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Unexpectedly hard ground meant increasing the number and power of driving rigs for a wind farm extension.

Green energy provider Ecotricity and its foundation contractor Aarsleff Piling faced unexpectedly stiff ground when installing foundation piles to support wind turbines in the English East Midlands. The work was needed for the extension to the Bambers Farm Wind Park at Mablethorpe, Lincolnshire.

The ground was so hard that during piling the timber packers between pile head and hammer driving cap were smoking heavily by the time the final blows were struck.

Based on ground conditions from the site investigation Aarsleff had planned to install the precast concrete piles with one of its own Banut fixed leader rigs with a 5t hammer.

But early refusal of the piles led it to change to using two rigs in unison with larger hammers.

Ecotricity is erecting six German Enercon 800MW turbines - with a 65m hub height and 48m diameter three blade rotors - on circular piled foundations. When commissioned, the additional turbines, alongside the original 10, will provide a further 14M. kWh/year of green electricity to an extra 4,250 local homes.

For the £3.5M (e5.2) Bambers Farm extension, Ecotricity is adopting a foundation specification and construction technique similar to that which turbine supplier Enercon has used on many other wind farms. Aarsleff is no stranger to the system. Its Danish parent company, Per Aarsleff, has installed thousands of precast concrete foundation piles - made by its sister company Centrum Pæle A/S - to an Enercon geotechnical and structural design.

'Enercon designs foundation solutions in Germany based on piles from Centrum and now Aarsleff has extended that to the UK, ' says Ecotricity project manager Simon Vince. 'Bambers is a follow on from Aarsleff's work for us at Reading [west of London], driving the same section piles, but from the soil survey we didn't anticipate problems installing them with the single rig, ' he says.

The foundations for each of the turbines require 16 equally spaced piles, which are installed in a circle and driven at a 1 in 4 rake. The completed 400mm square section piles, which are 20m long and made by Aarsleff's UK subsidiary Centrum Pile, will be capped with a reinforced concrete slab with cast in holding down bolts to take the turbine towers.

'Judging from the pile installation history on the initial adjacent wind farm and the site investigation information, we expected ground conditions on the Bamber extension to be similar, ' says Aarsleff piling manager Philip Chippindale.

'We took a 5t hammer to site and although piles started refusing early it wasn't initially considered a problem because the site investigation suggested we may be able to achieve the required pile capacity. However, we tested the piles and they failed to reach it.'

The ground conditions proved to be different from expectations with soft clay overlaying stiff clay over sand and gravel, with piles just toeing into the gravel. 'We tried a 6t hammer with an 800mm drop and that again didn't get piles to level. So we tried a 1m drop which got them down, but it was a very hard, slow drive, taking two to three hours to install a pile. However, this proved the 20m long piles would perform, ' says Chippindale.

'Our final solution involved pitching and driving piles down to about 15m penetration with a 6t Banut and then completing the drive with a Junttan rig with a 7t accelerated hammer.'

This operation was refined by pitching and pinning about a third of the piles on each base at a time while the other rig back drove the piles on another base. Aarsleff swapped the rigs from base to base until it completed all the piles.

Turbine erection on the project is still ongoing with two of the six installed so far.

'We're still learning as these are big piles which drive differently. But we came up with some options and agreed a format to move forward using two different rigs with bigger hammers and different driving capabilities. We will now work together on our next wind farm project in Bristol.'

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