Excavation for the bypass tunnel at the top end of Glendoe headrace is underway to overcome a rockfall discovered almost a year ago, barely eight months after the 100MW hydro project began operations. UK contractor BAM Nuttall has been brought in to the project to excavate a short access adit and the water diversion tunnel.

The owner, Scottish and Southern Energy (SSE), said that a 900m long bypass tunnel is being bored by drill and blast but the scale of the recovery work for the project is unlikely to see the plant supplied and generating again before the middle of 2011 – almost two years after the problem was discovered.

In its 2009-10 Annual Report, issued in late May, SSE said that BAM Nuttall had been retained to drive the two tunnels and that work was already underway. However, no specific details have been given about the local geology, tunnel build or cause of the rockfall.

The Glendoe scheme near Fort Augustus, Scotland, has approximately 16km of tunnel network, including the 6.2km long headrace. It was constructed under a design and build contract by the Hochtief Glendoe JV, led by Hochtief and including Poyry as the designer. The contract was awarded in late 2005. The client’s adviser is Jacobs.

Geology along the alignment of the headrace comprises quartz mica schist and quartzite with some minor faults. Most of the headrace was excavated using a 5.03m diameter open gripper TBM and some drill and blast, while the rest of the network was bored by drill and blast.

Depending on local ground conditions, four classes of support were available for the headrace ranging from minimum of rock bolts plus mesh and shotcrete, if needed, up to using all of those plus a steel set full ring. The UCS of the rock at the top end was 30MPa-130MPa. The successful tunnelling works were completed in early 2008.

When reporting the plant had to be shut down, SSE noted that no equipment had been damaged in the underground powerhouse as a consequence of the rockfall at the top of the headrace, which is fed by reservoir. The plant has a single, six-jet vertical Pelton turbine and operates under a gross head of 608m with a flow of 18.62m3/sec.

Investigations in August 2009 revealed the rockfall to be ‘very substantial’, said SSE. Then, it was anticipating that the plant might remain shutdown until well into 2010 at the earliest. But by then end of 2009 it had become clear the scale of repairs would have the plant out of action until well into 2011. The recovery options all required significant programmes of work.

In a presentation to the British Tunnelling Society (BTS) during the construction phase, SSE noted that for risk management at Glendoe it had a degree of geotechnical risk on the project, three geologists on site, and the contractor’s tender was based on a reference ground classification system. It worked alongside the JV contractor, which produced its own design and was responsible for the tunnelling.