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Basement construction: Red carpet arena

Delivering a basement excavation in London’s cinema district of Leicester Square has called for the red carpet treatment to prevent the work coming under the focus of too many camera lenses

Facades around London’s Leicester Square are world famous with tourists and film fans alike and current redevelopment at the western side of the square should mean the vista remains unchanged. However, retaining the 1920s exterior on Communications House while excavating a new two storey basement behind the historic frontage has been a significant challenge.

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LSQ London

The building – to be known as LSQ London – is being redeveloped by Brookfield Multiplex for Linseed Assets to a design by Make Architects to transform the building for office and retail space

The building – to be known as LSQ London – is being redeveloped by Brookfield Multiplex for Linseed Assets to a design by Make Architects to transform the building for office and retail space. While the above ground works is now underway, the focus until recently has been the basement excavation by Amery Construction which called for specialist 3D modelling and bespoke propping from Groundforce.

The basement for the £40M project will measure 55m by 30m in plan and be 8m deep when completed.

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Behind the facade

The 1920s exterior of Communications House has been retained while a new two storey basement behind the historic frontage has been excavated

The perimeter has been formed within the building’s existing façade and footings using a secant pile wall formed from 750mm diameter piles installed to 29m below ground level. The piles are topped with a reinforced concrete capping beam, and Amery has cast two raft slabs and central core within the excavation.

Temporary propping support was needed to support the structure and the building above during the excavation but tolerances were tight due to the age of the retained façade and surrounding infrastructure. Amery needed a solution that would prevent any deflection greater than 15mm throughout the work.

The basement contractor brought in Groundforce to work on the propping solution, which led to installation of 150 linear metres of Mega Brace hydraulic bracing system around the perimeter of the rectangular excavation. This was supplemented by two levels of cross bracing consisting of a range of modular props installed at slightly closer centres than usual, to make the system as stiff as possible and reduce deflection.

The struts were fitted with Groundforce’s wireless load monitoring system, which – together with inclinometers attached to the basement walls – were set with pre-determined trigger levels to give early warning of any increase in load or deflection.

The internal structure of the basement was constructed around the props, and the logistics of prop installation and removal were carefully planned in conjunction with Amery to ensure that the subcontractor could carry out its work without the props getting in the way, and also that the props could be removed easily once the structure had been built.

Planning for the installation of the props and how the construction would be carried out around them was undertaken by Groundforce’s recently developed digital engineering team. The team used 3D modelling to ensure the props could “thread” through lift shaft openings and miss as many obstacles as possible.

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Planned propping

The basement for the £40M project will measure 55m by 30m in plan and be 8m deep when completed

Amery asked Groundforce to demonstrate where the props would clash with the lift pit walls. “We were able to take the supplied drawings and recreate the lift shaft in 3D to show where our prop clashed, and gave recommended opening dimensions so that a pocket could be left in the lift pit wall to thread the prop through once it was no longer required,” explains Groundforce major projects engineer Adam Fletcher.

Following the pre-planning effort, the work on site has gone smoothly and basements works are now complete. The rest of the building structure is expected to be finished later this year.

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