Putting wind

Building a wind farm on Cambridgeshire's boggy fenland required some creative thinking from main contractor Morrison and foundations specialist May Gurney. Helen McCormick reports.

Windmills have been a feature of Cambridgeshire fenland for centuries, and in recent years the distinctive sight of modern wind turbines has become increasingly common across the county.
The flatness of the landscape is an obvious advantage for groundwork contractors, but this is more than offset by the challenge of securing these vast machines in the unstable ground.
Morrison Construction Services teamed up with piling specialist May Gurney to come up with a successful solution to this soggy problem.
The 4.5m design and
construct project involved the construction of eight turbine bases in fenland at Coldham, near Wisbech, plus a meteorological mast base and a sub-station as well as the access roads and installation of both electrical and control cabling.
In total, the wind farm will contribute approximately 16mW of electricity to the national grid - enough to power almost 10,000 homes. Turbine specialist Vestas supplied and erected the turbines in an independent contract with the client, which is a joint venture between Scottish Power and the Co-operative Group.

Soft conditions
"The biggest challenge was the ground conditions," says Morrison special projects director Ken Jones. "The fenland where the site is located represents a significant geotechnical challenge which meant constructing on piled foundations."
Morrison regards renewable energy as an important business sector, and is also building windfarms in Ross-shire and Aberdeenshire this year.
Coldham, however, presented some unique problems. The soft ground wasn't the only obstacle. The site is a 14,085ha working arable farm, which has been operated by the Co-op since 1916. Apart from having to work around the usual farm traffic, harvest fell in the middle of the project period.
This could not be avoided as the turbines could only be erected in wind speeds of less than 10mph, which is most likely in August. There was only one road in and one road out, both of which crossed the farmyard.

Design process
The design element of the job was therefore crucial, explains Morrison project manager Richard Bakewell. "Design is a continuous process all the way through," he says. "The idea that you spend three or four weeks designing it and then build it is not realistic.
"Co-operation between everyone involved in this job has been particularly important, it's a team effort between ourselves, the client and particularly May Gurney, which was a key part of it all the way through.
"This level of teamwork is the only way you can get a job like this moving, co-operating together right through the design stage, particularly the piling, through to construction."
Part of the preparatory work included strengthening the main tracks to and from the site to enable them to carry construction traffic and to allow safe passage for the turbines. A bridge also had to be installed. Each turbine arrived in seven parts: three sections of the base, three blades and one nacelle (the hub for the blades).

Careful planning
The blades are 45m long and the crane needed for the turbines weighs 800t, so the bends and strength of the track had to be carefully planned. The floating roads also had to accommodate the weight of the piling rig on the low loaders.
"One of the key targets was to have all the stone roads in place before harvest, which was of major concern to the farm," says Bakewell. "Farm traffic increased significantly then, and they didn't want us bringing in 2,000t of stone in a day as well as the foundations and piles.
"This would not be too bad on a normal site, but was tricky when we had to negotiate through the farmyard, plus the single-track roads with lay-bys.
"Crops were also being sprayed regularly, which we had to work around."
The contractors had to closely co-operate with the farm management to ensure there were no clashes. Farm manager Mike Shapland held weekly health and safety meetings with Morrison. "We spoke several times a day, keeping each other updated," he says. "As a result, it has gone extremely smoothly without disrupting us too much."
Both the Co-op and Morrison kept in touch with local residents, making sure the muck-away lorries and tippers used the designated routes and didn't congest any of the local villages.
There was little opposition from the community, partly thanks to the farm's fairly isolated position. "One local guy said windmills have been here for hundreds of years, so a few more wouldn't hurt," says Bakewell.

Site investigation
As well as piling, May Gurney's 800,000 contract involved
carrying out site investigation, plate- bearing tests and pile-load tests. The work took just eight weeks.
"We undertook a full mock pile test mimicking the installation of one of the finished turbine piles," says Steve Bursnell, piling projects manager at May Gurney. "We wanted to make sure that what we said we could do was actually feasible on site."
This testing process was crucial given the instability of the ground. Underneath a barely 2m-thick crust of hard soil lies about 16m of river alluvial deposits, common to fenland. The first rock is 18m to 20m below the surface. "It's unusual to build in this sort of ground," admits Bakewell.
May Gurney's work included 64 x 1.2m-diameter, 25m-long rotary bored piles for the bases of the eight wind turbines and 128 x 45cm-diameter continuous flight auger (CFA) piles for the cranes being used to erect the 130m-high turbines.
This involves putting in the concrete as the hole is dug. Both required heavy reinforcement due to the forces involved and the high-tension loads being applied to them.
The rotary piles were bored through temporary casings, which had been vibrated in by an attendant crane. The casings were socketed 1m into clay to prevent collapse of the bore due to the unstable silts, sands and gravels.
The casings were then removed once the reinforcement and concrete were placed.
Bursnell says the tight timescale did not faze them, despite never having put in piles for turbines before.
"We were extremely pleased with the progress of the job," he says. "The eight turbine bases were built in succession, but there was considerable overlap. We completed the rotary piles for one base in just two days, which is quite a feat. Co-ordinating the work from one pile to another was crucial."
May Gurney set up a central steelyard just by the compound. "We had trombone trailer lorries taking the cages to each base ahead of us; the co-ordination of it worked very well," adds Bursnell.
Both companies agree that they would undertake this type of work again. "It's been an interesting job," says Bakewell. "It was a very tight timescale considering the amount of design work involved. And of course, the ground conditions were unusual. Turbines are usually put up on a rock base; these are possibly the first turbines to have this type of piling."
The scheme is Scottish Power's first south of the border. It now generates 158mW of wind power and plans to invest 1bn in UK wind farms by 2010, eventually producing more than 1,000mW of electricity. <25A0>