With prolonged repairs to tunnels in the closing stages at the 100MW Glendoe station in Scotland – the biggest hydropower project in the UK for decades – the owner and operator, Scottish and Southern Energy (SSE) expects the plant to be generating again before the middle of this year.

The station, near Fort Augustus, will have been out of action for almost three years when it begins generating again with the extent of tunnel repairs eventually undertaken being more extensive than expected when investigations began into the blockage in the upper headrace tunnel. No equipment repairs were required as a result of the tunnel collapse.

In the end, a bypass tunnel was constructed around the worst of the rockfall blockage and fault zone in the top half of the 6.2km long headrace blockage area. A second bypass tunnel was constructed around the power cavern to provide access from the downstream side to remove about half of the extensive debris pile, extending more than half a kilometre down the tunnel.

About half of the headrace tunnel was lined with shotcrete, and extra lining has been installed in the tailrace tunnel in the latter stage of the remedial programme.

In anticipation of the re-start, SSE is soon to commence refilling the upper reservoir. At the lower end of the scheme, the tailrace discharges into Loch Ness.

The scheme

The Glendoe scheme has catchment area of approximately 75km2 and with its single, six-jet vertical Pelton turbine, supplied by andritz, is designed to generate about 180GWh of electricity per year.

The gross head on the turbine is 608m and flow is 18.6m3/s, and the plant is designed to achieve a fast loading rate of zero to 100MW in one minute, and from standstill to full load in four minutes. The plant is fully remotely controlled from Perth.

Glendoe was constructed with a 35m high CFRD dam to impound the upper reservoir, which has a crest length of 920m, and an extensive tunnel network totalling approximately 16km. The largest elements of the tunnel network were the headrace and the 3.4m wide aqueduct tunel, totalling 7km in length. In addition there were the powerhouse tunnels and cavern.

Contractor on the scheme was Hochtief Glendoe JV, led by Hochtief and including Poyry as the designer. The JV was awarded a design and build contract in 2005. Jacobs was retained as the client’s adviser.

The 5m wide headrace tunnel was by excavated using both TBM and drill and blast methods in geology of quartz mica schist and quartzite with UCS of 30MPa – 130MPa, and with some minor faults. Four classes of support were available from bolts, mesh and shotcrete of 50mm thickness if required up to steel set full ring with 200mm thick shotcrete.

During construction, SSE acknowledged that for risk management it had a degree of geotechnical risk on the project, had geologists on site alongside the contractor and was involved in decision-making.

The two-stage procurement process saw, first, four bidders work with a supplied reference ground classification system for design, and second, two tenders based on contractors’ designs. Awarded the contract, the Hochtief Glendoe JV had its own design refined for the scheme and was responsible for tunnelling.

The majority of the excavation of the headrace was undertaken uphill by TBM with the drill and blast method at the top section of the tunnel. The TBM was also used for the tailrace tunnel. Drill and blast construction was used for the cavern and aqueduct tunnels.

Tunnelling on the scheme began in 2006, got underway on the tailrace later that year and on the headrace in early 2007. By early 2008 the main excavation work on that tunnel had been completed. The power plant was commissioned and began operations less than a year later. However, by August 2009 the problem of the partial blockage of the headrace arose.

Repair works

Following the discovery of the partial blockage problem in the headrace, the upper reservoir was drained and investigations in late 2009 revealed that the collapse happened at the fault zone, located about two-thirds of the way up the tunnel and just over 2km from the reservoir intake at the top end. The fault zone is approximately 100m wide and the debris from the severe rockfall extended approximately 600m down the tunnel.

By early 2010 it was expected that the repair works could be completed and the power plant brought back online around a year later, by about the second quarter of 2011. However, it has taken a further year to execute the works that, in addition to the excavations, have involved a large amount of new tunnel lining work.

Excavation

The recovery plan called for excavation works to be undertaken from the bottom and top ends of the headrace.

From the bottom, a 550m, horseshoe-shaped tunnel was excavated by drill and blast from the existing access tunnel around the powerhouse cavern to gain access to the headrace just upstream. The repair contractor – BAM Nuttall – used the route to clear about half of the debris field.

Upstream of that section the blockage would remain all the way to the fault zone, and a massive 14m long concrete plug would be placed at the edge of the remaining blockage.

From the top end, the contractor excavated an approximately 900m long tunnel to cut through the fault zone and bypass the blockage. The new tunnel branched off from the headrace a little way upstream of the fault zone before passing through it and then rejoining the main tunnel at the cleared lower stretch of the debris field.

At the upstream end, in the remaining stub of headrace above the fault zone, next to the bifurcation with the bypass, a second massive 14m long concrete plug would be placed.

Bypassed, the damaged section of headrace would be isolated and sealed off permanently.

While undertaking the final stage preparations get the plant ready to go back into operation, SSE noted in its most recent financial report, covering the half year to 30 September 2011 and issued in November 2011, that restoration work was ‘progressing well’.

Following completion of the bypass tunnel around the collapse area, it said ‘the timely achievement of this major milestone’ meant the key focus was ‘on the repair of the defects identified in the rest of the tunnels.’

Lining

In the report for H1 of FY11-12, SSE added: ‘There has been an ongoing review and refinement of the design of the tunnel repairs to ensure the specification is optimal,’ results in the introduction of significant lining works to the recovery plan.

In addition to using rockbolts, half of the length of the entire tunnel (upper headrace, bypass, lower headrace and tailrace) has had a 100mm thick lining of steel mesh with shotcrete installed. This was done ‘where the rock structure has been found to require additional support,’ says SSE.

The upper bypass tunnel, at the fault zone, also had extra 500mm thick waterproof lining of shotcrete.

SSE said that a 100mm thick mesh and shotcrete lining was also built in the tailrace tunnel ‘to increase its durability and prevent erosion.’

The utility said that approximately 60,000 tonnes of shotcrete was used for the ‘extensive period’ of lining works.

Next steps

Following the extensive recovery works, and beyond the restart of the Glendoe station in the near future, it remains to be determined between the client and construction parties as to legal and financial responsibilities related to the collapse and recovery works.