground level, is suspended on 200mm thick tension walls running down from the lower ground floor slab above. These are cast tight to the perimeter piling, have limited shutter access, and were formed using self compacting concrete.

Lower down, half way between plant room and basement slab, heavy preloaded steel waling beams and struts provide temporary perimeter wall support until the basement slab is complete.

Grosvenor Crescent: Basement construction sequence

Grosvenor Crescent: Basement construction sequence

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