The peat Moorlands of the Pennines have posed several challenges for foundations contractors at Britain’s latest wind farm development. Declan Lynch reports
Constructing a wind farm involves successfully negotiating a series of barriers. Not only are there many stringent planning regulations - with local residents frequently opposed to such developments - but they are often located in remote sites, with difficult ground conditions.
Hyndburn wind farm, situated near Blackburn on Oswaldtwistle Moor, is a case in point. Client Energie Kontour had to steer its wind farm proposal through a tricky planning process, which involved halving the number of turbines, and now contractor Galliford Try is faced with constructing it on difficult peat ground.
Construction of the foundations for the 12, 2.5MW capacity wind turbines on the hills overlooking Blackburn demands more flexibility than a “one size fits all” approach.
“In some ways it would be easier to construct one type of foundation. But it wouldn’t be an appropriate solution - With peat ground conditions there is no consistency”
Costas Pantelidakis, Galliford Try
Where the glacial till is more than 10m deep, designer Donaldson Associates has opted for piled foundations, but where it is much closer to the surface, gravityfoundations are being used.
“In some ways it would be easier to construct one type of foundation,” says Galliford Try’s project manager Costas Pantelidakis. “But it wouldn’t be an appropriate solution.”
The site geology is divided into two distinct areas. At the southern end - where three turbines are to be located along with the substation - the ground is glacial till made up of gravelly clay. At the northern end of the site the remaining nine wind turbines are constructed on peat up to 4m deep.
“With peat ground conditions there is no consistency,” says Pantelidakis. “No two samples are the same.”
But the geotechnical issues with the peat are not the only challenges. Peat lands are protected by Natural England and there are strict limits on how much can be excavated, with specified locations for where the material must be transferred to within the construction site.
The piled foundations consist of 600mm diameter continuous flight auger (CFA) piles. “We used CFA piling because we were expecting to find big boulders in the ground,” says Pantelidakis, adding the likelihood of hitting boulders ruled out using driven piles. “Of course, we didn’t hit any boulders when installing the piles.”
To construct the piled foundations, engineers from subcontractor Wind Farm Civils excavated down about 2m from ground level and then cast a 50mm thick concrete blinding level to create a working platform. Wind Farm Civils then installed 20, 11.5m to 12m long CFA piles incorporating a 3m to 4m socket to reach the gravelly clay bedrock. The steel reinforced piles are being formed with C25/30 concrete and are being cropped, leaving a 600mm long reinforcement ready to be tied in with the lower base.
“Dynamic testing will be performed on one pile per foundation to ensure its integrity,” adds Pantelidakis.
In addition to the main construction work Galliford Try is also constructing 6.5km of trenches and piping for the power cable connecting the wind farm into the electricity network at Blackburn.
Subcontractor Wind Farm Civils is constructing the 6.5km 33kV power cable route from the Hyndburn wind farm’s substation to Electricity Northwest’s substation at Blackburn. This cable, known as the contestable cable, is where the developer builds the asset but it’s maintained by the regulator. In this case the regulator is Electricity Northwest which will be adopting the cable in May.
The 200mm diameter copper power cable is being laid at a depth of 900mm for the majority of its length.
Linking the site substation to the wind turbines are a series of 33kV power cables running along the access road network. These cables will be laid after the wind turbines have been installed.
The foundation itself splits into an upper and lower base. Steel fixers create a 14m2, 2m high reinforcement cage consisting of 16mm and 25mm diameter rebar at 200mm centres to create the cast insitu C35 concrete lower base. The cage has space for seven 150mm wide UPVC pipes to carry electrical cables from the turbine to the electricity network, via the substation.
Once the concrete has been cured, engineers can then lift in the 10t foundation anchor can which supports the turbine tower. “It’s critical that we have the height of the can correct,” says Pantelidakis. “The height of the can determines the height of the wind turbine, which is strictly 80m above ground level. Any different and we don’t meet our planning conditions.”
To connect the foundation anchor can into the base, alternate 32mm and 25mm diameter rebar at 125mm spacing is linked between the lower base and foundation anchor can to form the upper base. “There are 200mm diameter holes in the can where the reinforcement is meant to pass through,” says Pantelidakis.
A stronger C50 concrete is then used to create the hexagonal upper layer. In all, each piled foundation takes about 20 working days to construct.
For the nine remaining bases Galliford Try is constructing gravity base foundations. Each gravity base measures 17.5m by 17.5m and is constructed on 1m to 1.5m of engineered granular fill. “We dug down up to 5m to find the top of the bedrock,” recalls Pantelidakis.
On top of the fill a layer of 50mm blinding is placed ready for the reinforcement steel cage. Steel fixers are used in constructing the steel cage, with 16mm to 25mm steel bars at 200mm centres to create the lower base before casting using C35 concrete. Once the lower base is completed, the foundation anchor can is constructed in the same manner to that of the piled foundations.
“Progress on the site has been good so far,” says Galliford Try works manager Jim Brannan. “But the weather is a real issue. It can completely change in 10 minutes but it generally doesn’t stop work.”
Work is ongoing at the site. Foundations are due for completion this month, ready for a summer installation of the wind towers and turbines.
Turbine manufacturer RE Power will install the turbines and towers, but Galliford Try will remain as main contractor. On each foundation RE Power will install its MM82 wind turbines with a hub height of 80m and an overall height of 122m. The turbines will produce a significant loading on the foundations which have an axial design load of 1,600kN.
But there are still challenges to be resolved before work reaches this stage. Next to each foundation is a lifting area where RE Power will set its crane to install the wind turbine and tower. “We are still looking at ways to stabilise the peat ground next to the bases,” says Pantelidakis.
Each lifting area, measuring 40m by 25m, must withstand a design pressure of 200kPa at any point to support the crane and vehicles without the use of piles. Galliford Try is working with Donaldson Associates to come up with a solution. “We are considering a floating rock platform or some sort of peat stabilisation,” says Pantelidakis.
Once complete Hyndburn is expected to generate up to 72.2M/kWh of electricity a year based on a 27.5% capacity factor. That means it will meet 52% of the electricity needs of the households in Hyndburn - thanks to the innovative foundations.
A key part of the project is constructing the 4.5m wide, 6.5km long access roads in rugged terrain to be used both during the construction and maintenance of the wind farm’s 25-year life.
It was the first job for Galliford Try when it started on site last December. “It was a completely greenfield site when we started,” says Galliford Try project manager, Costas Pantelidakis.
Access road construction began just before Christmas 2011 with Galliford Try taking two weeks to construct the 500m road from the existing Hasledon Road to the site compound - marking the beginning of the construction site.
Using three excavators, bulldozers and rollers, engineers began by removing up to 2.5m of topsoil. “The ground conditions are very onerous,” says Pantelidakis. “The topography was very hilly, but at least with this first area we didn’t have the peat to worry about.”
After excavation, engineers laid down a geo-grade mat topped by 400mm to 1m of aggregate to create the access road. The design of the road, agreed with the hauliers constructing the foundations and installing the wind turbine, limited the gradient to 12% - a challenge given the hilly conditions.
“We’ve had to widen the corners of the roads to allow for the large flatbed trucks that will access the site,” says Pantelidakis.
Once the site compound was constructed, work could begin on the access roads to reach each of the 12 turbine locations.
The wet and windy conditions on top of the moor have been an ongoing challenge for the team though.
“Drainage problems are causing havoc with the roads,” says Galliford Try works manager Jim Brannan. The amount of rain means the contractors are continually adding new aggregate to the road.