Glendoe recovery - TunnelTalk
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Recovery contract for failed headrace at Glendoe Feb 2010
Shani Wallis, TunnelTalk
Official award is still awaited, but BAM Nuttall is known to be closing out final negotiations for the emergency recovery contract of the failed Glendoe headrace tunnel in Scotland. A drill+blast heading of up to 200m long, or more by some accounts, is needed to bypass a collapse that allowed an estimated 20,000 tonne of rock to be drawn into the 5m diameter unlined waterway.
A dramatic drop in water pressure at the turbines of the commissioned system back in August of last year (2009) indicated trouble and caused shut down and dewatering of the system just eight months after it came on-line in December 2008. Man-entry inspection confirmed complete blockage of the waterway at about 2,000m from the upper reservoir intake of the 6.2km long headrace and with debris lying in a reach of up to 700m-800m on the down stream side of the 5m diameter unlined TBM drive.
p1

Drill+blast launch chamber of the original TBM drive with continuous conveyor muck hauling system

The 100MW project for hydropower generator Scottish and Southern Engergy (SSE) was constructed by Germany contractor Hochtief under a £150 million ($US 250 million) design-build contract with design engineer the Energy & Hydropower Division in Switzerland of Pöyry.
It was said that there was no indication in the tunnel during TBM excavation, or in the months after breakthrough and before commissioning of the completed works, of any weakness or instability of the rock. TunnelTalk did learn however that a planned shaft at the upper end of the headrace for intake of a small stream was cancelled when the pilot hole for the proposed raisebore failed. The pilot drill went down a troubled 200m or so before it hung up the 250m deep intake was abandoned. This pilot hole and the zone of the rock falls are at a part of greatest cover over the long 6,200m inclined headrace TBM drive. There were no significant sections of poorer rock conditions on the TBM drive alignment and only spot bolting and some mesh was reported as being needed for occasional rock support.
An added consideration of the situation is the affect on the host rock of the watering up process to full hydraulic pressure plus to water flow during operation and the subsequent dewatering process, which relieves the tunnel of that imposed water pressure. Much of the debris seen in the tunnel by man-entry inspection might have entered after dewatering of the waterway. Investigations into the exact causes of the failure continue. Part of the recovery bypass tunnel contract will be to backfill and shut off the failed section of the tunnel. The dimensions of the bypass tunnel, its distance parallel to the failed tunnel, the price of the contract, whether the contract is a design-build procurement, and whether the bypass is lined with an in-situ concrete lining are all questions that await answers. SSE has said in it 2010 interim management statement however that "it remains unlikely that any electricity will be generated at Glendoe until the financial year 2011/12". A report also quotes a spokesman of SSE as saying that "a significant proportion of the financial consequences of the situation at Glendoe will be covered by contractual arrangements and insurance provision."
BAM Nuttall in joint venture with AMEC and with Halcrow as contract designer was one of five prequalified contractors to compete for the original Glendoe construction contract and drill+blast was being considered for excavation of the long inclined headrace. The other four prequalifiers were Balfour Beatty; the Skanska-Morgan Est JV; Impregilo; and the successful bidder, Hochtief. It is unknown when the drill+blast recovery bypass tunnel will begin.
References
Rock falls shut down Glendoe power plant - TunnelTalk, Aug 2009
Glendoe rockfalls more serious than initial fears - TunnelTalk, Oct 2009

           

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