Contractor Bam Nuttall was this week poised to carry out critical repair work on the 100MW Glendoe hydro electric station in Scotland following a dramatic collapse in one of its vital tunnels last year.
The contract award, expected imminently, will put an end to months of speculation over the true extent of the damage in the scheme’s headrace tunnel, which its owner Scottish and Southern Energy (SSE) has been keeping a closely guarded secret since the disaster.
The collapse happened in the upper third of the 6.2km long headrace tunnel that carries water from the reservoir in the Monadhliath Mountains to the power house near the southern tip of Loch Ness.
Industry insiders tipped Bam Nuttall as favourite to win the £20M contract, which will involve drilling and blasting a new tunnel to bypass the affected portion of tunnel.
A 200m long Robbins tunnel boring machine (TBM) hired from Herrenknecht originally created the 5m diameter power tunnels. These comprise the headrace and tailrace tunnels, built in a continuous drive from near Loch Ness up to the reservoir.
“For whatever reason, the tunnel wasn’t able to cope with it - but whether it was a complete failure or a fracture, we don’t know”
The mostly unlined headrace tunnel falls more than 600m from the reservoir down to the power house. Steel fibre reinforced spray concrete was used in areas with the poorest rock class, concrete lining on the 230m approach to the power house and steel lining in the final 19m.
The new diversion tunnel - reported to be 200m long - will be created using drill and blast. “You can’t bring a TBM to create this kind of a curvy circuit,” said a source close to the project. Bam Nuttall sister firm Bam Ritchies will infill the blocked section of tunnel.
SSE has so far refused to comment on the cause of the collapse, but sources said ground conditions could have played a part.
“The tunnel went through an old fault,” the source told NCE. “For whatever reason, the tunnel wasn’t able to cope with it - but whether it was a complete failure or a fracture, we don’t know.”
Hochtief was design and build contractor on the project. During construction in 2007, the project team told NCE sister publication European Foundations that geology had posed problems.
Ground conditions comprised peat over glacial till on variable rock including quartzite, granite and mica schist among others.
One of the known certainties was the presence of a geological fault traversing the area of the power station.
As well as variable rock, the team also encountered unexpectedly poor ground in other parts of the project. The 7km long aqueduct tunnel that supplies the reservoir had to be relocated by up to 200m in places because of a lack of competent rock cover.
In other places of poor rock cover tunnellers had to switch from drill and blast to cut and cover.
SSE reported the tunnel collapse in August, just two weeks after its formal opening. At the time it said the plant would be operating again in 2010 “at the earliest”.
In November it confirmed the rock fall as “very substantial” with industry sources putting the volume of the blockage at up to 20,000t. SSE said it was “unlikely” that the plant would be up and running again before the 2011/12 financial year.
The £160M scheme has been billed as the first large-scale hydro station to be built in Scotland for over 50 years
The Queen officially opened the £160M scheme to great fanfare on 26 June last year. It was billed as the first large-scale hydro station to be built in Scotland for over 50 years and had already begun generating electricity after the project completed two months early.
SSE refused to comment further on the causes of collapse or the contract award. It said in a statement that it expected a “significant proportion” of the costs to be covered by contractual arrangements and insurance. Hochtief also had not commented as NCE went to press.