There is an adage that foundation engineers can achieve any solution - given the budget. But for the budget conscious a high water table, soft alluvium and extensive river gravel combine to make cost effective pile design and basement construction especially onerous at London's increasingly fashionable Isle of Dogs. Frequent hard bands within the underlying Lambeth Group only add to the problem.
Faced with these conditions, piling contractor Westpile developed special augers to install a basement and load bearing piles for a new flagship construction for Weston Homes at 41 Millharbour.
The design for the approximately 85m by 45m site is for a 14 storey, quality residential building with some commercial use, and with a two storey basement to provide car parking.
In retaining wall terms, this was a complex job, says Westpile. 'At the client's specific request, the deep basement was to be free standing for the duration of the basement construction works.
The northern wall in particular had to act as an unpropped cantilever to a height of 8.5m, taking into account deep pile cap excavations directly adjacent to the wall.'
With the original ground level at approximately +5.5m OD, Westpile first recommended a pre-excavation of the site to about +4.0 to + 4.5m OD, reducing the retained height to 7.0m and ensuring that the secant walls would not require temporary internal propping during excavation.
In the first phase of the contract, Westpile installed a total of 622, 600mm and 750mm alternate interlocking hard piles and soft piles. The hard piles were reinforced predominantly to their maximum depth of up to 16m.
The contract period was 56 piling days using two large Soilmec hydraulic rigs.
Following the excavation of the basement, Westpile returned to install the 500mm diameter load bearing piles, which penetrated up to 16.5m into the Thanet Sand.
The Millharbour project, says Westpile, depended especially on successfully getting through the hard bands at the right price. It was completed on time and within budget leaving the development on schedule - and on track for occupation by 2004.