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Loch solid energy

Energy - The Glendoe hydroelectric scheme will play a key role in Scotland achieving its 40% renewable target. Damon Sch³nmann reports.

The residents of Fort Augustus village, at the southern tip of Scotland's Loch Ness, are likely to see an increase in visitors over the next couple of years. This will not be solely down to a rise in monster hunters. It will be driven by the workers building the nearby Glendoe hydroelectric project, which is starting to take shape in the Monadhliath Mountains overlooking the area.

The scheme forms part of the Scottish Executive's strategy to generate 40% of Scottish electricity from renewable sources by 2020.

Once nished, in late 2008 or early 2009, the Glendoe plant will produce up to 100MW for Scottish & Southern Energy (SSE); enough power for 37,000 homes when operating at normal capacity and up to 250,000 at its peak.

SSE awarded the £126M design and build contract for the project, the UK's rst largescale conventional hydroelectric scheme in 50 years, to principal contractor Hochtief Glendoe (an internal joint venture between Hochtief UK and Hochtief Construction) in December last year, following four years of development and granting of planning consent in July 2005.

The main designer is Finnish company Poyry, with Halcrow as planning supervisor for Hochtief.

Jacobs Babtie is consultant and supervisor for SSE.

The system will be supplied by a drill and blast aqueduct pipeline linking to a 7km aqueduct tunnel.

This 4.5m horseshoe section tunnel will outfall into a reservoir, which means that the reservoir, which has a 15km 2 catchment, will also be fed by the aqueduct's 60km 2 catchment.

From the reservoir, a 6.2km headrace tunnel will carry water to the power station, from which point a 1.8km tailrace will outfall into Loch Ness.

The headrace tunnel, reservoir, aqueduct tunnel and aqueduct pipeline will be fed at various points by a total of 19 catchment-feed intakes that are generally placed in places where a river crosses the network.

The aqueduct tunnel can also act as a reservoir for when the reservoir's upper storage limit has been reached. The live range means the reservoir's operational water level will fluctuate between 624m and 630m above sea level.

'The 6m operating range should be sufficient to run the turbine for four hours, ' says Hochtief project director Hans Dieter Imhof. 'The intention is not to have it running continuously; it is a system that will feed electricity into the network at peak times.' Construction is well under way with Hochtief using drill and blast for the 1.2km main access tunnel to the power station. As the contractor will use a tunnel boring machine (TBM) to bore the power tunnels from the Loch end, it is also blasting a short 120m adit at the bottom of the tailrace for a conveyor to remove TBM spoil, and a launch area for installing the machine.

'For the launch shaft and access tunnel we will use 2.5m rock bolts after drilling and grouting, ' says Hochtief tunnel engineer Jorg Steppuhn. 'The main access tunnel is for getting into the power house during excavation of the 23,000m 3 cavern, and will be used [after project completion] for maintenance.' A errenknecht BM ill eate the 5m diameter power tunnels in a continuous drive beginning at the Loch, passing the power cavern and continuing up to the reservoir.

The TBM will reach the powerhouse at the beginning of next year, ' says the contractor's planning manager Armin Krimpmann. 'It may then be shut down for two weeks while we do some drill and blast to connect the power house to this tunnel, then it will begin again towards the reservoir where it should arrive by the end of 2007, although it may be sooner as we have built in some contingency.' Imhof says: 'We do not intend to line the tunnels except with [steel fibre reinforced] shotcrete in places, depending on the rock class. In the best rock, class one, we will only shotcrete the arch. In the power tunnels there may be places where we will do nothing.

We hope to get the TBM tunnelling by 1 September [this year].' One area that will need concrete lining is the 100m of headrace before the power station with the last 18m requiring steel to withstand the high pressures there.

Hochtief will use extra support than that needed elsewhere at the 12m wide junction of the adit and the TBM driven tailrace tunnel. Site workers will install 150mm thick shotcrete and 4m rockbolts here in a 1.5m grid. This compares to the standard 2.5m rockbolts at about 2m to 2.5m spacing.

The contractor is using cartridged nitroglycerine with nonelectric detonators, which are safer as there is no risk of accidental detonation from mobile phones or other electrical equipment.

Explosives supplier Exchem special projects manager Malcolm Ingry says: 'We are supplying 38mm diameter explosives for the bulk blasting for the main tunnel work and 28mm diameter for the smooth blast arch profiles to initiate 80g detonating chord. It's the detonating chord that actually forms the arches. The 1.2km access tunnel to the powerhouse has a 7m diameter horseshoe section.

'After we reach the cavern area we will excavate the roof for the 19.5m wide, about 50m long and 30m deep chamber.' While excavating the upper part of the cavern from the access tunnel, Hochtief plans to start on the lower section from the tailrace tunnel. 'In February we will build a 220t crane beam in the upper cavern section and anchor it, then we can bench downward, ' says Imhoff.

The scheme requires just one dam for the reservoir and it will be filled with a maximum 16M.

m3 of water. 'It's a shallow but long dam at almost 1km long and reaching a maximum 35m height.

It will have 400,000m 3 of rockfill and its construction will use about 16,000m 3 of concrete, ' says Hochtief dam manager Michael Raeder.

'It will have a concrete conduit running underneath to empty the reservoir should it need maintenance and a side channel spillway safety feature will be able to release 159m 3/s of water if needed. Although it's a tiny dam compared with others around the world, however big you build one, you still need all the elements.' A challenge facing Hochtief is the Highlands peat covering many construction areas. It will require additional planning to strip it and the underlying glacial till out, so structures can found on the mica schist and granite bedrock beneath.

Once the overburden is removed the contractor will build a concrete plinth against what will be the upstream toe of the dam, to support the concrete face slab.

Site workers will then place the rockfill, progressing along the dam's centreline from south to north, before adding the concrete face. The underlying concrete outlet and spillway will be built in tandem with this work.

'The plinth and rockfill should begin at the start of next year and should complete by the summer of 2008 when we will do impounding, testing and commissioning if the powerhouse cavern is ready, ' says Krimpmann.

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