St. Savior's House, Walton Street, Knightsbridge, London SW1

Date Completed
1998
Architects
Smith Caradoc-Hodgkins
Consulting Engineers
Barton Engineers
Main Contractor
O'Rouke Limited (part of the Laing O'Rouke Group)
Basement Contractor
Room Below

Some fifteen years ago Room Below were contracted by O'Rouke, now part of the Laing O'Rouke Group) to design and construct the enabling and permanent works which were ultimately to form two basements, one partially under St. Savior's Church and the other on the adjoining land which now forms Hastings House.

The development comprised converting a substantial part of St. Savior's Church into an exclusive home and adding a retrofit basement. Underpinning the foundations of the Church was considered by the Main Contractor to be too expensive and they turned to Room Below for an engineered alternative. To form the basement a contiguous piled wall was constructed inside the line of the outer walls by pre-drilling and, in a separate operation, driving bottom driven thick walled segmental threaded tubes to a designed depth. A reinforced concrete capping beam was then cast and the area inside the contiguous piled wall excavated to form the basement substructure.

Supporting the nave columns in the temporary state proved to be the most difficult challenge. The original foundations were mass concrete founded on dense gravels, and these needed to be extended downwards by over 5 metres. The brickwork foundation at each nave column base was core drilled and needle beams inserted. A reinforced concrete collar was then cast around the column incorporating the needle beams. Four bored reinforced concrete piles were constructed around each column and temporary steel beams cast into each pile head effectively forming a tabletop below the level of the collar. Hydraulic jacks were placed between the temporary steel beams and the reinforced collar above, such that the nave columns could be jacked upwards to transfer load from the foundation bearing on the gravels to the bored piles. When piles came under load they would settle and the hydraulic jacks operated with equal and opposite action to ensure that there was no separation or movement to the stone columns above. The columns were then extended downwards to basement level.