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The Little Boltons

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The Little Boltons

Home improvement took on a completely new meaning when the owners of a semi-detached family residence in an exclusive area of west London decided to make some changes to the property.

Resisting the usual temptations to add an extension or simply refit the existing structure, in consultation with architects, Powell Tuck Associates, the owners decided to build a new basement and totally rebuild the interior of the existing structure.

The new six metre deep basement would extend under the front and rear gardens as well as beneath the house itself, a total area of around 315m2.
The rebuilding of the interior required the removal of all internal elements from ground floor to roof level, including floors, ceilings and stairs. Front, rear and side elevations were retained and temporarily supported during construction.

Supporting role
Access to the site was only possible from the front of the property and so large openings were formed through the front and rear walls, with sway frames being installed to facilitate plant access.

Before demolition and construction commenced in earnest, Ischebeck Titan micropiles with reinforced concrete pile caps were installed at the four corners and two sets in the centre of the property to support the sway frames. The piles were temporarily braced at intervals as the basement’s excavation progressed to assure stability of the support.

Space maximised
Main contractor Westgreen Construction Ltd, in conjunction with engineer CDS Ltd, were succesful in changing the basement’s support from a contiguous bored piled solution to silent sheet piling, installed by Giken Europe BV.

To maximise the space gained, Zero piles were installed adjacent to party and garden boundary walls. The sheet piles were internally braced with steel frames which were preloaded to limit ground movements.The sheet piles were also used to provide temporary support to the existing retained structure and to provide the lateral restraint for the Ischebeck Titan micropiles.

Ground forces
Ground conditions comprised approximately five metres of sandy silts overlying three metres of medium-dense sandy gravels which, in turn, overlaid the London Clay. Ground water level was at approximately 6 metres below existing ground level.
The existing and neighbouring structures were monitored twice weekly throughout the critical stages of the basement excavation and construction, and settlements were recorded in line with expectations (< 10mm).

Prior to excavation, and following a successful early testing programme to assure feasibility, Ischebeck Titan self-drilling micropiles, designed to work in tension and compression, were installed to enable the basement floor to withstand gravitational and uplift forces.

In what was the first use of a bayonet coupling in the UK, the micropiles were installed from the original ground level, some 6 metres above the intended finished depth of the basement.

Rather than leaving the entire pile in place, as would be required using temporary cased piles, the 6 metre drill string is uncoupled at the bayonet fixing by simply reversing the drilling direction, leaving the slab’s reinforcing micropile in place at the 6 metre depth and below, but enabling the drill string to be removed, and re-used, from the slab to ground level.

Team benefits
This method of micropile installation delivered huge benefits to both the excavation and concrete substructure programme, and also for health and safety issues. Other benefits include no trimming down of the piles, no risk of pile damage from plant impact and no obstructions to excavation plant. Lorry movements are minimal, which offers enviromental benefits, as there is very limited spoil created during installation and the cement grout is delivered dry and mixed on site. The method of installation is very quick and was ideal for these ground conditions, as conventional piles would have required temporary casing.

All in all, the methods devised by CDS Ltd. in conjunction with Ischebeck Titan have provided a time and cost-efficient solution to what could have been a very tricky project. Early consultation and the ability to think ‘outside the box’ has allowed the entire project team to bring the architect’s and owners’ plans to reality.

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