Conversion and Refurbishment
- St Saviour’s Church, Knightsbridge

St Saviours Church was built in two phases between 1870 and 1890, and is Grade Two listed. Located in Wilton Street, just behind Harrods department store in Knightsbridge, the church had a dwindling congregation and a significant redevelopment potential. Rather than close the church and church hall, the diocese decided on a compromise strategy to reduce the size of the church and community centre, to free up 70 % of the site for residential development.

Our clients, The Raven Group, bought an 80 year lease on the site and, based on studies by SCH Architects, opted to create two house, each in excess of 800 square metres, rather a number of much smaller flats, on the basis that this solution would create the most appropriate development, given the huge constraints of the confined site and the listed fabric.

As well as the Church, the property also included a church hall built in the late 1960’s on a narrow strip of land next to a row of Georgian cottages. The client’s brief was to maximise the usage of the site and so it was decided to place one house on the site of the hall (The Courtyard House), and the other in the rear half of the church (The Church House). The Courtyard House was to comprise a four storeys, including a new basement with swimming pool, while the Church House was also four storeys built up inside the volume of the church. Our initial site surveys revealed that the church had a relatively deep raised timber floor, and so we proposed a new basement within this building as well, to which was also added a swimming pool.

Extensive geotechical surveys established the size and depth of the church foundations and also those of the adjoining properties. We found that the Georgian cottages would need to be underpinned by 4.5 metres, that the church would need 2.5 metres of underpinning, and that the six central nave columns within the church would need underpinning by 5.5 metres.

We carried out a series of studies looking at the various methods of underpinning, and constructing basements, trying to optimise the construction costs with basement size. Local land values being so high meant that normally regarded methods often performed less well than expensive one that yielded more basement plan area. The scheme was eventually tendered on a combination of conventional mass concrete hand dug underpinning, and bored mini-piling within the church.

The nave column underpinning provided the most difficult change. The original foundations were mass concrete founded on dense gravel, and these needed to be extended down 5.5 metres lower. The presence of gravel, and the relatively small plan size of the footing precluded any sort of undermining, especially given the relative slenderness of the stone columns. The final tendered sequence of construction, was as follows;

1. Four 15 metre deep concrete piles were to be bored around each nave column.
   

2. The brickwork at each nave column base was then core drilled and four steel tubular needles were inserted.

3. A reinforced concrete collar was cast around the column and the projecting needles, to provide a strong point of connection.
   

4. Temporary steel beams were cast into the pile heads, under the collars, and resin filled hydraulic jacks were inserted between them.
   

5. The nave columns were then jacked upwards to transfer load from their foundation bearing on the gravel to the bored piles. This stage was carefully monitored.
   

6. Excavation proceeded and the mass concrete foundations of the nave columns were removed.
   

7. New reinforced concrete pads were constructed below the new basement floor level, and columns built up under the nave columns.
   

8. A new ground floor slab was constructed, supported by the new lower section of column, and laterally bracing the collars.

9. The bored piles and their capping beams were removed, and basement construction was completed.

The successful tenderer, O’Rourke Ltd, proposed an alternative pile type of bored and bottom driven steel tubes, which was accepted after some investigation. Otherwise the original sequence was used in the final construction.

Both houses were successfully completed in 1999 after a two year construction period, and each was sold for in excess of £8 million. For us the project will be remembered for the intense working relationship that we had with the contractor, for us an absolute requirement on such a technically complex project.

Architect SCH Architects and Trevor Lahif Architects