Shani Wallis, TunnelTalk

Inspection has confirmed that the rock fall in the headrace of the Glendoe power station in Scotland presents a far more serious situation than originally believed. A news release by owner Scottish and Southern Energy (SSE) states that "...the fall of rock in the tunnel is very substantial..." giving rise to suggestions that a bypass tunnel of up to 200m maybe needed to fix the problem and "...a resumption of electricity generation at the site....not [taking] place until well into 2010 at the earliest."

Elevation of the waterway

Electricity generation by the new power station was halted in August when a drop in water pressure through the turbine warned of trouble and internal rock falls near the top of the 6.2km long headrace were identified as the cause of the pressure drop. TunnelTalk has since learned that an estimated 20,000 tonne of rock has broken into the tunnel and that debris lies in a reach of up to 7-800m of the 5m diameter, unlined TBM driven waterway.

Through conversations with those connected with the current inspection processes and with the initial construction phase, it is said that man-entry access into the fully dewatered system from the intake at the scheme's reservoir was only possible for about 2,000m until the rock fall completely blocked any further advance. Access from the downstream powerhouse end confirmed that rock debris was evident for several hundred meters into the tunnel with advance again soon blocked off.

A geologist, who worked on the original construction phase, said that there was no indication in the tunnel during the TBM drive to indicate any weakness or instability of the rock. "After TBM breakthrough, the tunnel stood open for quite a while before being watered up - to allow for stripping out the continuous conveyor system, the rail

Smooth walls of the TBM drive

tracks and other construction support services, and also for installation of the steel lining and the turbine and generator equipment at the powerhouse. Any indications of weakness or excessive stress relief would have been evident and would have initiated further investigation and study at the time."

One fact explained was that a planned shaft at the upper end of the headrace to capture intake from a small local stream was cancelled when the pilot hole for the proposed raisebore failed. The pilot drill for Intake 15 went down a troubled 200m or so before it hung up completely. The pilot was abandoned and the parties to the project's contracts agreed to cancel the approximately 250m deep intake.

This pilot hole and the zone of the rock falls are at a part of greatest cover over the long 6,000m inclined headrace TBM drive. A joint walk through inspection by the owner and the design-build contractor Hochtief, would have followed clear-out and wash-down of the tunnel before acceptance and handover of the completed facility. There were no significant sections of poorer rock conditions through the 6km long TBM drive, and only spot bolting and some mesh reported as being needed for occasional rock support. The high pressure water tunnel, through predicted good quality rock, is designed by Hochtief's engineer, the Energy & Hydropower Division of Pöyry, to remain unlined, except for the 300m of steel lining at the powerhouse end. At the end of the drive there was little cause to suspect unseen weaknesses or potential trouble. The Babtie division of Jacobs Engineering as SSE's project design engineer, completed the site investigation studies and was also supervising engineer through the construction phase.

Inspection before TBM breakthrough

An added consideration of the situation is the affect on the host rock of the watering up process to full hydraulic pressure and water flow (a maximum 608m head delivering about 18.62m3/sec of water at about 1.5m/sec velocity to the turbine) 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 of the exact quality and condition of the rock at the location of the rock failure and the exact causes of this embarrassing, costly and serious situation for a brand new hydroelectric power station project continue.

Since close down of the plant in August, various companies have become involved with the investigation, recovery and repair process. These include SSE's framework contractor AMCO, which made safe man-entry into the tunnel; an independent tunnel consultant engaged to provide an independent review of the situation for the client; a consultant engaged to provide advise to insurance companies that provided risk coverage for the project; and contractors invited to present offers for clearing the tunnel of the many thousand-tonne rock pile and undertake the repair, which could involve building a drill+blast bypass. As SSE admitted in its press release "a resumption of electricity generation at the site .... will not take place until well into 2010 at the earliest".

References

Glendoe rockfalls shut down - TunnelTalk, Aug 09