Dinesh Chandra Patel
Director
Arup, London

Jim Mackey
Project Director
McGee Group Limited, London

Business as Usual—Upgrading Without Operational Interruption

To increase capacity and upgrade services at London’s storied Claridge’s Hotel, a five-story basement was mined below the listed Victorian structure, without ever disrupting the live operation of the hotel above. To construct new foundations and basement beneath the 90-year-old raft, 400 meters of horizontal tunnels were constructed to each of the 61 columns that support the building above. Vertical, 1.8-meter-diameter shafts were then hand-dug beneath each column with supports for caisson foundations. Five-story columns were built within each shaft with self-compacting C60/75N concrete, up to the underside of the existing raft, to provide vertical support. Once that was completed, the basement was excavated by miners, top down, around the new columns, to 22 meters beneath the “floating” structure. All 25,000 cubic meters of material was removed through one window at the rear of the building.

Investigative works showed that the old raft sat on an alluvial silt clay layer with high groundwater, which, when under water, behaved like toothpaste, and endangered mining operations. This almost brought the project to a stop, but a vacuum dewatering system was developed so that the strength of the material was improved prior to digging. With no previous precedent for such a project, staged 2D and 3D finite-element (FE) geotechnical analysis modeled the various activities across the complete construction sequence to predict load distribution, ground settlement, and heave from the dig. Inelastic cracking behavior of the raft required that it be jacked up or down to avoid damage from construction movements. As the work progressed, noise and vibration effects did not impact the hotel or neighbors. The project was delivered four months early and within the original budget.

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