Shani Wallis, Editor

Power generation at Scotland's new Glendoe hydro scheme is blacked out by a rock fall in the headrace tunnel. A drop in water pressure through the turbine warned of trouble and initiated the shut down and dewatering of the system last week for man-entry inspection.

Owner Scottish and Southern Energy (SSE) said identified internal rock falls near the top of the 6.2km long headrace reduced the water flow and had caused the pressure drop.

Plan of the Glendoe scheme

TBM drive at Glendoe

The blockage is believed to have caused a pressure drop of about 14%, or 38-39m of the scheme's gross 608m head, resulting in a reduced output of the 100MW turbine to about 90MW. A large rock trap built into the system by design-build contractor Hochtief and its contract engineers Pöyry performed well and prevented any damage to the turbine.

Specific details about the extent of the rockfalls were not offered in phone calls by TunnelTalk to SSE and others involved on the project. The response was that the incident, the possible causes, and the design and execution of the repair works, as well as a thorough investigation of the full 6.2km long, largely unlined headrace power tunnel are all under investigation with the assistance of Hochtief which is into the defects liability period of the contract after hand-over of substantial project completion at the end of last year. Jacobs confirmed it still has staff resident on the project site and Hochtief also has staff at the site finishing construction work and completing landscaping and final reinstatement work ahead of a contract close out by the end of the year.

Detailed design of the civil works by the Energy & Hydropower Division of Pöyry involved no responsibility on site for confirmation of geological conditions or construction supervision but a spokesman for the engineering firm confirmed that there were no known issues of rock quality. "Conditions were as expected with only minimal support required and that was mostly in the drill+blast headings. The TBM headrace tunnel is largely unlined with the 300m lower section of the power tunnel fully concrete lined and 90m of that with a steel liner. There was never any significant issues of poor rock conditions and certainly nothing that caused any stoppage of the TBM excavation."

SSE, a listed company on the London Stock Exchange, said an update on investigations and the progress of repair work would be published with its half-year results in November. It could not predict when electricity generation would resume. The shut down comes within a year of the plant starting electricity generation in December last year. Loss of production at Glendoe is said not to have a significant impact on supply. The scheme is a peak period installation and this can be made up by other sources in SSE's total 11,000MW generating capacity until the scheme is back on-line.

TBM and drill+blast adit junction

The £150 million (US$250 million) scheme at Glendoe built into the hills adjacent to Scotland's famous Lock Ness collects water from a wide catchment and conveys it from the new reservoir along the 6.2km headrace tunnel to drive the single 100MW Pelton turbine in the underground powerhouse. The gross 608m head delivers water at about 18.62m³/sec and about 1.5m/sec velocity to the turbine located about 300m underground. A 1.78m long tailrace releases the water into the Loch. The rapid response plant has a loading rate of 0-100MW in a minute and a start-up through turbine and generator synchronisation to full load in four minutes.

The design-build contract under the NEC, 2^nd Ed, Option A terms and conditions, was awarded to Hochtief, with support engineers Pöyry Energy, VA Tech Hydro and Andritz, in December 2005.

TBM and drill+blast adit junction

At a meeting of the British Tunnelling Society (BTS) in March 2007, it was explained that SSE took the major geotechnical risk on the job. SSE administrated the contract with its engineering adviser Jacobs in an integrated construction management and supervision team. The management team employed three geologists on site.

Site investigation data for the reference design was based on desk study work and a walk over the site. Further investigation for contractor submissions was completed by Fugro using ground based geophysics and aerial geophysics and seven vertical and inclined boreholes of between 25m and 350m, some of the deepest boreholes drilled in the UK, were also sunk to target suspected locations of faults and fracture zones.

TBM tailrace drive

As well as a deep underground powerhouse, the project's reference design included a steep inclined headrace tunnel rather than the more common pressure shaft between more gentle gradient headrace and lower power tunnels. In its submission Hochtief and its engineer Pöyry stayed with the reference design and planned on using a 5m diameter Robbins main-beam gripper TBM, refurbished and rented to the project by Herrenknecht, for the majority of the tailrace and for most of the almost 12% headrace incline. Extensive drill+blast was used for excavation of the underground powerhouse and for the associated tunnels including the 1,159m long main access tunnel to the machine hall cavern and a 7km long aqueduct tunnel that catches and delivers extra water resources to the reservoir.

The project is located near Fort Augustus beside Lock Ness in an area south east of the Great Glen Fault zone. The geology consists of metamorphosed sedimentary rocks that are folded and sheared with a sequence of interbedded quartzites, quartz schist and quartz mica schists with strengths range largely between 130MPa and 150MPa. Cover to the tunnels is between 250m and 350m and little groundwater was anticipated.

The Barton Q system was used for the reference design and support measures were defined for the contract on the basis of four rock classes down from five in the reference design, with steel fibre reinforced shotcrete, rockbolts and mesh as required providing the support and lining elements.

Predictions for rock quality in the drill+blast tunnels were reported as 10% in Class 1, the best, and 75%, 13% and 2% respectively in Classes 2-4. The BTS meeting heard that discussions with the client's geologists for work to that date resulted in an aggregate split of 86% in Class 2 and 14% in Class 3 with no reaches in Classes 1 or 4. Contractor payments it was said were based on the agreed rock class and not the support required or installed.

Drill+blast heading profile

Conditions for the TBM work to March 2007 was reported as being good. The TBM part of the tailrace tunnel was complete, a 108m long bridging tunnel to the headrace was also complete and more than 1,000m of the headrace was bored. Rock class predictions for the headrace drive were 60%, 25%, 10% and 5% in Classes 1-4 respectively. For the 1,000m completed to March 2008, about 90% was in Class 1, much better than expected and producing TBM advance rates of about 32m/day, well ahead of the 24m/day programme average, and a best day and week to date of almost 74m and 273m respectively.

In the predominantly good rock, minimum support of 50mm of shotcrete was applied as necessary. Muck haulage on the steep 11-12% uphill drive was via a continuous conveyor suspended from the crown and supplied by Marti Technik of Switzerland. Materials were delivered to the heading by low profile diesel-powered vehicles.

TBM launch on the steep incline

During the discussion session of the BTS meeting, and as reported in T&T International, Colin Mackenzie, Retired Director of UK contractor Amec Tunnelling (formerly Fairclough Tunnelling), commented that in his experience, rock adjacent to the Great Glenn Fault often exhibited micro cracking in a plane parallel to the fault. Asking if such features had been experienced the response was that maintaining the profile of the drill+blast headings was difficult in some locations.

The TBM headrace broke through in January 2008 and once the reservoir behind the new rockfill dam was impounded and the M&E works completed, the scheme came on-line in December 2008. Glendoe is the first hydro scheme built in the UK for several decades and attracted much publicity when then Prime Minister, Tony Blair marked the start of works in February 2006 and again in June 2009 when the Queen commemorated the official opening.

Trailrace construction site

In the SSE statement, Ian Marchant, Chief Executive, said: "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." Marchant added that "extreme care will be taken with the investigation and repairs to make sure the scheme emerges from this difficulty in a strong position to operate successfully over the long term."

Jacobs confirmed it still has staff resident on the project site and Hochtief also has staff at the site finishing construction work and completing landscaping and final reinstatement work ahead of a contract close out by the end of the year. Remedial work to repair the headrace tunnel may see that presence for both firms extended beyond that end date. A comment from the podium during the BTS meeting suggested the design of a large rock trap upstream of the powerhouse was a conservative feature.