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Claridge's epic basement demands five-star solution

McGee has taken on the mammoth task of building a five-storey basement underneath Claridge’s – while the five-star hotel remains fully open and with just a single window for access. Lucy Alderson finds out how the contractor did it.

Client: Claridge’s
Contract value: £35m
Main contractor: McGee
Region: London
Structural and geotechnical engineer: Arup 
Temporary works engineering support: RKD
Start date: November 2015
Completion date: December 2018

After 40 years at McGee, working up from site worker to managing director, Jim Mackey finally retired in 2015.

It wasn’t his first attempt; he had already tried to hang up his boots several times previously, but found he couldn’t quite let go. This time however looked like it would be the last, with Mr Mackey travelling with his wife to Ireland to their retirement home.

With his several unsuccessful attempts still fresh in the memory, his wife bet he would barely last five minutes. “No,” Mr Mackey insisted. “I promise you nothing in this world will make me go back to work… except for one project.”

That project was the Claridge’s basement extension.

Back in 2007, McGee was invited along with other contractors to submit a proposal to extend the iconic London hotel’s basement by two storeys. McGee won the contract, but the project was mothballed following the financial crisis in 2008.

“It’s a crazy job,” Mr Mackey said to his wife. “It will never happen.” And so they happily retired… for a grand total of eight weeks, until he received a rather unexpected phone call.

He recalls sitting in his rocking chair when the phone run. It was Claridge’s.

“They said to me, ‘Oh you know that job you were going to do with us in 2007? We want you to do it and it starts next week’.”

claridges mc gee 3 credit mc gee

Claridge’s McGee 3

Source: McGee

But Claridge’s no longer wanted a basement extension across just two storeys; they now wanted five. And the other catch? The five-star hotel had to remain open and construction works could not affect or disturb its day-to-day operations.

Mr Mackey initially came back to work on the design and help get the project started. “But then I was too involved and it was difficult to stay away,” he says. “I became more engrossed and there were more and more challenges to fight through.”

Enlisting the help of his daughter Michelle, who became senior engineer on the job, the father and daughter duo set about tackling this monster project.

Dewatering the site

McGee is building over 6,000 sq m of new floorspace underneath the hotel.

Basement level one will house a gym with a suspended swimming pool, a restaurant and retail space; basement level two will contain a swimming pool and meeting rooms; and operational space for hotel services, including food preparation will be on basement level three.

Plant rooms located at the top of the hotel will be moved to basement levels three and four. This will free up space at the top of the hotel for up to 40 new rooms.

Works began in November 2015.

claridges mc gee swimming pool slab credit nick carter

Claridge’s McGee swimming pool slab

Source: Nick Carter

Swimming pool slab

The first major hurdle, according to Mr Mackey, was addressing the widely varying stratum beneath the hotel. In some places the ground has the consistency of gravel. At the west end of the site a high water table meant a wet, silty, sandy material was “presenting enormous difficulties” for the team, he explains.

The initial challenge was to prevent further ingress to the footprint of the site. To the north end, operatives constructed a reinforced concrete beam to cut off water. To the south, an installed secant wall comprising 600 mm-diameter interlocking piles did the same.

With the perimeter sealed, dewatering of the site could begin via a pump inserted into an excavated well.

‘A room and a window’

With abstraction complete, the team could set about putting Mr Mackey’s method of constructing the basement extension into practice.

Claridge’s sits on top of a 50 m x 25 m reinforced concrete raft slab, which acts as the foundations for the building. Founding on this slab and carrying the building’s load are 61 steel columns.

Mr Mackey’s solution was to elongate the columns 30 m below the raft slab by a cleverly co-ordinated system of mining. The elongated columns could then come down onto installed piles, with the basement levels created after excavation was completed.

“They said to me, ‘Oh you know that job you were going to do with us in 2007? We want you to do it and it starts next week’.”

Jim Mackey, McGee

Sounds simple? Far from it – especially considering McGee was initially given only one room with a window to service the entire site.

The window opened up onto the street at the back of Claridge’s, through which equipment and material could be brought in. Within this room, the team cut a 2 m x 2 m hole for two reasons. 

“One was to provide access to get underneath the building,” Mr Mackey says. “The other was so we could take that piece of the raft slab away to a laboratory for testing by Arup. We needed to study the quality of the existing concrete to determine whether this ambitious adventure was feasible.”

Piles and shafts

Arup determined that the raft slab was strong enough to withstand construction works occurring below without being unduly stressed.

This gave the green light for operatives to continue to dig down 2 m below the raft slab before tunnelling to just beneath the first steel column nearest the access shaft. The tunnel was supported every 500 mm by steel frames that took the load of the building and prevented the raft from deforming.

Once the team hit the point directly underneath the steel column, a two-strong team began to dig down, using steel rings to construct a 1.8 m circular sleeve as they went. On average the gang could install three rings every 24 hours to a depth of 27 m.

At this point, the shaft width was extended to 2.4 m in diameter and lined with precast concrete rings to a further 6 m depth. This wider section would eventually form the pile upon which the steel column would sit, providing a new foundation for the building.

At this depth the pile was extended out again, this time into an underreamed bell-shape, creating more surface area to carry the load of the building. Site workers could then infill the entire pile with concrete.

Colossal columns

With the shaft constructed and pile filled with concrete, the gangs could set about installing 600 mm-diameter circular steel columns. This entailed placing a cylindrical lattice of steel reinforcement into the shaft, around which two semi-circle steel shutters were bolted together.

Column created, the team could then fill it with concrete as high as possible to the underside of the raft slab of the building. To ensure a load-bearing connection, a 200 kN hydraulic jack fixed inside the columns bridged any gap caused by concrete shrinkage as it cured. 

This process needed to be repeated for the remaining 61 columns. 

claridges mc gee material movement credit nick carter

Claridge’s McGee material movement

Source: Nick Carter

Moving the materials across the site

To do this, three headings (named Tom, Dick and Harry after the tunnels in The Great Escape) were opened up and traversed the footprint of the site. From these three main headings, splinter tunnels could access the points where the columns above the raft slab were located. “It was like a scene out of an Indiana Jones movie,” Michelle Mackey says with a laugh. “There were so many tunnels.”

These headings were created after the 10th column had been completed, at which point a second access shaft at the opposite end of the site had also been opened up at the northern end of the site.

After all 61 columns were braced with jacks, the team began to excavate the ground below to create the basements. The excavation of basement level one was completed in February this year.

Logistical challenges

“The initial mining operation to create the tunnels was very intensive,” Mr Mackey says. “I have to remind myself and others that everything has been done through a window and a hole on this job.”

Because of the limited space through which the team could access the site, it had no storage space in which to keep equipment. This meant meticulous planning was needed to ensure the correct gear was brought on site for each day.

“Any mistake you make in this could mean you have 14 miners waiting around for that equipment,” Ms Mackey points out. “It was that side of it [logistics] which was where it could all fall apart.”

“In my 40 years working in London and on city projects involving demolition, this was by far the most challenging I had ever seen”

Jim Mackey, McGee

But with careful planning in place, the team avoided falling short of materials and equipment. Indeed, it was six months ahead of schedule by the time the first basement had been excavated in February.

Thanks to the rigorous construction method devised by Mr Mackey, the project has run smoothly – despite the challenges. “In my 40 years working in London and on city projects involving demolition, this was by far the most challenging I had ever seen,” he says. “But I suppose it was one of those things I thought was worth doing.”

Standing in the middle of the completed level one basement, looking at the result of such a monumental team effort, it’s easy to see why Mr Mackey thought this project was worth coming out of retirement for.

claridges mc gee 11 mackeys

Claridge’s McGee 11 mackeys

Source: McGee

Meet the Mackeys

Mr Mackey unwraps a hardboiled sweet, pops it in his mouth, and ponders the question: was the job worth coming out of retirement for?

“Yes,” he says, looking over at his joint project leader – and daughter – Michelle. “We’ve had some pretty hectic days and some challenging ones problem-solving together,” he says. “But I’ve really enjoyed it. When this job is done, I think I’ll look back on it and say, ‘I’ve enjoyed 99.9 per cent of it’.”

Ms Mackey nods in agreement. “We said to each other: we’ll do one job together and it’ll be nice,” she says. “It’s been great.”

I ask whether Ms Mackey entered construction to walk in her father’s footsteps. “No,” she says. “It’s funny, it wasn’t like that at all. I always thought construction was what dad did, and I guess back then it was a job that boys did.”

She remembers travelling with him years ago on Saturday mornings when he was doing site visits – she even recognised some familiar faces when she started working at McGee in 2004.

But Ms Mackey had an unconventional route into construction. She studied maths at the University of Southampton from 1999 to 2002, staying on another year to study for a Master’s in statistics. After graduating, she found a job in contract researching. “I hated it,” she says. “I sat there all day, didn’t speak to anyone and just did programming.”

It was at this point Mr Mackey suggested she might like engineering. Two weeks later she joined McGee and was working on site as an engineer. She also enrolled at London South Bank University to complete a civil engineering degree part-time.

Ms Mackey explains how, during her early days gaining experience as an engineer, she would always ask her dad for advice if she came across a problem on site. But 14 years later they’re working side by side solving problems as a team.

I ask them both what it’s like working as a father and daughter duo. “It’s been great working together, but we’ve had some… err… good constructive debates over the time,” he says.

Ms Mackey laughs. “Last time dad was on site working at this level was in the 80s and 90s, and things have changed so much,” she says. “Sometimes in that respect we have different perspectives. The debates we have are thought-provoking anyway, even if we may be coming from two different angles.”

But they’re united when it comes to rolling out and implementing the construction process, Mr Mackey says. “It’s very much a team effort.”

 

Readers' comments (2)

  • Fantastic project and a fantastic team. Well done Jim, Michelle and the rest of the team!

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  • Who carried out the tunnelling?

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