Working with architect Powell Tuck Associates, the owners of the property decided to build a new basement and rebuild the interior of the existing structure.
It is an ambitious project given that the property is semi-detached and, therefore, any construction and excavation work needs to take into consideration neighbouring properties. It also meant that access to the site was extremely limited.
The new 6m deep basement extends under the front and rear gardens of the property, as well as beneath the house itself, over a total area of 315m2. Rebuilding 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.
Access to the site was only possible from the front of the property so large openings had to be formed through the front and rear walls, with sway frames installed to allow for plant access.
Temporary bracings installed at intervals between piles assured their stability as the basement excavation progressed.
Main contractor Westgreen Construction, in conjunction with engineer CDS, devised an imaginative scheme for the project, changing the basement's support from a contiguous bored piled solution, to silent sheet piling installed by Giken Europe.
To maximise the space gained, the Giken Zero piles have been installed adjacent to party and garden boundary walls. These sheet piles are internally braced with steel frames that have been pre-loaded to limit ground movements. They also provide temporary support to the existing retained structure and lateral restraint for the micropiles.
Ground conditions comprise about 5m of sandy silts, overlying 3m of medium to dense sandy gravels which, in turn, overlays London Clay. The groundwater level is about 6m below existing ground level.
The house and its neighbouring structures were monitored twice-weekly throughout the critical stages of basement excavation and construction, and settlements were recorded in line with expectations of less than 10mm.
Prior to excavation, and following a successful early testing programme to assure feasibility, site workers installed the second batch of micropiles. These are designed to work in tension and compression. They enable the basement floor to withstand gravitational and uplift forces.
It is these micropiles that have seen the first use of a bayonet coupling in the UK. The micropiles are installed from the original ground level, some 6m 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 6m drill string is uncoupled at the bayonet fixing by reversing the drilling direction, leaving the slab's reinforcing micropile in place extending down from 6m below ground level. The drill string is then removed and re-used.
A total of 53 micropiles were needed for the project in a variety of sizes and ranged in length from 8m to 12m. Safe working loads in compression ranged from 150kN to 500kN and in tension from 150kN to 300kN.
This method of micropile installation created benefits for the excavation and concrete substructure programme as well as for health and safety. These 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 environmental benefits, as there is very limited spoil created during installation and the cement grout is delivered dry and mixed on site. The project team claim the method of installation is quick and was ideal for these ground conditions, as conventional piles would have required temporary casing.
Following piling work, Westgreen installed the basement slab in August.