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Fresh start

Identifying, re-using and even removing old piles is a growing market.

Special measures to avoid clashing with piles, some dating back to Roman times, enabled Cementation Foundations Skanska to install piles for a new office development on a brownfield site in the City of London.

The 12 week contract on behalf of main contractor HBG at 12 Arthur Street, EC4, involved the installation of 57 large diameter rotary piles. Working from existing basement level, piles ranged in diameter from 600mm to 1,800mm diameter, up to maximum depth of 40m, typical of many large building developments in the capital.

Less typically, the contract also involved the removal of old piles to make space on the congested site close to London Bridge and the Monument.

At tender stage, it was anticipated that the removal of 15 concrete and 15 timber piles would be necessary, as these could not be reused due to lack of information on concrete strengths, reinforcement details and pile depths. But probing the pile positions ahead of the main works revealed that the co-ordinates did not coincide with layout drawings.

By assessing the likely verticality of the old piles, Cementation Foundations together with HBG and consultant Arup, designated a 'zone of confidence' within which new piling could be installed without clashing. Outside this zone, additional piles were removed.

In all, 40 concrete piles and two timber piles were taken out by a standard Bauer BG30 piling rig with 880mm segmental casing, using a specially stiffened kelly bar in conjunction with a rock auger.

The segmental casing was positioned over the pile and drilled into the ground, with the auger then rotated inside to break up the pile before extracting the debris.

The choice of material used to backfill the empty bores was equally important. It had to match as closely as possible the existing soil strength and stiffness, and a bentonite and cement mix was developed in trials before the main works began.

Working closely as a team, Cementation and HBG completed the removal and then installation of the new piles within the original 12 week contract period.

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