15 Sloane Square (Cadogan Mansions).

The gleaming masonry façade of Cadogan Mansions gives away little of the structure behind or the complexity below. As part of a long-term project to restore and remodel the period properties around Sloane Square, this 19th-century building was stripped down to its foundations and then rebuilt.

Location

London, UK

Client

Cadogan Estates

Architect

PDP London

Project Value

£21 million

Floor Area

6,100 m²

Status

completed 2004

Expertise

Structures

In restructuring the building, we introduced a basement area complete with sub-basement storage and five storeys of office and retail space above. Though this may seem like a straightforward development, a Victorian tube tunnel running diagonally across the site, an old sewer and the crumbling condition of the façade all needed to be considered.

The 13 m-wide Circle line tunnel which runs below is one of London Underground’s shallowest, at points being only 150 mm below ground level. The existing building had rested directly on its roof, but it was a requirement that ours did not. The demolition strategy therefore needed to be methodical and even, in tandem with 24-hour monitoring of tunnel movements. Part of the building bears onto 1.5 m-deep transfer beams, supported on deep capping beams on contiguous piled walls positioned close to the tunnel, thus allowing basements to be constructed on either side.

Vibration was a major issue, and was overcome by our innovative ‘bouncing’ floor slab, comprising around 150 sprung steel bearings situated between the foundation structure and the slab, which gave the building the freedom to move up to 20 mm vertically.

The one-brick-thick façade was stitched to the rubble behind with 6 mm steel rods in areas where collapse was imminent, with resin grout injected into the holes, and then tied to the inner frame. There was a risk of differential movement since the façade had its own foundations, so we introduced a sliding anchor to allow for this.