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Power base

Wind power - Techniques from Germany are being used for turbine foundations.

Aarsleff Piling has broken into the UK's onshore wind energy market with its first piling contract for an 85m high wind energy converter at the Green Park industrial complex at Reading, Berkshire.

German turbine supplier Enercon is adopting a foundation specification and construction technique that is well established in Germany. It requires 30 equally spaced piles set round a 14m diameter circle, and fanning out on a 1 in 8 rake. Aarsleff has installed precast concrete foundation piles, made by its Danish sister company Centrum Pæle. The completed piles are capped with a 2.5m deep, 15.5m diameter reinforced concrete slab with cast-in holding down bolts to take the turbine tower's 4.5m diameter tubular steel base section.

Site investigation suggested piles of up to 17m long were needed to punch through the site's soft clay, Thames gravels and London clay, Lambeth sands and gravels and into the underlying stiff clay. Combined end bearing and skin friction would accommodate cyclical working loads of 1,030kN compression and 278kN tension.

Using a 6t drop hammer, Aarsleff drove the first two piles diametrically opposite each other. But both refused in the dense sand and gravel at around 10m penetration. Enercon and Aarsleff called in Precision Monitoring Control (PMC) to carry out load tests. PMC found the piles could take the required loads.

Aarsleff continued piling with heavier reinforced concrete piles of shorter length. Ground works contractor Terraform followed on, breaking away concrete from the pile tops to expose the reinforcement and complete the pile cap.

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