Parts of London’s Claridge’s hotel will soon extend as far below ground as it does above. This called for complex numerical modelling to make the scheme possible.
Claridge’s hotel is a favourite haunt of the rich and famous, and for the last few years it has also been home to, what is arguably, London’s most complex basement scheme.
Much care has been taken to ensure guests enjoying the five-star hospitality have been unaware of the work that will deliver even more luxurious facilities. The scheme is believed to be a world-first, with basement construction under a live hotel. It is being delivered by contractor McGee and consultant Arup.
The project has been a long time in the making – discussions with Paddy McKillen, who heads up the Maybourne Group that also owns the Berkeley and Connaught hotels, started a decade ago. The recession and ownership issues caused part of the delay but working out how to deliver the ambitious scheme has also taken time.
According to Arup director Dinesh Patel, complex numerical modelling has been key to the current success of the work. When completed, the project will deliver a five-level basement below the 1920s extension of the hotel that will house a spa, gym, retail outlets, plant and backroom operations.
The work also involves the relocation of equipment on the top two floors to freeing up roof space for up to 40 new penthouse rooms.
“In 2007 we were invited to prequalify for the two-level basement along with other contractors,” says McGee project director Jim Mackey. The aim was to bring the hotel into the 21st century with a gym and spa.
“I don’t believe anyone has built a basement below a live hotel, let alone one of this size”
Most submitted proposals but said the hotel had to close to deliver the scheme. McKillen said they were not listening.“We were the only one to put forward a scheme to keep the hotel open.”
Mackey brought Arup on board to work out the detailed delivery of the mining technique-based scheme. Arup used numerical modelling and analysis to prove the Mackey’s theory.
The feasibility study started in 2015 and Patel’s team worked on the geotechnical and structural issues with close liaison with RKD Consultant director Adam Pellew on the temporary works elements of the scheme.
“This process is a first as far as I’m aware – I don’t believe anyone has built a basement below a live hotel, let alone one of this size,” says Patel. “The investigation was needed to prove that the solution could work, and the exercise took almost five months.
“At the start, we didn’t know if there would be a project or not. There was little known about the raft foundation which the 1920s extension sat on.
“We needed to know if there was any reinforcement and, if so, how much? Also how poor the ground under question was, as the hotel is sat above the route of the Tyburn River and we didn’t know if it would be possible to dewater the soft sediments sufficiently to excavate the basement,” explains Patel.
The existing foundation raft supports 61 columns but is almost 100 years old.
“The raft is not constructed totoday’s standards so it was hard to know how it would react to the excavation work,” adds Patel. “How will it distribute the load? Would it be strong enough to span over an excavation below one of the columns?
“It was frightening how many things could go wrong on this scheme. The risks needed to be carefully managed.”
The concept involved breaking through the raft to excavate by hand below each column to install a 1.8m steel caisson grouted into position and widened at depth to a diameter of 2.4 musing precast concrete rings. An underream was then excavated for the foundation, which extends up to 7m below the 22m deep basement, and the column – with starter bars for each basement floor slab – was cast with twin jacks placed between the top of the new column and the base of the existing column.
Mining techniques were to be used to carry out this process under all 61 columns before the basement excavation could get underway. Arup carried out a series of finite element analyses using 3D LS-Dyna to simulate the proposed construction works with the aim of understanding the movement of the existing structure, forces induced in the new caissons and stresses, and bending moments and shear forces induced in the existing raft and new structure.
“These models were crucial to our understanding of the behaviour of the existing raft that supports the weight of the hotel,” says Patel.“Essentially, it showed that the scheme was viable.”
At this stage the project was letunder a lump sum, which Mackey says is worth between £30M and £40M. The model formed the strategy for monitoring the movements in real time, the actual loads in each column, and the jacking strategy at each stage of the construction activity. This ensured that the raft was kept from deflecting too much.
To understand the ductility of the existing raft, an advanced 3D FE structural model of the raft with the actual reinforcement was also created. This model provided information on the structural capacity of the raft and how the reinforcement would yield under different loading conditions.
Now work has progressed to basement excavation (see box), the model is also being used to understand and predict the heave so that loads within the jacks can be adjusted accordingly.
“The hotel is effectively floating on oil at the moment,” says Patel. Work on site has just progressed to excavation of the second basement level in the top down scheme and the structure is on schedule to be handed over for fit out later this year.
“The sequencing based on thenumerical modelling has been key,” Patel concludes.
The construction process
Excavation work started on site in February 2016 from a hole in the raft measuring just 2m by 2m, which was later doubled in size.
In total 400m of hand excavated tunnels were constructed with three headings – named“Tom”, “Dick” and “Harry” – to access all of the columns, with the headings threaded between the columns with adits used to access the columns to help manage the ground movement.
“Initially we had just one room to work from, with everything going out of a ground floor window, but we gained a second work area on the other side as work progressed to speed up the operation,” says McGee project engineer Michelle Mackey.
“By working on two face swith three columns being excavated and three being concreted at a time, we were able to reduce the programme by six months.
“We have advanced quicker and the ground movements have been smaller than expected.”
Mackey says that, in total, 6,400m³ material has been taken out for the column construction. Mining the columns was not the only challenge, Mackey has also had to manage low headroom piling work and complex temporary works to create piled walls for groundwater cut off.
Despite this work, the groundwater still presented issues and McGee worked with WJGroundwater to undertake vacuum dewatering. “Vacuum dewatering is not commonly used in the UK but was essential for this project,” she says. “Ground freezing and grouting was considered but considered too difficult, given the access issues.”