Groundforce Shorco props up famous Mayfair club

12 Nov
Groundforce Shorco equipment is playing a central role in one of the UK’s most ambitious and high-profile city-centre redevelopments: Cambridge House, in London’s Piccadilly.

Cambridge House is a Grade I listed former townhouse built in the mid-18th Century for the Duke of Cambridge, the seventh son of King George III.

Occupying a prominent position on the corner of Piccadilly and Half Moon Street, the building operated as the Naval & Military Club from 1865 to 1999. During this time, the site became known colloquially as the “In & Out Club” due to the prominent gatepost signs directing vehicles through the courtyard at the front of the building.

Cambridge House is now at the centre of a technically ambitious scheme by developer Motcomb Estates to create a new hotel and associated private residences. In addition to Cambridge House itself, the site incorporates 90-93 and 95 Piccadilly (both Grade II listed) and the Grade I listed 94 Piccadilly, 42 Half Moon Street and 10-12 White Horse Street.

The development will combine these structures into three new buildings erected above multi-level basements behind the retained facades. It also includes a new five-storey extension to 12 White Horse Street.

The extensive L-shaped basement excavation poses numerous challenges. It is formed within a secant-piled retaining wall constructed around the perimeter of the site with the original facades retained along two elevations. This means that the bespoke steelwork supporting the fragile facades is assembled above the 16m-deep basement void.

Rigid structural support is essential to the stability of the entire scheme and the proprietary Groundforce equipment used to brace the secant-piled retaining wall also provides support to the proprietary steelwork erected at the intersection of the two legs of the L-shaped basement and supporting the façade retention.

The depth of the excavation means there are six levels of Groundforce props employed. These comprise MP150, MP250 and MP375 hydraulic props (150, 250 and 375-tonne capacity respectively) spanning the excavation between Maxi, Mega and Super Mega Brace waling beams located around the perimeter.

The size and capacity of the Groundforce components generally increases with the depth of the excavation.

The upper-level props along the Half Moon Street ‘leg’ of the excavation are braced against the underpinning of the existing buildings. However, the construction sequence has meant that there are several points around the perimeter where the permanent lining wall needs to be cast behind the waling beams.

Consequently, at these points the Groundforce waling beams cannot bear directly against the secant piles themselves. This has been solved by either casting a small section of lining wall before installing the beam, or casting offstand posts into the piles to offset the waling beam.

A further complication is that the original main staircase of the listed Cambridge House must be preserved, even though all the other internal parts of the building have been demolished.

This staircase is therefore supported by Deconstruct UK Ltd’s bespoke steelwork and the whole thing suspended over the basement excavation with steel beams resting on concrete piles.

The complex intersection between the Groundforce components and the steelwork resulted in the need for numerous specially-designed sections of beam to tie into the proprietary walers.

Where the heavier (in this case, MP250/MP375) props are used over long spans, Groundforce employs its large diameter ‘Super Tube’ Extensions to ensure sufficient stiffness and strength. But on this project, propping levels in close proximity to floor-slab levels prevented the use of these larger tube diameters.

This was overcome by designing bespoke prop extensions using thicker tubes. There were many other smaller bespoke items designed to provide vertical support and lateral restraint to props and waling beams.

With the site located in central London, there was very little room for storage of equipment, so deliveries were timed to allow equipment to be directly installed from the wagon where possible.

Deconstruct UK Ltd Project Director Mark Makinson says that careful design and close collaboration within the supply chain has been key to delivering this complex temporary works solution:

“The design work began in November 2019 and Groundforce had a designer working full-time with us to draw up design calculation packages and over 150 drawings.

“The scheme was continually amended by Deconstruct UK Ltd, Groundforce and the temporary and permanent works engineers to work around the site restrictions and construction sequence,” added Mark.

The design work for the scheme was finalised in March 2021 and the excavation has now been completed. Construction of the permanent basement box is now underway and the first of the Groundforce props will soon be dismantled and removed.