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Glendoe Tunnel [Inlet]

Glendoe Dam, Glen Tarff, Fort Augustus, Inverness-shire.

Tunnel system, Hydro electric scheme.

8km of tunnels including the headrace, each some 5m in diameter, conveys water from the [Glendoe] reservoir to the underground power station, and from there out into Loch Ness. In addition, a 1250m long access tunnel, 7m in diameter, leads to the power station cavern. [NS Energy]

"After a year, the tunnel collapsed, blocking a 71-m length, which necessitated construction of a by-pass tunnel." [Hencher, 2019]

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"The main tunnel is 5.3 miles (8.5 km) long, of which 4.3 miles (7 km) were constructed by the drill-and-blast method and lined with shotcrete. The rest was built using the cut-and-cover technique... the power station was constructed underground, in a cavern 125 x 59 ft (38 x 18 m) and 105 ft (32 m) high, excavated from solid rock by drill-and-blast. The cavern is 820 ft (250 m) below the surface, at a level where the tailrace to Loch Ness is almost horizontal. A 5.3-mile (8.5 km) tunnel connects the reservoir to Loch Ness, with the turbines about 1.2 miles (2 km) from the loch. Some of it was cut by drill-and-blast, but the majority was created by a 16-ft (5 m) diameter tunnel boring machine, 720 ft (220 m) long..

The collapse occurred in the Conagleann Fault Zone, around 1.2 miles (2 km) from the top end of the tunnel. The fault was known from its appearance at surface level, but during tunnelling, the tunnel boring machine had passed through it, and SSE recorded that its existence was imperceptible. A contract for repair work was awarded to BAM Nuttall, who built a diversion tunnel around the rockfall by drill-and-blast. This was full-lined with 20 inches (500 mm) of waterproof concrete, and they also applied a 4-inch (100 mm) concrete lining to almost half of the rest of the tunnel. A downstream access tunnel was also built as part of the repair contract. Energy generation at the scheme restarted in August 2012." [Wikipedia]

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"the Glendoe scheme involves collecting water from about 75 km2 - either directly or via 8 km of underground tunnels - in a new reservoir more than 600 meters above Loch Ness at the head of Glen Tarff... A tunnel boring machine, named Eliza Jane by local children, was used to create the more than 8 km-long pressure tunnel. This tunnel is 5 meters in diameter and has a maximum slope of 11.5%. A 6.2 km-long headrace tunnel leads from the reservoir to the powerhouse, while a 1.8 km-long tailrace tunnel discharges into Loch Ness...

The natural catchment of the reservoir is restricted to 15 km2, but an additional catchment of 68 km2 is obtained by diverting water from other basins into an aqueduct system comprising 19 intakes, an aqueduct pipeline and an aqueduct tunnel. It includes an 8 km tunnel constructed using the drill and blast technique, more than 4 km of underground ducts, numerous water inlets and penstock shafts. Overall, about 16 km of tunnels were created as part of the Glendoe development. The drill and blast method was also applied to construct the access to the 30 meter-high powerhouse cavern, as well as the cavern itself...

A scant eight months after it began operating, in August 2009, owners Scottish and Southern Energy identified a blockage at Glendoe caused by a fall of rock in the tunnel carrying water from the reservoir to the power station... The tunnel was drained and SSE, working with Hochtief and others, undertook a detailed investigation of its entire length. It was determined that to recover the scheme, two new tunnels were required with a bypass tunnel, together with a downstream tunnel to provide access to clear debris from the collapsed area...

... the bypass tunnel has had an extra 500 mm-thick secondary concrete waterproof lining applied as it also lies within the fault zone. The extensive lining works were expected to require about 60,000 tons of concrete. The bypass tunnel will be permanently sealed with massive 14 meter-long solid concrete plugs at both ends. Work is continuing on construction of the two remaining concrete plugs where the new access tunnel meets the lower headrace tunnel and on the original headrace tunnel above where the collapse occurred. The larger of these massive plugs is at the lower end of the tunnel and will be about 20 meters long" [Factor This, 2012]

Alternative Names: Glendoe Headrace Tunnel, Glendoe Tailrace Tunnel, Glendoe Power Tunnel, Glendoe Hydro Scheme

Notes: "Engineers began planning the Glendoe project, on the eastern shores of Loch Ness, as far back as October 2001. Formal construction work at the site started over four years later, in January 2006. It involved constructing a 960 metre-wide dam on the River Tarff to create a new reservoir some 600 metres above the power station, giving it the greatest head of any hydro electric power station in the UK.

An 8.6 kilometre tunnel connects the reservoir to the power station that is built 250 metres below ground level, about two kilometres from the banks of Loch Ness. Although some of the tunnel was created using traditional drill and blast techniques that would have been familiar to the Tunnel Tigers of the last century, much of its length was bored out using a massive tunnelling machine" [SSE]

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"At its height, over 750 workers were employed on the scheme. And, as in the days of schemes constructed in the decade following World War Two, those workers came from many different countries across Europe and beyond. Most lived in specially constructed camps high in the hills above Loch Ness, braving brutal weather conditions in winter, and the fearsome Scottish midges in the summer... Today, the main operating feature of Glendoe is its ability to react quickly to changing demand for electricity, being able to reach full output in just 90 seconds." [SSE]

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The £145 million power station, owned by Scottish and Southern Energy plc, was designed to produce about 180 GWh in an average rainfall year.

A substantial spoil bing (presumably associated with the access tunnel) is located at NH 4015 0878, near to an electric substation

Given coordinates are for the inflow mouth of the headrace tunnel. Length is approx.

Hydrographic Feeds: Glendoe Reservoir

Hydrographic Resurgences: Loch Ness