Sitting discretely in the Monadhliath Mountains, at 100 MW the Glendoe hydroelectric scheme is not large in comparison with many of the world's hydropower installations. Indeed, tucked away alongside Loch Ness in the folds of the Scottish highlands, the installation seems content to hide its light under a bushel.

But what it lacks in size it perhaps makes up for in punch. As the first major hydropower plant to be built in Scotland in decades - since 1957 when the 75 MW Errochty station in Perthshire was opened - Glendoe boasts the biggest head of any hydro station in the UK at more than 600 meters, and as such its developers proclaim it to be the country's most efficient hydro plant too.

Located about 200 km north of Edinburgh near Fort Augustus, 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.

Building the project

The £145 million (US$235 million) power station, owned by Scottish and Southern Energy plc, was designed to produce about 180 GWh in an average rainfall year. In addition, when synchronized, the plant is able to start generating electricity at full capacity in 30 seconds.

The €184 million project was executed by Hochtief Construction AG Major European Projects, together with its UK, Construction AG Civil Engineering and Tunneling, and Construction AG Transportation Projects and tunneling subsidiaries. Up to 400 jobs were created on site during the construction phase.

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. The initial scope of work additionally included equipping the cavern with a single 100 MW Pelton turbine supplied by Andritz Hydro, complete with connecting generator and transformer and associated electrical installations.

To set up the construction site, about 50 km of road had to be built or extended, and some road sections had to be removed again after the construction phase had ended.

The reservoir is impounded by a concrete-faced rockfill dam shaped to suit the topography and geology of the area. The 980 meter-long dam is 35 meters at its highest point and impounds the river Tarff and water from 18 other influents. Hochtief described the construction of the dam and powerhouse as a special feature of the project, saying that after completion of the construction works, the reservoir is hardly distinguishable from a natural loch, given that the turbine is connected via a complex of tunnel and pipe systems in a cavern 250 meters beneath ground level.

The award of the contract for the longest dam in SSE's portfolio marked the culmination of a selection process lasting several years, during which the project was optimized in cooperation with the customer. Ultimately, it was Hochtief's technological concept that proved convincing, they state in a release.

Construction started in February 2006 and in September 2008 Scotland First Minister Alex Salmond closed the dam, which then began storage of water for hydropower generation. Electricity was first generated at the facility in December 2008. Queen Elizabeth, accompanied by Prince Philip, officially opened the project in June 2009.

Disaster strikes

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 company stopped operations at the power station.

Internal rock falls near the top of the tunnel meant that although the turbine was undamaged, significant repairs were required. Ian Marchant, chief executive of owner SSE, commented at the time that "it is hugely disappointing that this problem should have arisen at Glendoe, when it appeared that the development of the scheme had been completely successful."

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.

SSE appointed Kilsyth-based civil engineering contractor, BAM Nuttall, to carry out the necessary repair work and over the intervening period speculation has grown that SSE is in a dispute with Hochtief over who is responsible for the tunnel collapse, along with suggestions of damages claims. Neither company would comment specifically on these allegations.

Nonetheless, with good progress apparently maintained, the process of refilling the reservoir at Glendoe was expected to begin in the spring of 2012 and electricity generation is due to resume in the first half of 2012.

Drill and blast excavation of the 550 meter-long downstream tunnel was completed in January 2011, and since then workers have made progress up through the headrace tunnel clearing away debris.

The owners also took the opportunity to repair erosion damage, with work continuing to extend the life of the 6 km-long headrace tunnel between the intake at the reservoir and the underground power station cavern by installing a 100 mm-thick concrete lining. The lining was applied over about half the total length of the headrace tunnel to increase its durability and prevent erosion where the original collapse occurred and where the rock structure was found to require additional support. Complex meshing around the tunnel was used in preparation for spraying the concrete. To date, completion of the relining of the headrace tunnel is the principal work outstanding in the tunnels.

By May 2011, more than 200 people were working on site to bring the power station back to production, and a major milestone was reached in the second week of July 2011 when the contractors broke through from the new 600 meter-long bypass tunnel in the headrace tunnel, again using drill and blast.

In addition, 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 and require 400 to 500 m3 of concrete. It is crucial that this plug is completely watertight, so extensive preparation work is required, including injecting high-pressure grout into the walls of the tunnel.

Representatives of Andritz have been back on site to monitor the condition of the turbine-generator unit and have given it a clean bill of health. They returned in March 2012 to begin commissioning tests in preparation for return to service.

According to SSE work on the restoration is progressing well, and investment is being made to ensure it is carried out to a high design specification.

David Appleyard is chief editor of HRW-Hydro Review Worldwide.