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On The Renewable Road

Floating roads on timber brush and geotextiles are helping expand Scotland’s largest onshore wind farm. Report and pictures by Adrian Greeman.

As GE discovered on a rain-soaked site visit this summer, the hills and moors on the west side of Scotland have a waterlogged feel.

But while the exposed and sodden landscape is unsuitable for farming, sheep rearing, housing or much else except forestry, it does have one important new use, the generation of wind power.

That has become even more significant since the Scottish Parliament recently committed to producing 100% of its energy from renewable sources by year 2020.

The impact is already being seen with an expansion of the large Whitelee wind farm just 15km outside Glasgow, the biggest of its kind in the UK with already 140 wind generators.

But positioning a further 75 turbines on the 370m high rolling hills in this East Renfrewshire location is not so easy.

Damp and wind have combined over thousands of years to produce a layer of saturated peat across the landscape which is soft and sponge like.

In places it can be as much as 9m deep.

GalsgowWindfarm_21Jun2011_0013

To get tree cutters and maintenance crews in the owner and existing user of the land, the Forestry Commission, gets by mainly with brushwood access roads.

But for the contractors building the new extension this is not enough.

They have to bring get in heavy plant and equipment to the various turbine locations, to build heavy concrete bases and later to fit the towers, deliver turbine units and the long blades.

Tackling that has fallen to a joint venture of contractors Sisk and Roadbridge, both from Ireland.

They are currently installing the big bases for the new wind towers which are being fitted in a follow on contract, and have also been developing a network of haul roads across the site, both for their own work and for the follow-on installation and subsequent long-term maintenance needs.

Some 28km of new road is required and another 22km of strengthening for existing forestry corridors.

Big bases

“The bases are big because we are installing larger units here than previously,” says Gary Parker, project manager for the client Scottish Power Renewables. “Instead of the 2.3MW turbines of the first phase the new ones will be 3MW Eco 100 type, 69 in total with a half dozen smaller 1.67MW units.”

Alongside each base there must also be a hard standing area required for the installation work, and possible future maintenance, on the new units, which are made by Ecotechia, now owned by Alsthom.

“The installation requires some very large cranes to be brought in eventually to lift the heavy turbine units to the top of the towers, which are about 100m high,” says Parker. “Blades to be fitted as well are less heavy, being manufactured from GRP, but they are longer than before at just under 50m in length.”

Delivery can therefore be awkward.

“There will be a 1,000t and two 400t capacity cranes working together,” explains Parker. “That is not so difficult - just a question of preparing a hard standing with a capacity of 200kN/m2.”

“It is a far more complex operation to set the steel, than for a conventional base but the design does save on concrete”

The tower bases are a relatively new buttress design foundation.

For these a battered side circular excavation is prepared down to the firmer boulder clay which underlies the site.

If needed the hole is brought back to about an 8m depth with a selected fill material before placing a thin blinding concrete layer.

The main foundation is then prepared in three stages with a total 490m3 of concrete.

First, reinforcement is laid out to a specific octagonal pattern and around this an eight-sided base slab is cast, using around 20m3 of concrete.

“It is a far more complex operation to set the steel, than for a conventional base but the design does save on concrete,” says Parker.

The second stage is a radiating star pattern of eight tapering buttress walls sitting on the slab, with a circular central core.

On this central part the base “can” of the steel tower is positioned and concreted in with a final pour.

The can, weighing some 20t, is a pretty large piece in itself, says Roadbridge contract manager Cathal Docherty: “We need a 100t crane for that.”

Mobilising strength

Once that is in place the excavation is backfilled, compacting material carefully and precisely between the buttress wing sections.

“That way you mobilise the strength of the fill as part of the foundation, which saves concrete,” says Docherty.

“Placing and compacting the fill is absolutely critical,” confirms Parker.

“essentially just as the Romans built them but with some modern additions”

Fill comes from a number of borrow pits, four new ones and two from the original wind farm development some years ago.

It is a crushed igneous rock, volcanic tuff which is located near the surface and, although a little weathered, suitable for fill purposes.

The roads to bring the articulated haul trucks and all the other pieces of plant required, steel cans and the like were originally to be built with a cut and fill method.

But following talks with Tensar and the engineers it was decided to go with “floating” roads across the peat “essentially just as the Romans built them but with some modern additions”, says Tensar engineer Mike Horton, who looks after ground stabilisation projects in Scotland.

Tensar has helped design the roads.

The basic design is to use brush from the spruce forest that lines the route, which is laid flat on the ground.

Above this goes a layer of Tensar TriAx geogrid, unrolled and overlapped which then supports a layer of the crushed quarry material 800mm thick.

Horton explains that the purpose of the grid is not reinforcement as such but to stabilise the fill material: “It prevents the stone rotating and therefore helps it lock together.”

The exact dimensions of the road depend on the varying thickness and firmness of the underlying peat and the number of vehicles that will have to pass.

For some roads this will not be many as they serve a single plot and are required mainly for a one-off construction effort, but others are spine roads for the fill haulage.

The 5m width of the smaller roads is increased for these to 8m.

The idea is to minimise the thickness of the road and its load on the underlying peat, and prevent settlement as much as possible, says Horton.

GalsgowWindfarm_21Jun2011_0094

Surprisingly, perhaps, he says, it is not the large cranes which are the constraining issue despite their size, but the number of axle passes for the haul trucks which causes the most wear and the possibility of rutting and damage.

In some few areas, where needed, a second layer of TriAx 150 is added into the road layer halfway down for additional capacity.

“There are a few parts which appear to be on drainage paths which are particularly sodden and where the peat’s CBR index is particularly low,” says Horton.

Another section of the project which has used three layers is a compound near to the site offices which will be used as a laydown area for the big turbine blades as they arrive on site.

They are brought in from the docks in Glasgow along the M8 motorway and the road to site.

By August most of the road work was complete and a large proportion of the base structures had been cast.

First components were coming in for the turbines themselves.

Once in place they will create Scotland’s largest onshore wind farm and a crucial location for Scottish Power Renewables, which has located its national control and monitoring centre here.

The area is also being used increasing for leisure activity, such as walking and cycling.

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