A £38M wind farm is under construction at Lissett, east Yorkshire, on the site of a former Second World War aerodrome. Contractor McNicholas is installing 12 turbines for Norvera Energy UK, which will generate electricity for about 13,400 homes.
Thirty piles are needed for each of the 12 bases – 360 in total. The Junttan PM26 LC is Aarsleff Piling’s largest rig. The firm specifically bought it to drive the large-section precast concrete piles required to take cyclical loadings for wind turbine foundation bases.
The piles are being driven at an angle to a depth of between 20m and 30m through clay with bands of sand and gravel into the underlying chalk.
The piles are made up from two 400mm2 section precast concrete piles jointed together. These will be strengthened with additional reinforcement to withstand compressive working loads of 1,200kN and tension loads of 170kN.
As the piles are 15m long and very heavy, Aarsleff believes the only rig in the UK that can handle and drive them at a backward and forward rake is the Finnish Junttan PM26LC with its HHK9A accelerated hammer. The HHK9A hammer is one of a new generation of Junttan hydraulic impact hammers with adjustable segmental drop weights of 9t, 7t, and 5t, and a variable stroke of up to 1.2m. It is operated from the rig’s hydraulic system and accelerates the drop weight during the fall, boosting impact energy.
This increases the efficiency at full stroke by up to 20% over a conventional free fall drop hammer of the same weight. The hammer produces a maximum impact energy of 106kN/m at full 1.2m stroke when equipped with the largest 9t drop weight, but at Lissett the hammer is in the mid-size 7t mode.
Impact energy, drop height and blow rate can be adjusted by the rig operator to suit ground conditions and pile type as the hammer is suitable for driving all types of precast concrete, steel tube and sheet and timber piles. All relevant pile-driving data is displayed on the rig’s computer screen in the operator’s cab.
As Aarsleff finishes the piling for each base, McNicholas follows on, cutting piles to level to expose the reinforcement and completing the 17m-diameter heavily reinforced concrete foundation bases and other ancillary works and access roads.
The 2.5MW Nordex N90 wind turbines, which each have a hub height of 80m and 90m-diameter blades, will then be erected, tested and commissioned by Nordex, with the first two expected to be operational by the end of this year.
As a reminder of Lissett Airfield’s wartime role, Norvera will erect a memorial and name the 12 individual turbines after the names of some of the Halifax bombers that flew from the base.