Close cooperation between Green energy provider Ecotricity and Aarsleff as their chosen specialist piling contractor was the key to overcoming unexpectedly stiff ground to successfully install foundation piles to support wind turbines for the extension to the Bambers Farm Wind Park at Mablethorpe, Lincolnshire. Based on ground conditions from the site investigation Aarsleff had intended to install the precast concrete piles with one of our own Banut fixed leader rigs with a 5 t hammer. But early refusal of the piles led Aarsleff, working in close agreement with Ecotricity, to change to using two rigs in unison with larger hammers.
Ecotricity erected 6 German Enercon 800 MW turbines with a hub height of 65 m and 48 m diameter three blade rotors on circular piled foundations installed by Aarsleff. Once commissioned the additional turbines, alongside the 10 existing Bambers Farm turbines, now provide a further 14MkWh/year of green electricity to an extra 4,250 local homes.
For the £3.5M Bambers Farm extension Ecotricity adopted a similar foundation specification and construction technique that turbine supplier Enercon used on many wind farms in Germany. Aarsleff’s Danish parent company, Per Aarsleff A/S, had previously installed thousands of precast concrete foundation piles, made by its sister company Centrum Pæle A/S, to the German Enercon geotechnical and structural design.
“Enercon design foundations 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 successful work for us at Reading, driving the same section piles, but judging from the soil survey we didn’t anticipate problems installing them with the single rig. We’re still learning as these are big piles which drive differently. But we have a good working relationship with Aarsleff and worked closely together to come up with some viable options and agreed a format to move forward using two different rigs with bigger hammers and different driving capabilities. I’m very impressed with Aarsleff’s professional manner in dealing with the unexpected and we will now work together on our next wind farm project in Bristol.”
The foundations for each of the Bambers Farm turbines required 16 equally spaced piles, which were installed in a circle and driven at a 1 in 4 rake. The completed 400 mm square section piles, which are 20m long and made by Aarsleff’s UK subsidiary Centrum Pile, were 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,” said Aarsleff piling manager Philip Chippindale. “We took a 5 t 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 which failed to reach capacity.”
The ground conditions proved to be different with soft clay overlaying stiff clay over sand and gravel with piles just toeing into to gravel. “We tried a 6 t hammer with an 800 mm drop and that again didn’t get piles to level. So we tried a 1 m drop which got them down, but it was a very hard, slow drive, taking 2-3 hours to install a pile. However, this proved the 20 m long piles would perform,” adds Philip Chippindale. “Our final solution involved pitching and driving piles down to about 15 m penetration with a 6 t Banut and then completing the drive with a Junttan rig with a 7 t accelerated hammer. This operation was refined by pitching and pinning approximately one third of the piles on each base at a time whilst the other rig back drove the piles on another base. The rigs were swapped from base to base until all the piles were completed. This ensured that the rigs worked safely and efficiently.”
Piling Contractor: Aarsleff Ground Engineering Ltd
Construction Date: August 2006
Scope of Works: 96 x 400mm sq x 20m precast reinforced concrete piles
Project value: £118k
Equipment: 1 Banut & 1 Junttan drop hammer fixed leader rigsDownload Case Study