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Piles double as basement props

Temporary support to existing basement walls has been provided by new basement retaining piles at a central London redevelopment.

FOUNDATION CONTRACTOR Stent recently finished a major piling contract in London for insurance giant Legal & General. Three of its rigs worked flat out on the 15 week fast track contract to install 740 secant wall piles and 114 load bearing piles for new basement and load bearing piles for an office development in Gresham Street in the heart of the City.

Most of the work was carried out from existing basement level, while demolition and other groundworks were under way. The exception was the northern part of the retaining wall for the new basement, which was put down from a temporary berm outside the site at street level.

Because space was tight, it was decided to replace raking props bracing the existing basement walls with some of the piles that form the hard/hard secant pile retaining wall for the new basement. These were installed in advance of the main wall and abutted the existing wall at 9m intervals, with 1,015mm diameter thick wall casing extending upward to form a temporary brace against the wall above. Each pile was installed through two soft cement bentonite female piles to ensure continuity of the completed secant wall.

Stent was working for main contractor O'Rourke Civil Engineering and construction manager Bovis Construction on the design and build project. Piling was split into three main areas: the up to 21m deep, 600mm to 750mm thick, hard/hard secant retaining wall surrounding the site, abutting the existing basement walls and deepening it; an internal core area, surrounded by a 900mm thick hard/soft secant wall; and load bearing piles between 900mm and 1,500mm in diameter.

The hard/hard secant wall was built using an adapted high powered CFA rig. However obstructions in the ground required one side to be built by conventional rotary methods. Because of low cut-off levels, load bearing piles were also installed by rotary methods. To allow steel erection to continue on piles under the temporary berms supporting the secant wall, 28 piles were built as plunge columns with 1mm tolerance in plan and elevation.

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