Hidden Wonders of London’s Piccadilly

It is in the very nature of facade retention schemes that passers-by rarely have the great idea of ​​what is going on behind the fragile facade, beyond what they can glimpse through its empty window openings.

Such projects are quite common, especially in our city centers, where the preservation of the historic architecture is a priority. But it is inevitably also complex tasks that require a high degree of construction design and engineering.

This article was first published in the November 2021 issue of The Construction Index Magazine. Please register online

They are not much more complex and demanding than the redevelopment of Cambridge House on Piccadilly – right in the heart of London’s West End.

Cambridge House is a listed former townhouse built in the mid 18’sth century for the Duke of Cambridge, the seventh son of King George III.

The building occupied a prominent position on the corner of Piccadilly and Half Moon Street and served as the Naval & Military Club from 1865 to 1999. During this time the place became known in everyday speech as the “In & Out Club” due to the prominent gatepost signs , directing vehicles through the yard in front of the building.

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

The buildings will combine these structures in three new buildings built above basements on several levels behind the retained facades. It also includes a new five-story extension to 12 White Horse Street.

The extensive L-shaped basement excavation presents several challenges. It is formed within a secant-stacked retaining wall built around the perimeter of the area with the original facades retained along two elevations. This means that the bespoke steelwork that supports the fragile facades is gathered over the 20 m deep basement cavity.

The main contractor for this project is Deconstruct, a highly specialized enabling work contractor that focuses almost exclusively on projects like this in central London, where it is headquartered.

The company’s commitment to Cambridge House dates back to 2018, shortly after the building, which had stood empty for almost 20 years, was acquired by brothers David and Simon Reuben with a view to remodeling. Years of neglect had resulted in widespread deterioration of the structure, and Deconstruct’s first task was to erect a temporary roof to keep the weather out.

Deconstruct was subsequently appointed to carry out the enabling works involving the demolition of the interior and the maintenance of the elegant facades, plus the excavation and construction of huge basements extending down to four storeys.

The fact that the basement excavation extends all the way up to the base boundary means that significant support is required to stabilize the secant pile retaining walls and prevent any differential movement.

This support has been provided by the specialist rental company Groundforce Shorco in the form of its modular hydraulic support legs, braced against beams installed around the perimeter of the excavation (see Correct job).

This article was first published in the November 2021 issue of The Construction Index Magazine. Please register online

Deconstruct installed its own bespoke steelwork for the facade retention system – and it was no straightforward task. In addition to the facades themselves, the steel must support the building’s magnificent cantilevered stone staircase inside the building – or rather inside where the building used to be, as everything except the staircase has been removed.

“We designed and manufactured all the steelworks,” says Mark Makinson, Deconstruct’s project director. “That’s what we do, except that this is the greatest work of this kind we have ever done.” Deconstruct also installed a special steel portal platform to carry a 360o excavator equipped with a 20m long telescopic boom to remove coils from the excavation.

The staircase itself has no structural integrity – Makinson describes it as being a ‘giant Jenga tower’ – and therefore it is encased in a rigid steel frame tied into the steelwork that supports the facades.

But the stairs start on the ground floor, and at the moment there is no ground floor – only a 20 m deep cavity. Vertical, as well as lateral, support was therefore required.

“We used our own mini-pile rig inside the building to install fourteen 26 m long concrete piles to support the stairs,” says Makinson. As the excavation progressed, steelworks consisting of three levels of heavy rafters were installed under the stairs to complete the rigid frame. “We have 250 tons of steel under it all,” says Makinson.

“We also installed hydraulic jacks under the frame to remove any clay leaks,” he adds. “We pumped them to 50% of their stroke length to give us adjustment in both directions, but we have not had to adjust them at all so far – there has been no movement,” he explains.

No one walking past the place on Piccadilly would have any idea that there was a gaping void just across the sidewalk – let alone that there was a Class I-listed stone staircase floating in the air. Most of the original facade remains in place, hiding the enormous transformation that is taking place inside.

Some parts of the facade have actually been dismantled to give the space access. These will be stored off-site and will be reinstated in their original positions at a much later date – which raises the question: why not also dismantle the precious staircase and store the components for later reinstatement?

“The client and the team pressed for it, but consent for dismantling was not given by Westminster [the relevant planning authority]”,” Makinson confirms. “I think because of the fact that we had a solution to keep it in place, Westminster wanted it to keep it in its original state!”

This article was first published in the November 2021 issue of The Construction Index Magazine. Please register online

Proper work

The L-shaped basement excavation under Cambridge House is formed within a secant-stacked retaining wall constructed around the perimeter of the site with the original facades retained along two elevations. This means that the bespoke steelwork that supports the fragile facades is assembled over the 16m-20m deep basement cavity.

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

The depth of the excavation means that six levels of Groundforce props have been used. These include MP150, MP250 and MP375 hydraulic props (150, 250 and 375 ton capacity respectively) spanning the excavation between Maxi, Mega and Super Mega Brace beams placed around the perimeter.

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

The upper-level props along Half Moon Street ‘legs’ of the excavation are braced against the foundation of the existing buildings. However, the order of construction has meant that there are several points around the perimeter where the permanent cladding wall must be cast behind the beams.

At these points, therefore, the Groundforce wall beams cannot abut directly against the secant piles themselves. This has been solved by either casting a small section of the cladding wall before mounting the beam or casting free-standing posts into the piles to displace the beam.

The complex crossing between the Groundforce components and the steelworks resulted in the need for several specially designed sections of beam to bind into the proprietary walers.

Where the heavier (in this case MP250 / MP375) props are used over long spans, Groundforce usually uses its large diameter ‘Super Tube’ extensions to ensure sufficient rigidity and strength. However, on this project, support levels in the immediate vicinity of floor slab levels prevented the use of these larger pipe diameters.

This was overcome by designing tailor-made plug extensions with thicker pipes. There were many other less bespoke items designed to provide vertical support and lateral retention of props and beams.

The design work for the scheme was completed 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.

This article was first published in the November 2021 issue of The Construction Index Magazine. Please register online

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