Aarsleff lays tubular steel piles for West Durham wind farm

Aarsleff is installing steel piles at the West Durham Wind Farm for the UK's first wind turbines to be supported on tubular steel foundations.

The UK's first onshore wind turbines to be supported on tubular steel piled foundations will form part of a new wind farm being built in County Durham.

A backfilled and reinstated former opencast coal site near Tow Law is being transformed by Banks Developments into the 24MW West Durham Wind Farm, supplying renewable energy into the national grid with five of the 12 turbines supported on drill casing recycled from the oil industry.

The steel tubes for five of the turbines' concrete bases are needed to punch through the unpredictable backfill, with its obstructions, boulders and voids, to toe into the underlying sandstone bedrock. The remaining seven bases are gravity reinforced concrete structures founded directly onto sandstone at normal ground level.

Hanson Contracting, with its consulting engineer Grontmij, is the main £6m design-and-build civil works contractor working for Banks Developments, which is using renewable energy consultant Natural Power to oversee the overall project construction. Hanson and Grontmij proposed using steel tubes after it was agreed that precast concrete piles were not suitable for the project.

"We had used Aarsleff before on other projects and knew we would work well together," says project manager Peter Hetherington. "We had discussions about the suitability of steel tubes and various sizes were evaluated, ranging from 244mm to 508mm in diameter. Aarsleff eventually suggested 340mm-diameter tubes, which were considered the optimum solution to cater for the ground conditions and loadings."

The 340mm-diameter tubes, which are oil well casings manufactured from high-grade steel, would be stiffer and could be hit harder by the hammer and be less likely to deviate in the backfill than the other options.

"Aarsleff believed the tubes had better overall capability and reduced the risk for Hanson, as any delay in the piling, due to obstructions, would be our problem and cost," says Hetherington.

"However, recycling and using a second-hand by-product from the oil industry presented some initial problems for us as there was no quality assurance certification for the tubes. We worked closely with Aarsleff and arranged for some independent materials testing to prove the quality of the material, which provided the certified quality needed. As a result, Aarsleff was awarded the contract for supplying and installing the steel tubes."

Aarsleff continued to work with Grontmij and Hanson to refine the type of pile load-testing required. Originally, static load tests had been specified, which would have been expensive due to all the necessary kentledge required and practically difficult due to the limited room on site. Instead, dynamic load tests were carried out on three piles in each base and approved by Grontmij before the remainder of the piles were installed in each base. It was agreed that if the dynamic load testing did not prove the design factor of safety, then static load tests would have to be carried out before the remainder of the piles were installed.

Aarsleff started on site using one of its 56t Junttan PM20L self-erecting hydraulic piling rigs with a 4t Junttan accelerated impact hammer. It is installing 36 tubular steel piles, equally spaced on a 15.6m pitch circle diameter in each of the five bases. The threaded and collared oil well casing tubes vary in length from 11m to 14m to cater for the 10m to 21m-long piles driven through the varying depth of backfill and into the underlying sandstone bedrock. Piles are driven to a sett of 25mm from 10 blows of the 4t hammer on a 1m drop.

Steel foundations

Aarsleff, which believes these are the first onshore turbines in the UK to be supported on steel piled foundations, started on the main piling in the third week of July and completed the 180 piles by the end of August.

After driving the open-ended tubes in each base, Hanson follows on cutting to length and filling the piles with an inert material. The top metre is filled with a concrete plug and reinforcement to tie into the main 17.3m-diameter reinforced concrete turbine base, which contains a cast in-situ 5m diameter flanged steel can to take the turbine tower.

Each base will support a 2MW REpower MM82 turbine with a hub height of 59m and three bladed rotor of 82m diameter. The first turbines are due to arrive on site early in 2009 and when fully operational next summer, West Durham Wind Farm will produce 24MW of electricity from the 12 turbines, enough to meet the needs of 13,400 homes.