Arup, McGee Group, RKD Consultant
Maybourne Hotel Group
Engineers overcame seemingly impossible constraints to design and build this huge five-storey underground structure unbeknown to passers-by, whilst the famous hotel above remained operational.
The 1920s steel-framed hotel building above the new basement, with its original reinforced concrete raft, was supported on soft alluvial soils with the added complication of ground water being present right up to the underside of the old raft.
First, the area was dewatered, which improved the strength of the underlying soil, making it safer to excavate. Next, horizontal tunnels were formed from a single 7m x 7m room starting point to 62 locations aligning precisely with the existing columns in the old building above.
At each location, a 1.8m diameter steel lined shaft was hand-excavated to a depth of 22m and then increased in diameter at the base to form 62 bases. From each new base, a 0.6m diameter column was extended to support the original raft. Incredibly, all materials in and out of the site were passed through a single 2m2 window.
The judges were extremely impressed by the adaptation of the design as challenges emerged, with the engineers working collaboratively with the contractor to make it more practical and faster to build.
The project was finished four months ahead of programme, on budget and all with zero disturbance to hotel guests.