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Glendoe hydro scheme, the real monster of Loch Ness

The £140 million Glendoe hydro scheme is expected to generate enough electricity to power every home in a city the size of Glasgow. By Simon Ellery

Main contractorHochtief Glendoe JV
TBM manufacturerHerrenknecht
Generation plant subcontractorVA Tech Hydro
Hochtief’s designerPöyry
SSE’s EngineerJacobs
Roads and dam subcontractorHQC
Concrete supplierTarmac Aggregate Products

It is a monster of a job. Excuse the cliché, but if you had to build a hydro-electric power station, putting it next to the remote Loch Ness would not be your first choice. And it’s not because of the mythical creature.

The dam is one of the largest civil engineering projects ongoing in Scotland and the first new conventional hydro-electric scheme in the country for almost 50 years.

Hochtief UK and Hochtief Construction AG form the Glendoe Hochtief joint venture, which is the main contractor.

Tarmac Aggregate Products was drafted in to supply the scheme with mammoth amounts of concrete.

The £140 million Glendoe hydro scheme is being built by the German giant for Scottish & Southern Electricity. In March next year it is expected to start generating up to 100 MW – enough to power every home in a city the size of Glasgow.

Although an ideal location for hydro-electric power, with a high reservoir and plenty of rainfall, building on unpredictable ground in a tough climate and on top of a mountain made construction a challenge.

Tarmac’s Caledonian business manager Gordon Liney was in charge of a project described as a “moving target”.

Rock steady

At first the team couldn’t employ the inappropriate rock that came out from drilling for aggregates in the concrete, then much later in the project it became useable. On top of this the weather conditions were often severe.

“They had real issues in the winter. Workers got snowed in – they might not get out for two or three days. It was quite tough.”

On a technical level there were problems getting the consistency right for the massive amounts of sprayed concrete used to create the dam – for the reservoir that holds 16 million cu m of water.

“It’s hard to do trials for shotcrete as every job you go to you’re using local products and they have their own peculiarities,” says Mr Liney.

“Every time you put them together again you are starting from the beginning. We worked 24 hours a day but it took weeks to get it ready.”

Tarmac Aggregate Products’ Alasdair Stables spells out the challenge. He says that in 2006 Tarmac established a concrete batching plant on site to supply structural and sprayed concrete for the access tunnel to the power house.

“But this proved problematic as the cements available locally were not suited to the specific technical requirements for spraying. We started getting feedback on how the shotcrete was holding.

“There are two factors in this. The first is how well it sticks to the wall and the second is the rebound. These things are very easy to see in practice.”

He said that admixtures can be added to the spraycrete during manufacture while an accelerator can also be added during spraying.

“This is important as it determines how spraycrete will behaves as it hits the wall. Otherwise it could run off the wall,” says Mr Stables.

But Tarmac and Hochtief found a solution between them with the help of two ad mixture suppliers that continually evolved their chemicals to suit.

Monster project

Work on the reservoir began in August 2006, more than 600 m above Loch Ness, with construction of a 905 m dam on the River Tarff.

The dam – which is a concrete-faced rock-filled embankment – is long and narrow with a maximum height of 35 metres.

An 8.6 km aqueduct tunnel will collect water and bring it to the reservoir while a further 8 km of headrace tunnels will take water from the reservoir to the underground power station, and a 2 km tailrace will finally empty into Loch Ness.

The third, shorter tunnel provides access to the power station cavern for maintenance.

A 200 m-long tunnel boring machine (TBM) called Eliza Jane has now completed the drilling of the headrace tunnel.

It started work in September 2006 and has travelled from the banks of Loch Ness upwards through the mountain, drilling a 5 m diameter tunnel through solid rock for over 8 km, climbing more than 600 m in height.

Mr Stables says that once Eliza Jane, which was shipped over from Germany, was turned on it ran continuously for 16 months.

“Once the boring machine was turned on it could not be turned off,” he says.

This was partly because of its high fees, but also because switching it off and on is also a complex procedure.

Breakthrough – the moment the TBM sees daylight again – happened in February this year. By the time Eliza Jane had bored the 8 km of headrace tunnel it had climbed more than 600 metres in height.

The actual power station is housed in an underground cavern located 1.8 km from the banks of Loch Ness.

Hochtief had hoped to find aggregate on site suitable for producing concrete but investigations initially showed there was nothing useful there.

Mr Stables says: “I think the main challenge was not knowing what the land would be supplying.”

Tarmac was asked to supply coarse and fine concrete aggregates. Initially this was supplied from the company’s Blackcastle Quarry but Tarmac opted to accelerate the opening of the new Dunain Mains Quarry in a move that costs hundreds of thousands of pounds.

Ground force

“We hoped that what came out of the ground would supply us with concrete aggregate,” says Mr Stables. “For Tarmac that meant accelerating the opening of a planned quarry to supply aggregate from Dunain Mains.

“It significantly reduced lorry movements as we were hauling from Blackcastle, which was 55 km from the site, but then we ended up 28 km from site [with the opening of the new quarry].

“We did look at the possibility of barging material down Loch Ness but the infrastructure for that was pretty huge so we didn’t take up the option.”

Toward the end of the dam’s construction Hochtief did discover aggregates on site, meaning Tarmac only supplied project regions close to the main roads as the Hochtief quarry is 10 km from the site entrance.

Tarmac built a second concrete plant on site to support the final stages of construction.

Mr Stables says that working with Hochtief was a “rewarding experience”, despite the problems.

“There were two different plants to manufacture concrete. Sometimes their plant broke down and there would be a lot of pressure on our plant.”

Once turned on the dam will provide a huge amount of electricity, in one of the most environmentally friendly ways.

SAFETY CONCERNS

Health and safety is a major issue on massive projects such as hydro-electric dams. The Glendoe scheme has had its difficulties in this area, with up to 700 workers on the site at the peak of activity.

The Health & Safety Executive is still investigating one recent fatality. That was Czech worker Ondrej Hladik, 46, who was killed in a forklift truck accident on 22 September.According to the HSE, there were 10 breaches of health and safety legislation over the past two years and it has served the site with four enforcement notices including a demand to “improve tunnel entry procedures”.

Hochtief told Construction News that its priority is to ensure the safety of workers: “We have complied fully with all requirements of HSE notices served on the site.”

But a Polish worker has also been seriously injured at the scheme just weeks after the fatality. The man, an employee of Hochtief, is understood to have broken both legs after the vehicle in which he was travelling overturned and rolled down the dam wall. A prohibition notice was served following the accident, ordering the contractor to stop use of its vehicles until safety checks were carried out.