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For peat's sake

Eaglesham Moor in Scotland might be an ideal location for a new wind farm, but it is a site full of geotechnical challenges.

The volume and type of peat found at the home of the new Whitelee wind farm, about 17km south of Glasgow, is at the top of the obstacles facing the project team.

A joint venture between Morrison Construction and Balfour Kilpatrick is responsible for completing the £80M infrastructure work to prepare the 5.5ha site for the arrival of 140 turbines.

"The ground conditions have been the biggest challenge on this project," says Morrison's and the joint venture's project manager Tony Windle. "We are building foundations and access roads where peat ranges in depth from 1m to 7m and goes from what can be a quite fibrous consistency to being like peat soup."

Donaldson Associates is the designer working for Morrison and has had to come up with three designs for the turbine bases to cope with the variable ground.

"The simplest foundation type is a reinforced concrete gravity foundation excavated to a suitable depth," says Donaldson associate Willie Cannon. This design supports and prevents the turbine overturning. But he says much of the land is less than ideal for founding on. Only about a third of the locations comprise competent basalt rock and in other areas there is up to 7m of peat overlying between 2m to 3m of glacial till.

Each of the turbine foundations consists of a reinforced concrete base. These are square in plan with bases tapering to a central octagonal pedestal that supports two rings of holding-down bolts cast into the concrete.

The variable ground meant three sizes are required: 15m2, 15.85m2 and 16.75m2, using between 240m3 and 300m3 of concrete. Cannon says the smallest of the bases is the simplest of the three designs because it is socketed 2m into the basalt. The other two types are designed for locations where there is weaker glacial till. Here the foundations have to go down to 3m into the till to provide enough support for the 110m high turbines (including the 45m long blade height).

Following initial site investigations at each of the turbine sites, designers came up with a further option for a piled foundation for 13 bases. This was because the glacial till appeared to be of poor strength and stiffness to accommodate the turbines. This option involves installing eight large 1.2m diameter bored piles into the basalt at depths from 8m to 20m or, if there is no basalt, down to 25m into glacial till.

But since starting work began on eight of these, further ground investigation by Land-Drill Geotechnics and testing by Professor Barry Clarke at Newcastle University proved the till had the required minimum sheer strength. This was 100kPa for the medium base and 150kPa at the largest.

Five of these locations are still to undergo further testing. Cannon says using piled foundations, therefore, remains a possibility.

The peat posed a further problem because the softer soil tended to collapse during the excavation for each base. In locations where there is a large volume of peat the team devised a temporary works support – called a rock doughnut – to counter the problem.

It involves using 40t excavators to push large rocks up to 1m in diameter into the ground in a 4m wide ring around the turbine area. Once this is complete, the peat inside can be excavated while the doughnut forms a retaining structure to hold back surrounding material.

Windle says this option was favoured over temporary piles to form a cofferdam because this would have meant more rigs to install sheet piles and a larger excavation.

All of the 1.5M.m3 of rock fill needed for the project is being generated by 13 onsite borrow pits. Three are being operated at the moment and the remaining 10 will be opened up as the job progresses. Once excavation is complete, a 50mm concrete blinding will be used to level the site before the base can be built up.

Site workers from subcontractors Barbour Structures, Osprey and Henderson are installing the 45t of 40mm diameter steel reinforcement bars needed for each foundation before placing a 600mm steel shutter around the base and casting the concrete. The work completes at a rate of four bases and four pedestals a week, after which site workers backfill the excavated area with general rock fill.

Rock fill is also being used beside each of the turbine bases capped with 200mm processed material to form crane hard standings with a 200kN/m2 load bearing capacity.

As well as turbine foundations, the site needs nearly 90km of access roads to allow delivery and maintenance. A 16.5km spine road cuts the site from east to west with access roads leading off to each of the turbine locations.

As part of this work, 10km of existing forestry roads need upgrading. This work is important because steep gradients and hairpin bends need levelling and straightening to allow access for the 38m long vehicles that will deliver the turbine structures.

About two-thirds of the total road length is using, what Windle calls, a floating road system. Once again the volume of peat along the site dictated the design. Elsewhere it has been possible to do a simple cut excavation before site workers compact 600mm layers of general rock fill, capped with 200mm of site-processed material. A crusher is on site to produce fines down to 127mm in diameter.

A geotextile separator layer and geogrids are being used to form the floating road where there is peat at depths of 1.2m or more. Site workers are installing a total of 1.4M.m2 of separator layers on top of the peat – hence the term "floating road". A combination of Geolay 609, Geolon PP60 and Swiftec PPS90 geotextiles provide this first level.

Windle says this part of the work is important because it is key to keeping the vegetation intact in places where the peat has a low strength. He says: "The minute you go through the crust, you could find you get a 2.5m settlement."

On top of the separator layer, workers roll out a combination of Naue Geosynthetics Secugrid 40/40, Maccaferri Enkagrid 40 and Tensar SS40 for the base geogrid layer. A dozer compacts 200mm thick layers of the 127mm processed fill to reach a total thickness of 600mm.

The same manufacturers are supplying Secugrid 30/30, Enkagrid 30 and SS30 for site workers to use above this layer, before another 300mm of rock fill is placed on top.

Once the floating road is installed, peat excavated from elsewhere on-site is being used along the sides of the road to create extra support. Windle says this is one of the only occasions where the peat works in the team's favour.

Site workers are completing up to 1.5km long stretches of the new road each week. Work on the turbine bases and roads is being completed in tandem with the installation of 950km of electrical and control cabling by Balfour Kilpatrick.

Infrastructure work began on site in October last year and the first turbines are expected to arrive on site next month. During the works a total of 633ha will be cleared of trees. Work is due to be complete in July 2009 when client Scottish Power will take over operation of the Siemens-supplied turbines.

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