Palace Gardens Terrace

Kensington, London W8

Temporary Works: Façade retention and basement construction
 City Basements
Project value: £5m

This project is located at Palace Gardens Terrace, Kensington, London W8 and comprised the conversion of two terraced properties into one high class residence for a wealthy overseas client.

The site is located in an exclusive residential area and this placed demanding conditions on the design brief.

Broadly, the project involved the rebuilding of two existing buildings within retained front and rear façades together with the construction of a new basement level underneath the whole demise including the pavement vaults and the raised rear garden.

There were a large number of constraints placed upon the team making this a very complex project from a temporary works and sequencing perspective. These may be summarised as follows:-

  • First and foremost, CDS had to provide lateral and vertical support to both the front and rear elevations
  • No fixings into the Party Walls were permitted;
  • The visual impact to the front elevation was to be minimised to comply with Planning conditions;
  • Minimal impact on the existing and new structural framing was required;
  • The proposal had to accommodate basement works i.e. piling, underpinning and excavation operations;
  • The tower crane located in the rear garden close to the rear façade had to be taken into account.

CDS, together with City Basements, had the benefit of becoming involved in the project at an early stage and, therefore, had an invaluable opportunity to influence the way the project was sequenced. This was considered essential given the complexity of the project.
CDS played a key role in developing the construction methodology which then impacted favourably on the overall project programme and cost.

The main element of temporary works involved the design and installation of an internal steel frame that offered support to both façades and both adjoining properties. The restriction on fixings into the latter effectively ruled out the use of traditional flying shores and led the solution to a ground supported frame. This frame had to be positioned to minimise impact on the existing and future structural frames to avoid later time-consuming modifications. Furthermore, it had to allow reasonable access for the proposed basement works to progress.

Ordinarily, vertical diagonal bracing would be employed to ensure lateral stability in both planes, but given the above constraints, this had to be avoided. This led to the key decision to provide large external raking, tubular steel struts located in the rear garden founded off a section of new ground floor slab that was to be constructed in a top-down fashion.

Lateral stability of the adjoining properties was provided by merely butting the frame members tight against each Party Wall. Trench props were used for this purpose and these were secured to prevent dislodgement.
The steel frame was founded on mini piles to allow the basement works to proceed without fear of undermining the vertical supports. Each pile was capped and utilised in the temporary propping scheme for the basement excavation; this provided lateral restraint to their heads as the piles become exposed.

The ‘top-down’ slab constructed within the rear garden also provided valuable storage space during the works. This was supported on perimeter bored piles and a number of temporary bearing piles, all designed and detailed by CDS. Scheme feasibility relied on the early construction of this slab and so the construction programme focused on gaining entry to the rear for the piling works to be undertaken.

An added complication was that both front and rear façades had to be supported vertically to allow construction of the new basement. For the front, the ‘Pynford’ beam technique was employed which comprised the installation of sacrificial steel stools and construction of a permanent reinforced concrete beam that would become part of the new structure. This was supported via steel needles off temporary bored piles. The rear façade was subject to planning changes during the works, but still required temporary needling at various levels. This was accomplished by
using the main temporary steel frame.

Both Party Walls required underpinning. Secant piles were then installed inboard of the walls. Both these elements needed to be temporarily propped during the basement excavation works.
In some areas, where space was restricted and reinforcement prohibited under the Party Wall Act, innovative strapping of underpins was designed and detailed.

A tower crane also had to be accommodated on the project. This was sited close to the rear façade founded on a piled base which was also utilised for other temporary works support.