McGee's Mega Basement
With the new Claridge's basement spa set to open, I take a look back
The basement of Claridge's hotel has been transformed with a new spa and 20m swimming pool set to open to guests in coming weeks. The huge 'iceberg' basement extends five storeys below ground, housing not just the spa, but also relocated plant from the roof and new facilities for staff to change, dine and cook.
The new basement lies below the (relatively new) 1920’s Art Deco section of the hotel. It’s creation has unlocked the previously unsightly and plant-riddled top floors of the hotel which have also now been re-built and extended, adding an extra 40 rooms and suites within the new mansards.
OK so this is a big basement, but just wait until you hear how it was constructed.
The image below symbolises the approach. Contractor McGee only took ONE bedroom out of service and their operatives effectively mined their way through the soil under the hotel. They tunnelled laterally (under the building) to 61 column positions and then dug vertical shafts down to form the columns and piles, before hollowing out the rest of the soil in a ‘top-down’ approach to form the basement. All this happened from Brooks Mews, whilst guests continued to enjoy the hotel, coming and going through the main entrance on Brook Street, oblivious to the works beneath them.
I visited the site in 2018 when the excavation was around B4 and it was bizarre to walk through the Lower Ground floor of the existing hotel where black-tie waiting staff walked the same corridors as McGee’s miners in hi-vis jackets - but somehow it worked.
The image above shows a worker excavating down to form one of the 61 columns. The internal dimensions of the steel-cased shaft were 1700mm (1800mm externally). This seems positively spacious, when you bear in mind that the column to be constructed back up within this shaft was 600mm. Allowing for formwork of the column, that left only around 450mm working space between the internal edge of the shaft and the outside of the column formwork!
Column and pile sizing was a careful balance. The columns couldn’t be more than 600mm otherwise they were un-buildable, but the piles needed to be much larger to transfer the loads to the ground. So as they continued digging down, once past B5 level the pile was widened to 2400mm diameter and then under-reamed up to 4600mm.
There were so many challenges on this project, it is hard to summarise them all here - from groundwater to redesigned piling rigs to jacking the building to coupling of steel reinforcement. If you are interested in the engineering, I recommend the NCE article linked at the base of this which captures more detail. If you’ve understood the basics, the info-graphic below is a useful tool to put some of the elements together in a few illustrations.
This story cannot be told without a mention of Jim Mackey. He was MD for McGee and initially advised on the idea (at the time a 2 storey basement) before the GFC in 2008, after which it was mothballed. Then in 2015, weeks after retiring, he received a call saying the job was back on, this time aiming for a 5 storey basement - and he was the man to lead it. Unable to resist the seemingly impossible challenge (one which the rest of the industry believed couldn’t be done), Mackey returned.
His daughter, Michelle, an engineer for McGee also joined him to form an impressive father and daughter duo, ready to tackle the c.£40m D&B basement box project. Arup were also a key part in the project, working extremely closely with McGee to understand not just the permanent, but also temporary works designs.
The project finished four months ahead of programme, on budget and all with zero disturbance to hotel guests. Quite the engineering feat.
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