Balancing act


May 03, 2019 = Amberwood House is possibly the last mega-basement project in Knightsbridge, and it’s a stunner. David Taylor reports
There’s nothing especially notable about façade retention projects, especially in London where the need to preserve architectural integrity makes the practice almost routine.

But most façade retentions are essentially straightforward: erect steelwork to support your façade, demolish everything else behind it, build the new structure and tie it into the façade.

But what if you also have to demolish not only everything behind the façade but also everything beneath it?

This was the challenge facing S Walsh & Sons, main contractor on the redevelopment of Amberwood House in Knightsbridge, formerly the Panamanian Embassy and home of legendary ballerina Dame Margo Fonteyn and her ambassador husband Dr Roberto Emilio Arias.

To say Amberwood House is in a prime location is a gross understatement. Its address is 17A Thurloe Place which puts it just over the road from the Victoria & Albert Museum with Harrods and Sloane Street just around the corner.

High-end property developer K10 Group is transforming the three-storey brick mansion into a palatial modern residence extending to 15,300ft2 (1,420m2) and incorporating a cinema-cum-clubroom and a 12m swimming pool – “the largest in Knightsbridge,” according to K10 CEO Kam Babaee. Amberwood House is soon to go on the market at an estimated price of £75m.

Of course to fit all this into the available plot of land, there’s only one way to go, and that is down. Walsh has not merely demolished everything inside the shell of Amberwood House, it has also excavated a 12m-deep, three-storey basement underneath it.

The difficulty is that this basement is not confined within the footprint of the house itself but occupies almost the entire footprint of the site, extending outwards beyond the foundations of the existing house (see photo).
“Façade retention isn’t remarkable in London,” confirms Paul McGeady, Walsh’s construction operations manager. “But here we’ve got the façade suspended over our heads. That’s the unusual bit,” he adds.

If this were not challenging enough, the site is extremely inaccessible. Amberwood House is tucked away down a private drive – the only way in and out of the site – and is hemmed in on three sides by neighbouring properties. It shares a party wall with one of them, the Grade II-listed Alexander House to the north.

“When it comes to safeguarding neighbouring properties you’ve got to go up a notch,” comments McGeady, somewhat cryptically.

The scheme involves so-called “flying freehold” issues. In other words, Walsh is excavating to create new basement space underneath Alexander House. “The neighbour owns the air above; our client owns the ground below. It’s a peculiarity of London,” says McGeady. “We’ve had to underpin Alexander House and excavate very accurately beneath it. I was about 50mm away from waving at them,” he says.

Walsh has also had to underpin the back walls of four terraced properties in Thurloe Mews, backing onto Amberwood House. “After we’d underpinned them, we installed a secant piled wall just millimetres away from them,” he says.

This pales into insignificance, though, compared with the underpinning and piling work required by Amberwood House itself.



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