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Basement brilliance

Spotlight - Geotechnical temporary works are aiding a west London basement project.

Temporary works more akin to ground engineering propping have been used on redevelopment of two properties near to the Old Brompton Road in London's South Kensington.

The project involves converting two end of terrace properties into a single residence. In most circumstances this would have been relatively straightforward.

But here, in addition to normal construction work, the developer wanted to create a 13m by 8m basement under the house on the end of the terrace.

To achieve this, the house had to be demolished and the adjoining house and neighbouring walls temporarily supported.

Ground conditions are typical for the area, comprising 5m to 6m of dense terrace sand and gravels overlying London Clay.

The water table is below the level of the new basement.

Initial considerations suggested an underpinning solution for the adjoining property and boundary wall. This would have required sacricial earthworks support and temporary propping at several levels to resist horizontal earth pressures.

For safety reasons, underpinning would have had to be constructed in a minimum of two vertical lifts due to the overall depth of the excavation.

In addition, the front, central and rear walls of the adjoining property would also have required underpinning using transition pins.

In short, underpinning would have been costly and potentially risky, a fact recognised by temporary works consultants Contractors Design Services (CDS).

CDS managing director, Simon Smith, explains: 'Underpinning is probably the first thought when faced with a situation like this, but with the complexities of this project, we very soon realised that this was a non-starter.

'So, with specialist groundworks contractors WT Group and anchor supplier Ischebeck Titan, we devised a solution using Pali Radice piles.' Piles were created using Ischebeck Titan's 127/111 hollow self-injecting anchors.

Each anchor has an external diameter of 127mm and an internal diameter of 111mm through which grout is injected under pressure as drilling proceeds.

The grout exudes from apertures in the drill bit, creating a continuous low pressure scouring action against the sides of the drill hole. This action exposes harder parts of the hole whilst ushing and removing softer materials.

A repeated withdrawal and redrilling procedure reams and improves the hole, which fills with grout to create a strong, irregular grout body reinforced by the steel anchor that runs through its centre.

The piles were installed at 450mm centres using 225mm sacricial drill bits through the existing masonry party and garden walls.

Holes were inclined at 15° to the vertical.

This meant that the piles provided vertical support to the walls and horizontal support to the ground below, allowing for excavation of the basement without the need for further temporary works.

A single level of temporary bracing was installed at the pile heads to maintain overall horizontal stability until the basement had been completed.

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